Acknowledgement

This thesis, performed at the Institut Charles Sadron (ICS, CNRS UPR 22) in the University of Strasbourg, was supervised by Dr. Jérôme Combet.

First and foremost, I would like to express my deepest gratitude to my supervisor, Jérôme Combet for believing in me and providing me this opportunity to learn, progress and to contribute to this research project. I'm thankful for his time and effort for the manuscript. I have greatly appreciated all his patient guidance and valuable advices not only for the scientific research but also for quotidian life, who supported me to accomplish this work throughout the three years.

Second, I wish to thank the jury members, Pr. Christophe CHASSENIEUX, Dr. Fabrice COUSIN, Dr. Natalie MALIKOVA, Pr. Sylvie FERLAY and Pr. Philippe M ÉSINI for taking their time to review my thesis.

Third, I'm also thankful to Pr. Philippe Mésini for his kind guidance and professional advices for the synthesis part of my thesis and for the time he spent to the correction of my manuscript. I have learned a lot about organic synthesis, polymerization, and analysis technics.

Then, my sincere thanks go to my colleagues at ICS for their contribution, support, training or aid on different instruments. I would like to thank M. Jean-Philippe Lamps for his training and advices for all the synthesis experiments and for the synthesis of poly(styrenesulfonate). I would like to thank Guillaume Fleith for his training and support for SAXS measurements, his help for SANS experiments at LLB and for lending me LEGO bricks. I want to thank Mélanie Legros and Catherine Foussat for all the SEC measurements; Leandro Jacomine and Wiebke Drenckhan for the training on the tensiometer and advices about surface tension measurements; Marc Schutz for the Cryo-TEM investigations; Delphine Chan-Seng for her advice on RAFT polymerization, Laurent Herrmann for the electronics of the irradiation devices, and Francois Courtier for the design of the SAXS sample holder. I would like to thank Michel Rawiso for his advices, his encouraging words and sharing his knowledge about everything.

My gratitude also goes to all the collaborators. I would like to thank Alexis Chennevière, Arnaud Helary, Fabrice Cousin and other colleagues at LLB for their help during SANS experiments and Mériem Er-Rafik at DTU (Denmark) for the CryoTEM measurements.

To my friends and colleagues at ICS, especially my coffee pals for their accompany and inspiration. I would like to thank especially my best friends Zheng LIU, Lu YU and Tongtong YANG since junior high school for their supporting and accompany. To finish, I express my sincere gratitude and appreciation for my parents who offer me this opportunity to study in France and my husband Da SHI, who has supported, encouraged and accompanied me to accomplish this thesis.

1

Résumé de thèse Chapter I. Introduction

Les matériaux "intelligents" sont des matériaux dont les propriétés peuvent être modifiées de manière significative par un stimulus externe (comme un stress mécanique, un champ électrique ou magnétique, la lumière, la température, le pH, etc.). Le développement de systèmes "intelligents" en solution aqueuse a suscité un grand enthousiasme pour leurs applications industrielles et académiques. Parmi les stimuli, la lumière présente un intérêt particulier car c'est une source propre et facile à manipuler.

Une grande variété de chromophores a été incorporée dans des molécules hydrosolubles conduisant à différents systèmes photosensibles pour lesquels les changements de morphologie, de propriétés interfaciales ou de propriétés en solution ont été largement étudiés.

Dans ce projet, nous nous intéressons aux solutions aqueuses de systèmes photostimulables modèles formés à partir de polymères chargés (polyélectrolytes, PEs) et de tensioactifs.

Les PEs sont des polymères qui possèdent des groupements ionisables. Dans un solvant polaire tel que l'eau, ces groupements se dissocient conduisant à l'apparition de polyions chargés et de contre-ions libres en solution. Les solutions de PEs combinent des propriétés caractéristiques des solutions de polymères et des électrolytes. Elles font l'objet d'études intensives tant au niveau fondamental qu'au niveau industriel. Elles présentent des propriétés physico-chimiques intéressantes, telles qu'une viscosité élevée, une capacité à l'auto-assemblage et la capacité d'interagir avec différentes (macro)molécules (tensioactif, protéine, ADN, PE). La combinaison de chromophores photoréactifs et de systèmes macromoléculaires à base de PEs conduit à de nouvelles organisations structurales en solution aqueuse dont les propriétés sont modifiables par simple irradiation.

Le chromophore azobenzène (Azo) est utilisé dans notre étude (Figure 1). Le groupement Azo présente une isomérisation trans → cis sous irradiation UV. Cette transition s'accompagne d'une diminution significative de la distance entre deux phényles, et de l'augmentation du moment dipolaire (de  0 à 3 D pour un groupement Azo sans substituant). Au-delà du simple aspect conformationnel, cette transition modifie également la balance hydrophile/hydrophobe dans nos systèmes. La transformation inverse (cis → trans) peut se produire par simple relaxation thermique dans l'obscurité, ou par photoisomérisation en utilisant une lumière bleue. Le premier système étudié concerne des chaînes de polyacrylate de sodium contenant des groupements Azo. Le chromophore est ici greffé sur des chaînes latérales du squelette macromoléculaire des polyions.

Le deuxième système est composé de tensioactifs contenant des groupements Azo (Azo-tensioactif) et de PEs « classiques » de charge opposée. Ce sujet est étroitement lié à la complexation tensioactif/PE, thématique que l'on retrouve de nombreuses formulations industrielles.

Enfin, les Azo-tensioactifs purs présentent également des propriétés d'autoassemblage extrêmement intéressantes. En présence de co-tensioactifs, de nouvelles organisations structurales peuvent se former. Le troisième système étudié est constitué de mélanges d'Azo-tensioactifs de co-tensioactifs et montre une variation réversible importante de la viscosité sous irradiation. D'un point de vue fondamental, ces systèmes peuvent également apporter un regard nouveau sur les solutions de PEs (transition hydrophile/hydrophobe, rôle des contre-ions, PEs associatifs…).

Chapter II. Polyélectrolytes Azo-modifiés

Les polymères hydrophobes et amphiphiles sont capables d'adopter une conformation particulière, mais aussi de s'auto-assembler pour former des particules colloïdales, des vésicules... La balance hydrophile/ hydrophobe est un paramètre clé pour expliquer la formation de ces structures. L'ajout d'un groupement Azo sur le PE (groupement latéral) offre la possibilité de modifier l'hydrophobie du système et de provoquer une réorganisation structurale par simple irradiation lumineuse.

II.2 Photoisomérisation

La photoconversion des (PAA-Azoy)n en solution aqueuse a été étudiée par spectroscopie d'absorption UV-Visible.

II.2.1 Rapport trans/cis

Le rapport trans/cis est un paramètre important qui influence l'organisation finale des

II.2.2 Réversibilité

Plusieurs cycles d'irradiation UV-Blue ont été appliqués sur des solutions d'Azopolymères afin de tester la réversibilité de la photoconversion. La variation de xtrans a été suivie par absorption UV-vis. Une bonne réversibilité après 3 cycles d'irradiation est observée.

II.2.3 Comparaison des spectres UV-Vis des Azo-polymères

II.3 Organisation structurale

Nous avons étudié l'organisation structurale des quatre Azo-polymères en solution par diffusion des rayons X aux petits angles (SAXS). Le poly(acrylate de sodium) NaPAA non modifié a également été étudié comme système de référence.

II.3.1 Organisation structurale des solutions de (PAA-Azo0.11)738RAFT :

chaînes longues, 𝝉azo = 0.11

De façon générale, les courbes de diffusion montrent un pic de corrélation à une position q* caractéristique des répulsions électrostatiques et de l'organisation des chaînes en solution. q* varie comme une loi de puissance avec la concentration, c α , où l'exposant α dépend de l'état de dispersion des polyions, ou plus généralement, des objets en solution.

Pour NaPAA738RAFT (sans groupement Azo), q* varie comme c 1/2 (Figure 5c, courbe noire) :

on est en régime semi-dilué (régime caractéristique d'un PE hydrophile en bon solvant).

La position du maximum est reliée à la maille du réseau enchevêtré (longueur de corrélation).

Les courbes de diffusion de (PAA-Azo0.11)738RAFT à différentes concentrations sont présentées sur la Figure 5 (dark : a, UV : b). La position du pic varie selon une loi de puissance avec la concentration (q*∝c α ) mais l'exposant α est maintenant égal à 0. Une diminution inattendue de l'intensité en q -2 est observée autour de 0.2 Å -1 . Cette valeur est proche de la diffusion observée dans les solutions de NaPAA pur. Elle indique clairement la présence de segments de PAA chargés dans le système. En d'autres termes, les particules denses n'incorporent pas tous les monomères de PAA : certaines parties des polyions restent en solution. Ce type de comportement se retrouve souvent avec des particules décorées de polymères ou avec des polymères ramifiés en étoile. Dans ces systèmes, des chaînes sont attachées à un coeur (core) et forment une couche plus ou moins dense en périphérie (shell). Ce phénomène est concevable avec nos PEs modifiés car ils sont fortement amphiphiles : certaines parties de chaînes (contenant les groupements Azo) peuvent se regrouper en un coeur hydrophobe tandis que des segments chargés restent en solution. Une couche (shell) formée d'un réseau de polyions (bouts de chaînes, boucles...) se forme alors autour d'un coeur (core) contenant les groupements photosensibles.

D'un point de vue diffusion, ce système peut être traité comme une particule « homogène » core-shell tant que les fluctuations de concentration du réseau de polymère (à l'intérieur du shell) sont négligeables (ce qui n'est généralement pas le cas à grand q).

En première approximation, ces fluctuations de concentration peuvent être prises en compte en rajoutant une contribution spécifique à l'intensité théorique prévue par un simple modèle core-shell. 1 En pratique, toutes les données ont été corrigées de cette contribution spécifique avant d'être analysées.

Nous avons analysé les courbes de diffusion (0.015 M -0.5 M) avec un modèle classique core-shell homogène (facteur de forme) combiné à un facteur de structure S(q) développé pour les colloïdes chargés dans un potentiel de Coulomb répulsif. 2,3 Nous pouvons anticiper que la densité de longueur de diffusion moyenne (SLD) de la couche sera proche de celle du solvant puisqu'elle est assimilable à une solution de PE semidiluée. Un exemple d'affinement est présenté dans la Figure 7 (dark, UV, c = 0.5 mol L -1 ). Nous pouvons maintenant donner une description schématique de la structure des particules (Figure 8). En absence de lumière (état dark), elles présentent un coeur hydrophobe dense de rayon environ 29 Å (contenant principalement les groupements Azo) entouré d'une couche (shell) formée par des segments de PAA chargés (solution semidiluée). La taille du coeur ne change pas avec la concentration (sauf pour la plus faible, 0.015 mol L -1 , où la statistique des courbes n'est plus très bonne). Ceci est cohérent avec les mesures Cryo-TEM qui montrent des particules d'un diamètre de l'ordre de 6 nm (le shell n'est pas visible par Cryo-TEM pour des raisons de contraste). Ce résultat important est en accord avec la variation de q* avec la concentration c observée précédemment. En effet, l'exposant de la loi de puissance (q*∝c α ) α ≈ 1/3 est caractéristique d'une simple loi de dilution, c'est-à-dire sans variation du nombre d'agrégation (i.e., du nombre de chaines formant les agrégats : le volume du coeur des particules est constant).

La couche externe des particules (shell) contient des segments de polyions (bouts de Il est donc possible par simple stimuli lumineux de contrôler la taille des particules en modifiant leur nombre d'agrégation (nombre de chaînes par particule). Ce nombre est toutefois assez limité est n'excède pas quelques unités (< 4).

II.3.2 Organisation structurale des solutions de PAA-Azo : chaînes courtes, 𝝉azo = 0.11

Pour les solutions de NaPAA non modifiés (NaPAA23sigma et NaPAA54RAFT), q* varie en c 1/3 : on est en régime dilué sur toute la gamme de concentrations. Les chaînes de NaPAA sont solubles à l'échelle moléculaire, q* est relié à la distance moyenne inter-chaînes.

Les courbes de diffusion et la variation de q* avec la concentration pour les PAA-Azo sont présentées sur la Figure 9. Contrairement aux chaînes longues, différents régimes en concentration (i.e. différent exposants  ) apparaissent. Comme les solutions de NaPAA non modifiés sont en régime dilué sur toute la gamme de concentrations, on peut raisonnablement supposer que c'est également le cas pour (PAA-Azo0.11)42Sigma. L'écart par rapport à une simple loi de dilution c 1/3 peut donc révéler une association (ou agrégation) entre polymères.

Pour les faibles concentrations ( = 1/3, régime I), les positions des pics de diffusion du PAA-Azo sont identiques à celles des PAA non modifiés (sans Azo). On observe la loi de dilution associée au régime dilué, les chaînes sont solubles à l'échelle moléculaire.

Pour les concentrations entre 0.03 et 0.1 mol L -1 (régime II), les positions q* de NaPAA sont superposables à celles de PAA-Azo sous UV : les chaînes modifiées sont toujours solubilisées à l'échelle moléculaire. En absence de lumière (dark), ce n'est plus le cas. Les valeurs de q* sont désormais plus faibles tandis que  < 1/3 : les chaînes commencent à s'agréger, on sonde des objets chargés de plus en plus distants. Ce régime est assez intéressant car un simple stimuli lumineux permet de « casser » les agrégats (irradiation UV) ou au contraire, de les créer (lumière bleue) de façon complètement réversible.

Pour les concentrations encore plus élevées (régime III), q* ne varie plus avec c. Nous sommes dans un régime où la distance inter-objets reste constante lorsque la concentration augmente : les polyions ajoutés contribuent à faire grossir les agrégats existants. Le profil de diffusion suggère une organisation étoilée (dark) partiellement modifiée sous UV. Pour les PEs en absence de lumière (dark), la variation de q* peut se décrire par une loi de puissance (q*∝c α ) avec un seul exposant α sur toute la gamme de concentrations (α = 0.25 et 0.23 pour (PAA-Azo0.15)39Sigma et (PAA-Azo0.15)54RAFT respectivement). Ce comportement correspond au régime II précédent, les chaînes sont agrégées.

Sous UV, deux régimes apparaissent. En dessous de 0.03 mol L -1 (comme pour (PAA-Azo0.11)42Sigma, Figure 9c), q* atteint les valeurs du NaPAA pur (régime I). Dans cette gamme de concentrations les agrégats peuvent être désassemblés de façon réversible par simple irradiation UV. Ce comportement est assez similaire à celui observé pour (PAA-Azo0.11)42Sigma, mais la gamme de concentrations est désormais beaucoup plus faible. Ceci est une conséquence directe de la teneur plus élevée en Azo.

Au-delà de 0.03 mol L -1 , q* suit toujours une loi de puissance avec des exposants de 0.12 ((PAA-Azo0.15)39Sigma) et 0.16 ((PAA-Azo0.15)54RAFT). Le maximum s'affine tandis qu'un épaulement bien défini apparaît autour de 0.15 Å -1 . Nous sommes en présence d'objets de type core-shell constitués d'un coeur formé par les groupements Azo et une couche constituée de segments de PAA en régime semi-dilué.

Chapter III. Complexes de Azo-tensioactifs et polyélectrolytes de charge opposée

Les complexes tensioactif/PE sont utilisés dans de très nombreux domaines tels que les peintures, la conservation des aliments, le contrôle rhéologique...

Les Azo-tensioactifs présentent des propriétés modulables sous irradiation. La combinaison des PEs et des Azo-tensioactifs offre de nouvelles possibilités pour contrôler l'organisation structurale des complexes à l'aide d'un simple stimulus lumineux.

III.1 Azo-tensioactifs

Les Azo-tensioactifs sont composés d'une tête polaire, d'un espaceur OR2, d'un groupement Azo et d'une queue alkyle R1 (Figure 10). Pour simplifier les notations, ils sont appelés xAzoyTAB, où x est le nombre de carbones de la queue alkyle R1 et y est le nombre de carbones de l'espaceur R2. La position du groupement Azo au sein du tensioactif varie suivant la combinaison (R1, R2). Dans ce travail, nous avons synthétisé deux Azotensioactifs : 4Azo2TAB et 8Azo2TAB. La concentration micellaire critique (CMC) est définie comme la concentration audessus de laquelle des micelles commencent à se former. Les CMC des Azo-tensioactifs ont été déterminées par tensiométrie (tensiomètre Kibron) à 21°C dans l'obscurité et sous irradiation UV. La CMC est estimée à 2.6 mM pour la forme trans-4Azo2TAB (dark) et à 8.2 mM pour la forme cis-4Azo2TAB (sous UV). Le 8Azo2TAB est plus hydrophobe, la CMC de la forme trans est de 0.29 mM tandis que celle de la forme cis est de 0.82 mM.

III.1.1 Photoisomérisation

La photoisomérisation des Azo-tensioactifs a été étudiée par absorption UV

III.1.2 Étude de l'organisation structurale

L'organisation des solutions micellaires a été étudiée par SAXS et SANS (Figure 11). Les isomères trans-8Azo2TAB (chaîne alkyle deux fois plus plus longue), s'autoassemblent en agrégats plus volumineux (Ra = 66 Å, Rb = 24 -28 Å). La taille des agrégats diminue de 70% sous UV : Ra = 31 Å, Rb = 22 Å.

III.2.1 Photoisomérisation

Deux types d'échantillons ont été étudiés : le système de référence de concentration C (4Azo2TAB/PSS, 50 mM/100 mM), et une solution très diluée (C/1000, 0.05 mM/0.1 mM).

La cinétique de photoisomerisation a été étudiée par spectroscopie d'absorption.

Pour la solution très diluée (C/1000), l'état photo-stationnaire sous UV est atteint après

III.2.2 Étude par RMN

La RMN a été utilisée pour confirmer l'association entre les Azo-tensioactifs et les chaînes de PSS (Figure 13a). Pour l'analyse des données, nous supposons que l'intensité diffusée est dominée par le facteur de forme du collier de perles P(q) (S(q) = 1) aux grands q ( q > q min avec q min = 0.05 Å -1 pour 4Azo2TAB/PSS et 0.04 Å -1 pour 8Azo2TAB/PSS). Le facteur de forme d'une telle organisation a été décrit dans le cadre des PEs hydrophobes. 6 Dans notre situation, nous utilisons une version simplifiée et négligeons la contribution des segments de polyions qui relient les perles (Schéma 3). Les perles sont quant à elles modélisées par un assemblage core-shell ellipsoidal. Les paramètres géométriques déterminés pour les deux conditions de contraste sont très similaires. Les polyions ne décorent pas seulement les micelles préexistantes, mais sont intimement mixés avec les groupements Azo au niveau du shell (co-micellisation). La taille globale de l'agrégat est plus petite que celle des micelles de tensioactifs purs (à concentration égale) malgré l'incorporation des polyions.

Sous irradiation UV, l'oscillation caractéristique dans la fonction de structure des polyions est déplacée vers les grands q : la taille de l'agrégat est réduite (Figure 15b et Figure 15d). Ce phénomène est également visible sur la fonction de structure des tensioactifs (D2O) et plus spécifiquement, sur l'épaulement associé aux corrélations interperles. Le déplacement aux grands q sous UV indique une diminution de la distance interperles.

Le volume des perles sous UV est réduit de 29%. Cette variation est cependant beaucoup plus faible que celle observée pour les micelles pures (69% pour les micelles 4Azo2TAB et 83% pour les micelles 8Azo2TAB).

III.2.4 Étude structurale par SAXS

L'analyse SAXS est similaire à celle réalisée par SANS. Le modèle reproduit relativement bien les données (Figure 16).

III.2.5 Étude par microscopie électronique (Cryo-TEM)

Des mesures de Cryo-TEM ont été effectuées sur les complexes Azo-surfactant/PSS (en absence de lumière) à une concentration de C/10 (Figure 17). Dans ces expériences seuls les rayons externes des perles (rayons du shell) sont potentiellement observables.

La structure interne des agrégats n'est pas accessible.

Pour 4Azo2TAB/PSS (Figure 17a), les objets sont très difficiles à observer. Tout au plus, on peut remarquer la présence de petits agrégats qui ne sont pas répartis de manière homogène dans le film et ont tendance à s'assembler entre eux (cercle de la Figure 17a).

Pour 8Azo2TAB/PSS (Figure 17b), des petites particules apparaissent clairement.

Elles ne sont pas réparties de façon homogène et forment des assemblages en colliers de perles. Cette observation valide définitivement notre approche. La taille des perles (≈ 7 nm) coïncide également avec les mesures par diffusion de rayonnement. Le nombre exact de perles est plus compliqué à déterminer et peut également varier d'un collier à l'autre.

Cet exemple met en évidence la complémentarité entre la microscopie électronique et la diffusion pour révéler l'organisation structurale dans les systèmes complexes.

IV.1.2 Études structurales

Les propriétés rhéologiques macroscopiques dépendent de la microstructure du système. La diffusion du rayonnement ainsi que la microscopie électronique sont des techniques complémentaires qui permettent de sonder l'organisation structurale à différentes échelles spatiales.

IV.1.3 Mesures Cryo-TEM

Des mesures de Cryo-TEM ont été effectuées sur 8Azo2TAB/NaSal C/4 (Figure 21

IV.1.4 SANS

Sur la Figure 22, nous présentons les mesures SANS réalisées à la concentration C/4.

En absence de lumière (dark), la longueur de persistance des micelles allongées (évaluée sur les images de Cryo-TEM) est supérieure aux distances caractéristiques observables par diffusion de neutrons : on sonde uniquement la structure des « segments » et leur terme de section. Aux faibles vecteurs de diffusion, l'intensité varie quasiment en q -1 . Ce comportement est caractéristique de particules en bâtonnet et est cohérent avec les observations de Cryo-TEM. On note cependant une petite contribution du facteur de structure dans cette région angulaire. Dans le domaine q > 0.03 Å -1 , nous sommes sensibles à la section des particules. L'absence d'oscillation caractéristique (ou de minimum) peut être liée à un problème de polydispersité ou à la présence d'une section elliptique. Un modèle cylindrique de section elliptique nous a semblé plus approprié et a été utilisé pour affiner les données.

Sous irradiation UV, nous constatons une forte diminution de l'intensité aux petits q indiquant une modification importante de la forme des particules. Curieusement, nous ne trouvons pas de régime de Guinier (plateau) aux petits angles. Ceci est lié à la forte polydispersité des objets suggérée par Cryo-TEM. La modélisation est donc assez compliquée. Les petits objets globulaires étant plus abondants, nous avons utilisé un modèle d'ellipsoïde de révolution homogène et restreint le domaine en q affiné. Le rayon majeur (suivant l'axe de révolution) a été fixé de façon arbitraire.

Les SLD des particules sont quasiment identiques aux valeurs théoriques attendues.

IV.2.2 Études structurales par SANS et SAXS

Seule la concentration C/4 a été mesurée par SANS (Figure 26) et SAXS (Figure 27).

Dans l'obscurité, l'intensité varie quasiment en q -1 aux plus faibles valeurs de q. Sous irradiation UV, on distingue désormais un régime de Guinier (la polydispersité observée avec 8Azo2TAB/NaSal semble avoir disparu). La présence d'un facteur de structure reste toujours un peu visible en dessous de q = 0.03 Å -1 .

Nous avons modélisé l'intensité avec un cylindre infini de section elliptique pour les mesures en absence de lumière (dark) et un ellipsoïde prolate pour les mesures sous UV (Figure 26). Nous avons considéré des particules homogènes pour la diffusion de neutrons et une organisation core-shell pour la diffusion de rayons-X (Figure 27). The second system is composed of Azo-containing surfactants (Azosurfactant) and oppositely charged PEs. Azo units are incorporated in the surfactants molecules and not grafted on PEs. This topic is closely related to that of surfactant/PE complexation which is often found in industrial formulations.

Pure Azo-surfactants also present interesting self-assembly properties. In the presence of co-surfactants, different organizations may be formed. The third photo-responsive system investigated in this work is made of Azo-surfactant/cosurfactant mixtures. Their solutions show particular rheological properties and an interesting reversible gel to fluid transition under light irradiation.

This thesis contains seven chapters. The first chapter introduces general concepts about polyelectrolytes.

The second chapter presents general characteristics of azobenzene and summarize works of azo-containing aqueous solution systems.

In the third chapter, we describe the synthesis and the characterization of different Azo-containing compounds as well as the samples preparation. The synthesis of azobenzene, Azo-surfactants and Azo-containing polymers are presented.

The fourth chapter discusses the problem of light sources, the photostationary state and all analytical technics.

In the next three chapters, we present the results on our three studied systems: Azo-containing sodium polyacrylates, Azo-surfactant/PE complexes and Azo-surfactant/co-surfactant systems. We aim at figuring out the structural organization of these systems, their photo-behaviors and the structural variation under light stimulus. Small-angle scattering technics (SAXS and SANS), NMR and The presence of electrostatic charges along the chain is at the origin of the water solubility and the stability of the solution. Many efforts have been done in the field of theoretical studies, simulations and experiments to describe PE systems. This is however challenging since many variables are involved and highly correlated. Theoretical approaches have been carried out starting from works of Katchalsky and Kuhn 1,2 , in which PE chains were described as rod-like charged entities. Scaling models were later introduced by de Gennes et al. 3 where PEs were modelized as an extended array of electrostatic blobs. For PE in poor solvent condition, Dobrynin et al. 4 proposed a necklace-like structure made of beads connected by narrow strings. Later, theoretical models were extended to more complicated situations such as PEs in semidilute regime, in the presence of multivalent ions or in addition of salts. Because of the particularity of each PE systems and the various parameters involved, the theories that describe the conformational and dynamic properties of charged polymer chains are quite diverse. They are well reviewed in an extensive literature [5][6][7][8][9] . Thanks to the scattering techniques, which are able to probe the structural organization, a lot of progress has been made [10][11][12][13][14][15][16][17] and a wealth of information on PEs organizations in dilute and semi-dilute solutions has been obtained. Other approaches by computer simulation have been developed to understand structural properties 18,19 , counterions distribution [20][21][22] and salt effect 23,24 . Some simulations has also helped to predict and support scattering experiments 25 .

Even if PEs are used in many fields, we still don't have a full comprehensive understanding of their solutions. The objective of this chapter is to give a general introduction on these systems. We will present briefly a classification for PEs, some basic definitions, and the usual models for the conformation of hydrophilic PEs. Then the hydrophobic PEs will be shortly described. The complexation of PEs and oppositely charged surfactants is an important subject of the thesis. The binding process and the structural organization will also be discussed.

I.1 Basics of polyelectrolytes I.1.1 Classification of polyelectrolytes

PEs can be classified in different ways. Based on the sign of the charge along the backbone, we may have polyanions or polycations. According to the acid-base properties of the ionizable groups, we can distinguish between strong and weak PEs. In analogy to strong acids and bases, strong PEs are fully ionized in solution.

The number of charges and their positions along the chain are fixed or ''quenched'' (pH-independent charge distribution). On the other side, weak polyacids or polybases can be partially ionized. Their ionisation degree varies with the pH and charges fluctuate along the chain (''annealed'' PEs).

A PE is said to be hydrophilic when its backbone is in a good solvent condition in water. When the skeleton is in poor solvent, we usually talk about hydrophobic PEs. The hydrophilic/hydrophobic character of a PE can be gradually varied by introducing hydrophilic/hydrophobic monomers along the backbone (copolymer).

The final structural organization will be highly dependent on the hydrophilic/hydrophobic balance in the systems. Partially sulfonated polystyrene (PSS) in which sulfonated styrene and neutral styrene are present is a good example for hydrophobic PE. NaPAA (sodium polyacrylate) is commonly referred as a hydrophilic PE. Ionene polymers represent a unique class of cationic polymers in which ionic sites are located on the skeletons and where the charge distribution is regular and can be carefully adjusted 26 .

The natural/synthetic origin also provides a way to classify PEs. Natural PEs include for instance polysaccharides (alginates, hyaluronic acid or hyaluronate, carrageenan, chitosan), polypeptides, DNA and proteins. The chemical structure of natural PEs is often very complex and often irregular (polydispersity, branching).

Synthetic PE are usually much simpler. Classical examples are poly(acrylic acid), poly(methacrylic acid), poly(acrylamide-2-methyl-propane sulfonate), poly(sodium styrene sulfonate) and polydiallyldimethylammonium chloride. Some PEs are given in Figure 1. Other classifications based on the stiffness (flexible, rigid), the architecture (linear, branched, crosslinked, pendant groups) and the composition (homopolymer, copolymer) can also be found in the literature.

In the present work, we are more especially interested in highly charged, linear, flexible synthetic polyelectrolytes (NaPAA and NaPSS).

I.1.2 Basic energy and length scales

PEs are composed of many repeating units. They can be obtained by polymerization of ionizable monomers or by post-functionalization of neutral polymers with ionizable groups. The size 𝑎 of monomer is in the order of 2.5 Å for vinyl polymers. The average number of monomers in the chain (degree of polymerization) is referred to N. The concentration of PEs is generally defined in term of monomer concentration c (mol L -1 ). The average distance between two successive charges along the PE chain is denoted 𝑙 . After ionization in polar solvent, polyions carry a chemical charge fraction 𝑓 ( 𝑓 = 𝑎/𝑙 ) which is also determined by the ratio of ionized monomers to the total number of monomers.

For highly charged PE, we may observe a counterion condensation phenomena leading to a reduction of the net charge along the chain. The chemical charge fraction is then replaced by an effective charge fraction feff.

Electrostatic interaction brings also a characteristic distance. Consider a pair of ions owning charges 𝑧 𝑖 𝑒 and 𝑧 𝑗 𝑒 separated by a distance 𝑑 𝑖𝑗 in a solvent with uniform dielectric constant 𝜖, the electrostatic interaction energy 𝑈 𝑖𝑗 is given by the Coulomb law as:

𝑈 𝑖𝑗 𝑘 𝐵 𝑇 = 𝑧 𝑖 𝑧 𝑗 𝑒 2 4𝜋𝜖 0 𝜖𝑘 𝐵 𝑇 1 𝑑 𝑖𝑗 (1)
where 𝑘 𝐵 is the Boltzmann constant, T the absolute temperature (in Kelvin), 𝑧 𝑖 and 𝑧 𝑗 the valency of the ions, 𝑒 the electronic charge, and 𝜖 0 is the vacuum permittivity.

We define the Bjerrum length 𝑙 𝐵 as the distance at which the electrostatic interaction energy between two elementary charges is comparable to the thermal agitation energy 𝑘 𝐵 𝑇. In standard units, 𝑙 𝐵 is given by:

𝑙 𝐵 ≡ 𝑒 2 4𝜋𝜖 0 𝜖𝑘 𝐵 𝑇 (2)
For oppositely charged ions separated by a distance shorter than 𝑙 𝐵 , the attraction between them is stronger than 𝑘 𝐵 𝑇 and the ions are more likely to be paired instead of completely dissociated. The surrounding solvent should be sufficiently polar to ionize PEs. For pure water at 25 °C, 𝜖 ≈ 78.4 so that 𝑙 𝐵 ≈ 7.14 Å

I.1.3 Electric screening: influence of the ionic strength

The range of the electrostatic interaction between ions is given by the Debye length 𝜅 𝐷 -1 , also known as the screening distance. To put it simply, at a distance smaller than 𝜅 𝐷 -1 , charges interact via an unscreened Coulomb potential. At larger distancesn interaction vanishes. 𝜅 𝐷 -1 can be written:

𝜅 𝐷 -1 = ( 1000𝑁 𝐴 𝑒 2 𝜖 0 𝜖𝑘 𝐵 𝑇 ∑ 𝑧 𝑖 2 𝑐 𝑖 𝑖 ) - 1 2 = (4𝜋𝑙 𝐵 𝐼) -1 2 (3)
Where 𝑐 𝑖 is the concentration of ions 𝑖 in mol L -1 . The term ∑ 𝑧 𝑖 2 𝑐 𝑖 corresponds to the ionic strength 𝐼. The Debye length is inversely proportional to the ionic strength: when the concentration of ions is increased, the electrostatic interactions are shielded, and the Debye length is reduced.

For monovalent salts in water (𝜖 ≈ 78.4, at 25 °C and 𝑙 𝐵 ≈ 7.14 Å ), the Debye length 𝜅 𝐷 -1 depends only on the salt concentration 𝑐 𝑠 :

𝜅 𝐷 -1 ≈ 0.304 √𝑐 𝑠 𝑛𝑚 (4)
For pure water (pH 7 without added salt), the self-dissociation leads to the presence of H + and OH -ions, with a concentration of 10 -7 mol L -1 . Thus, even in this ideal situation, the electrostatic interaction will be screened (approximatively over a distance of 700 nm). For significant salt concentrations (counterions or added salts), the Debye length varies from a few tens of nanometers to a few angstroms.

I.1.4 Persistence length

The persistence length 𝑙 𝑝 quantifies the stiffness of a polymer. On length scales shorter than 𝑙 𝑝 , the polymer behaves like a rigid rod. For charged polymers in solution, 𝑙 𝑝 is increased due to repulsive interaction between monomers. It is often considered that it can be written as the sum of an intrinsic contribution 𝑙 𝑝0

(persistence length in absence of charge) and an electrostatic contribution 𝑙 𝑝𝑒 .

𝑙 𝑝 = 𝑙 𝑝0 + 𝑙 𝑝𝑒 (5)
The first calculation was proposed by Odijk 27 , Skolnick and Fixman 28 . On the condition that 𝜅 𝐷 𝑙 𝑝 ≫ 1 and for a highly charged rod-like chain, the electrostatic persistence length 𝑙 𝑝𝑒 scales as 𝜅 𝐷 -2 . Therefore, the electrostatic persistence length can be significantly large for salt-free conditions (𝜅 𝐷 -1 → ∞).

𝑙 𝑝𝑒 = 1 4𝜅 𝐷 2 𝑙 𝐵 (6) This model considers the electrostatic interaction as a perturbation of the elastic free energy, it is therefore limited to semi-flexible or rigid PEs (𝑙 𝑝𝑒 ≪ 𝑙 𝑝 ).

Theories developed for flexible PEs showed that in absence of added salt, 𝑙 𝑝 varied as the Debye length 𝜅 𝐷 -1 . 29,30

I.2 Structural organization of hydrophilic polyelectrolytes in aqueous solution

I.2.1 Isolated polyelectrolytes chains

We first consider a PE chain as a connected sequence of charged and neutral monomers in a salt-free solution at infinite dilution. This simplified model helps to introduce some important concepts relative to PEs.

I.2.1.1 Strongly charged polyelectrolytes: Counterion condensation and effective charge

In the case of a highly charged PE (𝑓 → 1), counterions are supposed to be dissociated in aqueous solution. However, if the charge fraction is too high, the electrostatic potential around the chain is so large that part of these ions are trapped in the vicinity of the chains (Manning-Oosawa condensation [31][32][33] ). The chemical charge fraction 𝑓 has to be replaced by the effective charge 𝑓 𝑒𝑓𝑓 . If the distance 𝑙 between two chemical charges along the polyion is larger than 𝑧𝑙 𝐵 (𝑧 is the valency of the ions), that is to say 𝑓 is small, no condensation is observed: the final charge fraction 𝑓 is equal to the chemical charge fraction. If the distance between two consecutive charges 𝑙 along the polyion is lower than 𝑧𝑙 𝐵 , the counterion condensation takes place until the effective charge 𝑓 𝑒𝑓𝑓 is equal to:

𝑓 𝑒𝑓𝑓 = 𝑎 𝑧𝑙 𝐵 ( 7
)
The minimum distance between two effective charges along the macroion is then given by the Bjerrum length 𝑙 𝐵 for monovalent counterions, and 𝑧𝑙 𝐵 for multivalent counterions. As given in Figure 2, once Manning's condensation threshold is reached (f=a/zlB), an the increase in chemical charge fraction does not longer modify the net charge of PEs. we might expect a 'fully extended' conformation for PEs at very small concentration. However, since the swelling causes entropy loss, this is not favourable. Based on the Flory theory for neutral polymer chains, Katchalsky and coworkers 2 proposed the first model for charged polyions. A PE with N monomers and a charge fraction 𝑓 is described as a random coil with end-to-end distance 𝑅. The free energy 𝐸(𝑅) contains two contributions: the conformational entropy (in the Gaussian approximation) and the electrostatic energy.

𝐸(𝑅) = 𝑘 𝐵 𝑇 ( 𝑅 2 𝑁𝑎 2 + (𝑁𝑓) 2 𝑙 𝐵 𝑅 ) ( 8
)
The minimization of the free energy leads to:

𝑅~𝑁𝑓 2 3( 𝑙 𝐵 𝑎 2 ) 1 3 (9)
The end-to-end distance 𝑅 is proportional to 𝑁 suggesting that a flexible PE chain without salt would adopt an 'extended' conformation. A more sophisticated model based on the electrostatic blob concept was later introduced by de Gennes et al. 3 . PE chain can be viewed as a linear string of 'blobs'. Each blob is composed of a flexible subchain containing several monomers 𝑔. The chain inside the blob is not perturbed by the electrostatic interactions. Due to the electrostatic repulsion between blobs, they tend to align: a PE chain is elongated and form an 1D array of blobs with an overall size R (Figure 3).

Assuming that the electrostatic energy of each blob is comparable to the thermal energy 𝑘 𝐵 𝑇, the blob size 𝐷 𝑒 is given by:

𝑙 𝐵 (𝑓𝑔) 2 𝐷 𝑒 ~1 ( 10
)
Where 𝑓 is the fraction of charged monomers and 𝑔 is the number of monomers in a blob.

If the subchain in the blob can be considered as a Gaussian chain, 𝐷 𝑒 2 ~𝑔𝑙 2 .

Combining with eq (8), we have:

𝐷 𝑒 ~𝑏 ( 𝑓 2 𝑙 𝐵 𝑎 ) - 1 3 (11) 𝑔~( 𝑓 2 𝑙 𝐵 𝑎 ) - 2 3 ( 12
)
The total length 𝑅 of the PE chain is 𝑅~𝑁𝑓 𝜈 . The exponent 𝜈 depends on the solvent quality (𝜈 = 2/3 for θ solvent and 𝜈 = 4/7 for good solvent).

I.2.2 Chains in semidilute regime

So far, we only considered the conformation of isolated PE chains in salt-free dilute solution. When the concentration increases, the distance between chains decreases. At a certain concentration 𝑐 * , chains start to overlap and form a network of mesh size 𝜉. This critical concentration 𝑐 * can be estimated from the degree of polymerization and the typical volume of the chain (θ solvent):

𝑐 * ≈ 𝑎 -3 𝑁 -2 ( 𝑙 𝐵 𝑎 ) -1 𝑓 -2 (13)
To be concrete, let us consider a highly charged vinyl polymer chain (𝑎 = 2.5 Å) with an effective charge fraction of 0.35 in aqueous solution (𝑙 𝐵 = 7.14 Å). The calculated 𝑐 * values for different degree of polymerization N are listed in Table 1.

𝑁 100 1000 10000

𝑐 * (mol L -1 ) 3•10 -2 3•10 -4 3•10 -6
Table 1 Estimated critical overlap concentration 𝑐 * for chains of different lengths according to eq (13).

The critical concentration decreases strongly with the length of the PE chain.

Hence, most of the experiments with long PE are usually performed in a concentration beyond the overlap concentration c* (i.e., in the semi-dilute regime).

This semi-dilute regime has been described by de Gennes 3 and Dobrynin 4 as an isotropic entangled system as depicted in Figure 4. Such a system is characterized by a single correlation length 𝜉. 𝜉 represents both the mesh size of the network and the screening length of the electrostatic interaction.

For length scales smaller than 𝜉, electrostatic interactions dominate, each polyion segment behaves like a single chain in dilute regime (extended conformation of electrostatic blobs of size 𝐷 𝑒 ) . At distances larger than 𝜉 , the interactions are screened out and the chain behaves like a random walk of correlation blobs of size 𝜉 . The correlation length 𝜉 in a good solvent can be written as:

𝜉 = (𝑎𝑐 𝑝 ) - 1 2 ( 𝑙 𝐵 𝑎 ) - 1 7 𝑓 -2 7 ( 14
)
Many experimental works agree with this scaling law 11,15,17,34 and therefore to the isotropic model. If the concentration increases even further, the network is more compact, and we reach the concentrated regime. This range is out of the scope of our investigation and won't be presented here. In small-angle scattering measurements, PEs solutions present a broad peak at a scattering vector 𝑞 * . This maximum originates from the electrostatic interaction between polyions. It disappears when salt is added due to screening effects 35 . The position of the peak varies with the concentration according to different scaling laws 𝑞 * ∝ 𝑐 𝛼 . In dilute regime (𝑐 < 𝑐 * ), 𝑞 * varies as c 1/3 and is related to the distance between the center of mass of the polyions (𝑞 * = 2𝜋/𝑑). In semidilute regime (𝑐 > 𝑐 * ), chains are entangled and form an overlapping network characterized by a mesh size 𝜉. 𝑞 * now varies as c 1/2 and probes the mesh size 𝜉 (𝑞 * = 2𝜋/𝜉).

I.3 Hydrophobic Polyelectrolytes

I.3.1 Definition and examples

PEs are called 'hydrophobic' when the backbone or parts of the monomers are in poor solvent condition. The water solubility and the structural organization of the chain result from the balance between the electrostatic repulsions (that tend to stretch the polyions and stabilize the solution) and the hydrophobic interactions (that leads to their collapse). Hydrophobic PEs can be classified according to their architecture (Figure 5). Ionic groups can be introduced as pendant groups onto hydrophobic monomers of the main chain. A classic example is the polystyrene sulfonate obtained by partial sulfonation of a polystyrene chain 36 . Water is a bad solvent for styrene but it is a good solvent for styrene sulfonate. In that case, changing the sulfonation degree does not only modify the charge fraction (and the electrostatic interaction) but also the hydrophobicity of the sample. Similarly, based on an ionic backbone, a low fraction of hydrophobic groups can be added onto the sidechains or the two extremities (hydrophobically modified polyelectrolyte). The most studied family 37,38 is a derivative of poly(sodium acrylate), on which hydrophobic moieties can be readily grafted. If the fraction of hydrophobic groups is high, they are usually named 'polysoap'. This situation will correspond to one of our model systems.

The notion of amphiphilic PEs often appears when we talk about hydrophobic PEs, due to the coexistence of hydrophilic and hydrophobic parts. We also hear about associative PEs referring to amphiphilic polymers that can form supramolecular assemblies (micelles, lamellar phases, gels). We should note that it may be sometimes difficult to find which category the hydrophobic PEs belong to since there are no sharp boundaries among the different classifications.

In this present work, one basic idea was to modulate the hydrophobicity of

I.3.2 Conformation of hydrophobic polyelectrolyte

I.3.2.1 Necklace conformation in dilute solutions

Hydrophobic PEs but also, the hydrophilic/hydrophobic transition in these systems are still very active field of study, both on a theoretical and experimental level. The hydrophobic interaction in hydrophobic PE favours the collapse of the chain into globule, while the electrostatic repulsion between charged groups tends to stretch the polyion and improves its solubility in water. The conformation of PE chains in poor solvent results from the balance between electrostatic and hydrophobic interactions. The reflection on the shape of hydrophobic PEs was inspired from the problem of charged droplet by Rayleigh 40 . A charged droplet is unstable beyond a critical charge (> 𝑒(𝛾𝑅 3 /𝑘𝑇𝑙 𝐵 ) 1/2 , where 𝛾 is the surface tension) and breaks into smaller droplets, which are in equilibrium at a finite distance. The instability appears when the electrostatic energy is comparable to the surface tension energy. For PEs, we are in a very similar situation. We can imagine a neutral polymer in bad solvent condition which is therefore collapsed (droplet). The question is what happens when charges are added along the chain.

It is impossible to tear apart the polyions due to the chemical bonds. But the chain can minimize its free energy by adopting a particular shape made of small globules connected by narrow strings: the necklace conformation. This model was first proposed by Kantor and Kardar 41 to describe polyampholytes chains (random sequences of positive and negatively charged monomers). Then it was applied to hydrophobic PE by Dobrynin and coworkers 42,43 .

Consider a hydrophobic polyelectrolyte chain with 𝑁 monomers of size 𝑎 and a charge fraction 𝑓 in poor solvent of dielectric constant 𝜖 . The reduced temperature is 𝜏 = (Θ -𝑇)/Θ, where Θ is the theta temperature of the polymersolvent system.

When the charge fraction 𝑓 is higher than (𝜏𝑎/𝑙 𝐵 𝑁) 1/2 , the chain turns into a necklace conformation of 𝑁 𝑏𝑒𝑎𝑑 beads of size 𝐷 𝑏 and 𝑁 𝑏𝑒𝑎𝑑 -1 strings as schematized in Figure 6. Just above the critical overlap concentration, an isotropic network is formed and described by a correlation length 𝜉.

𝜉 ≈ 𝑎𝑔 𝑏 1 4( 𝑐𝑎 3 ) -1 2 (15)
For length scales smaller than 𝜉, the segment behaves like a single necklace chain in dilute solution. At a distance larger than 𝜉, chains adopt a random walk conformation. This concentration range corresponds to the string-controlled regime (Figure 7 a). The correlation length 𝜉 decreases as 𝑐 -1/2 until it is smaller than the string length 𝑙 𝑠𝑡𝑟 . A threshold concentration 𝑐 𝑏 is defined when these two lengths have the same order of magnitude (𝜉 ≈ 𝑙 𝑠𝑡𝑟 ).

For concentrations 𝑐 > 𝑐 𝑏 , the electric repulsion between beads is screened, only one bead is present per correlation volume 𝜉 3 . We suppose that most of the polymer mass is located in the beads. The correlation length is then inversely

proportional to 𝑐 1/3 : 𝜉 ≈ ( 𝑎𝜏 𝑙 𝐵 𝑓 2 ) 1 3 𝑐 -1 3 ≈ ( 𝑔 𝑏 𝑐 ) 1 3 ( 16
)
This regime is referred to the bead-controlled regime (Figure 7 b). This hydrophilic/hydrophobic transition therefore modifies both the chain conformation (the size of the bead is changing), but also the dispersion state of the polyions in semi-dilute regime (in the string-controlled regime 𝜉 ∝ 𝑐 -1/2 , 𝜉 > 𝑙 𝑠𝑡𝑟 while in the bead-controlled regime 𝜉 ∝ 𝑐 -1/3 , 𝜉 < 𝑙 𝑠𝑡𝑟 ).

The pearl-necklace configuration of this theoretical description is quite difficult to evidence experimentally. Scattering experiments are well suited techniques to study the structural organization in these systems. Since the transition modifies the dispersion state, characteristic variations should be found for the variation 𝑞 * = 𝑓(𝑐) . Since 𝑞 * = 2𝜋/𝜉 , one should find 𝑞 * ∝ 𝑐 1/2 in the string-controlled regime and 𝑞 * ∝ 𝑐 1/3 in the bead-controlled regime.

Different concentration regimes were indeed found in partially sulfonated poly(styrenesulfonate) [44][45][46] . If 𝑞 * still varies as 𝑐 𝛼 , now 𝛼 (in between 1/2 and 1/3) depends on the hydrophobicity of the sample. Form factor measurements can also be measured and compared to theoretical model 47 . These experiments evidenced the formation of string and globules. However, these kinds of investigation are very rare.

One idea behind this thesis was to trigger the transition by simple UV stimulus (that changes the hydrophilic/hydrophobic balance in the azo-containing PEs), without changing the chemistry of the polyions.

I.4 Surfactant/Polyelectrolyte complexes

Complexes can be formed readily between polyelectrolytes (PEs) and oppositely charged surfactants. Surfactants are known to modify the surface tension, the phase behaviour and the rheology of a solution 48,49 . The addition of PEs has been proved to enhance further the bulk and interfacial properties leading to enormous potential applications. Various combinations of PEs and surfactants result in unique behaviours and structural arrangements due to the formation of surfactant/PE complexes. We present here a brief summary on their formation and their structural organization in solution.

I.4.1 Formation of complexes

Binding of surfactants to PEs chains is the first step leading to the formation From a thermodynamic point of view, the complexation between a polyion and an oppositely charged surfactant is entropically favoured 51 . The positive contribution to the entropy variation due to the release of counterions is more important than the negative contribution from the loss of configurational entropy of PEs. The enthalpy change can be positive or negative depending on the charge density of PEs and the salt concentration. The complexation process is generally endothermic for highly charged PEs and exothermic for weakly charged PEs 52 .

The addition of salts weakens the electrostatic interactions and reduces the enthalpy of the binding process. Thus, the complexation may become endothermic at high ionic strength 52,53 .

The complexation process depends on the surfactant concentration as illustrated in Figure 9. In a salt-free solution, at low surfactant concentrations, the charged polar head of surfactant is attracted electrostatically by the PE chain. With increasing the surfactant concentration, the CAC is reached. It corresponds to the point where the PE associates with the surfactant to a measurable degree. In general, the CAC value is a few orders of magnitude lower than CMC of the pure surfactant solution. Above the CAC, PEs start to form aggregates with the surfactants. One PE chain may interact with many micellar aggregates or inversely one micelle may interact with many chains. When the molar ratio reaches 1:1, charges of the polyions are neutralized by that of surfactants. It corresponds to the charge neutralization concentration (CNC). The stoichiometric mixture of PEs and surfactants usually leads to phase separation forming precipitates or ordered structures. In some systems, increasing further the surfactant concentration may allow the re-solubilization of the complex and the formation of micelles if the concentration of free surfactants in solution is higher than CMC. PE/surfactant complexes display a rather complicated phase behaviour, which depends largely on the choice of PE and surfactant. The investigation of phase behaviour has been well reviewed in many articles 51,54,55 .

I.4.2 Structural organization of the complexes

The structure of surfactant/PE complexes and their phase behaviour result from the balance between electrostatic and hydrophobic interactions. It also depends on the intrinsic properties of PEs (stiffness of the polyion, charge density, degree of polymerization) and surfactants (length of the hydrophobic tail, critical packing parameter, number of charged groups) 50,56,57 . The complexes are also responsive to chemical environment such as the ionic strength, the pH of the solution (weak PE) or the addition of co-solvent [58][59][60] .

Several typical structures are illustrated in Figure 10. For weakly charged PEs, the system is mainly influenced by hydrophobic interactions, the behaviour is similar to the association of surfactants with neutral polymers. The hydrophobic tail of surfactant has a tendency to interact with PE backbone. We may observe a bottle-brush conformation. We can also find micelle-PE complexes associated in a pearl-necklace structure in which the polyions can penetrate inside the micellar aggregates. Depending on the charge density of PE, the micelles will be densely packed together or separately distributed along the polyion. For more hydrophobic PEs, spherical or worm-like micelles may be formed with the PEs incorporated in the hydrophobic core 61 . When the charge density is higher, the complexation process is dominated by electrostatic interactions. The necklace structure is formed on the condition that the PE chains decorate the micelles rather than penetrating into the core 58 . The stretch extent depends on the flexibility of PE chains. In the case of more rigid PEs or block copolymers, it's possible to obtain a bilayer like structure 62 .

Even if the necklace organization is a globally accepted structure, experiments proving its existence are rather rare. In this thesis, we intend to study the complexes with surfactant of variable hydrophobicity (and packing parameter) by small-angle scattering. We plan to evidence the structural organization of both components separately and to follow the variations triggered by UV irradiation.

Chapter II. The state of the art of "smart" photosensitive systems in aqueous solution

Chromophores are usually conjugated systems that absorb light. Under a suitable light source irradiation, photoactive molecules can undergo photoisomerization (i.e., stilbene 1,2 , azobenzene 3 ), photocleavage (i.e., onitrobenzyl ester 4,5 , coumarin 6 ) or ring opening/closing reaction (i.e., dithienylethene 7,8 , spiropyran [9][10][11] ) in a reversible or irreversible way.

In this chapter, we will focus on azobenzene derivatives. We will give a general presentation of the structural and physical properties, as well as their response to light irradiation. Remarkable works of these photo-tunable systems will be summarized.

II.1 Azobenzene

II.1.1 Overview of azobenzene properties

Azobenzene (Azo) is an extensively studied chromophore showing a very efficient photochemical isomerization. Azo family was firstly discovered as commercial synthetic colouring agents in 1800s and used in textile, printing and paper manufacturing 12 . In 1937, Hartley and coworkers observed for the first time the trans ⇌ cis photoisomerization of azobenzene and could successfully isolate the cis isomer 13 . Since then, the photochemistry of the Azo family has gained great attention and a large variety of azo-containing photoresponsive systems have been developed. The trans/cis photoconversion is a totally reversible process, triggered simply by a light stimulus within the large absorption band of the trans configuration. Azo, is a robust molecule, which shows a remarkable photostability even after prolonged irradiation. Azo compounds can be synthesized from the well-known azo-coupling reaction, including the first diazotization of primary aromatic amine to form its diazonium salt and then the coupling reaction between the diazonium salt and another aromatic compound (Scheme 1). Methods of Azo family synthesis and their functionalization have been well developed and described in a large literature [14][15][16] .

Scheme 1 Synthesis route of azobenzene.

The thermal stable trans conformation adopts a planar structure with a nearzero dipole moment (Figure 1a).

II.1.2 Isomerization mechanism

The mechanism of photoisomerization has been a controversial subject [18][19][20][21] .

Two main mechanisms -rotation and inversion-have been proposed (Scheme 2).

The rotation pathway begins with the rupture of the N=N double bond allowing a free rotation around the N-N single bound. The C-N-N-C dihedral angle is changed by an out of plane torsion, whereas the N-N-C angle remains fixed at about 120°.

Finally, the reformation of the double N=N bond leads to the cis isomer. 19 For the inversion mechanism, the isomerization occurs via an in-plane

II.1.3 Effect of the substitution on azobenzene isomerization

The absorption band (and therefore, the optimal wavelengths for photoisomerization) can be strongly influenced by the substituents on the phenyl rings. An electron donor group at the ortho or para positions of the benzene rings can lead to a red-shift (i.e., longer wavelength) of the absorption spectra of trans-Azo 22 . This effect is highly desirable for photo-switch biological applications where long wavelengths are needed for the irradiation 17,[22][23][24][25] . The red-shift can be even more important with an acceptor group at one para position and a donor group at the other para position (push-pull type Azo) 26 . Because of their polarized structure, the absorption properties of 'push-pull' Azo are also solvent dependent 24 .

The ring-substituents also affects the lifetime of the metastable cis isomer (generally on the order of hours). Introducing electron-donor or 'push-pull' type groups can lower the thermal energy barrier of the trans ⇌ cis photoconversion and thus accelerate the rate of thermal isomerization. Fast thermal relaxation means that it is difficult to maintain a steady fraction of cis isomer for a long period of time. The Woolley group have designed a series of red-shifted Azo compounds 22,27,28 , presented in Figure 2. The para-substituted Azo compound ( 1) and ( 2) shifts the maximum absorbance to 382 nm and 450 nm respectively with a half-life on the order of seconds. The para-and ortho-substituted ( 3) is further red-shifted, but the lifetime of its cis isomer is much shortened. The incorporation of bulky substituent or more generally, the introduction of conformational strain on the macrocyclic compounds can increase the thermal energy barrier for the back-isomerization and hence, improve the stability of the cis-form. An azo derivative with para-substituted bulky groups created by Shirota et al. 29 has a lifetime of 60 days. Rau et al. 30 reported an azobenzenophanes with three-carbon bridges for which the cis-form is stable at room temperature with a lifetime of about a year.

II.1.4 Photostationary state

After a certain irradiation time, Azo containing systems (bulk or solution) reach a photostationary state. In this equilibrium state, the rates of the photoinduced trans-to-cis isomerization is identical to the rate of photo-and heatinduced cis-to-trans isomerization. The composition in cis and trans isomers remains constant. The UV-Vis absorption spectroscopy and NMR are common technics to determine the trans/cis fraction in the stationary state.

The trans ⇆ cis isomerization is always a dynamic equilibrium. For most of Azo derivatives, the photoconversion under light irradiation will never lead to a final state containing 100% cis-Azo or 100% trans-Azo. The composition at the steady state depends on the excitation properties (wavelength and intensity) but also on the absorption spectra of the compound 31,32 . Light sources with a narrow wavelength distribution minimize the inverse conversion caused by undesired wavelengths, are used preferably. For classical Azo compounds, the stationary state contains predominantly the cis isomers under UV irradiation. For red-shifted Azo derivatives, the absorption spectra of cis and trans isomers overlap, the stationary state is a mixture that may contain more or less cis isomers. In some cases, a single wavelength can promote both the forward and the reverse photoreactions, leading to a continual interconversion between the two isomers.

Tabor et al. 33 reported that the concentration of Azo compounds also affects the trans/cis ratio at the stationary state. He found that the kinetic of trans ↔ cis conversion was not first order, suggesting a diffusion limited process. The penetration of irradiation light may be limited by the strong absorption of the solution, which results in a rapid isomerisation of uppermost Azo molecules, followed by a diffusion-limited mixing and photoconversion of the remaining Azo molecules. This situation is very important in this thesis, since we usually work on thick samples of large absorption. The typical time needed to reach the stationary state will highly depend on the diffusion coefficient of the Azo molecules (or Azo containing particles).

Since trans isomer is a thermally stable isomer, the fraction of cis isomers in the dark state is lower than most detection limits. Thus, without visible light irradiation, the thermal cis-to-trans relaxation can lead to an all-trans steady state.

A sample kept in dark for long enough time can be considered as composed of 100% trans isomers.

II.2 The state of the art of azobenzene based materials

Azo derivatives show a large flexibility and versatility in molecular design. Azo moiety can be inserted into a matrix, attached by covalent bond or mixed in a noncovalent way (electrostatic, hydrophobic or π-π interactions). Many Azo derivatives have been used in different types of materials to provoke a photosensitive response: polymer systems 34 , liquid crystals 35 , colloidal assemblies 36,37 , layer-by-layer thin films. Notable achievements have been done in biological systems in which Azo compounds were used as light-triggered ligands or cross-linkers and allowed the control of the structure and the function of peptides, proteins, DNA etc 17,38 . In this work, we will exclusively focus on photoresponsive systems in aqueous solution, including Azo-containing macromolecules, Azo-containing surfactants and Azo-surfactant/Polyelectrolyte systems.

II.2.1 Amphiphilic photoresponsive polymers

Amphiphilic polymers are bearing both hydrophilic and hydrophobic units.

The hydrophilic substituent can be charged, ionisable or neutral. Typically, sulfonate, pyridyl, carboxyl, hydroxyl and amino groups are used as hydrophilic group. Water soluble poly(ethylene glycol) (PEG) can be used for the polymer design as well. A large variety of architectures can be obtained by varying the polymer architecture, the monomer structure, the type and ratio of hydrophobic substituent, the molecular weight of polymer. Owing to their amphiphilic properties, amphiphilic polymers are widely used both in daily living and industry as rheology modifiers, adhesives, stabilizers and suspending agents for drug delivery systems.

Azo-containing amphiphilic polymers (Azo-polymers) have gained an increasing attention for their excellent photo-responsive ability. The photoisomerization from trans-apolar isomer to cis-polar isomer may affect the polymer structure leading to large modifications such as phase transition, the swelling/shrinking of polymer, micelles deformation. We introduce hereafter some typical examples. The presentation of synthesis strategies for amphiphilic Azopolymers can be found in Appendix d).

Amphiphilic Azo-polymers is widely used to prepare multilayer films through a layer-by-layer process. Highly light sensitive hollow capsules were prepared by multilayer films, using an alternative deposition of poly[1-[4-(3-carboxy-4hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) and polydiallyldimethylammonium chloride (PDADMAC) on silicon dioxide templates 39 .

After removing the template (using 0.2 M NH4F and HF buffer solution at pH = 4.5), the capsules were illuminated with UV radiation. The AZO moieties in the multilayers tend to self-organize in the form of J-aggregates. The re-orientation of Azo moieties within shell formations destroyed progressively the integrity of capsule until the final breakage (Figure 3). This UV induced capsule breakage offered a new way to modulate the release of encapsulated drugs. Colloidal particles can be constructed from random amphiphilic Azo-polymers in selective solvents. The general preparation process consists of dissolving the polymer in an organic solvent and adding gradually water in the polymer solution.

By increasing the water content, the solubility of polymers in the mixed aqueousorganic dispersion media is decreased, which promotes its gradual aggregation 40 .

For example, the random copolymer shown in Figure 4, can form micro-particles in aqueous media 41 . The formation of colloidal particles was provoked by gradual addition of different NaOH solutions of different concentrations to the THF copolymer solutions. They found that the particle size depends on the Azo molar fraction in the copolymers and the preparation conditions (such as the nature and the evaporation rate of the solvents). However, they did not investigate the structural modifications under UV irradiation. Li et al. 42,43 reported that uniform colloidal spheres could be formed from polyacrylic acid based random Azo-polymer (Figure 5, R = H) in THF/H2O mixed solution. Aggregates started to form above a critical water content (25%), and the colloids grew as the water content increased (Figure 6). The photoisomerization behavior and the morphology modification upon UV irradiation were investigated.

Aggregates totally disrupted when the water content was below 37.5 vol%. For higher values (50-80 vol%), colloidal spheres could be partially disaggregated by UV light irradiation. A quite similar amphiphilic Azo-polymer shown in Figure 5 (with R = OCH2CH3), was found to form hollow spheres by mixing polymer aqueous solutions (in THF/water media (v:v = 1:1)), and slowly evaporating THF 44 . Factors such as the polymer initial concentration in THF, water addition rate, and diluent volume affected the size and shell thickness of the hollow spheres. "Vesicle-like"

spheres underwent an obvious morphological transition when exposed to the UV light (Figure 7). Diblock and triblock Azo-polymers have gained an increasing attention due to their well-controlled structure and the wide morphological possibilities. More precisely the size and the form of the self-assemblies can be varied by the nature and the length of the blocks as well as the nature of the solvent (and co-solvent).

The preparation process is also an important factor that influences the final morphology of the system. As an example, diblock copolymer PAzoMA-b-PAA (Figure 8) can form micellar aggregates in H2O/dioxane solvent 45 . The morphology (vesicles or micelles) depends on the water fraction. The photoisomerization of Azo units can lead to the deformation and the fusion of the vesicles under UV light illumination (Figure 9). Finally, random amphiphilic Azo-polymers can also be used as a lighttriggered modulator in complex systems with surfactants and proteins. When mixed with Bovine Serum Albumin (BSA), random amphiphilic Azo-polymers (Figure 10) form complexes with photoresponsive properties 46 . Reversible light triggered association is observed in water solution leading to large variations of the viscosity. The affinity of BSA for the polymer depends both the length of the hydrophobic spacer on the azobenzene and the presence of additional n-alkyl side groups. The light-responsive association/dissociation of complexes between neutral surfactant (TX100) and random Azo-modified polymers derived from polyacrylic acid was investigated (Figure 11) 47 . The binding isotherms of TX100 were determined by capillary electrophoresis. Micelle-like clusters were formed due to hydrophobic interactions between Azo-containing side groups and micelles of TX100 (above the CMC) as shown in Figure 12. It is proposed that some Azo groups may be anchored in the TX100 micelles. This process depends on the distance between adjacent chromophores and is controlled by the free energy of created loop. Exposure to UV light decreases the free-energy of Azo association and thus Azo-anchors detach from TX100 micelles. The maximum response was obtained when the molar ratio of bound azobenzene to bound surfactant is low, most typically below 20:100. CmAzoCnTAB is the most studied structure of Azo-surfactants (Figure 13). The relation between the total length of the hydrophobic segment (R1 + R2) and the CMC value was investigated by Hayashita et al. 48 (moderate total length, 4 ≤ R1+R2 ≤ 8) and Blayo et al. 36 (from moderate to long total length, 8 ≤ R1+R2 ≤ 14).

The driving force for the micellization is the presence of hydrophobic segment.

Increasing the tail or the spacer length can decrease the CMC. Additionally, the tail length has a stronger influence on the CMC than the spacer. For a fixed total hydrophobic length, a longer tail leads to a lower CMC.

Under UV irradiation, most of the Azo adopt a cis conformation, the surfactants become more hydrophilic (while the CPP is changed). Consequently, the CMC of the cis conformation (CMCcis) is always higher than that of the trans state (CMCtrans). The difference between the CMC of trans and cis isomers (ΔCMC) is maximum for a total hydrophobic length of 6 carbons (ΔCMC = 5.9 mmol L -1 for C2AzoC4TAB). ΔCMC represents the range of concentrations in which micelles are formed only for the trans isomers but also, where micelles are disassociated into single surfactant molecules under UV light exposure. A large ΔCMC range may be especially interesting for the reversible release of drug or reversible encapsulation. The size and shape of various cationic azo-surfactants micelles above the CMC have been measured by SANS and reported on different systems 36,[49][50][51] . Blayo et al. 36 Shang et al. 53 investigated a class of nonionic photoresponsive surfactants composed of a polar di(ethyleneoxide) head and a spacer varying between 2 and 8 methylene groups (C4AzoCnE2) as given in Figure 15. For the trans-form, they found a linear decrease of the CMCtrans when increasing the spacer length from two to six methylene groups. The variation of the CMC cis is not monotonic as in the case of charged Azo-surfactants. For the cis-form, the CMCcis of C4AzoC2E2 is three times larger than that of C4AzoC4E2. However, a further increase of spacer length to 6 CH2 results in a decrease of the CMCcis value.

For surfactants with 8 CH2 spacers, the hydrophobic interaction predominates resulting in immediate aggregation and precipitation in the trans-form.

The maximum ∆CMC was 21.1 µM with C4AzoC4E2. And the largest surface tension modification was found with four CH2 and six CH2, approximately 10 mM m -1 . The structure of the aggregates formed by C4AzoC4E2 and C4AzoC6E2 was further investigated. The trans-C4AzoC4E2 and trans-C4AzoC6E2 self-assemble into bilamellar and unilamellar vesicles respectively, while both cis isomers form a bicontinuous phases under UV light 37 .

Figure 15 Structure of nonionic Azo-surfactant C4AzoCnE2.

Tabor et al. 33 have designed three photoswitchable carbohydrate-based surfactants (Figure 16). Structural modifications under radiation were monitored in situ by time-resolved small-angle neutron scattering. The micelle morphology and the aggregation number depend on surfactant concentration. At a concentration near the CMCtrans value, UV irradiation causes a complete disaggregation of the micelles. At a concentration slightly higher than the CMCcis, the rod-like micelles formed by trans isomers convert into smaller spherical micelles formed by cis isomers upon UV irradiation. For a concentration much higher than the CMCcis, trans/cis isomerization only leads to a decrease of the diameter of rod-like micelles, whereas the length is unaffected.

II.2.2.2 Wormlike micelles

It is well known that certain cationic surfactants with long alkyl chain and aromatic salts (e.g., tosylate, cinnamate, salicylate) can form thread-like micelles in aqueous solution resulting in an entangled network with a pronounced viscoelasticity 54 . Typical systems are cetyltrimethylammonium bromide (CTAB) with sodium salicylate (NaSal) 55 or dichlorobenzoate 56 and dodecyltrimethylammonium chloride (DTAC) with NaSal 57 .

The impact of additives on the micelles morphology can be rationalized on the basis of the critical packing parameter (CPP) (Figure 17). The CPP value is defined as 𝐶𝑃𝑃 = 𝑉/𝑎𝑙, where 𝑉 is the volume of the hydrophobic part of the surfactant, 𝑎 is the effective area of the headgroup at the micelle/water interface and 𝑙 is the length of the hydrophobic tail. Globular micelles are formed with P < 1/3. Surfactants with 1/3 < P < 1/2 rather form cylindrical assemblies, such as wormlike micelles. When P > 1/2, we usually observe vesicles or bilayers. Addition of aromatic anions can screen the electrostatic repulsion between hydrophilic headgroups, and thus decrease the effective headgroup surface 𝑎. Aromatic salts embedded in the micelles modifies the volume of single surfactant as well, resulting in the increase of the packing parameter. As a result, micellar aggregates grow into long, flexible micelles, referred to as "wormlike" or "threadlike" micelles. Another systems composed of a cationic surfactant erucyl bis(2hydroxyethyl)methyl ammonium chloride (EHAC) and an Azo derivative 4azobenzenecarboxylic acid (ACA) (in its salt form) were investigated by Oh et al. 60 (Figure 19). The strong binding between trans-ACA and EHAC favours a cylinder like geometry leading to the formation of unilamellar vesicles with low viscosity.

After UV light irradiation, cis-ACA interacts with the hydrophilic region of EHAC, leading to a truncated-cone geometry. Elongated micelles were thus formed resulting in a 5 orders of magnitude increase in viscosity (from 3 mPa•s to 500 Pa•s).

II.2.3 Oppositely charged Polymer/Azo-surfactant complexes

Surfactants have been widely used in association with oppositely charged macromolecules in aqueous solution. As mentioned in Chap I.4, the complexation originates from the electrostatic and the hydrophobic interactions, resulting in various physicochemical properties and structural organizations, such as necklace, bilayer and spherical micelles.

The complexes are readily formed in a non-covalent way by simply mixing the two components; this strategy offers a facile access to photo-tunable 'smart' systems. The phase behaviour of the complexes usually depends on the charge ratio. Stoichiometric complexes (1:1) are generally highly stable (but not soluble) and precipitate. Interesting liquid crystal properties for these systems have been reported 61,62 .

Nevertheless, there is little work of Azo-surfactant/PE complexes in aqueous solution. Lee et al. [START_REF] Lee | Photoreversible Viscosity Changes and Gelation in Mixtures of Hydrophobically Modified Polyelectrolytes and Photosensitive Surfactants[END_REF] These polymers were synthesized by the synthetic route shown in Scheme 2.

The main chain polymer was prepared by controlled polymerization of tert-butyl acrylate (tBA) by RAFT to generate well-defined poly(tert-butyl acrylate) (PtBA) polymers. Subsequently, the acidolysis of PtBA yielded the polyacrylic acid (PAA).

The salt form (NaPAA) was obtained by the dialysis of PAA in excess of NaOH.

PAA was esterified by AzoC4Br, a brominated derivative of azobenzene.

Scheme 2 Synthetic route to the polymers (PAA-Azoy)n.

We will first give a brief overview of the RAFT polymerization, and then, present the results of our polymerizations and the characterization of the targeted polymers. In the following part we will expose the synthesis of Azo derivatives and their grafting on the polymer.

III.1.2 Controlled radical polymerization RAFT

III.1.2.1 Principle

As said before, the main chain polymer is synthesized by RAFT. We remind briefly the principles and the properties of this method of polymerization. there is no creation of radicals. Therefore, an external source of radicals is required to maintain a constant rate of polymerization, which is very different from other RDRP methods. Typically, radical initiators such as azobisisobutyronitrile (AIBN) and 4,4'-azobis(4-cyanovaleric acid) (ACVA) can be used 2,3 . RAFT can also to be initiated by redox agents 4,5 or by light irradiation 6 . R-group should be an excellent leaving group when the propagating radical is added to the C=S bond of RAFT agent and enables to reinitiate polymerization rapidly and efficiently to avoid rate retardation. Guides to select the Z-and Rgroups for different monomers are available in many reviews [7][8][9][10] .

Scheme 4 The overall RAFT process.

III.1.2.2 Reaction conditions

The RAFT polymerization conditions are the same as those of conventional free radical polymerization. The same initiator, temperature and solvent are used, in addition to an appropriate RAFT agent. Three stoichiometric ratios must be adjusted for a successful RAFT polymerization. This is how DPn and hence, the number-average molar mass Mn can be predicted and targeted before polymerization.

In the course of the reaction, before completion, the expressions for Mn and DPn are given by the following equations. 11,12 𝐷𝑃 𝑛 (𝑡) =

[𝑀] 0 -[𝑀] 𝑡 [𝑅𝐴𝐹𝑇] 0 = 𝑝 [𝑀] 0 [𝑅𝐴𝐹𝑇] 0 (1)
𝑀 𝑛 = [𝑀] 0 -[𝑀] 𝑡 [𝑅𝐴𝐹𝑇] 0 × 𝑚 𝑀 + 𝑚 𝑅𝐴𝐹𝑇 (2)
where [𝑀] 𝑡 is the concentration of monomer at time t, p the conversion rate of polymer, mM and mRAFT the molar weight of the monomer and the RAFT agent.

These equations indicate that if the reaction is controlled, DPn and Mn should vary linearly with monomer conversion. RAFT process offers a good molecular weight control over a range from 1000 to 100000 g mol -1 . Higher molecular weights are also accessible with specific reaction conditions.

2)

[𝑅𝐴𝐹𝑇] 0 /[𝐼𝑛𝑖𝑡𝑖𝑎𝑡𝑜𝑟] 0
The ratio of RAFT agent to initial radical source affects the control of the molecular-weight evolution. The initiator concentration should be chosen to offer a balance between an acceptable rate of polymerization and an acceptable level of dead chains 12 . Usually [𝑅𝐴𝐹𝑇] 0 /[𝐼𝑛𝑖𝑡𝑖𝑎𝑡𝑜𝑟] varies from 5 to 10. 10

3)

[𝑀] 0 /[𝐼𝑛𝑖𝑡𝑖𝑎𝑡𝑜𝑟]
As for conventional free radical polymerization, the rate of polymerization depends on the ratio of monomer to initiator. These ratios are usually reported as

[𝑀]/[𝑅𝐴𝐹𝑇 𝑎𝑔𝑒𝑛𝑡]/[𝐼𝑛𝑖𝑡𝑖𝑎𝑡𝑜𝑟]

III.1.3 Synthesis of the PAA chains

Polymerization procedure is based on a previously reported method 13 .

Scheme 5 Synthesis of PtBA 1 by RAFT

The inherent properties of the RAFT agent (choice of Z-and R-group in Scheme 4) and the reactivity of monomers both affect the equilibrium of this addition-fragmentation process 14 . We have chosen to work with trithiocarbonates since they have proved to be efficient RAFT agents for the polymerization of acrylates. We have also chosen to work with a commercially available trithiocarbonate type agent (Scheme 6).

III.1.3.1 Kinetics measurements of the polymerization

The kinetic of polymerization was monitored by 1 H NMR and SEC. As shown in Figure 1, peaks at 6.26 ppm, 6.01 ppm and 5.68 ppm correspond to the three acrylate protons. The integrals of these peaks were normalized to the intensity of an internal standard. The normalized integrals decrease with the consumption of monomers during the polymerization. To determine the conversion rate of tBA monomers, we followed the normalized integral of the peak at 5.68 ppm.

[𝑀] t

[𝑀] 𝑜 = 𝐼 t 𝐼 0 (3) 𝑝 = 1 - [𝑀] t [𝑀] 0 = 1 - 𝐼 t 𝐼 0 (4)
where [𝑀] 0 initial concentration of tBA, 𝐼 0 integral of the peak at t0

[𝑀] 𝑡 concentration of tBA at time t, 𝐼 𝑡 integral of the peak at t 𝑝 conversion rate The evolution of the molar mass was monitored by SEC. When combining this information with 1 H NMR kinetic results, we can plot the evolution of the apparent number-average molar mass Mn as a function of monomer conversion p.

The kinetic plots for short and long PtBA chains are presented in Figure 2 and Figure 3 respectively. The kinetic rates (Figure 2a and Figure 3a) display a firstorder behaviour after an induction period of 40 min, which means that the global polymerization rate is constant after 40 min. The induction period is often observed at the beginning of RAFT polymerization and is attributed to the preequilibrium step of RAFT process as discussed in the introduction on RAFT 14 . Figure 2c and Figure 3c show the evolution of the molar mass Mn,SEC measured by SEC. It increases linearly with the monomer conversion (Figure 2b and Figure 3b), which confirms the controlled growth of chain length. It can be compared with the theoretical Mn,theo as calculated by Eq. ( 2).

For the short polymer, there is a good agreement between the measured molar mass Mn,SEC values and the theoretical Mn,theo values. For the long polymer, the masses measured by SEC are lower than the theoretical ones. This effect may be attributed to the difference in hydrodynamic volume between PtBA and the molecular weight standard poly(methyl methacrylate). It could be also attributed to the loss of living radicals due to a higher rate of initiator.

Finally, Figure 2d and Figure 3d show that the polydispersity is constant and remains low, thus confirming a controlled radical polymerization. It is around 1.1

for the low masses and 1.2 for the high masses.

In conclusion, the kinetics studies show that the polymerization is controlled.

The molar mass can be reached by adjusting the [𝑀] 0 /[𝑅𝐴𝐹𝑇] 0 ratio and the conversion rate; the polydispersities are quite low.

The polymerization advancement p was followed by NMR, and the reactions were stopped at p ~ 60%. The expected theoretical degrees of polymerization are therefore:

𝐷𝑃 𝑛 = 0.6 [𝑀] 0 [𝑅𝐴𝐹𝑇] 0 (5)
This gives a degree of polymerization of 54 for short chains and 738 for long chains. Hence the synthesized PtBA are labelled after the theoretical masses PtBA54RAFT and PtBA738RAFT. The measurements of the masses by SEC are summarized in Table 1.

III.1.3.2 Acidolysis of PtBA

This acidolysis is carried out by TFA in MeOH at room temperature. This step does not necessitate purification since the solvent and TFA can be evaporated under vacuum. As shown in the Scheme 2, PAA was modified with bromide AzoC4Br. This reagent was synthesized by the procedure reported by Wang et al. 15 (Scheme 8).

The phenol-azobenzene (Azo-H) was synthesized by azoic coupling between phenol and aniline. The resulting phenol was etherified by butylene dibromide to form AzoC4Br. The final compound is formed with an overall yield of 50 % at the scale of 5 g.

𝑥 + 𝑦 = 1 (6) 𝑅 = 𝐴𝑟𝑒𝑎2 𝐴𝑟𝑒𝑎1 = 4𝑦 3 + 4𝑦 (7)
The molar fraction of Azo units 𝜏 𝑎𝑧𝑜 is given by:

𝜏 𝑎𝑧𝑜 = 𝑦 = 3 4 × 𝑅 1 -𝑅 (8)

Determination of the azo content by UV-Vis:

UV-Vis spectroscopy is a more accurate technique to determine the molar fraction of azo units. The Azo groups of the polymers are the only UV-active part of the polymer. So, the absorbance (A) of Azo-containing polyions solution is directly proportional to the concentration of azo units (Beer-Lamber law).

𝐴 = 𝜀𝑙𝑐

where A is the measured absorbance; 𝑙 (cm) is the path length through the sample (𝑙 = 1 cm); c (mol L -1 ) is the concentration of the absorbing species; 𝜀 (L mol -1 cm -1 ) is the molar absorption coefficient of the azo group. 𝜀 at the maximum absorption (349 nm) was determined in a mixture of THF/H2O (1/1 in v/v) to ensure the solubility of both AzoC4Br and azo-containing sodium polyacrylates. The found value is 21278 ± 221 (L mol -1 cm -1 ).

Solutions of (PAA-Azoy)n were prepared at precise and appropriate concentrations and their absorbance at 349 nm were measured. The molar fraction of Azo unit in the polymer can be calculated by equation:

𝜏 𝑎𝑧𝑜 = 1 1 + 1 𝑀 1 ( 𝑚𝜀𝑙 𝐴𝑉 -𝑀 2 ) ( 9
)
where m (mg) is the mass of azo containing polymer used for solution preparation; V (mL) is the total volume of the prepared solution; A is the measured absorbance; M1 (g.mol -1 ) is the molar mass of the non-functionalized monomers; M2 (g.mol -1 ) is the molar mass of the functionalized monomers.

The details of these two calculation methods are given in appendix a) and b).

Measured 𝝉 𝒂𝒛𝒐 values for Azo-modified polymers:

The values are summarized in Table 3. The NMR results are consistent with that of UV-Vis. 𝜏 𝑎𝑧𝑜 values determined by UV-Vis spectroscopy are more accurate and thus used in the nomenclature of azo-containing polymers (PAA-Azoy)n (y = 𝜏 𝑎𝑧𝑜 ).

Synthesized

For the synthesized chains, the Azo fraction in the final product is equal to the equivalent number of Azo units introduced initially in the reaction media, which means a quantitative grafting reaction and no loss of Azo during purification.

Concerning the Azo-polymers obtained from commercial PAA with a high polydispersity, the final molar fraction of Azo units is lower than the number of equivalents added in the reaction. This is probably related to the fact that part of Azo units were grafted to oligomers that were removed during the dialysis procedure.

III.1.1.3 Measurements of the molar masses of (PAA-Azoy)n

All the azo modified polymers in acid form were analysed by SEC (Table 4). Since the starting polymer PAA23Sigma has two main populations and certainly contains oligomers, (PAA-Azo0.11)42Sigma also has a large polydispersity and shows two populations as well as seen in Figure 6a. However, part of small oligomers were certainly eliminated during the dialysis procedure. That explains why the average degree of polymerization is larger for the modified polymers (PAA-Azo0.11)42Sigma (DPn = 42) than for the initial PAA23Sigma chains (DPn = 23). For the same reason the polydispersity decreases after grafting (from 1.4 to 1.3).

(PAA-Azo0.15)54RAFT and (PAA-Azo0.11)738RAFT polymers synthesized from welldefined PAA retain a narrow polydispersity as shown in Figure 6b and Figure 6c.

The difference in elution volumes is directly related to the grafting of azo groups.

PAA738RAFT is not shown in Figure 6c, since the high molecular weight PAA chains cannot be measured correctly by SEC (carboxylic acid groups interacted strongly with the SEC column or the solubility weakens for high molar mass PAA).

III.1.2 Conclusion

The targeted polymers were synthesized with a controlled mass and a controlled polymerization rate at the scale of 1 g. The backbone was synthesized by RAFT polymerization of tBA which yielded a soluble polymer, easy to characterize by SEC at this step. Acidolysis of the polymer and neutralization occurs with conservation of DPn. Finally, the grafting rate can be achieved quantitatively for grafting rates lower than 50 % per mol. Analysis by SEC: the 50 µL aliquot was analyzed by SEC, according to the experimental procedure described in chapter IV (techniques).

III.1.3 Experimental procedures

III.1.3.4 aciolysis and dialysis

PtBA 1 (1 g) was dissolved in a mixture of TFA (20 ml) and MeOH (10 ml).

The mixture was stirred at room temperature for 2 days. Most of TFA and THF were evaporated and the residue lyophilized to afford Polyacrylic acid (PAA) 2. 1 was transferred into a pretreated dialysis membrane tube (MWCO 1 kD). The dialysis tube was put in a beaker containing 1 L Milli-Q water. Milli-Q water was refreshed every day until the conductivity of water was constant. The final product 6 in ionic form was obtained by lyophilization (yellow powder, 0.44 g, 53% yield).

III.2 Azo-containing surfactants

Azo-containing surfactants (Azo-surfactant) were obtained according to a published procedure, [17][18][19] indicated in Scheme 10. The synthesis started with the azo coupling reaction between alkyl-aniline and phenol, followed by the addition of a two-carbon spacer and the introduction of a quaternary ammonium head. Two Azo-surfactants with different chain lengths (4Azo2TAB and 8Azo2TAB) were synthesized.

Scheme 10 General synthesis procedure of the azobenzene surfactants

III.2.1 4Azo2TAB

Scheme

III.2.2 8Azo2TAB

Scheme Hz, 3H, H20). 13 (m, 10H, H15-H19), 0.88 (t, J = 7.1 Hz, 3H, H20). 13

III.3 Sample preparations

III.3.1 Polyelectrolyte aqueous solutions

Salt-free aqueous solutions of PEs were prepared by dissolving PEs powders in ultrapure water. Very low concentrations were obtained from the dilution of concentrated solutions. The mass of residual water in dry PEs was considered during concentration calculation. Solutions were stirred one hour and then stored in dark place at room temperature at least 2 days before measurements. The same procedure was followed for surfactant solution preparation.

III.3.2 Polyelectrolyte/Surfactant complexes solutions

PEs and azo-surfactants solutions were prepared separately by dissolving powders in ultrapure water. PE/surfactant solutions were obtained by mixing initial PEs and surfactant solutions at desired concentrations under strong stirring.

Solutions were kept in dark place at room temperature at least 3 days before measurements.

III.3.3 Azo-surfactant/Aromatic salt solutions

Azo-surfactant and salt solutions were prepared separately and mixed under They offer a large range of wavelength, with a narrow wavelength distribution.

Even though the lighting area of UV LED is not very large, it's possible to assemble several units and to create a suitable setup for the experiments.

A filtered UV tubes (365 nm) with a large wavelength distribution was used at the beginning. The lightened area is sufficiently large for illuminating NMR tubes and UV-Vis cuvettes. One limitation of UV tubes lamp is that the irradiation can only be applied on one side of the sample. Another limitation is the heat dissipation which might warm up the sample after a long irradiation period (that can affect the isomerization process of azobenzene). Hence, samples were illuminated at 10 cm away from UV tubes in practice. UV tubes are more suitable for dilute solutions and short time irradiation.

UV LEDs with a narrow viewing angle (15°) were used in most of the experiments. The spectra distribution is centred at 365 nm with a narrow wavelength distribution (363 -370 nm at half-maximum). They were arranged in different ways according to the nature of the measurements. Different setups will be presented in the following part. No heating problems were observed. Samples can be put very close to the LEDs to improve the irradiance.

The radiant flux received per unit area (irradiance) were measured with a photodiode power meter (ThorLabs PM130D) for different sources and typical setups (Table 1).

UV sources

IV.1.2 Comparison between UV tubes and UV LEDs irradiation

The trans/cis ratio is highly sensitive to the irradiation setup (intensity, wavelength distribution). In order to find the most adapted UV illumination conditions, comparisons between UV tube and UV LEDs irradiation were realized Figure 1 shows the NMR spectra of 4Azo2TAB 50 mM in D2O before and after light stimulus (UV or blue LEDs). Pure trans and their cis isomers solutions show clearly distinct peaks (Figure 1 red curve for dark and green curve for UV).

Therefore, the molar fraction of trans isomers 𝑥 𝑡𝑟𝑎𝑛𝑠 at photostationary state is estimated by the integral of the trans and the corresponding cis peaks. The interpretation of NMR peaks and the method of 𝑥 𝑡𝑟𝑎𝑛𝑠 calculation is given in Chap VI.2.2.2.

By UV-Vis

We plot the variation of the UV absorption spectra of 4Azo2TAB/PSS complexes using different light irradiations setups in Figure 2. The concentration (PSS/4Azo2TAB 100/50 mM) used for structural studies is too high to be measured directly by UV-Vis spectroscopy directly, a dilution is needed prior to the experiments. The trans form presents a maximum adsorption band at 347 nm.

Upon UV irradiation, this band decreases and shifts progressively to lower wavelength and a band increases at 438 nm. Two isosbestic points are present, which reflect that the absorption spectrum is a linear combination of the pure trans and pure cis absorption spectra. We can therefore extract the trans/cis ratio from the UV-Vis spectrum at any irradiation time. The details of the method are given in Chap VI.3.2. The sample concentration may also affect the trans fraction because the Azo moiety has a very strong absorbance. This will be discussed in Chapter VI.

Blue light is used to induce the phototransition from cis to trans state. Several tests were performed with incandescent lamp. Heating problems were observed.

We finally used blue LEDs (465 nm) in the same way as UV LEDs for the back transformation from cis to trans isomers. The final composition 𝑥 𝑡𝑟𝑎𝑛𝑠 after blue light irradiation is estimated in the same way as for UV irradiation.

IV.2 Small-Angle X-ray and Neutron Scattering

IV.2.1 Generalities

Small-Angle Neutron Scattering (SANS) and Small-Angle X-ray Scattering techniques (SAXS) are well-suited techniques to determine the structural organization in soft matter, from self-assembled systems to colloidal and polymer solutions 9,18 . SAXS and SANS can probe distances from a few angstroms to tens of hundred nanometres, giving information about the shape and size of the objects as well as their mutual organization.

For PEs and surfactants in solution, we measure the total differential scattering cross section of the solute Σ(𝑞 ⃑) that contains all the information about the structural organization, and a background (solvent scattering and incoherent scattering in SANS) that needs to be subtracted. The intensity is measured as a function of the scattering vector 𝑞 ⃑ defined as 𝑞 = 4𝜋 sin(𝜃 2 ⁄ ) /𝜆 where 𝜃 is the scattering angle and 𝜆 the wavelength.

The scattering cross section of the solute per unit volume Σ(𝑞 ⃑) (in cm -1 ) or more simply, the intensity, depends on the structural organization of the elementary scatterers (monomers, surfactants, ions…) and writes:

Σ(𝑞 ⃑) = 1 𝑉 ∑ 𝐾 𝑖 𝐾 𝑗 〈𝑒𝑥𝑝[𝑖𝑞 ⃑(𝑟 ⃑ 𝑖 -𝑟 ⃑ 𝑗 )]〉 𝐾 𝑖 = 𝑎 𝑖 -𝑎 𝑠 𝑣 𝑖 𝑣 𝑠 𝑖,𝑗 (1)
𝑟 ⃑ 𝑖 and 𝑟 ⃑ 𝑗 are the positions of the scatterers i and j, Ki and Kj are their contrast lengths (cm) relative to the solvent s. ai and as are the coherent scattering lengths (cm) of the scatterers i and the solvent, and vi and vs, are their partial molar volumes (cm 3 mol -1 ). These partial molar volumes can be determined from density measurements or estimated by incremental volumes. It is usually more convenient to introduce the scattering length density of the scatterers 𝜌 (or SLD in cm -2 ):

𝜌 = 𝑎 𝑣 𝑁 𝑎 (2)
with Na the Avogadro number.

For an isotropic two components system made of PEs and surfactants in aqueous solution (we neglect here the contribution of the counterions) and considering monomer-monomer, surfactant-surfactant and monomer-surfactants pairs in equation ( 1) , the scattering cross section split in three distinct terms

Σ(𝑞) =
By choosing the right composition 𝜙 𝐷 , it is possible to match the SLD of component A and therefore, to mask its contribution to the scattered intensity (𝜌 𝐻 2 𝑂/𝐷 2 𝑂 = 𝜌 𝐴 , KA = 0, KA 2 SAA(q) = 0, KAKBSAB(q) = 0). The structure function of the other component B is then revealed (∑ ( q)= KB 2 SBB(q)  0). This contrast matching method or more generally, the contrast variation is really unique in neutron scattering. With X-rays we are usually in a situation where all the structure functions contribute to the scattered intensity.

For polymeric chains, micelles and their complexes, each structure function contains two contributions. One originates from interferences between scatterers in the same "object" (form factor), and one from interferences between scatterers in different objects (structure factor). The form factor (associated to intra-object correlations) enables to characterize the size and the shape of the assembly while the structure factor (inter-object correlations) gives information about their dispersion state. The later therefore directly reflects the interaction between the objects.

In our systems composed of PEs, surfactants and their mixtures, we observe the formation of globular as well as cylindrical particles. The "colloidal description" is often more appropriate to analyse the scattered intensity

∑(𝑞) = 𝐼(𝑞) = 𝜙Δ𝜌 2 𝑉𝑃(𝑞)𝑆(𝑞) ( 6
)
where 𝜙 is the volume fraction of the particles, V their volume and Δ𝜌 2 the contrast factor (Δ𝜌 = 𝜌 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒 -𝜌 𝑠 ). P(q) is the normalized form factor (P(q=0) = 1). S(q) is the structure factor.

IV.2.2 Form factor

For very dilute systems, or more generally when inter-particles correlations are negligible, S(q)=1, only the form factor P(q) contributes to the intensity.

The spatial resolution is inversely proportional to the scattering vector q.

Different geometrical information can be found according to the q range of observation and the typical size Rg (radius of gyration) of the particle (Figure 4).

For q → 0 (violet zone in Figure 4), the object can be considered as punctual with respect to the spatial resolution: only global properties can be determined.

Since P(q)→1, ∑ ( q)= 𝜙Δ𝜌 2 𝑉 , we can extract the volume of the objects, and therefore the degree of polymerization for polymers in solution.

For qRg < 1 (red zone in Figure 4), we are in the Guinier regime. We find a plateau at low q. The reduction of the intensity around qRg  1 allows to determine the radius of gyration Rg.

For qRg>1, we enter in the intermediate regime (blue zone in Figure 4). The scattered intensity scales as q -α , where α is related to the shape of the object. For example, α = 1 for an infinite rod, 2 for a Gaussian chain. If the geometry of the object is well defined (sphere, ellipsoid, cylinder with low polydispersity), additional characteristic oscillations are observed in the regime and can be analysed on the basis of analytical form factors.

In the high-q regime (Porod regime, green zone in Figure 4), we probe the interface of the objects. For a smooth interface, the intensity usually varies as q -4 .

Figure 4 Form factor P(q) and corresponding range of observation in real space. Adapted from Ref 9.

IV.2.3 Scattering length densities (SLD)

The SLDs of the different components are key parameters in small-angle scattering. This knowledge is important to prepare the experiments, but also, to correctly analyse the data. The SLD determination is based on scattering length and the partial molar volume of the molecules.

IV.2.3.1 Scattering length of molecules or monomers

The X-ray scattering length bx is proportional to the total number of electrons Ze (that increases linearly with the atomic number Z) of the molecule.

𝑏 𝑥 𝑡𝑜𝑡𝑎𝑙 (10 -13 𝑐𝑚) = 𝑟 𝑒 × 𝑍 𝑒 = 2.82 × 𝑍 𝑒 (7) where re = 2.82 10 -13 cm is the classical radius of the electron.

Values for common elements are given in Table 4.

The

IV.2.3.2 Partial molar volumes of molecules or monomers

The partial molar volume is a thermodynamic quantity which is intimately related to the solute-solvent interactions. It corresponds basically to the overall volume variation of a solution induced by the dissolution of a solute. We should note that if the volume of a substance cannot be negative, the partial molar volume can be. An example is the dissolution of a strong electrolyte in water. The water molecules in the solvation shell around the ions are physically closer than in bulk pure water resulting in a global volume decrease.

The partial molar volume 𝜐 (cm 3 mol -1 ) of a molecule can be obtained by density measurements (when possible) or with the incremental volume method.

Molecules are then divided in different chemical parts of known partial molar volumes. By considering the additivity of the partial molar volumes 𝜐 0 𝑖 𝜐 = ∑ 𝜐 0𝑖 (8) Azo-surfactants can be divided into several parts as shown in Figure 5 (alkyl tail, azobenzene core, oxy group, small alkyl spacer and polar head). The partial molar volumes of each chemical group are summarized in Table 5. The volume associated to the polar head is deduced from the partial molar volume of the cetyltrimethyl ammonium bromide (CTAB) 10 in the micellar state by considering 𝑣 𝐶𝐸𝑇𝐴𝐵 = 𝑣 𝐶 16 𝐻 33 + 𝑣 (𝐶𝐻 3 ) 3 𝑁 + + 𝑣 𝐵𝑟 -= 365.4 𝑐𝑚 3 𝑚𝑜𝑙 -1 (9) 𝜈 (cm

IV.2.3.3 Scattering length density of molecules or monomers

The scattering length density ρ (cm -2 ) of a molecule is given by 𝜌 = 𝑎 𝜈 𝑁 𝑎 (10) or more explicitly 𝜌(10 10 𝑐𝑚 -2 ) = 𝑎(10 -13 𝑐𝑚) 𝜈(𝑐𝑚 3 𝑚𝑜𝑙 -1 ) × 6.022(10 23 𝑚𝑜𝑙 -1 ) (11) where NA (mol -1 ) is the Avogadro's number, a (cm -1 ) and 𝜈 (cm 3 mol -1 ) are the scattering length and the partial molar volume.

The scattering length densities, scattering lengths and partial molar volumes (X-rays and neutron) for all the molecules and monomers used in this thesis are presented in Table 6 [13][14][15] . 𝜈 (cm 3 mol -1 ) partial molar volume, bx (cm) X-ray scattering length, ρx (cm -2 ) X-ray scattering length density, bN (cm) neutron scattering length, ρN (cm -2 ) neutron scattering length density. Values for PAA -, azo-containg PAA are calculated according to the method described in IV.2.3.

IV.2.3.4 Fitting models

For the different systems forming aggregates, we analysed the scattered intensity on the basis of the general equation

𝐼(𝑞) = 𝜙Δ𝜌 2 𝑉𝑃(𝑞)𝑆(𝑞) (12)
Experimental data have been fitted on the absolute scale using the NCNR SANS package developed by NIST Center for Neutron Research and running with the Igor Pro software. Different models of the library have been used and/or modified. We briefly present here the most important form factors we used as well as their adjustable parameters.

Spherical and core-shell spherical models

The form factor for a homogenous sphere (also called one shell spherical model) of radius R is given by

𝑃 𝑆 (𝑞, 𝑅) = [3 sin(𝑞𝑅) -(𝑞𝑅) cos(𝑞𝑟) (𝑞𝑅) 3 ] 2 ( 13
)
For a core-shell model with a core radius Rc, a shell radius Rs, and SLD 𝜌 𝑐 The adjustable parameters for the fits are the volume fraction, the core radius and the shell thickness, the SLDs of the core, the shell and the solvent (fixed value), as well as a background.

The homogeneous sphere model has been used to fit the scattering of PAA-Azo copolymers (long chains). It has been combined with a Gaussian polydispersity and a structure factor S(q) developed for charged colloids in a repulsive screened Coulombic potential.

Ellipsoid of revolution and core-shell ellipsoids

For a uniform ellipsoid of revolution (also call one shell ellipsoid) with semiaxis R, R and 𝜀R (where 𝜀 is the ellipticity)

𝑃(𝑞, 𝑅, 𝜀) = ∫ 𝑃 𝑆 𝜋 2 0 (𝑞, 𝑟(𝑅, 𝜀, 𝛼))𝑠𝑖𝑛𝛼𝑑𝛼 (16)
with

𝑟(𝑅, 𝜀, 𝛼) = 𝑅√𝑠𝑖𝑛 2 𝛼 + 𝜀 2 𝑐𝑜𝑠 2 𝛼 ( 17
)
The adjustable parameters for the fits are the volume fraction, the semi-radius along the symmetry (or rotation) axis (𝜀R), the semi-radius in the equatorial plane (R), the SLDs of the particle and the solvent (fixed value) as well as a background.

This model applies to oblate (𝜀 < 1), prolate (𝜀 > 1), or spherical (𝜀 = 1) particles.

Semi-radius in the equatorial plane and along the symmetry axis (R and 𝜀𝑅) are also called major and minor (oblate situation) or minor and major (prolate situation) according the ellipticity value.

This model has been used to fit the scattering of surfactant micelles in combination with a structure factor S(q) developed for charged colloids in a repulsive screened Coulombic potential.

The model for the core-shell ellipsoid is a variant the homogeneous model.

The adjustable parameters for the fits are the volume fraction, the major and minor core radius, the major and minor shell radius, the SLDs of the core, the shell and the solvent (fixed value) and a background. The shell thickness was let free in the two directions (major, minor) or kept constant.

This model has been used to fit the scattering of surfactant micelles with neutron and X-rays and the intensity of PE/surfactant complexes. For the micelles, this model was combined to a structure factor S(q) developed for charged colloids in a repulsive screened Coulombic potential.

Cylindrical model with elliptic section and core-shell variant

For an elliptical cylinder on length H, a minor axis a and a major axis 𝜀 a (𝜀 is the ellipticity) with no preferred orientation the form factor is

𝑃(𝑞) = ∫ 𝐹 2 (
where J1(x) is the Bessel function of the first kind and first order.

The adjustable parameters for the fits are the volume fraction, the minor radius, the ellipticity e, the length of the cylinder, the SLDs of the particle and the solvent (fixed value), as well as a background.

This model has been used to fit the scattering of wormlike micelles. Due to the very long size of the aggregates, the length H cannot be determined and has been fixed to an arbitrary value.

The core-shell elliptic model is a variant the homogeneous model. The adjustable parameters for the fits are the volume fraction, the major core, the ellipticity, the SLDs of the core, the shell and the solvent (fixed value), and a background. A single ellipticity has been introduced in the model. The shell thickness is therefore not identical in the direction of the short and the long axis.

This model has been used to fit the scattering of surfactant and of wormlike micelles by SAXS.

Pearl-necklace model

In order to fit the scattering of PE/surfactant complexes, we need to determine the intensity scattered by a pearl-necklace assembly. Our simplified model is based on the form factor initially developed for hydrophobic PEs 16 . In this model, N pearls are located on the segments (size A) of a freely jointed chain. For simplicity, we ignore the scattering of the connecting segments and only consider the contribution of the pearls. If Ppearl(q) is the form factor a one pearl of volume

Vpearl, the intensity scattered by a pearl-necklace containing N pearls (considering S(q)=1) is given by

𝐼 𝑝𝑒𝑎𝑟𝑙 𝑛𝑒𝑐𝑘𝑙𝑎𝑐𝑒 (𝑞) = 𝜙Δ𝜌 2 𝑉 𝑝𝑒𝑎𝑟𝑙 𝑃 𝑝𝑒𝑎𝑟𝑙 (𝑞)𝐼𝑛𝑡𝑒𝑟𝑓(𝑞) (22)
with

𝐼𝑛𝑡𝑒𝑟𝑓(𝑞) = 2 𝑁 [ 𝑁 1 - sin(𝑞𝐴) 𝑞𝐴 - 𝑁 2 - 1 -( sin(𝑞𝐴) 𝑞𝐴 ) 𝑁 (1 - sin(𝑞𝐴) 𝑞𝐴 ) 2 sin(𝑞𝐴) 𝑞𝐴 ] ( 23
)
Interf(q) is the interference function between the pearls along a single necklace.

This model has been used to fit the scattering of PE/surfactant complexes.

We used the core-shell ellipsoid model for Ppearl(q). The adjustable parameters combine those of Ppearl(q) but also, those of the interference functions: the number of pearls and the average distance between them. Note that 𝑁 cannot really be fitted and must be an integer. It is therefore changed manually until the fit quality is improved.

IV.2.3.5 Structure factor S(q)

In our systems, electrostatic interaction plays a key role in the stabilisation of the solution. Repulsions are usually responsible for the presence of a peak in the scattered intensity. In order to analyse correctly the data, we have often combined the form factors with a structure factor S(q) developed for charged micelles and colloids in a repulsive screened Coulombic potential 16,17 .

The adjustable parameters combine those of P(q) and those of the structure factor S(q): the volume fraction, the radius of the particles, the salt concentration, the charge number (in e units), the temperature (fixed) and the dielectric constant (fixed).

This structure factor has been used to fit the scattered intensity of surfactant micelles (using an ellipsoid form factor) as well as the spherical particles found in PAA-Azo modified polymers(core-shell spherical particle). For this system, S(q)

was associated to the form factor or core-shell spherical particles. Optics TM (Osmics, Inc.) combined with a three pinholes system. The scattered intensity was measured as a function of the magnitude of the scattering vector with a 2D multiwires detector located at d = 0.81 m from the sample position (0.01 Å -1 < q < 0.33 Å -1 ). Scattering patterns were treated according to the usual procedures for isotropic scattering. Glass and quartz capillaries (1mm in diameter)

IV.2.4 SAXS experimental conditions and illumination protocol

were used as sample containers. Scattering from pure water and empty capillary were also measured and subtracted to the sample measurements. Intensities were converted into absolute scale with a calibrated Lupolen standard. Scattering vectors were calibrated using the diffraction peaks of a silver behenate powder.

Experimental resolution was measured on a Bragg peak of tricosane sample.

Except for relaxation studies, the illumination was always maintained during SAXS measurements. Experiments were performed in vacuum at room temperature.

IV.2.4.2 Illumination setup and protocols

Two UV or blue LEDs were fixed above and below the capillary (2 mm apart)

to illuminate the sample in situ (Figure 7). The concentration of Azo in solutions varies from 15 mM to 150 mM. In order to ensure the equilibrium state is reached, measurements were done regularly until the scattered intensity is stabilized. The irradiation time needed to reach the stationary state depends on the chromophore concentration, but also on the structure of the solutions. In this final state, the structure is believed to be homogeneous in the sample. Unintended effects may appear in highly viscous solutions or in thick samples for which the diffusion process is not able to homogenize the system in a reasonable time, or before the chromophore relax to their trans state. For low concentration chromophore solutions, an equilibrium period of 40 -60 min is necessary (10 min/measurement during the transition regime). For concentrated solutions or particular compositions, longer irradiation time is needed. To be more efficient, the whole capillary was pre-irradiated by 8 LEDs during 1 -2 hours before being introduced in the SAXS instrument (Figure 8). For thermal relaxation studies, we also need to probe an 'homogeneous irradiated system' on the time scale of the measurement. Using the "classical" irradiation device in the diffractometer (Figure 7), the initially irradiated zone is of the order of 5 mm along the capillary without pre-irradiation. We must avoid the diffusion effect between molecules initially in the irradiated zone (majority of cis conformation) and in the non-irradiated zone (trans conformation). For that reason, samples were pre-illuminated outside with 8 LED until the stationary state is reached, installed in the sample holder and then left to relax in dark.

We may wonder if the measurement using outside pre-illumination has the same final state as with a direct in-situ irradiation. The UV plots of wormlike micelles (8Azo2TAB/NaSal) obtained with the two irradiation ways are given in

IV.2.4.3 Experimental conditions

Borosilicate and quartz glass capillaries

Borosilicate and quartz glass capillaries were tested under UV by SAXS.

While the absorption of borosilicate is a little higher than that of quartz, the final state probed by SAXS is nearly identical (Figure 10). For cost reason, most of the SAXS experiments were done with borosilicate capillaries.

Diameter of the capillary and sample preparation

The diameter of the capillary (sample thickness) may influence the trans/cis composition at the final state because the Azo moiety absorbs enormously the incident UV light. The homogenization of the solution requires the diffusion of the chromophores (and therefore the diffusion of the micellar aggregates in which they are embedded) over the thickness of the sample. For high azobenzene content, high viscous systems and large diameter, the homogenization may be difficult to achieve. The apparent final state measured on surfactant/PE complexes at relatively high concentration (4Azo2TAB/PSS, 50 mM/100 mM) in 1.5 mm capillary show large difference at large q values with respect to the 1 mm capillary. The oscillation around 0.2 Å -1 (characteristic of the micellar aggregate) is shifted to low q upon UV irradiation (i.e., closer to the dark solution) for the largest diameter capillary (yellow curve in Figure 11). This is consistent with final state in which cis/trans ratio is reduced. This capillary diameter effect on the final state is also possible with the 1 mm diameter capillary. However, the final state is identical to that measured on the same sample at lower concentration. It seems therefore unlikely. 1 mm capillary appears as the best compromise for measurements under light stimulus.

We remark that the real diameters of capillaries vary from one to the other (0.85 mm to 1.2 mm). Their thicknesses were measured individually by optical microscopy. It is an important parameter for the conversion of the scattered intensity to the absolute scale.

IV.2.5 SANS experimental conditions and illumination protocol

IV.5 UV-Vis absorption spectroscopy

UV-Vis absorption measurements were carried out on a Perkin Elmer Lambda 97 25 spectrometer using standard quartz cells (Hellma OS) with an optical path length of 10 mm. Spectra were recorded in the range of 250 -700 nm. Absorbance from pure water was systematically subtracted from sample measurements. This technique is used to determine the trans/cis molar fraction of the chromophores and can only be used with very low absorbance solutions.

For higher concentration solutions (as for samples for structural studies), a dilution is needed prior to the measurements. A small fraction of the solution was collected, diluted 1000 to 2000 times and analysed immediately on the spectrophotometer.

UV-Visible spectroscopy was also used to investigate the photoisomerization process during the transition regime until the steady state is reached as well as during the thermal relaxation.

The irradiation setup for quartz cell is presented in Figure 14. The sample is illuminated by 4 LEDs on both sides of the cell. At the very beginning, several measurements were carried out with UV tubes in order to compare with UV LED irradiation. Sample were placed at 10 cm away from the UV tubes to avoid heat dissipation problems (Figure 15). However, we abandoned this illumination method and chose to use the more efficient UV LEDs setup.

Figure 15 Irradiation setup with UV tubes for UV-Vis measurement.

IV.6 Surface tension measurements

The surface tension measurements were performed on a Kibron tensiometer using a small sample cup (3 mL). Millipore water was used for calibration. Azosurfactant solutions with accurate concentrations were prepared and kept 2 days in the dark state for equilibrium. For the measurement of the cis-form, the solution was pre-irradiated by 8 UV LEDs at a distance of 5 mm in a quartz cell (Hellma OS, 10 mm path length) with stirring (Figure 14). The irradiation time depends on the sample concentration and ranged from 4 h to 20 h. The irradiated solution was transferred in the plastic cuvette just before the measurement. Four LEDs were set above the cuvette to maintain the cis-form state during measurement (Figure 16). The critical micellar concentration (CMC) value was determined by the plot of 𝛾 as a function of lnc.

V.1 Azo-containing polyelectrolytes

We Table 1.

Scheme 1 Structure of (PAA-Azoy)n. It is possible to extract the trans fraction 𝑥 𝑡𝑟𝑎𝑛𝑠 at any irradiation (or relaxation) time if the absorption spectra associated to the pure trans and pure cis forms are known. Since all the solutions are kept in dark for 2 days before measurements, the initial UV-Vis absorption spectra can be considered as composed of 100% trans isomers. Unfortunately, a 100% cis isomers state can never be reached experimentally. In a dilute solution for instance, the residual amount of the trans isomers at equilibrium is about 8%. This value is determined from NMR analysis of Azo-surfactants and is in in agreement with the value described in the thesis of Eloise Chevallier 1 . The pure cis absorption spectra can therefore be deduced from a linear combination of the spectrum at equilibrium and in dark state using:

𝐴 𝑒𝑞 (𝜆) = 𝑥 𝑡𝑟𝑎𝑛𝑠 𝑒𝑞 𝐴 𝑡𝑟𝑎𝑛𝑠 (λ) + (1 -𝑥 𝑡𝑟𝑎𝑛𝑠 𝑒𝑞 )𝐴 𝑐𝑖𝑠 (λ) (1)
⟹ 𝐴 𝑐𝑖𝑠 (𝜆) = 𝐴 𝑒𝑞 (𝜆) -𝑥 𝑡𝑟𝑎𝑛𝑠 𝑒𝑞 𝐴 𝑡𝑟𝑎𝑛𝑠 (𝜆) (1 -𝑥 𝑡𝑟𝑎𝑛𝑠 𝑒𝑞 ) (2)
𝑤𝑖𝑡ℎ 𝐴 𝑡𝑟𝑎𝑛𝑠 (𝜆) = 𝐴 𝑑𝑎𝑟𝑘 (𝜆) 𝑎𝑛𝑑 𝑥 𝑡𝑟𝑎𝑛𝑠 𝑒𝑞 = 8%

The trans/cis ratio for the 1 M solution (at any irradiation time), or more generally for any solutions can be deduced from the absorption spectrum using:

𝐴(𝜆) = 𝑥 𝑡𝑟𝑎𝑛𝑠 𝐴 𝑡𝑟𝑎𝑛𝑠 (𝜆) + (1 -𝑥 𝑡𝑟𝑎𝑛𝑠 )𝐴 𝑐𝑖𝑠 (𝜆) (3)
𝐴 𝑡𝑟𝑎𝑛𝑠 and 𝐴 𝑐𝑖𝑠 must be measured at the same concentration. This relationship is valid for any wavelength 𝜆 . The determination of 𝑥 𝑡𝑟𝑎𝑛𝑠 can be therefore estimated at a single wavelength measurement 𝜆.

𝑥 𝑡𝑟𝑎𝑛𝑠 = 𝐴(𝜆)-𝐴 𝑐𝑖𝑠 (𝜆) 𝐴 𝑡𝑟𝑎𝑛𝑠 (𝜆)-𝐴 𝑐𝑖𝑠 (𝜆) (4)
Here, we preferred to fit the whole absorption spectra. There are two advantages. First, we are less sensitive to the error measurements on 𝐴(𝜆) and 𝐴 𝑡𝑟𝑎𝑛𝑠 (𝜆). The main source of uncertainty in the calculation now only comes from the estimation of 𝑥 𝑡𝑟𝑎𝑛𝑠 𝑒𝑞 needed in the expression of 𝐴 𝑐𝑖𝑠 (𝜆). Second, for high concentration solutions, we need to dilute the samples. This inevitably introduces small errors in the final concentration, and therefore in the 𝑥 𝑡𝑟𝑎𝑛𝑠 determination if we use other reference solutions (for full cis and full trans spectra). To overcome these problems, every measurement was renormalized with regard to the full trans reference to reproduce the two isosbestic points. This can only be done if we work on the whole absorption spectra and not at a single selected wavelength.

This procedure was applied on the absorption spectra in Figure 2a and Figure 2b to extract the variation of the trans fraction with the UV irradiation time (Figure 2c). The trans/cis ratio for 1 M solution is 32% at the equilibrium state while it is close to 10% for very dilute systems. We see here a direct consequence of the local organization on the photoconversion process.

V.2.2 Reversibility

In order to probe the reversibility of the photoconversion on Azo-polymers, we

V.2.3 Comparison of Azo-polymers UV-Vis spectra

The UV-Vis spectra in dark state for the four synthesized Azo-polymers are given in Figure 5. The position of the absorption peak is very sensitive to the local environment of Azo groups. A blue shift is expected when the azobenzene is aggregated. 4 Indeed, for the same Azo fraction (𝜏 𝑎𝑧𝑜 = 0.11), the long polymer

V.3 Structural organization study

We have studied the four different Azo-polymers by SAXS at different concentrations, in the dark state and under UV irradiation.

The goal is to determine the structural organization of these systems and the modifications triggered by irradiation. The unmodified poly(sodium acrylate)

NaPAA was also studied for comparison.

V.3.1 Reference systems: poly(sodium acrylate) (NaPAA)

A commercial PAAH and two well-defined synthesized PAAH were ionized and purified by dialysis to obtain the salt form NaPAA. These three unmodified NaPAA are considered as reference systems for the structural study of azocontaining polyions. Figure 6 presents SAXS curves for different monomer concentrations (0.015 M to 1 M). They all exhibit a broad peak at a position q* which is characteristic of PE solutions and electrostatic repulsions. Log-log plots of q* as a function of the concentration c (monomer concentration) are presented in Figure 7 for the three PEs. q* varies as c α with different power laws exponents α (NaPAA738RAFT α = 0.43; NaPAA54RAFT α = 0.30;

NaPAA23sigma α = 0.29). For NaPAA23sigma and NaPAA54RAFT, q* varies as ≈ c 1/3 , we are in dilute regime in the whole concentration range (small chains). For NaPAA738RAFT, q* varies roughly as c 1/2 , we are in semidilute regime. These behaviours are expected for short and long chains of PE. NaPAA chains are therefore solubilized at the molecular level.

Figure 7 q* vs concentration for reference PEs: NaPAA23sigma, NaPAA54RAFT and NaPAA738RAFT.

V.3.2 Azo polymers: general observations

The four Azo-polymers have been measured at different concentrations in dark and under blue or UV light.

Typical scattering curves at 0.25 mol L -1 are presented in Figure 8 for each PE in dark and under UV (log-log and linear scales). The scattering of pure NaPAA is also presented for comparison. The different systems show similar behaviours.

First of all, a scattering peak is still present for modified NaPAA-Azo (in dark and under UV) but its position is shifted to lower q values compared to pure NaPAA.

Second, the application of the UV radiation provokes a displacement of the peak which moves closer to that of pure NaPAA.

Third, and this is certainly the most visible effect, the UV stimulus causes a very important decrease in intensity (by a factor ≈ 2, more visible on the linear scale). These observations demonstrate that our systems are highly sensitive to UV and that the photoisomerization at the molecular level scale can induce large structural modification at the mesoscopic scale.

A last point to mention concerns the shape of the scattered intensity.

Modifications are observed between long and short chains, but also according to the Azo-fraction content and the concentrations. These modifications will be discussed later.

The nature of the regime of the solution (dilute, semi-dilute) is important for understanding the scattering curves and the significance of the scattering peak.

According to the degree of polymerization, our unmodified NaPAA is in dilute (short chains) or in semi-dilute (long chains) regime in the whole concentration range. It is highly probable that short NaPAA-Azo will remain in dilute regime whatever their conformation and/or their association in solution. The situation is more complex for modified NaPAA-Azo with a high degree of polymerization.

Everything will depend on the conformation of the chains (collapsed or extended)

and their interaction with others.

In dilute regime, charged colloids usually show a correlation peak at a position q* related to the average distance d between the center of mass of nearest neighbours. By considering a well-defined organization in solution (cubic, facecentered-cubic...), this distance only depends on the colloid concentration. For self-assembled particles, the colloid concentration now depends on the aggregation number Nagg. It is therefore possible to derive a relationship between the position of the correlation peak q* and the aggregation number Nagg. For a face-centered-cubic structure 5 :

𝑞 * = 2𝜋 𝑑 , 𝑑 = 1 √2 ( 4000 × 𝑁 𝑎𝑔𝑔 𝑁 𝑎𝑣𝑜 × 𝑐 ) 1 3 × 10 8 Å ( 4
)
where Navo is the Avogadro number, and c is the molar concentration of elementary units forming the particles (mol L -1 ). This is only an approximation since the maximum is not really a Bragg peak (there is no long range order in the solution). Different relationships have been proposed to relate precisely q* to the average distance d between charged particles. However, from a phenomenological point of view, we will only consider here that 𝑑 ≈ 2𝜋 ∕ 𝑞 * and

𝑑 ∝ ( 𝑁 𝑎𝑔𝑔 𝑐 ) 1 3
, 𝑞 * ∝ ( 𝑐 𝑁 𝑎𝑔𝑔 )

1 3

(5)

The interesting point is that the position of the peak varies as c 1/3 if the aggregation number is constant (simple dilution law). If it not the case, this implies a variation of Nagg with c and therefore, a modification of the volume of the particle.

This classical behaviour certainly applies to our charged systems provided that they are in dilute regime. It will help us to explain the variation of q* with the concentration but also, during the irradiation process.

V.3.3 Structural reversibility upon irradiation and thermal relaxation

The reversibility of the structural properties upon UV and blue irradiation was tested with SAXS. We followed the position of the maximum q* in the photostationary states during several UV-blue irradiation cycles. The accuracy of the measurements is good enough to evidence very small variations. Typical scattering curves (dark, blue, UV) and q* variations on short and long chains ((PAA-Azo0.15)54RAFT, (PAA-Azo0.11)738RAFT ) at c = 0.25 mol L -1 are presented in Figure 9 and Figure 10. PEs therefore tend to collapse in water due to the presence of Azo groups along the backbone and form dense aggregates.

As already mentioned, q* under radiation is systematically larger that q* in dark. The difference is related to the variation of the number of chains embedded in the aggregate (aggregation number Nagg).

A close inspection of the scattering intensity around 0.2 Å -1 (especially for the highest concentrations) shows an unexpected q -2 decay that is not really compatible with homogeneous spherical particles.

Finally, no deep minimum is observed in the large q range (for which P(q) dominates). The particles are certainly polydisperse or not purely spherical. Cryo-TEM measurements in dark have been performed to provide another view of these polymers in solution. This technique has a limited resolution (≈ 2 nm) and cannot reveal small structural details in our systems (we are close to the detection limit). However, as long as the contrast is acceptable, it provides essential information (size, shape, location of the objects) that perfectly complements SAXS analysis.

Cryo-TEM experiments have been carried out at 0.03 mol L -1 . An image is presented in Figure 13. We find distinct spherical objects evenly distributed in the sample. Small and big particles (average diameter 5 -6 nm) coexist with a few rare elongated objects (long arrows). These results definitively validate our colloidal picture. According to these observations, we have considered a model based on well separated spherical polydisperse charged particles for the analysis of SAXS measurements. As previously mentioned, an unexpected q -2 decay of the intensity is observed around 0.2 Å -1 . This is close to the scattering signature of pure NaPAA in solution (this behaviour can be seen in Figure 6). That clearly indicates the presence of PAA segments in solution in our modified PAA-Azo system. In other words, we do not have a dense particle incorporating all PAA monomers. This scattering behaviour is often found with polymer-decorated particles or star branched polymers, in which chains are attached to the core and form a polymer network around it. This phenomenon is conceivable in our systems since our PEs are highly amphiphilic. Part of the chains may be embedded in a dense core while the other remains in solution (end of chains, loops…). A semi-dilute PE solution is then forming a shell around the core.

From a scattering point of view, this system can be treated as a homogeneous core-shell particle as long as the concentration fluctuations of the polymer network (forming the shell) are negligible (which is usually not the case at high q). As a first approximation, this "non-homogeneous shell" can be accounted for by adding another contribution to the classical core-shell scattered intensity of the form 6 :

𝐼 𝑓𝑙𝑢𝑐 (𝑞) = 𝐼 𝑓𝑙𝑢𝑐 (0) 1 + 𝑞 2 𝜉 2 ( 6
)
where 𝜉 is the correlation length of the PEs solution in the shell. 𝐼 𝑓𝑙𝑢𝑐 (0) and 𝜉 are usually treated as adjustable parameters during the fitting procedure. In the present case, we prefer not to fit these values and rather subtract a common «background» 𝐼 𝑓𝑙𝑢𝑐 (𝑞) to the data. 𝐼 𝑓𝑙𝑢𝑐 𝑟𝑒𝑓 (0) and 𝜉 𝑟𝑒𝑓 have been estimated from the scattering curve at cref = 0.5 mol L -1 in dark. For the correction of other 𝐼 𝑓𝑙𝑢𝑐 𝑟𝑒𝑓 (0, 𝑐 𝑟𝑒𝑓 ) = 0.08 cm -1 . Long dash line: q -2 decay, short dash line: q -4 decay.

The data have been fitted with a classical homogeneous core-shell model combined to a structure factor S(q) developed for charged colloids in a repulsive screened Coulomb potential 7,8 . We do not any longer consider concentration fluctuations in the shell. We estimate that the data have been corrected for these effects. We can anticipate that the average SLD of the shell will be close to that of the solvent since the layer is approximated to a semi-dilute PE solution. The fit parameters are the volume fraction, the average radius of the core, the average thickness of the shell, the radius polydispersity (standard deviation/average value) as well as the SLDs of the core and the shell. In order to correctly reproduce the structure factor S(q), the effective charge of the particles and the salt concentration are also considered as adjustable parameters. The fits are presented in Figure 15 and Figure 16 for dark and UV experiments. The theoretical curves fit well the experimental data. The geometrical parameters and the SLD are summarized in Table 2. We can now give a schematic description of the structure of the particles. In the dark state, we have a dense core of typical radius around 29 Å and a shell of ionized charged PAA segments. The size of the core does not change with the concentration (except maybe for the lowest one 0.015 mol L -1 where the statistics is very poor). This is consistent with Cryo-TEM measurements which have shown particles with a diameter of the order of 6 nm (the shell is not visible here due to a very low contrast). This is an important result in agreement with the variation of q* as a function of the concentration (Figure 12 c). Indeed, a power law exponent of 1/3 has been determined that is characteristic of a simple dilution law i.e., without variation of the aggregation number (the volume of the particle is therefore constant).

Under UV, q* is higher and the exponent is reduced (𝛼 = 0.26). For a given concentration, this indicates the formation of new particles and thus, a decrease of the average aggregation number. This is compatible with the observed reduction of the core size with the concentration (from 28 to 21 Å when c varies from 0.5 to 0.015 mol L -1 , which means a reduction in volume by a factor 2).

If we fix the concentration c, we can roughly estimate the ratio of the aggregation numbers in dark and under UV (N agg dark /N agg UV ) from the variation of q* (Equation 5). For c = 0.03 mol L -1 , we get

𝑁 𝑎𝑔𝑔 𝑑𝑎𝑟𝑘 𝑁 𝑎𝑔𝑔 𝑈𝑉 ⁄ = (𝑞 𝑈𝑉 * 𝑞 𝑑𝑎𝑟𝑘 * ⁄ ) 3 = 1.72 (7)
If we assume that the core is a good indicator to extract the aggregation number (i.e., each chain occupies the same volume in the core, and in absence of penetrating water), we can compare this value to that obtained from the fit by considering the ratio between the volumes of the core in dark and under UV:

𝑁 𝑎𝑔𝑔 𝑑𝑎𝑟𝑘 𝑁 𝑎𝑔𝑔 𝑈𝑉 ⁄ = 𝑉 𝑐𝑜𝑟𝑒 𝑑𝑎𝑟𝑘 𝑉 𝑐𝑜𝑟𝑒 𝑈𝑉 ⁄ = (𝑅 𝑐𝑜𝑟𝑒 𝑑𝑎𝑟𝑘 𝑅 𝑐𝑜𝑟𝑒 𝑈𝑉 ⁄ ) 3 ≈ 2 (8)
The aggregation number is divided by a factor 2 under UV.

These values (eq7, eq8) are nearly identical. The variation of q* is definitively related to the variation of the aggregation number in the particles. The PAA-Azo chains leaving the particles under UV radiation then reform smaller aggregates in solution.

Since the Azo group is highly hydrophobic, the majority should be confined in the core. The typical radius of the core is actually close to (but larger than) the size of one Azo modified monomer, i.e., around 20 Å (in its extended form). Some PAA monomers are therefore certainly inserted (or the shape of the particle is not perfectly spherical and closer to an ellipsoid). The exact proportion is quite difficult to determine. If we consider the SLD of the core, it varies in between 11.5•10 10 cm -2 (average value in dark) to 10.94•10 10 cm -2 (average value under UV radiation).

It is much lower than that of PAA -(22.18•10 10 cm -2 ) and close (but always below) that of the grafted Azo group (12.95•10 10 cm -2 ). It means that water penetrate in the core and decrease its average SLD. If we neglect the presence of PAA, the water volume fraction is given by

𝜙 𝐻 2 𝑂 = 𝜌 𝑐𝑜𝑟𝑒 -𝜌 𝐴𝑧𝑜 𝑔𝑟𝑜𝑢𝑝 𝜌 H 2 O -𝜌 𝐴𝑧𝑜 𝑔𝑟𝑜𝑢𝑝 (9)
leading to water content in between 40% (dark) to 56%. These values are surprisingly high for hydrophobic components. However, similar values have already been reported in certain Azobenzene micellar systems 9 .

To fix the ideas, we can give an order of magnitude of the aggregation number Nagg (i.e., the number of polymer chains in the aggregate) in dark and under UV. If we consider that all the Azo groups are present in the core, and that water can penetrate in the aggregate, Nagg is simply

𝑁 𝑎𝑔𝑔 = 𝑉 𝑐𝑜𝑟𝑒 (1 -𝜙 H 2 O ) 𝑁𝜏 𝐴𝑧𝑜 𝑉 𝐴𝑧𝑜 𝑔𝑟𝑜𝑢𝑝 ( 10
)
where N is the degree of polymerization of the PE (N = 738), 𝜏 𝑎𝑧𝑜 is the molar fraction in Azo-groups (𝜏 𝑎𝑧𝑜 = 0.11) and Vazo group is the volume of the Azo group (Vazo group = 293 Å 3 ). The numerator corresponds to the total volume associated to the Azo groups in the core while the denominator is the total volume of Azo in a PE chain. If we consider the concentration c = 0.03 mol L -1 (or any concentrations in the dark state), we find Nagg = 2.5 in dark, and 0.9 under UV (we should have a single chain in the aggregate). Whatever the accuracy of the calculation, the number of chains in the pearls is very limited. Inter, but also intra-chain interactions are therefore very important for the stability of the particles.

Around the core, we have a shell in which we find polyions that form loops on the surface, as well as chain ends. It is clear that the structure of the shell is more complex than the classical homogeneous semidilute polymer solution we have considered (and subtracted) here. It might have been better to envisage a PE solution instead of neutral polymers. The main difference is the shape of the contribution 𝐼 𝑓𝑙𝑢𝑐 (𝑞) that shows a maximum for charged systems.

According to the Azo fraction of the modified polymer (𝜏 𝑎𝑧𝑜 = 0.11), we have in average ≈ 9 PAA monomers between two Azo groups. The typical distance is of the order of 22 Å. This seems to be too short to create a loop compatible with our shell thickness (around 20 Å at least). However, it must keep in mind that this number is only an average value and hides many disparities in terms of grafting homogeneities along the chains. Some chains also may have less Azo groups than other and therefore more facilities to do form loops, but also, to leave the aggregate. In the previous paragraph, we have shown that long chains (azo = 0.11) selfassemble and form spherical core-shell particles. Intra and intermolecular interactions play a key role in the process. The situation becomes very interesting if we reduce the size of the polymer while keeping the Azo fraction constant.

We focus here on a short polymer with a degree of polymerisation N = 42, an

Azo fraction azo = 0.11 (identical to that of the long chains) and a high polydispersity (Ip = 1.33). Each chain has in average  5 Azo groups which is rather few compared to the long chains ( 74). Since the polydispersity is large, the disparity in number of Azo groups per chain can be very important.

SAXS measurements at 0.25 mol L -1 in (dark and UV) are presented in Figure 18 and compared to unmodified NaPAA. As already mentioned in Figure 8, the maximum in dark is at a lower q value compared to NaPAA solution. Under UV irradiation, the peak shifts to higher q (and approaches very close to that of pure NaPAA) reflecting a large structural reorganization triggered by UV stimulus. A series of solutions at different concentrations have been measured in dark and under UV (Figure 19a andb). The variation of q* as a function of the concentration is plotted in Figure 19c (q* values for pure NaPAA are indicated for comparison). Contrary to the long chains, for which a single power law q*c α is observed (  0.3 in dark and 0.26 under UV), different regimes in concentration with different exponents are now clearly visible. Since the unmodified parent NaPAA is in dilute regime in the whole concentration range, we can reasonably assume the same situation for PAA-Azo. Departure from a classical c 1/3 dilution law may therefore reflect an association (or an aggregation) process between polymers as roughly described by equation 5. The structural organization therefore varies gradually with the concentration in dark and under UV. Based on q* variations, four distinct regimes (Regimes I, II, III and IV) can be identified in dark, while three only are observed under UV (I, III and IV, regime II is absent). A close inspection of the scattering profiles in the different regimes shows subtle differences. These differences will help us to identify the structural organization and the origin of the different regimes. This can be summarized as follow.

Regime 1 (c  0.03 mol L -1 )

In this regime, q* values are identical for both dark and UV measurements but also for pure NaPAA. Since q* varies as c 1/3 , we just experience a dilution process of individual molecules, PEs are solubilised at the molecular level. For these very low concentrations, polyions are far apart (average distances between objects: dNaPAA, dDark, dUV), and do not associate with others. The possible intramolecular collapse (due the hydrophobicity of the azo) and the eventual conformational transition under UV (induced by the photoisomerization) do not change the average distance between the objects (d  2/q*) and the power law exponent. Only the shape of the scattering curve may reveal these aspects, but no differences are observed (this is anyway difficult to evidence since we do not only observe the form factor of the chains at low q). A schematic picture of the structural organisation is given in Figure 20. Regime II (0.03 mol L -1  c  0.125 mol L -1 )

When we increase the concentration, we are more sensitive to the intermolecular interactions.

q* values in dark are now lower than those under UV (or NaPAA): the average distance between the objects is larger (dDark > dUV = dNaPAA). This implies the association (or the aggregation) of polyions. In this regime   0.2 (which corresponds to a clear departure from the classical dilution behaviour   0.33), the average number of chains implicated in the association (Nagg) is larger than unity and increases with c.

This association is induced by the large hydrophobicity of the trans-azo units and/or the interaction between inter-chain aromatic cycles (𝜋-𝜋 stacking). Under UV irradiation, the azo units undergo the photoisomerization from the trans form (hydrophobic form) to the cis form (hydrophilic form). Since PEs are more hydrophilic and the ring interconversion disturbs the 𝜋-𝜋 stacking, we observe the disassociation of the aggregates (Figure 21) and we recover the classical q*c 1/3 behaviour under UV values as with pure NaPAA. Since irradiated PAA-Azo are now solubilized at the molecular level, regime II is not present under UV.

The exact nature of the aggregate in dark is quite difficult to determine since the scattering pattern is very close to that a classical PE (only the maximum is shifted). The number of polyions implicated in the association is certainly very limited. No signature for a dense hydrophobic core (as for long chains) is visible.

We believe that the interaction between polyions only gives very loose aggregates. The regime II is very interesting since we can form, and completely disassemble small aggregates on demand by simple light stimulus.

This behaviour persists until 0.125 mol L -1 where the shape of scattering curves starts to change (yellow and green curve in Figure 22).

Figure 22 Scattering curves in dark close to regimes II and III. Data have been shifted vertically to better evidence the modifications around the maximum. Long dash line: q -2 decay, short dash line: q -3 decay.

Regime III (0.125 mol L -1  c  0.5 mol L -1 )

In this concentration range, q* values are constant in the dark state. From a geometrical point of view, as the chain concentration increases, the distance between the aggregate doesn't change (dDark). The aggregates concentration in the solution is therefore constant, and their aggregation number Nagg is increased.

This regime corresponds to a simple growth of the particles.

Under UV irradiation, contrary to what is observed in regime II, q* values do not match that of pure PAA. UV stimulus therefore break the aggregates and form smaller ones. Since Nagg is reduced, q* is higher. Curiously, q* does not vary with the concentration meaning that the same fraction of chains is dissociated when the light stimulu is applied (eq7).

The shape of the scattered intensity in dark (Figure 22) starts to deviate below 0.013 Å -1 . An extra contribution appears (with a scaling behavior close to q -3 as indicated in Figure 22) that looks very similar to stars shape molecules 10 . The origin for such an exponent comes from the particular radial density profile in the stars. The term star-like is certainly excessive in our case but we can say that a core is certainly forming in dark, with some arms around. The fact that our system behaves as a star-like assembly in this regime also indicates that the center of the particles does not form a large and dense core (as for long chains of PAA-Azo).

The growth of the particles in regime III in dark then leads to a star-like arrangement: a core is gradually forming due to the assembling of the Azo groups with a shell of PAA segments around (Figure 23). This correspond to the first step of the formation of a core-shell particle. Upon UV irradiation, the scattering curves look different. We do no longer have the signature of star-like assembly. UV light stimulus is therefore able to disassemble the azo cores leading to the formation of loose aggregates.

Regime IV (c ≥ 0.5 mol L -1 )

For the largest concentration (1 mol L -1 ), q* is slightly increasing in the dark state. The average distance between aggregate is reduced. It is quite difficult to extract a power law tendency to judge the global variation of Nagg with the concentration. Under UV, q* is decreased and matches the value found in dark:

we have now very similar aggregation numbers. Compared to lower concentrations under UV, the aggregation is larger.

The scattering curves in dark and under UV (displayed in Figure 24) are very close to the "star-like" assembly observed in the dark state of regime III. The maximum is however more pronounced and sharper. In the meantime, a large shoulder is found around 0.2 Å -1 . We believe the aggregates are simply better organized, which a small and denser core of hydrophobic azobenzene and a corona of PAA loops and chain ends. The description is now close to that of long chain except that the core is certainly smaller (Figure 25).

The local concentration of PAA in the shell may be very large leading to a smaller correlation length and the shoulder formation in the high q range.

Photo-induced effect are less important since the core is better defined with a better compactness.

Figure 24 Scattering curves in dark and under UV irradiation in regime IV. Long dash line: q -2 decay, short dash line: q -3 decay. The validation of this structural description would require the refinement of the data with a specific model especially in the high concentration regime. Cryo-TEM measurements are not possible since the size of the particles is too small, but also, the interesting concentration is too large (dilutions cannot be done as they modify the structural organization).

UV-Vis measurements estimated the trans/cis fraction under UV for this sample at different concentrations (Figure 3). The structural study shows that for c = 0.03, 0.06 and 0.125 mol L -1 (regimes I and II), we have loose aggregates in dark that are completely disassembled under UV irradiation. In the meantime, UV-Vis measurements have determined a very low trans fraction under UV-irradiation (10% trans, 90% cis) for these concentrations. The high efficiency of the UV light stimulus is a consequence of the structural organization in loose aggregates observed in the dark state. The trans → cis photoisomerization is facilitated.

On the other hand, at 0.25, 0.5 and 1 mol L -1 for which a gradual structural evolution and the formation of a core is presumed, the final trans ratio under UV is very high (40% trans, 60% cis). The photoisomerization process and the photostationnary state in the solution clearly depends on the structural organization that can impose geometric constrains that hinder the trans → cis isomerization.

V.3.6 Structural organization in short chain PAA-Azo solutions (azo = 0.15)

We have shown previously that short chains with a moderate azo content (11%) exhibit different structural organizations, from isolated polyions in solution to a core-shell assembly. In order to evaluate the role of the Azo groups in the association, we have increased the Azo grafting ratio close to the limit of solubility in water.

We have considered two distinct PEs: (PAA-Azo0. SAXS measurements and q* variation in dark state and under UV irradiation are given in Figure 26 and Figure 27 for both PAA-Azo. The general trends are very similar. If we focus on q* variations, and according to the previous section, only regime I (UV) and II (dark, UV) are observed.

For PEs in dark, q* variation can be described a single exponent law on the whole concentration range ( = 0.25 and 0.23 for (PAA-Azo0.15)39Sigma and (PAA-Azo0.15)54RAFT respectively), this behaviour corresponds to regime II.

Under UV radiation, two regimes are observed. Below 0.03 mol L -1 (as for (PAA-Azo 0.11 ) 42Sigma , Figure 19), q* reaches the values of pure NaPAA (regime I).

Above 0.03 mol L -1 , q* follow a power law variation with exponents 0.25 ((PAA-Azo0.15)39Sigma) and 0.23 (regime II, (PAA-Azo0.15)54RAFT).

At 1 mol L -1 , q* are almost identical in dark and under UV: the aggregation number does no longer vary with the UV light stimulus.

More information can be gained from a rapid inspection of the shape of the scattered intensities. For simplicity we just discuss the case of (PAA-Azo 0.15 ) 54RAFT which behaves similarly to (PAA-Azo0.15)39Sigma.

For the lowest concentrations in dark (c  0.03 mol L -1 ), we identify the signature of loose aggregates that can be disassembled by UV stimulus. This is very similar to (PAA-Azo0.11)42Sigma, but the concentration limit is significantly lower than for (PAA-Azo0.11)42Sigma. This is a direct consequence of the larger Azo content.

At large q, we find a q -2 intensity variation characteristic of PAA scattering in solution.

For c  0.06 mol L -1 in dark, we first find the signature of star-like particles that rapidly evolve with concentration. The maximum sharpens while a well-defined shoulder clearly appears around 0.15 Å -1 (Figure 27). The shoulder is more pronounced than with (PAA-Azo0.11)42Sigma and still originates from the scattering of PAA segments in solution. The correlation length associated to the PAA lattice is therefore smaller. Finally, the power low exponent around the correlation peak is larger than the previous characteristic q -3 variation observed for "star-like" (PAA-Azo0.11)42Sigma systems. This announces a well-defined structure factor and a more compact core. In conclusion, we still have objects with a core made of Azo moiety and a shell made of semidilute PE segments.

Figure 28 Scattering curves of (PAA-Azo0.15)54RAFT in dark at 1 mol L -1 . Long dash line: q -2 decay, intermediate dash line: q -3 decay, short dash line: q -4 decay.

This description is also valid for (PAA-Azo0.15)39Sigma, with a larger polydispersity (Figure 26). The only difference is the presence of a large upturn at low q value in the dark state that almost completely hides the scattering peak. This upturn indicates the formation of very large clusters in solution that disappears upon UV radiation. This effect is reversible and is therefore related to the presence of azobenzene groups. The origin is still unknown and the link with the polydispersity needs to be clarified. This is however a very interesting effect since UV stimulus allows here to modulate the structural organization at a larger length scale.

V.3.7 Conclusion

Four Azo-polymers with different lengths and different Azo fractions have been investigated. These systems show an important and reversible response upon light stimulus. The photoisomerization at the molecular level is responsible for a large structural reversible reorganization.

For all these systems, the addition of hydrophobic Azo groups to the hydrophilic NaPAA leads to the formation of spherical aggregates.

For long chains (PAA-Azo0.11)738RAFT, we find a compact core-shell structure in which the core is composed of Azo groups and the shell mainly contains PAA segments. Only a few chains are implicated in the assembly.

Under UV irradiation, the size of aggregates decreases, chains are released and form new aggregates. This behaviour is very similar to that of classical Azosurfactants in which UV irradiation leads to the formation of smaller micelles. Our system behaves similarly. It is therefore not "frozen" and can be partly assembled and disassembled on demand by simple light stimulus (UV, Blue).

The situation is more complex for small chains (PAA-Azo0.11)42Sigma, (PAA-Azo0.15)39Sigma, (PAA-Azo0.15)54RAFT. If we still have small aggregates in which Azo groups are located, we observe different regimes in concentration.

In particular, we have identified a small concentration range in which it is possible to completely disassemble small aggregates under UV radiation. This situation is still comparable to that of pure micellar solutions of Azo-surfactants. In these systems, the change of hydrophobicity (and critical packing parameter) induced by the photoisomerization shifts the CMC to higher values under UV. For concentrations in between CMCdark and CMCUV, UV irradiation simply breaks the micellar aggregates. We therefore have a concentration range in which micelles can be disassembled under radiation. In our system, the situation looks more complicated since the Azo groups are chemically linked to the polymer, but the underlying process is probably identical. Within this crude description, the

VI.1 Azo-surfactants

Azo-surfactants are composed of a polar head, an alkoxy spacer OR 2 , a lightresponsive Azo group and a hydrophobic alkyl tail R1. To simplify the notation, cationic Azo-surfactants are denoted xAzoyTAB, where x is the carbon number of the alkyl tail R1 and y is the carbon number of the spacer R2. The position of Azo unit along the surfactant architecture varies with the combination of R1 and R2.

Scheme 1 Structure of cationic Azo-surfactant, where R1 is the alkyl tail and R2 is the spacer chain.

According to the studies of Hayashita 4 and Blayo 1 groups, the micellar morphology and the extent of structural modification under light irradiation depend on the total length of hydrophobic segment and the position of Azo moiety. It also appears that the combination of short spacers and long alkyl chains has more influence on the hydrophobicity of Azo-surfactants. 1,4 In the present work, we chose a short two-carbon spacer for R2 (m = 2). For the alkyl R1, two chain lengths were considered: a butyl chain (moderate length, n = 4) frequently used as hydrophobic tail of Azo-surfactant 2,3,5-7 and a longer octyl chain (n = 8) is which is supposed to form more elongated micellar aggregates 1 .

Hence, 4Azo2TAB and 8Azo2TAB Azo-surfactants (Scheme 2) have been synthesized as described in Chap III.2.

Scheme 2 Structure of Azo-surfactants used in Azo-surfactant/PE systems.

VI.2 Characterization of micelles VI.2.1 Surface tension and critical micellar concentration (CMC)

The critical micellar concentration (CMC) is an important characteristic for surfactants in solution. It is defined as the surfactant concentration, above which micelles start to form. The CMCs of Azo-surfactants for both trans and cis isomers have been determined on a Kibron tensiometer by measuring the surface tension of a concentration series at 21 °C (in dark and under UV irradiation). The surface tension varies linearly with the logarithm of the concentration just below the CMC.

Above the CMC, it reaches a plateau. The CMC is located at the intersection of these two concentration regimes.

As shown in Figure 1 and Figure 2, the Azo-surfactants follow this general behaviour. The CMC is estimated at 2.6 mM for trans-4Azo2TAB and 8.2 mM for cis-4Azo2TAB. 8Azo2TAB is more hydrophobic, the CMC is found at 0.29 mM for the trans form and 0.82 mM for the cis form. All the determined CMCs agree with previously reported values 1,4 . The amount of surfactant molecules adsorbed per unit area at aqueous/air interface, also known as the surface excess concentration 𝛤 can be evaluated from the plots and the equation:

Γ = - 1 𝑘𝑇 ( 𝜕𝛾 𝜕𝑙𝑛𝐶 ) 𝑇,𝑃 (1)
where k is the Boltzmann constant (1.38 10 -23 J K -1 ), T is the absolute temperature (K). The area per molecule A at surface is defined as 1/𝛤. As shown in Table 1, the cis form of both 4Azo2TAB and 8Azo2TAB has a larger area per molecule value than the trans form. This is consistent with a bent conformation for the cis form. The area per molecule A of 8Azo2TAB is expected to be smaller than that of 4Azo2TAB, since the van der Waals interaction increases with increasing the chain length 8 . However, considering the accuracy of the measurements, we cannot conclude here.

Only the area per molecule of a similar Azo-surfactant 2Azo4TAB (with an ethyl tail R 1 = 2 and a butyl spacer R 2 = 4) can be found in the literature 9

VI.2.2 Photoisomerization study

VI.2.2.1 By UV-Vis spectroscopy

The photoinduced behaviour was probed by UV-Vis spectroscopy for both 4Azo2TAB and 8Azo2TAB solutions at a very low concentration (c = 0.08 mM < CMC). Absorption spectra are presented in Figure 3 and Figure 4 for different irradiation durations (UV and blue light).

The trans isomers present a maximum adsorption band at 350 nm, while the cis isomers are characterized by two adsorption bands at 312 nm and 435 nm (Figure 3a and Figure 4a). When exposed to UV light, the trans isomers progressively convert to the cis forms leading to a decrease of the band at 350 nm and the appearance of the bands at 312 nm and at 435 nm. For any irradiation time, the absorption spectrum is a linear combination of the pure trans and pure cis absorption spectra weighted by the trans/cis ratio. Two isosbestic points are present at 302 nm and 426 nm and simply reflect the superposition of the absorption spectra of the pure isomers. Reversibly, cis isomers convert to trans isomers quickly under blue light irradiation as shown in Figure 3b and Figure 4b.

We have extracted the trans/cis ratios during light irradiation by using the method described in Chap V.2.1. The trans molar fraction 𝑥 𝑡𝑟𝑎𝑛𝑠 as a function of the irradiation time is presented in Figure 3c and Figure 4c (during UV and blue exposure). The isomerization rates of 4Azo2TAB and 8Azo2TAB are very similar.

Reversibility

To characterize the switching efficiency and the reversibility of the process, Azo-surfactant solutions were irradiated alternatively by UV and blue light until they reach their equilibrium state. The trans fraction determined from UV-Vis absorption spectroscopy as a function of the irradiation cycles is presented in Figure 5. We find a good reversibility after 3 cycles. at the peak position. This leads to similar conclusions (𝐼 ∝ (𝜌 -𝜌 𝑠 ) 2 ) provided that the form factor and the structure factor are not modified when changing the contrast.

The root of the scattering intensity as a function of SLD solvent (different H2O/D2O mixtures) is presented in Figure 8b. The red and blue curves correspond to the fit with the theoretical model by considering all the measurements (blue) or only the three highest intensities (red). The SLD of 8Azo2TAB is estimated in between 0.87 and 1.00 10 10 cm -2 , which is very close to the theoretical expectation (1.00 10 10 cm -2 ). Theoretical values are therefore fairly reliable and will be used in the data analysis. The theoretical molar volume will also be taken for SAXS SLD calculation.

Attention should be paid about the physical meaning of the scattering length density. The estimated SLD for each surfactant is an average value (valid for 𝑞 → 0 where the molecule can be considered as point like). In fact, large difference in SLD can be found when considering different parts of the molecule (Figure 10 a and b). The Azo group has a SLD far from that of alkyl chains. When the surfactants form micellar aggregates, the micelles may have a heterogeneous organization from a SLD point of view: the surfactants may contribute to the scattered intensity at large q even if the contrast factor (𝜌 -𝜌 𝑠 ) 2 is equal to 0. From a small-angle scattering point of view, this system is equivalent to a core-shell organization with two distinct scattering length densities  core and  shell in a solvent solvent . Since water may penetrate in the aggregate and part of the counterions are released in the solution, but also, due to the presence of nonassembled surfactants in the solution, the volume fraction of the core-shell particles of the model may be slightly different than the real volume fraction of the solute.

core and shell depend on the composition of the micelles and are sensitive to the aggregation number, the counterions condensation (and therefore, the effective charge of the micelle) as well as the hydration process. In absence of water penetration in the hydrophobic core, the aggregation number Nagg can be estimated from the ratio between the volume of the core Vcore and the volume of the tail Vtail (Nagg = Vcore/Vtail ).

When increasing the surfactant concentration, the micelles grow and change their shape from spherical (just above the CMC) to ellipsoidal. However, their dimension is constrained in one direction by the size of the surfactant (in its fully extended conformation) to avoid the formation of a "hole" in the middle of the aggregate.

Behind its apparent simplicity, the intensity predicted by a core-shell organization can be very diverse and complex. It mainly depends on the radial SLD profile or more precisely, on the differences core -solvent and shell -solvent.

This particularity is at the origin of the differences usually observed in SAXS and SANS experiments on micellar solutions. As an example, if core -solvent <0 and shell = solvent (a common situation in SANS), only the core is visible (the intensity is similar to that of a 'one-shell' model, i.e., a single core). If core -solvent <0 and  shell - solvent > 0 (a common situation in SAXS), the first oscillation of the form factor can be largely amplified. If core tends to solvent, only the shell is visible. A very particular situation also occurs when the average scattering length density of the particle matches that of the solvent. The intensity tends to zero for q → 0 while the scattering at large q values in retained (zero average contrast situation).

For conventional cationic surfactants such as CTAB (cetyltrimethylammonium bromide) and DTAB (dodecyltrimethylammonium bromide) in SANS experiments, the excess scattering length density Δρ between C12 alkyl chain and D2O is -6.77•10 10 cm -2 . The TMA + head have a similar Δρ (-6.82•10 10 cm -2 ). Counterions close to the micelle surface are diluted with water. Their SLDs are not large enough to contribute to the intensity. As a result, the one-shell model (ellipsoid, homogeneous particle) is appropriate to analyze the data 11 . The one-shell ellipsoid model has also been applied to different Azo-surfactants 1,2,12 .

Nevertheless, in contrast to CTAB and DTAB, the Azo-surfactants contain an azobenzene group, whose SLD is quite different from that of alkyl chains (Table 2). A two-shells (core-shell) model (or a multishell model) may be more suitable to fit the data and could reveal a subtle organization within the micelles. In this description, contrary to the previous representation for "classical micelles" the shell is believed to contain the polar head but also hydrophobic parts of the surfactant.

Hence micellar aggregates have also been modeled by a core-shell ellipsoid with minor (Rb) and major (Ra) radius and two independent SLD as shown in Scheme 3. This model allows to explore a wide range of morphologies, from oblate to prolate and includes also spherical geometry. For SAXS measurements, the scattering length density can no longer be considered as uniform in the aggregate. Even for CTAB and DTAB the cloud of counterions now clearly contributes to the intensity and a core-shell organization is needed to analyze the data. For Azo-surfactants, the situation is even more We analyzed the data with different geometrical models, from spherical to ellipsoidal. All the form factors P(q) were combined to a structure factor S(q) developed for charged colloids in a repulsive screened Coulomb potential 13,14 .

This structure factor has been widely utilized with charged micelles and accurately reproduces the interference function between particles. No general "scale factor"

was introduced in the model to account for error in the absolute scale determination. Thus, the absolute intensities in SANS and SAXS experiments are supposed to be accurate.

The shape of the correlation peak depends on the electrostatic repulsions and varies with the effective charge of the objects (strength of the repulsive interactions) and the ionic strength of the solution (screening effect). Its position is related to the particle concentration or to their volume fraction . Due to the water penetration in the micelles or the counterion release, but also, to the presence of free surfactants above the CMC, the volume fraction of the model is close -but different -than the real volume fraction of the solute in the solution. In order to properly reproduce the correlation peak, the volume fraction  was therefore fitted during the analysis (and compared to the experimental one).

For similar reasons, the salt concentration in the solution and the charge of the micelles were also fitted. The first should be close to the CMC. The second should lead to a dissociation rate of the bromide ions of the order of 15 -30% as found for similar polar heads (dissociation rate = charge/Nagg). In any case, the final values of these parameters do not change the size and shape of the particles and are of little relevance in the present work.

For homogenous systems (one-shell model), the SLD of the particle directly impacts the global intensity (I(q) ~  2 ). The SLD value was first set to that of the surfactant (in its ionic form) and kept fixed during the refinement. This parameter was then relaxed and fitted in a second time. The difference between the fitted SLD and that of the surfactants may have a physical origin (indicating for instance a water penetration in the micelle), but could also come from experimental problems such as a wrong estimation of the theoretical molar volumes (leading to an erroneous surfactant SLD) or difficulties with the absolute scale determination procedure.

In many micellar systems, the shape of the aggregates is changing with the concentration, from spherical just above the CMC, to anisotropic (such as ellipsoidal particles). We therefore analyzed the data with different simple globular models.

The simplest spherical model associated to a large polydispersity lead to unphysical results and was definitively discarded (large standard deviation and very small SLD). Other models for monodispersed ellipsoids (oblate/prolate) with a single shell or a core-shell scattering length density profiles were tested and were quite successful. Only the best results obtained with one-shell and core-shell monodisperse oblate ellipsoids will be presented in this section.

Fits are shown in Figure 12 and Figure 13 for 4Azo2TAB and 8Azo2TAB using the one-shell and the core-shell models. Fits are rather satisfying.

Below 0.15 Å -1 , almost no difference can be observed in the fit quality with these two models.

For 4Azo2TAB micelles, a slight difference appears in the high q region around 0.15 -0.4 Å -1 (Figure 12 a). The deviations are even more pronounced for 8Azo2TAB micelles (Figure 13 a). In order to improve the fit quality at high q, data were also fitted with a core- One-shell and core-shell models inevitably lead to the same results. In the present situation, we believe that the core of the micelle is preferably composed of alkyl group, while the rest of volume (shell) mixes all the other chemical groups.

Due to the large difference between the SLD of the azo (and the oxygen) in the shell and the alkyl chain (in the core), we expect an average non uniform radius SLD profile that can be better modelized by a core-shell description. In any case, these effects are certainly not very large since the volume of the core is probably much smaller than that of the shell. During the fitting procedure, we imposed the SLD of the core to that of the C4H9 or C8H17. This model improves the quality of the fit in the large q range specially for 8Azo2TAB.

The fit parameters are summarized in Table 3 (one-shell model) and Table 4 (core-shell model).

One It is not evident to decide whether the polar head is visible or not with neutron.

Very often it is supposed that the ionic heads are mixed with solvent molecules reducing their contrast and making them 'invisible'. For trimethyl ammonium head groups, their hydrophobicity prevents water penetration. Since their contrast in D2O is important, they certainly contribute to the scattered intensity. 15 Other studies report high penetration of water inside Azo-surfactant aggregates. This argument is motivated by the variation of scattering length density of the fitted particle. 1 In the present work, we will consider that the ammonium contributes to the scattering and that water may penetrate in the micelles. The aggregation number can be determined from the volume of the dry aggregate (i.e., without water) and the surfactant (in its ionic form).

𝑁 𝑎𝑔𝑔 = 𝑉 𝑑𝑟𝑦 𝑚𝑖𝑐𝑒𝑙𝑙𝑒 𝑉 𝑠𝑢𝑟𝑓 = 𝑉 𝑚𝑖𝑐𝑒𝑙𝑙𝑒 (1 -∅ 𝑤𝑎𝑡𝑒𝑟 ) 𝑉 𝑠𝑢𝑟𝑓 (3)
∅ 𝑤𝑎𝑡𝑒𝑟 is the water content that can be extracted from the fitted SLD of the particle and the theoretical SLD of the surfactant :

𝜌 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒 = ∅ 𝑤𝑎𝑡𝑒𝑟 𝜌 𝑤𝑎𝑡𝑒𝑟 + (1 -∅ 𝑤𝑎𝑡𝑒𝑟 )𝜌 𝑠𝑢𝑟𝑓 (4)
⇒ ∅ 𝑤𝑎𝑡𝑒𝑟 = 𝜌 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒 -𝜌 𝑠𝑢𝑟𝑓 𝜌 𝑤𝑎𝑡𝑒𝑟 -𝜌 𝑠𝑢𝑟𝑓 (5) Under UV irradiation, an oblate ellipsoid model is still adapted. The effective length of Azo-surfactants decreases due to the trans to cis isomerization. The their aggregation number.

To reveal the structure of the micelles, SAXS curves were fitted with the same oblate ellipsoid core-shell model (except the trans-8Azo2TAB curve as explained later). It must be noted that the one-shell model doesn't work and can't reproduce the large intensity observed around 0.2 Å -1 . Only a core-shell structure in which the core contains alkyl chains (𝜌 𝑤𝑎𝑡𝑒𝑟 > 𝜌 𝑐𝑜𝑟𝑒 ≈ 𝜌 𝑎𝑙𝑘𝑦𝑙 , fixed parameter) and a shell with 𝜌 𝑠ℎ𝑒𝑙𝑙 > 𝜌 𝑤𝑎𝑡𝑒𝑟 (fitted parameter) can reproduce the data. In this approach, the shell contains the rest of the surfactant molecule, but also a layer of condensed counterions (that certainly explains slight differences in the shell radius). The aggregation number Nagg is calculated in the same way as for SANS data. But it is only an estimated value due to the presence of an additional layer of condensed counterions. The geometrical parameters of trans-4Azo2TAB micelles are consistent with that found by SANS. The trans-8Azo2TAB curve could not be fitted with the core-shell model due to the presence of heterogeneous aggregates in the solution (at the origin of a "strange" scattering contribution around 0.1 Å -1 ). SAXS measurement have been performed at a temperature very close to the Krafft temperature of 8Azo2TAB (the "equivalent" CTAB has a Krafft temperature around 20 -25 °C16 ).

VI.2.4 Conclusion

As a conclusion, 4Azo2TAB and 8azo2TAB self-assemble in oblate ellipsoidal aggregates above the CMC. SAXS and SANS data could be fitted with a coreshell model in which the core is composed of hydrophobic alkyl tail, and the shell, which is partially hydrophobic, contains the rest of the molecule. With a longer alkyl tail, the volume of trans-8azo2TAB aggregates is 4.5 times larger than that of trans-4azo2TAB aggregates. Under UV irradiation, 8Azo2TAB and 4Azo2TAB micelles undergo an important structural transition. Their shapes are still ellipsoidal, but their volumes are reduced by a fraction from 3 to 6. The phase diagram of Azo-surfactant/PE mixtures in aqueous solutions depends on several parameters, such as the Azo-surfactant/PE ratio, their concentrations, but also intrinsic properties of the PE (molecular weight, stiffness, charge) and of the surfactant (packing parameters).

VI.3 Azo-surfactant/Polyelectrolyte complexes

Different molar ratios r of Azo-surfactant/PSS from 0.15 to 1.0 have been tested. Mixtures of 4Azo2TAB and PSS are soluble for ratios smaller than 0.5 (Table 6). 8Azo2TAB and PSS gives homogeneous solution until r = 0.6 (Table 7).

Both mixtures precipitate when approaching the charge stoichiometry (charge neutralization r = 1). The scattering curves are presented in Figure 18 after normalization to the concentration.

Without going into the details of the structural organization, we recall that the scattered intensity contains both intramolecular (or intra-particular) and intermolecular (inter-particular) contributions. In the colloidal approach, I(q) is written as the product of the form factor P(q) by the structure factor S(q).

Curves in dark (Figure 18 a) are superposed above 0.05 A -1 , which means that the local structure doesn't vary with the concentration. Only the form factor is observed in this q range. This definitively validates our assumption that the oscillation around 0.2 A -1 is a characteristic of the form factor of the aggregates.

On the contrary, curves measured under UV (Figure 18 b) do not completely overlap. For the mixtures denoted C (4Azo2TAB/PSS 50 mM/100 mM) and C/2 (25 mM/50 mM) measurements are superposed: the final states at the local level are identical. This is no longer the case for 3C (150 mM/300 mM). The oscillation is less shifted to higher q, indicating a larger trans molar fraction at the stationary state. This effect is probably a consequence of the higher viscosity of the system that slows down the diffusion of the particles and the homogenization of the solution.

In conclusion, the mixture of Azo-surfactant/PSS 50 mM/100 mM (r = 0.5)

is the best compromise (larger scattering, larger modifications under UV, acceptable absorption) and will be considered as our reference system.

VI.3.1.3 Diameter of the capillary

Due to the strong UV absorption of azobenzene, the diameter of the capillary affects the photoconversion. 1 mm and 1.5 mm capillaries were tested under identical UV irradiation conditions (Figure 19). In the dark state, the experimental curves overlap indicating that the diameter of the capillary does not influence the scattering measurements. Under UV, the position of the oscillation for the 1.5 mm capillary is less shifted to higher q. This indicates that the trans/cis transformation is not optimal even after a long equilibrium time. It is therefore better to use a 1 mm diameter capillary for SAXS measurements.

VI.3.3 Structural study by NMR

NMR was used to confirm the association between Azo-surfactants and PSS chains. As shown in Figure 24, hydrogenated PSS (PSS-H) has two broad peaks at 6.5 ppm -7.5 ppm corresponding to the aromatic hydrogens (H c , H d ) and a broad peak at 1.74 ppm corresponding to hydrogens on the polymer backbone (Ha, Hb). 4Azo2TAB presents two peaks at 7.6 ppm and 6.8 ppm associated to azobenzene protons (H4-H7), and two peaks at 4.2 ppm and 3.6 ppm related to the hydrogens of the spacer (H2, H3). The intense sharp peak at 3.1 ppm corresponds to the quaternary ammonium head (H1). The alkyl tail of 4Azo2TAB has four peaks between 0.9 ppm and 2.7 ppm(H8-H11).

For the mixture of PSS-H and 4Azo2TAB, all the peaks of aliphatic protons are broader and slightly shifted. The peaks in the aromatic zone (6-8 ppm) are overlapped and difficult to distinguish. To resolve this problem and reveal only the contribution of 4Azo2TAB, we used deuterated PSS (PSS-D). As shown in Figure 25, the spectrum looks almost identical except that the broad peaks at 7.41 ppm and 6.10 ppm disappear. Hence, these two broad peaks belong to PSS. They are broader and shifted meaning that PSS chains in the mixture are not as mobile as in the pure PSS solution. The four broad peaks between 6 ppm and 8 ppm correspond mainly to the aromatic protons of 4Azo2TAB and are quite different from that of pure 4Azo2TAB (Figure 24, green curve) indicating that 4Azo2TAB interacts with PSS in mixture. We compare in Figure 26 the NMR spectra realized in dark, as well as under UV and blue light using PSS-D (only the peaks of the Azo-surfactants are visible).

Under UV, the peaks are split due to the presence of the trans/cis isomers in the mixture. Since most of the peaks are broad and overlapped, it is quite difficult to estimate their respective contributions. An interesting peak is the singlet at 3.01 ppm corresponding to the ammonium head. Under UV, it splits in two identifiable contributions at 3.10 ppm and 3.01 ppm (corresponding to the trans and cis isomers respectively). These peaks are partially superposed, but we can roughly estimate the proportion of trans/cis isomers. At the stationary state, the proportion for trans isomers is approximately 28% (72% for cis isomers). The variation is less important that that observed for pure Azo-surfactant. This transformation is reversible by simple blue light irradiation (20 % cis and 80 % trans). The experimental chemical shift displacement Δδ of the different peaks vary with their location along the surfactant (Table 8). The protons of hydrophobic tail H8 -H11 show downfield shifts (positive Δδ), while the protons of polar head H1 and of spacer H2, H3 are slightly shifted upfield (negative Δδ). It is more complicated for aromatic protons (for which the correspondence is difficult to establish). We can speculate that these protons are partially shielded (H4) and the others deshielded (H 5 , H 6 , H 7 ). The non-monotonic variation of the chemical shifts indicates that the nuclear environment varies along the surfactant molecule within the aggregate. The sign and the magnitude of Δδ can be explained by a ring current effect due to the presence of the aromatic rings of PSS chains. In the presence of an external magnetic field, the delocalized 𝜋 electrons of the aromatic ring generates a ring current, which creates its own magnetic field (Scheme 4). The induced field above and below the plane of the ring is in the opposite direction to the external field, thus protons located above and below the ring shift upfield (negative Δδ). At the edge of the ring, the induced field is in the same direction as the external magnetic field, and protons are shifted downfield (positive Δδ). At low q values and up to 0.03 Å -1 , each curve shows a similar plateau (as for X-rays). In this q range, we are dependent on the long-range intermolecular correlations. Above 0.03 Å -1 , the intensity profiles vary with the contrast: the partial structure functions of the polyions and the surfactants are therefore not identical.

In other words, we have a heterogeneous structural organization at the local scale.

The presence of a well-defined oscillation in the polyion partial structure function around 0.25 Å -1 (measured in H2O/D2O, D2O = 0.267) indicates a global reorganization of the polyions and the formation of nanometric objects. Pure solutions of NaPSS (even in presence of salt) never show this behaviour.

The partial structure function of the surfactant (measured in D2O, D2O = 1)

shows similarities with the scattering of pure surfactants in the high q range that indicates the presence of globular particles. Another important feature is the presence of a large shoulder in between 0.03 and 0.1 Å -1 . This characteristic signature is related to the short-range correlations between the particles. They are therefore not uniformly distributed in the solution (like simple micelles) and tend to assemble to form larger objects (aggregate of aggregates).

All these remarks suggest a picture where polyions wraps around globular aggregates and link them together. More specifically, polyions and surfactants form the small globular objects. The surfactants are in the core of the aggregate while the polyions are rather found at the surface. According to NMR measurements, part of the chain can be mixed with the surfactant in the aggregate.

For long enough PE, these globules are linked by polyion segments and form a pearl-necklace organization. This kind of structure is often claimed in the literature to describe the surfactant/PE complexes in solution. However, there is very little structural experimental evidence of its existence and even less of the mutual organization of the different components.

From a practical point view, a pearl can be modelled by a simple core-shell assembly. This is of course a very simplified description, but it captures the main characteristics of the structure. In our pearl necklace model (Figure 32), these pearls are supposed to form ellipsoidal core-shell aggregates and they are arranged on a freely jointed chain of adjustable segment size d. The form factor for such a system has been derived by Schweins et al. 21 for hydrophobic polymers and spherical uniform pearls. In the present situation we use a simplified version and neglect the link between the pearls (extended polyions segments). For the fits, we assume that the scattered intensity is dominated by the form factor of the pearl necklace P(q) (S(q)=1). Since experimental measurements always contain longrange intermolecular correlations in the low q region, the intensity is only fitted on a restricted q range (qmin = 0.05 Å -1 for 4Azo2TAB/PSS and 0.04 Å -1 for 8Azo2TAB/PSS). The fits are presented in Figure 33 and Figure 34 and polyions still form a necklace like structure. The volume of the pearls is now reduced by 29% compared to the dark situation. The variation is smaller than that observed for pure micelles (69% for 4Azo2TAB micelles and 83% for 8Azo2TAB micelles).

VI.3.4.3 SAXS results

The SAXS analysis is similar to the SANS study (same model). As for neutrons scattering the SLD of the core is fixed to that of the alkyl chain. The volume fraction is also set at its theoretical value. Fits for 4Azo2TAB/PSS and 8Azo2TAB/PSS are presented in Figure 36.

Fit parameters are given in Table 12. The model reproduces well the data.

The shell radii are however larger than those determined by SANS (by a few Å).

This effect could be due to the presence of a condensed counterions layer that is always more visible with X-rays than with neutrons.

Under UV, large modifications are observed at high q indicating a structural rearrangement (pearl size reduction). We observe a variation of the pearl's size

Reversibility

The reversibility of the photoinduced structural modifications was tested by SAXS on 4Azo2TAB/PSS and 8Azo2TAB/PSS at the concentration C (Figure 37).

We followed the position of the oscillation around 0.2 Å -1 in the steady state under UV and blue irradiation (during several UV  blue cycles). Both systems show a perfect reversibility. For 4Azo2TAB/PSS, small objects are present but difficult to observe. At the most, we can notice that these small aggregates are not homogeneously distributed in the film and tend to assemble with others (circle in Figure 39). For 8Azo2TAB/PSS, small particles are easily visible (Figure 40 and Figure 41). They are not homogenously distributed and clearly form pearl necklace assemblies. This definitively validate the necklace organization. The size of the pearls ( ≈ 7 nm) also coincide with small angle measurements. From these images, it is clear that the number of pearls per necklace can be larger than 2.

The exact number of pearls is complex to determine and may also vary from one necklace to another. This example highlights the complementarity between electronic microscopy and scattering technics to reveal the structural organization in soft matter systems.

VI.3.5 Conclusion

We have investigated the structural organization of Azo-surfactant/PSS complexes and their photoisomerization behaviour. Both NMR and SANS studies indicate that polyions interact with oppositely charged Azo-surfactants due to the electrostatic interaction but also, participate to the micellization (co-micellization).

The cis fraction at the photostationary state determined by NMR is around % (28% trans isomers, 72% cis isomers) under UV radiation. The system recovers 80% trans isomers (80% trans isomers, 20% cis isomers) under blue irradiation.

We have studied the structural organization by SAXS and SANS. We demonstrated the presence of micellar aggregates forming a necklace organization. Pearls are made of surfactants and polyions. The internal organization is however heterogenous and can be described as a core-shell structure. The core contains the hydrophobic tail of the surfactants, while the shell is an intimate mixture of polyions, Azo groups, C 2 H 4 spacers and polar heads. The complexation can rather be described by a co-micellization.

The aggregates formed by Azo-surfactant and PSS are smaller than micelles of pure Azo-surfactants. The distance between aggregates (pearls) is almost twice larger than the major shell radius. It seems like that 'pearls' are close to each other. This is further confirmed by the Cryo-TEM images. For 4Azo2TAB/PSS, the major and minor shell radius are 31 -26 Å and 22 -18 Å respectively. Larger major and minor shell radius are found for 8Azo2TAB/PSS: 35 -32 Å and 24 -20 Å.

support its own weight. It becomes fluid when the UV light is applied (Figure 1). As shown in Figure 3, pure NaSal solution presents well-defined narrow peaks. 8Azo2TAB solution whose concentration is above the CMC, has broader peaks due to the formation of micelles. Surfactant in the micelles and individual surfactants in solution are in thermal equilibrium: we always have dynamical exchanges between the two populations. The resulting peak is an average of all the peak positions corresponding to the free and self-assembled surfactants.

Since the concentration of 8azo2TAB is much higher than its CMC (CMCtrans = 0.29 mM), the detected signals almost correspond to the hydrogens of micelleforming molecule.

The resonance peaks of 8Azo2TAB/NaSal mixture (C) are much broader (blue curve in Figure 3) and the peaks of 8Azo2TAB at 4.23 ppm and 3.60 ppm corresponding to the spacer C2H4 even disappear (H2, H3) indicating the formation of larger aggregates. Generally, aromatic rings generate a negative contribution to the local magnetic field, resulting in upfield shifts for the adjacent molecules [1][2][3] .

The intense peak of ammonium head at 3.1 ppm is shifted to 2.9 ppm (upfield), which indicates that NaSal molecules approach the polar head of 8Azo2TAB surfactant. The overlapping peaks associated to the hydrophobic tail (around 0.8-1.5 ppm) are unaltered, which means their environment is not influenced upon the addition of NaSal. All these modifications suggest that NaSal molecules are packed in the hydrophilic region of 8Azo2TAB aggregates. The previous description only applies for the dark state. Since large photoinduced structural modification appears under UV, we expect to see a clear signature of the new arrangement by NMR. However, the spectra under UV light does not show much difference from that in the dark state even after 20 h of UV irradiation (Figure 4, green curve). We barely observe the splitting of peaks due to the coexistence of trans and cis isomers, but we cannot extract useful information.

A less concentrated sample, C/4 mixture was thus investigated (Figure 5). A similar spectrum for the dark state was observed. But the photostationary state was reached faster (after 8 h under UV irradiation). The UV curve shows the splitting of certain peaks which is typical for the coexistence of trans and cis isomers. The two peaks attributed to the spacer of 8Azo2TAB (H2, H3) reappear (Figure 5, green curve). The sharp intense peak (H1) located at 2.82 ppm is split 201 into two peaks, at 3.01 ppm for trans isomers and 2.93 ppm for cis isomers. It allows to determine the trans/cis ratio in the solution. At the equilibrium, the trans/cis ratio can be estimated to 26:74.

Under blue light irradiation, the reverse photoisomerization occurs (cis to trans). The spectrum (blue curve in Figure 5) is quite similar to the one in dark state (red curve in Figure 5), while the small peak at 2.72 ppm indicates that there are still cis isomers left. The trans/cis ratio is determined as 81:19.

VII.1.3 Structural study

The macroscopic rheological properties are governed by the microstructure of the system. Scattering methods as well as electronic microscopy are The images clearly show two distinct organizations. In the dark state, we find wormlike particles with a diameter between 5 to 7 nm and length of several micrometers. After 4 hours of UV irradiation, we find mainly spherical objects with a diameter around 4 nm but also small wormlike particles with the same diameter but with lengths in between 10 to 29 nm. This morphological transition under irradiation explains the variation of the rheological properties. In order to get a better picture of the local organization, these observations need to be combined with small angle scattering experiments.

To get the meaning of the fitted SLD values, it is necessary to recall the SLD of the different components in the system. They are presented in Table 4. The SLD of the particles are nearly identical to the expected one (Table 4). The scattering curves normalized to the concentration are presented in Figure 10 for C, C/2 and C/4 in the dark state and under UV irradiation. For both measurements, we find a large maximum around 0.2 Å -1 , that is characteristic of micellar aggregates.

ρ x (10 +10 cm -2 ) ρ N (
In the dark state, as for neutrons experiments, the highest concentration is not superposed to the others due to the contribution of the structure factor. C/2 and C/4 are superposed: the structure factor is negligible; we are only sensitive to the form factor.

Under UV, and for same reason, the concentration C is not superposed on the others. Furthermore, the minimum found around 0.1 Å -1 for C/4 is not as deep as the one found for C/2. This could reflect a variation of the ellipticity of the ellipsoid with the concentration or a small polydispersity. Additionally, as for neutron experiments, we never see the Guinier regime. Despite a more efficient irradiation setup, we cannot avoid the coexistence of globular and elongated particles in the system. We have fitted the scattered intensity with the same shapes (elliptic cylinder and prolate ellipsoid) as for SANS measurements. The only difference is the introduction of core-shell organization. With the exception of the shell thickness and its own SLD, everything is similar to the neutron analysis. In order to minimize the number of adjustable parameters, we have considered a constant shell thickness for the prolate ellipsoid and a constant ellipticity for the elliptic core-shell cylinder.

The fits are presented in Figure 8. Data are well reproduced in the selected q range. Final parameters are summarized in Table 5. SLD shell (10 10 cm -2 ) 11.67 10.5

Table 5 SAXS fit parameters. In the elliptic core shell cylinder model, the section has four radii: the major and the minor shells and the major and the minor core. The ellipticity is identical for the core and the shell. In the prolate core shell ellipsoid, the major axis corresponds to the rotation axis. The cross section perpendicular to the axis of symmetry has a circular core-shell geometry. The thickness is held constant. Asterisks correspond to fixed parameters. The parameters for the aggregate under UV irradiation should be taken with care (the system is polydisperse and the fit is restricted on a very particular q range).

First of all, in the dark state SAXS and SANS give exactly the same result for the cross section of the micelle (major and minor shell radius of the core-shell elliptic cylinder: 33.0 and 20.4 Å; major and minor radius of the elliptic cylinder:

32.8 and 20.1 Å).

Second, the SLD profile determined from the SAXS analysis is very particular.

The SLD of the core (8.35 10 10 cm -2 ) is lower to that of the water (9.40 10 10 cm -2 ) and close to that of the alkyl tail (C8H17, 7.56 10 10 cm -2 ). The SLD of the shell (11.67 10 10 cm -2 ) is larger to that of water and close the average 8Azo2TAB/NaSal value (11.88 10 10 cm -2 ). In other words everything happens as is the core was mainly composed of the C8H17 tails of the surfactant while the shell is made of azo and hydrophilic groups but also, salicylate molecules. This is in accordance with NMR results. The same trend is observed under UV (these results are however less relevant due to the absence of a well-defined form factor and the very limited q range for the fit).

Finally, we monitored the thermal relaxation by SAXS (C/4). The variation of the position of the maximum around 0.2 Å -1 (characteristic of the surfactant assembly) is given the Figure 13. A characteristic time of 22.2 h was obtained using an exponent decay relaxation.

VII.2 Self-assembly of 8Azo2TAB/sodium benzenesulfonate

The chemical structure of NaBenSulf is very close to that PSS monomer (Figure 14). But they show different phase behaviours when mixed with Azosurfactants.

Using the same concentrations as for Azo-surfactant/PSS complexes, the mixture (8Azo2TAB/NaBenSulf 50 mM/100 mM, r=0.5) precipitates. However, similarly to NaSal, for 8Azo2TAB/NaBenSulf 75 mM/62.5 mM, r = 1.2, reference concentration C), the mixture shows gel property (Figure 15). We can speculate that similar threadlike micelles are formed. But the differences in the chemical structure between NaSal and NaBenSulf may result in different micellar morphology that we propose to investigate hereafter.

VII.2.1 Study by NMR

The mixture 8Azo2TAB/NaBenSulf C/4 (18.75 mM/15.62 mM) has been analyzed by 1 H NMR (Figure 16). The 8Azo2TAB/NaBenSulf mixture shows more broader peaks than their individual spectra. We clearly observe that the protons in the hydrophilic region of 8Azo2TAB (H1, H2, H3) are shifted upfield for the mixture. While, the protons of hydrophobic region (H8 -H15) is not influenced by the addition of NaBenSulf. Hence, we can assume that NaBenSulf molecules are intercalated between hydrophilic ammonium head and participate to the micelle formation. This will further be proved by small-angle scattering study. In the elliptic cylinder model, the section has two radii: the major and the minor. The ratio between the major and the minor radius gives the ellipticity. In the prolate ellipsoid, the major axis corresponds to the rotation axis. The cross-section perpendicular to the axis of symmetry is circular. Asterisks correspond to fixed parameters.

The structural parameters are very close to that of 8Azo2TAB/NaSal C/4. The description of the aggregate under UV radiation is however more accurate since the particles are now monodisperse. The shape of the aggregate is not dramatically modified by changing the aromatic salt. The SLD of the particle (1.18 10 10 cm -2 ) is very close to that of the pure components (1.26 10 10 cm -2 ).

VII.2.2.2 SAXS measurements

Typical SAXS measurements for different concentrations C and C/4 are presented in Figure 21 and Figure 22. The mixture C/2 is not studied here because it precipitates. 8Azo2TAB/NaBenSulf presents a maximum in between 0.1 and 0.2 Å -1 which is characteristic of micellar aggregates. This maximum is clearly modified under UV irradiation indicating a large modification of the micellar shape.

Data normalized to the concentration are almost superposed at large q. The deepth of the minimum is however higher under UV reflecting slight modification of the ellipticity or a smaller polydispersity.

From a general point of view, the local structure of the aggregates is preserved at different concentrations. At low q, we find small differences (below 0.03 Å -1 ) due to the presence of a structure factor. The effects of structure factor almost disappear at C/4 but are still present. The fits have been only performed on the lowest concentrations using an adapted q range. SAXS data have been fitted using an elliptic core-shell cylinder (dark state) and a prolate core-shell ellipsoid (under UV irradiation). The fits are given in Figure 23, the final parameters are in Table 7. accordance with NMR results. Under UV irradiation, we note a large decrease of the shell thickness in which Azo groups are embedded.

We have also tested the structural reversibility of the system (concentration C) by using alternative UV and blue light irradiation. The variation of the position of the maximum (around 0.2 Å -1 ) is shown in Figure 24. A good reversibility is observed.

VII.3 Comparison of wormlike micelles systems

The growth of 8Azo2TAB micelles and the formation of wormlike structures (clearly demonstrated by cryo-TEM) is induced by the addition of both NaSal and NaBenSulf. This phenomenon results from the electrostatic interaction between the cationic headgroup of the surfactants and the anionic co-surfactants, but also, from the hydrophobic interaction and probably the 𝜋 -𝜋 interaction between aromatic groups. The addition of aromatic salts changes the "average" critical packing parameter of the surfactant (as presented in Chap II.2.2) and finally the shape of the micellar assembly. In the dark state, it favours the formation of wormlike micelles, while under UV it provokes the formation of globular particles.

A reversible gel↔fluid transition is observed under alternative UV/bleu irradiation.

According to NMR studies, protons of the hydrophilic region (H1-H3) are shifted upfield with addition of aromatic salts, while the hydrophobic tails were almost unchanged. Aromatic anions are therefore intercalated into the hydrophilic layer of the micelles due to the electrostatic interaction with cationic ammonium headgroups. The aromatic region is quite complicated to analyze due to the coexistence of protons of azobenzene and aromatic anions. But similar investigation on 8Azo2/PSS systems demonstrates that the external added aromatic rings of PSS interact with azobenzene moieties. We assume here that the aromatic salts (which is more mobile in the solution than the PSS monomer) may also penetrate in the aggregate even more deeply and interact with phenyl groups of azobenzene moiety due to the 𝜋-𝜋 interaction These arguments have been validated by SANS and SAXS experiments.

Wormlike micelles observed in the dark state have been modelled by elliptic cylinders. Globular particles found under UV radiation were modelled by prolate ellipsoids. SAXS have evidenced the presence of a core-shell organization within the objects. The core is made of alkyl tail (C8H17) while the shell contains mainly the Azo groups, the spacer (C2H4) and the hydrophilic headgroup of the surfactant, but also, the aromatic anions.

In the dark state, both wormlike micelles have a minor radius of 20 -23 Å and a major radius of 33 -37 Å. Under UV irradiation, the wormlike micelles break and form smaller aggregates resulting in a significant decrease of viscosity (Figure 26). At the photostationary state, the 8Azo2TAB/NaSal mixture (C/4) contains 74% of cis isomer (trans/cis ratio 26:74) resulting in a situation where globular shapes

General Conclusion and Perspectives

This thesis aims to study model photo-sensitive PEs systems in aqueous solutions. The photo-sensitivity arises from the presence of azobenzene groups.

We are more specifically interested in the photoconversion behaviour, the structural organization and the structural variation observed under UV irradiation.

Three different light-responsive systems have been synthesized and studied, they are:

➢ Azo-modified sodium polyacrylates ➢ Azo-surfactant/PSS complexes ➢ Azo-surfactant/co-surfactant wormlike micelles These three systems have been investigated by UV-Vis spectroscopy, NMR, small-angle scattering and cryo-TEM leading to the following conclusions.

➢ Azo-modified sodium polyacrylates

The grafting of Azo moiety on side group of sodium polyacrylates increases the hydrophobicity of PEs and leads to the formation of spherical photo-sensitive aggregates in aqueous solution. The structure of these aggregates depends on the polymer length and the Azo fraction as well as the concentration.

We find that compact core-shell aggregates are formed for the long chains ((PAA-Azo0.11)738RAFT). The dense core is made of the Azo groups while the shell is composed of polyion segments. Only a very limited number of chains are inserted in the aggregate. Upon UV irradiation, aggregates are partly disassembled and form smaller particles.

For short chains with 11% Azo, we still find macromolecular association, but the structural organization now varies with the concentration. Different regimes are observed. Short chains with 15% Azo show similar organization, but the core is more compact. In these systems, it is possible to find a concentration range in which loose aggregates can assemble and disassemble by simple light stimulus.

This regime could be interesting for certain applications such as active compound carrier. The hydrophobic core may encapsulate the active compound and release it under UV irradiation. The aggregate size can be adjusted by varying the polymer length and the Azo molar fraction.

In these systems, Azo units are grafted randomly on polydisperse PE chains.

The polydispersity can affect the structural organisation and increases the The synthesis of amphiphilic methacrylates containing hydrophilic oligo(oxyethylene) segments and Azo moieties has been attempted by radical homopolymerization. Methacrylates (MEn) were introduced in benzene at 60°C for 24h in the presence of AIBN (Figure 3). However, the homopolymerization of MEnME did not occur neither in the same condition of MEn nor in DMSO at 70°C for 24h. this is likely due to the formation of stable radicals that are unable to sustain the chain propagation 3 . The copolymerization of a functional monomer (containing the Azo) and a hydrophilic monomer may be the straightforward approach. Unfortunately, this method does not work for all the monomers such as the example of MEnMe shown in Figure 3. Azobenzene is known to have strong inhibition to the radical polymerization process.

The solvent, temperature, type and amount of initiator must be chosen carefully.

The radical copolymerization is badly controlled, copolymers have usually a large polydispersity. An alternative method is the post-functionalization of hydrophilic welldefined polymers. Precursor polymers with reactive groups can be readily prepared by classical polymerization process.

A series of raneoi copolyiers (Figure 4) containing a minor fraction of octadecylamine and/or a primary Azo-amine were synthesized by Pouliquen et al. 4 .

The grafting procedure was carried out in N-methyl-2-pyrrolidone through successive reactions of amines (octadecylamine and/or the azobenzene derivative) on poly(acrylic acid), in the presence of 1.2 eq of dicyclohexylcarbodiimide as activating agent. An example is given in Figure 6. A poly(ethylene glycol) block (PEGBr) as macroinitiator was first prepared by reaction between PEG monomethyl ether and 2bromo-2-methylpropionyl chloride 6 . Then the ATRP polymerization of Azo-monomer was carried out at 80°C by using Cu(I)Cl complexed with HMTETA as the catalyst and PEGBr as the initiator. The polymerization degree of Azo-monomer was well controlled in a range from 24 to 92. Block Azo-polymers with narrow polydispersity (PI = 1.08 -1.10) from 13 200 to 47 500 g mol -1 were obtained. Such block polymers were used to form thin films with liquid crystalline phases. The macro-RAFT agent, poly(N-isopropylacrylamide) capped with dithiobenzoate (PNIPAM-SC(S)Ph) was firstly synthesized by RAFT polymerization in THF at 70°C using dithiobenzoate derivatives as the chain transfer agent 8 . Then this agent was used for the RAFT polymerization of Azo containing monomer to yield the diblock amphiphilic copolymer PNIPAM-b-PAzoM (PI= 1.2) (Figure 8). A unique synthesis method was developed by Wang et al. 9 . The block copolymer was synthesized via the combination of a macromolecular azo coupling reaction and the RAFT polymerization method (Figure 9). The agent CTA-N-Ph was designed to

e.3.2 SAXS measurements

The characteristic scattering peak associated to electrostatic repulsion is absent in Azo-divalent/PSS solutions due to the presence of added salts (screening effect, Figure 12). A clear modification is observed under UV irradiation.

Flat curves are observed in pure divalent ions solution (Figure 13). This indicates that they are dispersed in solution and do not assemble into organized structures. No modification is observed for pure Azo-divalent under UV irradiation. Nous avons considéré dans un premier temps un PE hydrophile sur lequel nous avons greffé des groupements Azo. Ce système forme de agrégats globulaires en solution en raison du collapse des chaî nes et des associations inter/intramoléculaires. La taille des agrégats varie sous irradiation UV.

Elle dépend également de la masse molaire des PEs et du taux de greffage des groupements Azo.

Nous avons considéré dans un second temps des tensioactifs sur lesquels des groupements Azo ont été introduits (Azo-tensioactifs). Nous avons étudié leur complexation avec des PEs de charge opposée. Ces systèmes s'organisent en colliers de perles. Les polyions décorent les micelles et pénètrent à l'intérieur de celles-ci (co-micellisation). Sous irradiation UV, la taille des perles diminue sans vraiment modifier l'organisation des complexes.

Nous nous sommes enfin intér essés à l'auto-assemblage d'Azo-tensioactifs et de co-tensioactifs. Ce mélange conduit à la formation des micelles allongées et à la créa tion d'un gel. Sous irradiation UV, on observe une transition gel → fluide. L'origine ce phénomène est lié à un changement morphologique des agrégats micellaires (micelles allongées → micelles globulaires).

Mots-clés : Polyélectrolytes (PEs) photosensibles, Azo-tensioactif/PE complexes, Auto-assemblage Azo-tensioactif/co-tensioactif, diffusion des rayons X et des neutrons aux petits angles

Résumé en anglais

This thesis aims at designing, synthesizing and characterizing model photo-responsive systems in aqueous solution. These systems are based on polyelectrolytes (PEs) and surfactants. The photosensitivity arises from the presence of azobenzene (Azo) groups that undergo a photoisomerization from a trans to a cis isomer under UV irradiation.

We first considered a hydrophilic PE on which we grafted Azo groups. This system forms globular aggregates in solution due to chains collapse and inter/intramolecular associations. The size of the aggregates varies under UV irradiation. It also depends on the molar mass of the PEs and the Azo content.

We then considered surfactants in which Azo groups have been introduced (Azo-surfactants). We studied their complexation with oppositely charged PEs. These systems show a pearl necklace organization. The PEs decorate the micelles and penetrate inside (co-micellisation). Under UV irradiation, the size of the pearls decreases without really modifying the general organization of the complexes.

Finally, we were interested in the self-assembly of Azo-surfactants and co-surfactants. This mixture leads to the formation of wormlike micelles and the creation of a gel. Under UV irradiation, a gel → fluid transition is observed. The origin of this phenomenon is linked to a morphological transition of the micellar aggregates (wormlike micelles → globular micelles).

Keywords: Photoresponsitive polyelectrolytes (PEs), Azo-surfactant/PE complexes, Azosurfactant/co-surfactant self-assembly, small-angle X-ray and neutron scattering.

Figure 1

 1 Figure 1 Structure des isomères azobenzene trans et cis

II. 1 Tableau 1

 11 Azo-polyacrylates (PAA-Azo y ) n Nous avons synthétisé quatre Azo-polyacrylates (Schéma 1) de masses molaires distinctes (chaînes longues et chaînes courtes) contenant différentes fractions molaires en Azo (𝜏 𝑎𝑧𝑜 ). Ils sont nommés (PAA-Azoy)n, où y est la fraction molaire en unités Azo (ou encore, fraction en monomères modifiés) et n est le degré de polymérisation. (PAA-Azo0.11)42Sigma et (PAA-Azo0.15)39Sigma sont obtenus à partir d'un acide polyacrylique commercial. (PAA-Azo0.15)54RAFT et (PAA-Azo0.11)738RAFT sont obtenus à partir d'un acide polyacrylique bien défini (synthétisé par polymérisation RAFT). Les caractéristiques des Azo-polyacrylates utilisés sont résumées dans le Tableau 1. Schéma 1 Structure chimique des Azo-polyacrylates de sodium (PAA-Azoy)n. Caractéristiques des Azo-polyacrylates de sodium

  systèmes photosensibles. La fraction molaire en isomères trans (xtrans) peut être déterminée par des mesures d'absorption UV-Visible. La Figure 2a montre la variation des spectres UV-Vis d'une solution très diluée de (PAA-Azo0.11)42Sigma sous irradiation UV. L'isomère trans (courbe bleue, dark) présente une bande d'absorption forte à 338 nm correspondant à une transition π-π* et une bande faible à 440 nm attribuée à une transition n-π*. Sous irradiation UV, la bande d'absorption à 338 nm diminue tandis que sa position est déplacée à 319 nm. En outre, le pic d'absorption centré à 440 nm devient plus important. Deux points isosbestiques sont observés à 300 et 405 nm. Ceci est la signature de la coexistence des deux isomères trans et cis en solution (système à deux états) : tous les spectres UV-Vis peuvent être décomposés comme une combinaison linéaire du spectre trans pur et du spectre cis pur pondérés par leurs fractions molaires respectives. L'état pur trans peut être obtenu en laissant reposer l'échantillon deux jours dans le noir (« Dark »).

Figure 2

 2 Figure 2 Variation des spectres UV-Vis de (PAA-Azo0.11)42Sigma sous UV a) solution diluée (0.05 mM en Azo) ; b) solution concentrée (0.1 M en Azo après dilution rapide).

Figure 3

 3 Figure 3 Variation de la fraction xtrans en fonction du temps d'irradiation pour (PAA-Azo0.11)42Sigma, concentrations 0.0625 M -1 M en monomères.

  La position de la bande d'absorption (« dark » state) est très sensible à l'environnement local des groupements Azo. Un décalage vers les plus petites longueurs d'ondes (blue-shift) est attendu lorsque l'Azo est sous forme agrégée. On dispose ainsi d'un indicateur pour sonder l'organisation des chromophores en solution. Pour la même fraction en Azo (𝜏azo = 0.11), la chaîne longue (PAA-Azo0.11)738RAFT présente un décalage vers le bleu par rapport à la chaîne courte (PAA-Azo0.11)42Sigma (Figure 4a). Cela indique que l'organisation structurale locale est différente, et que les groupements Azo ne sont pas directement au contact de l'eau avec les chaînes longues (des agrégats contenant les chromophores sont présents même aux faibles concentrations mesurées). Les bandes d'absorption des deux Azo-polymères courts de plus forte composition en Azo (𝜏 azo = 0.15) sont quasiment à la même position : la structure locale est très similaire aux très faibles concentrations (Figure 4b). Elle est sans doute assez proche de celle rencontrée pour les chaînes courtes à 𝜏azo = 0.11.

Figure 4

 4 Figure 4 Spectres d'absorption normalisés de : a) Azo-polymères longs et courts, 𝜏azo = 0.11 ; b) Azopolymères courts, 𝜏azo = 0.15. Tous les spectres ont été mesurés à une concentration en Azo d'environ 0.05 mM.

3 (α ≈ 1 / 3 )

 313 dans l'obscurité (dark) et à 0.26 sous irradiation UV. Ce comportement est très différent de celui observé dans le cas du simple NaPAA (α ≈ 1/2) : nous ne sommes plus en régime semi-dilué comme observé précédemment. La forme des courbes de diffusion est également modifiée par rapport aux solutions de NaPAA. Elles sont maintenant caractéristiques de particules chargées en solution. En présence de groupements Azo le long du squelette, les polyions ont donc tendance à s'effondrer et à former des agrégats denses. Le régime en c 1/3 observé dans l'obscurité est alors caractéristique d'une simple loi de dilution des agrégats. La variation en c 0.26 observée sous UV préfigure un changement du nombre d'agrégation des particules sous irradiation.

Figure 5

 5 Figure 5 Résultats SAXS pour (PAA-Azo0.11)738RAFT. a) mesures dans l'obscurité (dark) ; b) mesures sous UV ; c) variation de q* en fonction de la concentration.

Figure 6

 6 Figure 6 Images CryoTEM pour (PAA-Azo0.11)738RAFT à 0.03 mol L -1 (dark). Petites flèches : petits objets sphériques. Longues flèches : objets allongés.

Figure 7

 7 Figure 7 Courbes SAXS et fits en absence de lumière (dark) et sous UV pour (PAA-Azo0.11)738RAFT pour une concentration de 0.5 mol L -1 .

  chaines et boucles à la surface du coeur). L'épaisseur de cette couche varie entre 23 Å à 63 Å suivant la concentration. L'organisation structurale de cette couche est néanmoins plus complexe que celle d'une solution de polymère neutre classique considérée pour corriger les données (fluctuations de concentration). Cet aspect pourrait être amélioré.

Figure 8

 8 Figure 8 Représentation schématique des agrégats de (PAA-Azo0.11)738RAFT

Figure 9 II. 3 . 3

 933 Figure 9 Résultats SAXS pour (PAA-Azo0.11)42Sigma. a) mesures dans l'obscurité (dark) ; b) mesures sous UV ; c) variation de q* en fonction de la concentration.

Figure 10

 10 Figure 10 Structure d'un Azo-tensioactif

L

  'analyse des données a été réalisée en combinant le facteur de forme d'un ellipsoïde de révolution et le facteur de structure de colloïdes chargés dans un potentiel Coulombien. Le groupement Azo ayant une densité de longueur de diffusion (SLD) très différente de celle des groupements alkyles, une organisation « hétérogène » de type core-shell (avec deux SLD distinctes 𝜌 𝑠ℎ𝑒𝑙𝑙 et 𝜌 𝑐𝑜𝑟𝑒 ) a également été testé pour décrire les agrégats micellaires. Les isomères trans-4Azo2TAB forment des micelles ellipsoïdales oblates d'axes (majeur Ra et mineur Rb) 35 Å et 20 Å. Sous irradiation UV (forme cis), le changement d'hydrophobie des chromophores et du paramètre d'empilement conduisent à un shift de la CMC et une réorganisation structurale importante : Ra = 23 Å et Rb =17 Å. L'affinement à l'aide d'un modèle core-shell conduit à un rayon mineur du coeur proche de la longueur de la chaîne alkyle étendue. Cette observation tend à valider une description plutôt hétérogène des particules.

Figure 11

 11 Figure 11 Courbes SANS. Fits avec un modèle core-shell pour a) 4Azo2TAB 50 mM ; b) 8Azo2TAB 50 mM

environ 5

 5 min d'irradiation et contient seulement 8% d'isomères trans (figure 12 a et c). Pour le système de référence "concentré" C, l'état stationnaire est atteint après plus de 2 h d'irradiation et contient encore 39% d'isomère trans (figure 12 b et c). La différence entre la Figure 12a et la Figure 12b montre que la concentration est un facteur important qui impacte directement l'efficacité de l'irradiation et la cinétique d'isomérisation. Sous irradiation bleue, l'isomérisation inverse cis → trans se produit, conduisant à un état stable composé de 89 % d'isomères trans et 11 % d'isomères cis pour le système de référence C.

Figure 12

 12 Figure 12 Variation du spectre UV-Vis pour 4Azo2TAB/PSS a) C/1000 ; 0.05 mM/0.1 mM ; b) C, 50 mM/100 mM, les mesures sont réalisées après prélèvement et dilution d'une partie de la solution irradiée. c) variation de xtrans en fonction du temps d'irradiation.

  Pour le mélange 4Azo2TAB/PSS-H, les pics des protons aliphatiques sont plus larges et légèrement décalés. Les pics aromatiques (6 -8 ppm) se superposent et sont difficiles à distinguer. Pour révéler uniquement la contribution de 4Azo2TAB, nous avons utilisé du PSS deutéré (PSS-D). Le spectre semble quasiment identique mais les larges pics à 7.41 ppm et 6.10 ppm disparaissent désormais (Figure 13b). Les quatre pics entre 6 ppm et 8 ppm (correspondant principalement aux protons aromatiques du 4Azo2TAB) sont très différents de ceux du 4Azo2TAB pur, ce qui indique que le tensioactif interagit fortement avec les polyions dans le mélange.

Figure 13 1 5 III. 2 . 3

 13523 Figure 13 1 H spectres RMN dans D2O de complexes 4Azo2TAB/PSS. La localisation probable du PSS dans les agrégats peut être déterminée en comparant le spectre de 4Azo2TAB pur à celui des mélanges 4Azo2TAB/PSS-D. La variation du déplacement chimique Δδ (Δδ = δcomplexes -δpure) dépend de l'environnement local des protons dans les complexes, et donc, de l'organisation des agrégats. La présence du cycle aromatique (styrène) induit généralement un blindage (Δδ négatif). Les protons de la tête polaire H1 et de l'espaceur (H2, H3) sont légèrement blindés (Δδ négatif), ce qui confirme une interaction importante entre le PSS et la zone polaire du tensioactif. La plus grande variation des déplacements Δδ est observée dans la région aromatique. On peut ainsi supposer que les cycles benzéniques du PSS se trouvent à l'intérieur des agrégats micellaires à proximité des groupement Azo. Ceci est cohérent avec d'autres études sur des systèmes tensioactifs / PEs sans Azo. 4,5III.2.3 Étude structurale par SANSLes mesures SANS (dark, UV) ont été effectuées à une seule concentration C et dans différentes conditions de contraste. Ces expériences visent à déterminer les organisations respectives des différentes espèces (polyions et tensioactifs). Nous avons utilisé du PSS deutéré (PSSD -) et des Azo-tensioactifs hydrogénés qui possèdent des SLD extrêmement différentes (8Azo2TA + : 𝜌8Azo2TA+ = 1.00 10 10 cm -2 , 4Azo2TA + : 𝜌4Azo2TA+ = 1.30 10 10 cm -2 , PSSD -: 𝜌PSSD-= 6.35 10 10 cm -2 ). Il est ainsi possible de trouver un mélange H2O/D2O dont la SLD est identique à celle de l'Azo-tensioactif (𝜌solvant = 𝜌azosurf) afin de révéler la fonction de structure partielle des polyions ou de trouver un mélange H2O/D2O tel que 𝜌solvant = 𝜌PSSpour masquer la contribution des polyions (méthode de l'effacement de contraste).Les mesures SANS dans différents mélanges H2O/D2O sont présentées sur la Figure14. Dans D2O (𝜙D2O = 1, 𝜌D2O = 6.38 10 10 cm -2 ) seuls les tensioactifs sont visibles. Au

Figure 14

 14 Figure 14 Mesures SANS dans différents mélanges H2O/D2O pour a) 4Azo2TAB/PSS; b) 8Azo2TAB/PSS. Pour q  0.03 Å -1 , les courbes de diffusion montrent toutes un plateau similaire. Dans cette plage de vecteurs de diffusion, nous sommes sensibles aux corrélations intermoléculaires à longues distances. Pour q  0.03 Å -1 , on est plus sensible au facteur de formes des objets en solution. Les profils de diffusion varient avec le contraste révélant une organisation structurale hétérogène à l'échelle locale. La présence d'une oscillation bien définie autour de 0.25 Å -1 (dans H2O/D2O, 𝜙D2O = 0.267 où l'on sonde la structure des chaînes) indique une réorganisation globale des polyions et la formation d'agrégats nanométrique relativement bien définis. Les solutions de NaPSS « pur » ne présentent jamais ce comportement. La fonction de structure partielle de 4Azo2TAB (dans D2O) est assez proche de ce que l'on observe aux grands angles sur des simples solutions micellaires. Elle est caractéristique de particules globulaires. Une autre particularité importante est la présence d'un épaulement entre 0.03 et 0.1 Å -1 . Cette signature originale est liée aux corrélations à courte portée entre les particules. Celles-ci ne sont donc pas uniformément réparties dans la solution (comme de simples micelles isolées) et ont tendance à s'assembler pour former des objets de plus grandes taille (agrégats d'agrégats). Toutes ces remarques suggèrent une image où les polyions s'enroulent autour d'agrégats globulaires et les relient les uns aux autres. Les complexes semblent donc s'organiser en collier de perles. Plus précisément, les polyions et les tensioactifs forment les petits objets globulaires, les tensioactifs sont au coeur de l'agrégat tandis que les polyions se trouvent plutôt en surface. Comme suggéré par les mesures RMN, une partie de la chaîne peut également pénétrer dans l'agrégat et interagir fortement avec les groupements Azo.

Schéma 3

 3 Représentation schématique du modèle collier de perles. Les fits reproduisent parfaitement les données pour les deux conditions de contraste (Figure 15), en absence de lumière (dark) et sous irradiation UV. Le rayon mineur du coeur est systématiquement proche de la longueur étirée des chaînes alkyles des tensioactifs : comme les simples solutions micellaires, les groupements alkyles forment le coeur des agrégats. Pour les complexes 4Azo2TAB/PSS, les rayons du shell (majeur, mineur) sont de 26 et 18 Å. Les complexes 8Azo2TAB/PSS forment des agrégats plus volumineux avec des rayons de 34 et 20 Å. L'épaulement observé dans les fonctions de structure des tensioactifs (entre 0.03 et 0.1 Å -1 , Figure 15a et Figure 15c) et lié aux corrélations inter-perles est parfaitement reproduit. Il nous permet de déterminer une valeur précise de la distance moyenne entre perles. Cette distance est quasiment le double du rayon du shell (majeur) : les perles sont très proches les unes des autres. Le modèle permet également de reproduire l'oscillation observée autour de 0.2 -0.3 Å -1 sur la fonction de structure des polyions. Celle-ci provient finalement de l'organisation des chaines de PSS dans la couche (shell) autour du coeur micellaire (Figure 15b et Figure 15d),.

Figure 15

 15 Figure 15 Mesures SANS pour 4Azo2TAB/PSS a) dans D2O et b) dans H2O/D2O, 𝜙D2O = 0.267; Mesures SANS pour 8Azo2TAB/PSS c) dans D2O et d) dans H2O/D2O, 𝜙D2O = 0.267.

  Les rayons du shell sont toutefois plus importants que ceux déterminés par SANS (de quelques Å : rayons majeur et mineur 31 Å -22 Å pour 4Azo2TAB/PSS et 35 Å -24 Å pour 8Azo2TAB/PSS). Cet effet pourrait être dû à la présence d'une couche de contre-ions condensée toujours plus visible par diffusion des rayons-X que par diffusion de neutrons. Sous UV, des modifications importantes sont observées aux grands q indiquant un réarrangement structural et une réduction de la taille des perles (24 % pour 4Azo2TAB/PSS et 26% pour 8Azo2TAB/PSS). Cette variation est comparable à celle observée par diffusion de neutron et est parfaitement réversible par simple irradiation de lumière bleue.

Figure 16

 16 Figure 16 Mesures SAXS pour a) 4Azo2TAB/PSS ; b) 8Azo2TAB/PSS.

Figure 17

 17 Figure 17 Images de Cryo-TEM mesurées à C/10 pour a) 4Azo2TAB/PSS, Flèche : particule isolée. Cercles : petits agrégats de particules (agrégat d'agrégats) ; b) 8Azo2TAB/PSS, Flèche courte : région sans agrégat micellaire. Flèche longue : région où on observe une densité élevée d'agrégats. Les micelles ne sont pas réparties de manière homogène dans le film. L'organisation en collier de perles est clairement visible.

Figure 18

 18 Figure 18 Photos du mélange 8Azo2TAB/NaSal (C, 75 mM/62.5 mM) dans l'obscurité (dark, gel) et et sous irradiation UV (fluide).

Figure 19 22 Figure 20 1 H

 1922201 Figure 19 Variation du spectre d'absorption UV-Vis du mélange 8Azo2TAB/NaSal sous UV pour différents temps d'irradiation.

,

  Dark et UV). Les images montrent clairement deux organisations distinctes. En absence de lumière (dark) nous observons des particules allongées de diamètre 5 -7 nm et de plusieurs micromètres de longueur. Après 4 heures d'irradiation UV, on observe majoritairement des petits objets globulaires de diamètre environ 4 nm ainsi que des particules allongées (diamètre environ 4 nm, longueurs comprises entre 10 et 30 nm). Cette transition morphologique explique la variation des propriétés rhéologiques (gel → fluide). Afin d'obtenir une meilleure description de l'organisation structurale, ces observations ont été combinées avec des mesures de diffusion aux petits angles.

Figure 21

 21 Figure 21 Image Cryo-TEM de 8azo2TAB/NaSal (C/4) a) Dark et b) UV.

Figure 22 Mesures

 22 Figure 22 Mesures SANS et fits. Solutions de 8azo2TAB/NaSal (C/4) a) dark (modèle core-shell cylindre avec section ellipsoï dale) et b) UV (ellipsoï de prolate).

Figure 23 Mesures

 23 Figure 23 Mesures SAXS et fit. Solutions de 8azo2TAB/NaSal C/4 a) dark (modèle cylindrique core-shell avec section ellipsoï dale) et b) UV (ellipsoï de prolate).

Figure 24 IV. 2 . 1

 2421 Figure 24 Photos du mélange 8Azo2TAB/NaBenSulf (C, 75 mM/62.5 mM) dark (gel) et sous UV (fluide).

Figure 25

 25 Figure 25 Spectres 1 H RMN dans D2O pour a) NaBenSulf, 8Azo2TAB, 8Azo2TAB/NaBenSulf C/4 (18.75 mM/15.62 mM) ; b) 8Azo2TAB/ NaBenSulf (C), dark, irradiation UV, irradiation bleue

  À l'état dark (micelles allongées), les mesures SANS et SAXS conduisent à des valeurs très similaires pour le terme de section (rayons majeur et mineur du shell par SANS : 33 et 20 Å ; rayons majeur et mineur du shell par SAXS : 37 et 23 Å). La différence de 3 -4 Å pourrait provenir de la présence d'une couche de contre-ions plus visible par SAXS. Sous UV (micelles globulaires), les mesures SANS et SAXS donnent des tailles comparables (rayons majeur et mineur de l'ellipsoïde par SANS : 40 et 21 Å ; rayon majeur et mineur de l'ellipsoïde par SAXS : 39 et 24 Å). La différence est négligeable. Le coeur des agrégats est constitué des chaînes alkyles du 8Azo2TAB. La couche (shell) est quant à elle constituée des groupements Azo et des groupements hydrophyles de l'Azotensioactifs mais aussi, des molécules de NaBenSulf comme suggéré par la RMN.

Figure 26 MesuresFigure 27

 2627 Figure 26 Mesures SANS et fits (dark et UV). Solutions de 8Azo2TAB/NaBenSulf C/4 (18.75 mM/15.62 mM)

Figure 1

 1 Figure 1 Chemical structure of sodium polyacrylate (NaPAA), polyacrylic acid (PAA), Ionene, sodium polystyrene sulfonate (NaPSS), Poly(diallyl-dimethylammonium chloride) (PDADMAC) and poly(acrylamide-co-sodium-2-acrylamido-2-methylpropane sulfonate) (AMAMPS).

Figure 2

 2 Figure 2 Evolution of the effective charge 𝑓 𝑒𝑓𝑓 as a function of chemical charge fraction 𝑓.

Figure 3

 3 Figure 3 Schematic representation of a weakly charged, flexible chain in a very dilute solution. Inspired from Ref 8.

Figure 4

 4 Figure 4 Schematic representation of a semi-dilute polyelectrolyte solution. Inspired from Ref 8.

  Azo-modified PE by light stimulus and to determine the induced structural modifications. Light-sensitive systems may provide an interesting way to follow the hydrophobic transition without having to use different PEs of distinct hydrophobic/hydrophilic properties.

Figure 5

 5 Figure 5 Schematic representation of various hydrophobic PEs. Reproduced from Ref 39.

Figure 6

 6 Figure 6 The necklace conformation of hydrophobic PE chain. Reproduced from Ref 40.

Figure 7 a

 7 Figure 7 a) String-controlled regime (𝑐 * < 𝑐 < 𝑐 𝑏 , 𝜉 > 𝑙 𝑠𝑡𝑟 ); b) Bead-controlled regime (𝑐 ≥ 𝑐 𝑏 , 𝜉 ≤ 𝑙 𝑠𝑡𝑟 ). Reproduced from Ref 8.

  of the complexes. According to the surfactant concentration, two mechanisms can be observed. When a very low amount of surfactants is added in the PE solution, they bound to the polyions in a non-cooperative manner. When the concentration is increased, the adsorption process becomes cooperative (adjacent sites are more likely filled with surfactants) leading to the formation of small PE/surfactant aggregates. This cooperative binding occurs at a "critical aggregation concentration" (CAC) that can be several orders of magnitude lower than the "classical" critical micelle concentration (CMC). Binding isotherms measured by potentiometric and calorimetric titrations may provide information on the binding mechanism. It is usually a sigmoidal curve that depicts the fraction of surfactants bound per monomer (degree of binding 𝛽 ) as a function of the surfactant concentration. The number of bound surfactants increases linearly with the surfactant concentration in the cooperative binding regime. The slope allows to determine the binding constant 𝐾 0 , which is directly related to the strength of the interaction between surfactants and PEs.

Figure 8 a

 8 Figure 8 a) Binding process of surfactant to polyelectrolyte b) Illustration of the binding isotherm: degree of binding vs surfactant concentration. Reproduced from Ref 50.

Figure 9

 9 Figure 9 Complexes structure with increasing surfactant concentration. Reproduced from Ref 50.

Figure 10

 10 Figure 10 Typical micellar or micelle-like structures formed with PEs. Reproduced from Ref 50.

  Upon irradiation with UV light, Azo undergoes a transformation from a trans to a cis isomer. This photoinduced transition is accompanied by a significant decrease of distance between the two phenyls, and the increase of the dipole moment from near zero to 3 Debye (for Azo without substituents). The cis-Azo state has a bent conformation with the phenyl rings twisted 55° out of the plane from the initial Azo group. The reverse transformation (cis → trans) can occur either by thermal relaxation in the dark or by irradiation with blue light (within the absorption band of the cis state).

Figure 1 a

 1 Figure 1 a) Structures of trans and cis isomers of azobenzene; b) Absorption spectra of trans and cis isomers of azobenzene dissolved in ethanol. Adapted from Ref 17.

  Scheme 2 Proposed mechanisms for azobenzene trans ↔ cis isomerization. Reproduced from Ref 21.

Figure 2

 2 Figure 2 Structures of red-shifted Azo compounds Ref 22,27,28.

Figure 3

 3 Figure 3 Schematic illustration of (PDADMAC/PAZO) microcapsule disruption induced by UV irradiation. Adapted from Ref 39 .

Figure 4

 4 Figure 4 Structure of amphiphilic copolymer.

Figure 5

 5 Figure 5 Scheme of poly(acrylic acid) based Azo-polyelectrolytes.

Figure 6

 6 Figure 6 Schematic representation of the sphere formation process and the TEM images of samples obtained with different water contents. Water content (vol %): a) 20%, b) 30%, c) 40% and d) 50%. The scale bars in the figures are 100 nm. Adapted from Ref 43 .

Figure 7

 7 Figure 7 Schematic illustration of the formation of the hollow microspheres. Adapted from Ref 44 .

Figure 8

 8 Figure 8 Synthesis route of diblock copolymer PAzoMA-b-PAA.

Figure 9

 9 Figure 9 Schematic representation of the disruption and the reformation of vesicles upon UV and visible light irradiation. Adapted from Ref 45.

Figure 10

 10 Figure 10 Structure of random Azo-modified poly(acryclic acid).

Figure 11

 11 Figure 11 structure of Azo-modified poly(acrylic acid).

Figure 12

 12 Figure 12 Illustration of AMP multiple association onto a hydrophobic cluster and the variation of the composition of complexes upon light irradiation. Adapted from Ref 47.

  found that azo-surfactants bearing hydrophobic segments from 8 to 14 carbons self-assemble into ellipsoidal or elliptical cylinder micelles. The aggregation number increases with the total length (R 1 + R 2 ). The structural modification under UV illumination varies with the Azo position along the surfactant molecule. The cis-aggregates adopt sphere or ellipsoidal shapes with smaller size and smaller aggregation number.

Figure 13

 13 Figure 13 Schematic illlustration of structural variation of Azo-surfactant micelles. Adapted from Ref 36.

Figure 14

 14 Figure 14 Structure of anionic Azo-surfactants CnSO3Na and CnCO2Na.

Figure 16 a

 16 Figure 16 a) Structure of carbohydrate-based Azo-surfactants; b) Volume fraction of micelles in solution (𝜙(𝑡)) as photoisomerization progresses (from fits of SANS data). Adapted from ref 33.

Figure 17

 17 Figure 17 Molecular shapes and critical packing parameter (CPP) of surfactants.

Figure 18

 18 Figure 18 Photoisomerization of C4AzoNa and possible mechanism for the formation and disruption of wormlike micelles by light irradiation. Adapted from Ref 59.

Figure 19

 19 Figure 19 Structure of EHAC, ACA and proposed mechanism for reversible photorheological fluids. Adapted from Ref 60.

  studied a photoresponsive system composed of a hydrophobically modified poly(acrylic acid) (HM-PAA) and a cationic Azosurfactant (Figure 20 a). In these systems, micellar aggregates started at the critical aggregation concentration (CAC). The cooperative binding continued until the viscosity of mixtures reached a maximum. This rise of viscosity is due to the solubilisation of dodecyl chains grafted on HM-PAA in micellar aggregates: the aggregates act as cross-linkers for HM-PAA chains. The reversible change of viscosity can be controlled by alternative UV and visible light irradiation. Later, they investigated the structural organization of pure micellar aggregates and HM-PAA/Azo-surfactant mixtures by small-angle neutron scattering (Figure 20 b) 50 .The pure surfactant trans isomers self-assembled into triaxial ellipsoid aggregates with axes of 20, 30 and 30-35 Å. The irradiated system (mostly cis isomers) formed disk shape particles with thicknesses of 6 -10 Å and radii of 10 -20 Å. Similar micellar morphologies were observed for the mixtures. Well-developed micellar aggregates were obtained for both trans and cis isomers with a diameter of approximatively 50 Å, which was smaller than that of pure surfactant assemblies at the same concentration. The structural variation was less pronounced for HM-PAA/Azo-surfactant mixtures under UV light irradiation, which can be attributed to the influence of the dodecyl chains on the aggregation. It must be noted that SANS measurements were carried out with fully hydrogenated samples in D2O. The respective organization of the different constituents could not be resolved.

Figure 20 a

 20 Figure 20 a) Chemical structure of HM-PAA and cationic photoresponsive surfactant azoTAB; b) Visualization of the self-assembly of azoTAB in the presence of HM-PAA. Adapted from Ref 50.

Figure 21 1

 211 Figure 21 Schematic representation of the self-assembly of the block ionomer complex vesicles. Adapted from Ref 64.

Scheme 3

 3 Scheme 3 Proposed Mechanism of Reversible Addition-Fragmentation Chain Transfer Polymerization

1 )

 1 [𝑀] 0 /[𝑅𝐴𝐹𝑇] 0 [𝑀] 0 is the initial concentration in monomer and [𝑅𝐴𝐹𝑇] 0 the concentration in RAFT agent. When the reaction is controlled, each RAFT agent generates one polymer chain. When the reaction is complete, all the monomers are incorporated in the chains Therefore the [𝑀] 0 /[𝑅𝐴𝐹𝑇] 0 ratio is the number of monomers per chain. It equals to the theoretical number-average degree of polymerization DPn.

Scheme 6

 6 Scheme 6 Structure of 2-(Dodecylthiocarbonothioylthio)-2-methylpropionic acid.

Figure 1 1 H

 1 Figure 1 1 H NMR in CDCl3 of polymerization of tBA.

Figure 2

 2 Figure 2 Kinetic plots for PtBA54RAFT ([𝑡𝐵𝐴]/[𝑅𝐴𝐹𝑇 𝑎𝑔𝑒𝑛𝑡]/[𝐴𝐼𝐵𝑁]=90:1:0.07). a) ln ([𝑀] 0 /[𝑀] 𝑡 ) as a function of polymerization time determined by 1 H NMR. b) 𝑀 𝑛 obtained by SEC as a function of monomer conversion. c) SEC traces for different polymerization times. d) Polydispersity index 𝐼 𝑝 as a function of monomer conversion.

Figure 3

 3 Figure 3 Kinetics plots for PtBA738RAFT ([𝑡𝐵𝐴]/[𝑅𝐴𝐹𝑇 𝑎𝑔𝑒𝑛𝑡]/[𝐴𝐼𝐵𝑁]=1250:1:0.5). a) ln ([𝑀] 0 /[𝑀] 𝑡 ) as a function of polymerization time determined by 1 H NMR. b) 𝑀 𝑛 obtained by SEC as a function of monomer conversion. c) SEC traces for different polymerization times. d) Polydispersity index 𝐼 𝑝 as a function of monomer conversion.

Scheme 8 58 Scheme 9 Figure 5 1 H

 858951 Scheme 8 Synthesis of AzoC4Br.

Figure 6

 6 Figure 6 SEC curves of a) PAA23sigma and (PAA-Azo0.11)42sigma. b) PAA54RAFT and (PAA-Azo0.15)54RAFT. c) (PAA-Azo0.11)738RAFT. All GPC data were done with acid forms.

13 C

 13 for the polyelectrolyte syntheses.III.1.3.1 GeneralNMR: NMR spectra were recorded on a Bruker Advance 400 spectrometer operating at 400 MHz for 1 H and 100 MHz for13 C.FTIR:The IR spectra were measure with a Bruker Vertex 70 spectrophotometer equipped with a DTGS detector and on a Smith ATR-diamond stage of type DuraSample IR II.Elemental analysis and mass spectra were performed at the "service d'analyses" of the "Faculté de Chimie", Strasbourg III.1.3.2 Polymerization of tert-butyl acrylate (tBA) tert-Butyl acrylate (tBA) was passed through a column of basic alumina to remove inhibitors prior to use. For the synthesis of PtBA with a low molecular weight (7900 g.mol -1 as determined by SEC), tBA (7.00 g, 54.62 mmol), CTA (220 mg, 610 µmol), AIBN (6.98 mg, 60 µmol) were dissolved in 1,4-dioxane (19 ml).Hexamethylcyclotrisiloxane (118 mg) was added as an internal standard. The mixture was deoxygenated by bubbled argon for 40 min. The reaction flask was then immersed in a preheated oil bath at 65 °C and allowed to stir for 3 hrs. Upon completion, the solution was opened to air and cooled to room temperature. The solvent 1,4-dioxane was removed by evaporation. The polymer was redissolved in 10 ml THF and precipitated in 150 ml MeOH/H2O 7/3, centrifuged and dried.The obtained polymer 1 is a yellow solid (3.62 g, 75% yield). A similar procedure was followed for the synthesis of PtBA with a high molecular weight (62788 g•mol -1 as determined by SEC), with the following amounts tBA (10 g, 78.02 mmol), CTA (22.9 mg, 0.062 mmol), AIBN (5.1 mg, 0.031 mmol), and stirring at 70 °C for 2 hrs. The resulting polymer was redissolved in 60 ml THF and precipitated in 450 ml MeOH / Water 9/1, centrifuged and dried. The obtained polymer 1 is a pale-yellow solid (4.77 g, 85% yield).1 H NMR (400 MHz, CDCl3) δ 2.21 (s, backbone CH), 1.83 -1.26 (m, backbone CH2, COOC(CH3)3). NMR (100 MHz, CDCl3) δ 174.31-173.82 (COOC(CH3)3), 80.47 -80.57 (COOC(CH3)3), 42.47 -41.72 (backbone CH), 37.53 -35.98 (backbone CH2), 28.09-28.23 (COOC(CH3)3). ATR-IR (cm -1 ) max: 2978 (as CH3), 2933 (as CH2), 1725 (C=O), 1479 ( CH2), 1457 (as CH3), 1448 (as CH3), 1393 and 1365 (s CH3 tBu), 1256 (C-O). III.1.3.3 Kinetic measurements by NMR and GPC The syringe was deoxygenated with argon 3 times before withdrawing. Aliquots of 0.1 ml of the reaction medium were withdrawn every 30 min (50 µL for NMR, 50 µL for SEC) and cooled immediately by liquid azote bath to stop the polymerization. Analysis by NMR: The 50 µL aliquot was diluted in CDCl3 and analyzed by NMR. Hexamethylcyclotrisiloxane was used as an internal standard.

1 H

 1 H NMR (400 MHz, D2O) δ 2.35 (s, backbone CH), 1.88 -1.20 (m, backbone CH2). ATR-IR (cm -1 ) max: 2921 (as CH2), 2850 (s CH2), 1698 ( C=O acid), 1453 ( CH2), 1161 (C-O). PAA 2 was dissolved in aqueous solution (15 mL) with an excess of NaOH (pH=9) and dialyzed against Milli-Q water for 3 days in a regenerated cellulose membrane (molecular weight-cutoff MWCO = 1 kD). Milli-Q water was refreshed every day. Sodium polyacrylate (NaPAA) 3 power was obtained by lyophilization. NMR (400 MHz, D2O) δ 3.71 (m, backbone CH, CH2), 2.42-1.29 (m, backbone CH, CH2). ATR-IR (neat solid) (cm -1 ) max: 2920 (as CH2), 2853 (s CH2), 1554 (as CO2 -), 1452 ( CH2), 1404 (s CO2 -). Part of NaPAA is kept without further modification and is used as reference for the study of azo containing sodium polyacrylate.

  Scheme 11 Synthesis of 9. a) NaNO2, HCl, 0 °C; b) K2CO3, Bu4NBr, DMF, 50 °C; c) NEt3, THF, 70 °C under argon.

: Compound 8

 8 (1.1 g, 3.1 mmol) and a solution of trimethylamine in ethanol (3.3 ml, 3 mol L -1 ) were dissolved in 10 ml THF. The mixture was stirred at 70 °C under argon during 24 hrs. After cooling to room temperature, the precipitate was filtrated and recrystallized from EtOAc/EtOH (8/3) at 90 °C to afford (9) an orange solid (1.0 g, 79% yield).1 H NMR (400 MHz, CDCl3) δ 7.90 (d, J = 9.0 Hz, 2H, H3-Ar, H5-Ar), 7.79 (d, J = 8.3 Hz, 2H, H8-Ar, H12-Ar), 7.29 (d, J = 8.4 Hz, 2H, H9-Ar, H11-Ar), 7.03 (dd, J = 9.1, 0.0 Hz, 2H, H2-Ar, H6-Ar), 4.62 -4.54 (m, 2H, ArO-CH2), 4.41 -4.32 (m, 2H, ArOCH2-CH2), 3.62 (s, 9H, N(CH3)3), 2.67 (t, J = 7.7 Hz, 2H, CH2Ar), 1.68 -1.58 (m, 2H, CH2-CH2Ar), 1.44 -1.30 (m, 2H, , CH2-CH2 CH2Ar), 0.93 (t, J = 7.3 Hz, 3H, CH3).

  Scheme 12 Synthesis of 12. a) NaNO2, HCl, 0 °C; b) K2CO3, Bu4NBr, DMF, 50 °C; c) NEt3, THF, 70 °C under argon. 4-((4-octylphenyl)diazenyl)phenol (C8AzoOH) (10): 4-octylaniline (2.9 g, 14.1 mmol) was dispersed in 80 ml of 1 M HCl solution and 20 ml EtOH at 0 °C. Sodium nitrite (1.0 g, 14.1 mmol) in 10 ml water was slowly added. The mixture was left to react for 30 min at 0 °C. Then 30 ml of phenol (1.3 g, 14.1 mmol) in 10% wt NaOH was introduced dropwise in the reaction mixture at 0 °C and stirred thoroughly during 10 hrs. The crude product was recovered by extraction EtOAc/H2O. Then the organic layers were washed with NaOH aqueous solution in order to remove the no reacted phenol. The combined organic layers were dried (MgSO4) and concentrated under vacuum. Chromatography (SiO2, Cyclohexane/ EtOAc, 0/100 to 7/3) afforded (10) an orange solid (0.53 g, 12% yield). 1 H NMR (400 MHz, CDCl3) δ 7.86 (d, J = 8.9 Hz, 2H, H3-Ar, H5-Ar), 7.79 (d, J = 8.4 Hz, 2H, H8-Ar, H12-Ar), 7.30 (d, J = 8.3 Hz, 2H, H9-Ar, H11-Ar), 6.94 (d, J = 8.9 Hz, 2H, H2-Ar, H6-Ar), 2.67 (t, J = 7.6 Hz, 2H, H13), 1.71 -1.59 (m, 2H, H14), 1.40 -1.19 (m, 10H, H15-H19), 0.88 (t, J = 6.8

N

  ,N,N-trimethyl-2-(4-((4-octylphenyl)diazenyl)phenoxy)ethan-1-aminium bromide (8Azo2TAB) (12): Compound 11 (0.50 g, 1.2 mmol) and a solution of trimethylamine in ethanol (1.2 ml, 3 mol L -1 ) were dissolved in 5 ml THF. The mixture was stirred at 70 °C under argon for two days. After cooling to room temperature, the precipitate was filtrated and recrystallized from EtOAc/MeOH (3/1) at 90 °C to afford (12) as an orange solid (0.37 g, 64% yield). 1 H NMR (400 MHz, CDCl3) δ 7.91 (d, J = 9.0 Hz, 2H, H3-Ar, H5-Ar), 7.80 (d, J = 8.4 Hz, 2H, H8-Ar, H12-Ar), 7.30 (d, J = 8.5 Hz, 2H, H9-Ar, H11-Ar), 7.04 (d, J = 9.1 Hz, 2H, H2-Ar, H6-Ar), 4.52 (m, 2H, H21), 4.33 (m, 2H, H22), 2.67 (t, J = 7.6 Hz, 2H, H13), 1.66 (m, 2H, H14), 1.37 -1.22

IV. 1 . 1

 11 photoconversion and the photostationary state. Most popular UV sources used with photoresponsive systems are mercury lamps[1][2][3] , filtered UV tubes4,5 , optical fibers[6][7][8] and UV LEDs 7 .Mercury arc lamps offer a large range of wavelengths (200 nm -2500 nm).Additional filters are needed to select specific wavelengths. Mercury arc lamps have a large lighting area and may emit dangerous levels of UV radiation. Hence manipulations must be done with care.Alternatively, UV tubes, optical fibbers coupled with laser (or diodes devices), or simple UV LEDs can deliver UV light in a safer way. Filtered UV tubes have a large spectral distribution. They have a sufficient lighting area and can be handled easily. Optical fibbers associated to laser have gained increasing interest because they can easily transmit UV light on very small areas. Nevertheless, their limited lighting surface can also be a disadvantage. UV LEDs own low energy consumption, a concentrated narrow beam and high effective luminous efficiency.

by 1 H

 1 NMR and UV-Vis measurements. We present here some examples of different systems. By NMR NMR measurements under light stimulus were carried out for two Azosurfactant solutions in D2O (4Azo2TAB at 50 mM and 8.6 mM). The LED irradiation setup is composed of 8 LEDs (4 LEDs on each side of the sample) put close to the sample. The UV tubes (that can only irradiate from one side of the sample) is set at a distance of 10 cm. Samples in glass NMR tubes with a diameter of 5 mm were irradiated by LEDs or UV tubes until the trans/cis composition remained unchanged.

Figure 1 1 H

 1 Figure 1 1 H NMR spectra of 4Azo2TAB 50mM in D2O (dark) before and after light irradiation (UV, blue LEDs).

Figure 2

 2 Figure 2 Variation of absorption under a) UV tube irradiation; b) UV LEDs irradiation.

Figure 3

 3 Figure 3 Evolution of the trans fraction for 4Azo2TAB/PSS complexes (50 mM/100 mM) under UV light irradiation. Red curve: irradiation by UV LEDs; Blue curve: irradiation by UV tubes. The concentrated solution was diluted 1000 times prior to measurements.

Figure 5

 5 Figure 5 Chemical structure of Azo-surfactant.

Figure 6

 6 Figure 6 Contrast variation for PSS/Azo-surfactant complexes. The cartoons represent what is mainly seen with SANS using deuterated PSS and hydrogenated surfactant for 𝜌 𝑠 =6.38 and 1.306 10 10 𝑐𝑚 -2 .

IV. 2 . 4 . 1

 241 SAXS instrument SAXS measurements were performed at the Institut Charles Sadron on a Rigaku diffractometer installed on a microfocus rotating anode generator (Micromax TM -007 HF) operating at 40 kV, 30 mA. The beam was monochromatized (wavelength = 1.54 Å) and collimated with a confocal Max-Flux

Figure 7

 7 Figure 7 In situ light irradiation setup for SAXS experiments (blue light).

Figure 8

 8 Figure 8 External pre-illumination by 8 LEDs for SAXS samples.

Figure 9 .

 9 Figure 9. No difference is observed between these illumination methods. Even if the exchange between the irradiated zone and the non-irradiated zone exists during the measurement, this does not seem to influence the trans/cis ratio at equilibrium state for this sample.

Figure 9

 9 Figure 9 Comparison of SAXS curves of 8Azo2TAB/NaSal (75 mM/62.5 mM) irradiated by different ways: only with two UV LEDs in situ (black curve) and pre-illumination of the whole capillary with UV LEDs plus two UV LEDs in situ (red curve).

Figure 10 SAXS

 10 Figure 10 SAXS curves of 4Azo2TAB 50 mM measured in glass (borosilicate) and quartz capillary.

Figure 11 SAXS

 11 Figure 11 SAXS curves of 4Azo2TAB/PSS (50mM/100mM) complexes with capillaries of different diameters.

IV. 2 . 5 . 1

 251 SANS instrumentSANS measurements were carried out on the PAXY diffractometer at Laboratoire Léon Brillouin (CEA Saclay, France). Three configurations (sample to detector distance/wavelength) were used (1 m/ 5 Å, 3 m/5 Å, 5 m/8.5 Å) covering a q range from 5 10 -3 to 0.45 Å -1 . Samples were loaded in quartz cell (Hellma) of 1 mm path length. The temperature was controlled and fixed to 22°C. Data were corrected and set in absolute scale using the PASINET software developed at LLB.IV.2.5.2 Illumination setupSample irradiation was performed in situ using a multi-LED setup developed at the LLB. This system is made of ten LEDs arranged circularly on one side of the sample (1 cm apart) and surrounding the neutron beam. Additional LEDs were installed at other positions of the sample changer to pre-illuminate the samples if required. Each measurement under UV were repeated several times and compared to ensure that the equilibrium state is reached.Additionally, some samples were pre-irradiated by UV tubes or LEDs outside the instrument and stirred several times before their installation on the sample changer. This helps to homogenise quickly the sample and thus reduce the irradiation time in situ. The irradiation time required to reach the equilibrium state was estimated from precise kinetic measurements on representative samples (micelles, PE-micelles).The presence of external light sources in the close environment might slightly influence the trans/cis ratio at the equilibrium. Two distinct experiments have been performed at the LLB on these systems. In the second experiment, the sample was sheltered by an umbrella with anti-UV coating.

Figure 12

 12 Figure 12 Illumination device for SANS measurements.

Figure 13

 13 Figure 13 Irradiation of NMR tubes.

Figure 14

 14 Figure 14 Irradiation of UV-Vis samples by LEDs.

Figure 16 IrradiationIV. 7 IV. 8

 1678 Figure 16 Irradiation Setup for surface tension measurements.

Figure 17

 17 Figure 17 Irradiation setup for Cryo-TEM experiments.

  have synthesized four Azo-containing sodium polyacrylates with different chain lengths and Azo molar fractions (Scheme 1). The synthesis and the characterization of Azo-polymers are presented in Chap III.1 (P 45). Just to recall that they are denoted (PAA-Azoy)n, where y is the molar fraction of azo units and n is the number average degree of polymerization. (PAA-Azo0.11)42Sigma and (PAA-Azo0.15)39Sigma are obtained from a commercial polyacrylic acid. (PAA-Azo0.15)54RAFT and (PAA-Azo0.11)738RAFT are obtained from a well-defined polyacrylic acid synthesized by RAFT polymerization. Their main characteristics are recalled in

Figure 1 a

 1 Figure 1 a) Variation of UV-Vis spectra of (PAA-Azo0.11)42Sigma for different UV irradiation time, with azo unit concentration of 0.05 mM.

Figure 2 UV-

 2 Figure 2 UV-Vis spectra of 1 M (PAA-Azo0.11)42Sigma solution as a function of the UV irradiation time t: a) 1 M solution under UV, diluted just before the measurement; b) dilute solution directly under UV; c) variation of the trans fraction xtrans vs irradiation time for a 1 M and 0.05 mM solution.

Figure 3

 3 Figure 3 Variation of 𝑥 𝑡𝑟𝑎𝑛𝑠 vs irradiation time for (PAA-Azo0.11)42Sigma solutions.

  have measured the trans fraction 𝑥 𝑡𝑟𝑎𝑛𝑠 𝑒𝑞 during several irradiation cycles on dilute solutions. The photoconversion from trans to cis is realized by UV light irradiation 365 nm while the reverse process, cis to trans, is triggered by Blue light illumination at 450 nm. Several UV-Blue irradiation cycles were applied on Azopolymer solutions directly in the UV-Vis cuvette (1 cm path length). The trans fraction at the photostationary state was determined by UV-Vis spectroscopy as described in the previous paragraph. The behaviour is very similar for both long and short length Azo-polymers with the same azo unit fraction (𝜏 𝑎𝑧𝑜 = 0.11). We found a good reversibility upon light stimulus (Figure4).

Figure 4

 4 Figure 4 Reversibility trans ⟷ cis measured by UV-Vis during UV-Blue irradiation cycles. The trans fraction at the stationary state is measured on: a) (PAA-Azo0.11)42Sigma (Azo unit concentration 0.056 mM); b) (PAA-Azo0.11)738RAFT (Azo unit concentration 0.05 mM).

(

  PAA-Azo0.11)738RAFT exhibits a blue shift compared to the short one (PAA-Azo0.11)42Sigma (Figure5a). This clearly indicates that the local structural organization of Azo groups is different, and that chromophores are certainly not in contact with water for long chains (Azo are probably inserted in aggregates even at low concentration). The absorption peak of the two others short Azo-polymers with 15% Azo units are almost at the same position: the local structure is very similar at very low concentration.

Figure 5

 5 Figure 5 Normalized absorption spectra of a) long and short Azo-polymers with 𝜏 𝑎𝑧𝑜 = 0.11; a) two short length Azo-polymers with 𝜏 𝑎𝑧𝑜 = 0.15. (all the spectra were measured at an Azo unit concentration around 0.05 mM ).

Figure 6

 6 Figure 6 SAXS curves for reference PEs: NaPAA23sigma, NaPAA54Raft, and NaPAA738Raft.

Figure 8

 8 Figure 8 Scattering patterns measured on (PAA-Azo0.11)42Sigma, (PAA-Azo0.15)39Sigma, (PAA-Azo0.15)54RAFT, (PAA-Azo0.11)738RAFT at c = 0.25 mol L -1 in log-log scale (up) and linear scale (down). Blue curve : dark; red curve : under UV radiation; black curve unmodified NaPAA.

Figure 9

 9 Figure 9 SAXS plots in Dark, under UV and Blue light a) 0.25 M (PAA-Azo0.15)54RAFT; b) 0.25 M (PAA-Azo0.11)738RAFT.

Figure 10

 10 Figure 10 Peak position q* in the photostationary state during UV and blue irradiation cycles a) 0.25 M (PAA-Azo0.15)54RAFT; b) 0.25 M (PAA-Azo0.11)738RAFT.

Figure 11 V. 3 . 4 11 )

 113411 Figure 11 Thermal relaxation measured by SAXS. Position of the peak q* as a function of time. a) 0.25 M (PAA-Azo0.15)54RAFT; b) 0.25 M (PAA-Azo0.11)738RAFT

Figure 12 SAXS

 12 Figure 12 SAXS data of (PAA-Azo0.11)738RAFT. a) measurements in Dark b) measurements under UV c) q* vs concentration.

Figure 13

 13 Figure 13 CryoTEM images of (PAA-Azo0.11)738RAFT at 0.03 mol L -1 . Small arrows: typical small spherical objects. Long arrows: elongated objects. The particles are homogeneously distributed in the film.

  data at concentration c, 𝜉 is considered constant (=𝜉 𝑟𝑒𝑓 ) while 𝐼 𝑓𝑙𝑢𝑐 (0, 𝑐)=(𝑐/ 𝑐 𝑟𝑒𝑓 )𝐼 𝑓𝑙𝑢𝑐 𝑟𝑒𝑓 (0, 𝑐 𝑟𝑒𝑓 ). Examples showing the scattered intensities and 𝐼 𝑓𝑙𝑢𝑐 (𝑞) are given in Figure 14. All the data have been corrected for the polymer network concentration fluctuations contribution before being analysed.

Figure 14 SAXS

 14 Figure 14 SAXS data measured at 0.5 mol L -1 (left) and 0.125 mol L -1 (right). The line corresponds to the contribution of the concentration fluctuations of the polymer network around the particles. (𝜉 𝑟𝑒𝑓 = 15 Å,

Figure 15 SAXS

 15 Figure 15 SAXS data and fits (lines) for measurements in dark.

Figure 16 SAXS

 16 Figure 16 SAXS data and fits (lines) for measurements under UV irradiation.

Figure 17 schematicV. 3 . 5

 1735 Figure 17 schematic representation of (PAA-Azo0.11)738RAFT aggregates.

Figure 18 SAXS

 18 Figure 18 SAXS data of (PAA-Azo0.11)42Sigma and NaPAA solutions at 0.25 molL -1 .

Figure 19 SAXS

 19 Figure 19 SAXS study of (PAA-Azo0.11)42Sigma. a) SAXS measurements in dark; b) SAXS measurements under UV irradiation; c) log-log plot of q* vs concentration.

Figure 20

 20 Figure 20 Schematic representation of the structural organization in regime I.

Figure 21

 21 Figure 21 Schematic representation of the structural organization in regime II.

Figure 23

 23 Figure 23 Schematic representation of the structural organization in regime III.

Figure 25

 25 Figure 25 Schematic representation of the structural organization in regime IV.

  15)39Sigma and (PAA-Azo0.15)54RAFT. Both have an Azo content of 15%. The main difference lies in the polydispersity. (PAA-Azo0.15)39Sigma (Ip = 1.26) has been obtained from a commercial PAA (Ip = 1.40) while the parent PAA of (PAA-Azo0.15)54RAFT has been synthesized in the laboratory by RAFT (Ip = 1.12).

Figure 1

 1 Figure 1 Surface tension as a function of lnc for 4Azo2TAB.

Figure 2

 2 Figure 2 Surface tension as a function of lnc for 8Azo2TAB.

Figure 3 UV-

 3 Figure 3 UV-Vis absorption spectra of a 0.08 mM 4Azo2TAB solution a) under UV light, b) under blue light; c) variation of 𝑥 𝑡𝑟𝑎𝑛𝑠 upon UV and blue light stimulus.

Figure 4

 4 Figure 4 UV-Vis absorption spectra of a 0.08 mM 8Azo2TAB solution a) under UV light, b) under blue light; c) variation of 𝑥 𝑡𝑟𝑎𝑛𝑠 upon UV and blue light stimulus.

Figure 5

 5 Figure 5 Isomerization reversibility measured with UV-Vis absorption spectroscopy: a) 4Azo2TAB; b) 8Azo2TAB.

Figure 6

 6 Figure 6 Thermal relaxation of Azo-surfactants by UV-Vis.

Figure 8

 8 Figure 8 Contrast variation with 8Azo2TAB a) Neutron scattering curves of 8Azo2TAB (50 mM) measured in different H2O/D2O mixtures; b) Plot of √𝐼(𝑞 → 0) versus 𝜌 𝑠 for 8Azo2TAB.

Figure 9

 9 Figure 9 Schematic representation of a micellar aggregate. Left: molecular organization. Right: equivalent core shell structure from a scattering length point of view.

Figure 10

 10 Figure 10 Schematic representation of SLD of different parts for Azo-surfactants. a) and b) for SANS; c) and d) for SAXS.

Scheme 3

 3 Scheme 3The one-shell model and the core-shell ellipsoidal model (oblate).

Figure 11 SANS

 11 Figure 11 SANS profiles of 50 mM 4Azo2TAB and 50 mM 8Azo2TAB solutions.

Figure 12 SANS

 12 Figure 12 SANS data of 50 mM 4Azo2TAB solution and fit with: a) one-shell model; b) core-shell model.

Figure 13 SANS

 13 Figure 13 SANS data of 50 mM 8Azo2TAB solutions and fit with: a) one-shell model; b) core-shell model.

  shell. (Figure 12 b and Figure 13 b). It is not common to use a core-shell model for classical micelles with neutron scattering. Most of the time, the different chemical groups of the surfactants have very similar SLD (without specific labelling). At the same time, counterions are often 'diluted' by the solvent and are not visible.

Trans-4Azo2TAB forms

  oblate ellipsoidal aggregates with a long radius (Ra) of 35 Å and a short radius (Rb) of 19 -20 Å. Trans-8Azo2TAB with a longer alkyl tail, self-assembles in larger aggregates (Ra = 66 Å, Rb = 24 -28 Å). The modification of the model (from a one-shell to a core-shell description) improves the quality of the fit at large q values without changing the external size of the micelle (major shell and minor shell). The minor core radius is close to the length of the alkyl chain, which seems to validate our core-shell approach. The minor shells (19 -20 Å for 4Azo2TAB and 24 -28 Å for 8Azo2TAB) are shorter than the fully extended surfactant lengths if we consider the molecules as a whole (24.7 Å and 29.2 Å) and close to their lengths if we remove the polar head (20.7 Å and 25.2 Å) as presented in Figure14.

Figure 14

 14 Figure 14 Azo-surfactants structure and model of their micelles (fully extended chain).

Figure 15 SAXS

 15 Figure 15 SAXS curves and fits. a) 4Azo2TAB 50 mM; b) 8Azo2TAB 50 mM. Solution in dark could not be fitted due a scattering pollution around 0.1 Å -1 .

Figure 16

 16 Figure16shows the SAXS profiles for solutions of different composition (constant PSS concentration 100 mM, 0 < r < 0.8). When the surfactant content is increased, we note the gradual appearance of a characteristic oscillation at around 0.2 A -1 . It is related to the presence of micellar aggregates on the system.

Figure 16 SAXS

 16 Figure 16 SAXS plots of a) 4Azo2TAB/PSS mixtures for molar ratios r = 0.15 -0.5 (dark state); b) 8Azo2TAB/PSS mixtures for molar ratios r = 0.15 -0.8 (dark state). The PSS concentration is constant [PSS] = 100 mM..

Figure 17 shows

 17 Figure17shows SAXS measurements of 4Azo2TAB/PSS in dark and under UV for r = 0.3 and 0.5. The displacement of the oscillation (0.2 Å -1 ) to highest q values is the key effect observed under UV. It indicates a modification of the form factor of the aggregates under irradiation. The large oscillation around 0.2 Å -1 is more intense for r = 0.5 and shows large modifications with UV. This composition was therefore chosen as the reference ratio for this study.

Figure 17 SAXS

 17 Figure 17 SAXS plots of 4Azo2TAB/PSS mixtures: a) r = 0.3; b) r = 0.5 in dark and under UV irradiation. The PSS concentration is constant [PSS] = 100 mM.

Figure 18 SAXS

 18 Figure 18 SAXS plots of 4Azo2TAB/PSS mixtures, r = 0.5 : a) curves normalized to concentration in the dark state; b) curves normalized to concentration under UV irradiation. C corresponds to the global concentration PSS 100 mM and 4Azo2TAB 50mM.

Figure 19 SAXS

 19 Figure 19 SAXS plots of 4Azo2TAB/PSS mixtures measured in 1mm and 1.5 mm capillaries.

Figure 20

 20 Figure 20 Variation of the UV-Vis absorbance of 4Azo2TAB/PSS mixtures for different UV irradiation times for a) the concentrated solution 50 mM/100 mM and b) the dilute solution 0.05 mM/0.1 mM.

Figure 21

 21 Figure 21 Variation of trans fraction for the concentrated (4Azo2TAB/PSS 50 mM/100 mM, blue curve) and diluted solutions (4Azo2TAB/PSS 0.05 mM/0.1 mM, red curve) as a function of the irradiation time.

Figure 22

 22 Figure 22 Reversibility of Azo-surfactant/PSS complexes measured by UV-Vis spectroscopy. 4Azo2TAB/PSS 0.05 mM/ 0.1 mM (left) and 8Azo2TAB/PSS 0.05 mM/ 0.1 mM (right).

Figure 23

 23 Figure 23 Thermal relaxation of Azo-surfactant/PSS (0.05 mM/ 0.1 mM) by UV-Vis measurements.

Figure 24 1 H

 1 Figure 24 1 H NMR in D2O of 4Azo2TAB/PSS-H mixture (blue, 100 mM/50 mM), 4Azo2TAB (green, 50 mM) and PSS-H (red, 100 mM).

Figure 25 1 H

 1 Figure 25 1 H NMR spectra in D2O of 4Azo2TAB/PSS-D mixture (green, 100 mM/50 mM) and 4Azo2TAB/PSS-H mixture (red, 100 mM/50 mM) in dark. The yellow areas correspond to signal of PSS-H chains, which is 'invisible' in the case of PSS-D.

Figure 26 1 H

 1 Figure 26 1 H NMR spectra of 4Azo2TAB/PSS-D (100 mM/50 mM) mixtures in D2O, in dark and under UV and blue irradiation.

Figure 27 1 H

 1 Figure 27 1 H NMR spectra in D2O: green line pure 4Azo2TAB 50 mM and red line 4Azo2TAB in 4Azo2TAB/PSS-D (100 mM/50 mM) mixtures.

Figure 28 1 H

 1 Figure 28 1 H NMR spectra in D2O: green line pure 8Azo2TAB 50 mM and red line 8Azo2TAB in 8Azo2TAB/PSS-D (100mM/50 mM) mixtures.

Scheme 4

 4 Scheme 4 Schematic illustration of ring current effect.

Figure 30 SAXS

 30 Figure 30 SAXS profiles of 8Azo2TAB/PSS normalized to the concentration a) in dark state b) under UV irradiation. The reference concentration is noted as C for 8Azo2TAB/PSS 50 mM/100 mM.

Figure 31 SANS

 31 Figure 31 SANS plots: a) 4Azo2TAB/PSS (50 mM/100 mM) in dark state (in D2O and in H2O/D2O); a) 8Azo2TAB/PSS (50 mM/100 mM) in dark state (in D2O and in H2O/D2O)

Figure 32

 32 Figure 32 Schematic representation of pearl necklace model.

Figure 34 SANS

 34 Figure 34 SANS profiles and fits for 8Azo2TAB/PSS (50 mM/100 mM): a) in D2O b) in H2O/D2O. Data are only fitted in between 0.04 Å -1 to 0.4 Å -1 .

Figure 37

 37 Figure 37 Reversibility of Azo-surfactant/PSS complexes (50 mM/100 mM) measured by SAXS.

Figure 38

 38 Figure 38 Thermal relaxation measured by SAXS. Position of the peak q* as a function of time for a) 4Azo2TAB/PSS; b) 8Azo2TAB/PSS.

Figure 39

 39 Figure 39 CryoTEM image of 4Azo2TAB/PSS (5 mM/10 mM). Arrow: apparent isolated particle. Circles: small aggregate of particles (aggregate of aggregates). The particles are not homogeneously distributed in the film.

Figure 40

 40 Figure 40 Cryo-TEM images for 8Azo2TAB/PSS (5 mM/10 mM) in dark. Particles are not homogeneously distributed in the film. Small arrow: region without micellar aggregate. Long arrow:region where the density in aggregates is higher. Pearl-necklace organisation is clearly visible.

Figure 41

 41 Figure 41 Cryo-TEM image of 8Azo2TAB/PSS (5 mM/10 mM). Particles are not homogeneously distributed in the film. Small arrow: region without micellar aggregate. Long arrow: region where the density in aggregates is higher. Pearl-necklace organisation is clearly visible.

Figure 1 VII. 1 . 2

 112 Figure 1 Photos of 8Azo2TAB/NaSal mixture (75 mM/62.5 mM) in dark state (gel) and under UV irradiation (fluid).

Figure 2

 2 Figure 2 UV-vis spectra of 8Azo2TAB/NaSal aqueous solution (0.0375 mM/0.0312 mM, C/2000) for different irradiation times.

Figure 3 1 H

 1 Figure 3 1 H NMR spectra in D2O of 8Azo2TAB (75 mM, red curve) NaSal (62.5 mM, green curve), and the 8Azo2TAB/NaSal (75 mM/62.5 mM, blue curve) mixture.

Figure 4 1 H

 1 Figure 4 1 H NMR spectra in D2O for 8Azo2TAB/NaSal (75 mM/62.5 mM) in dark and under UV.

Figure 5 1 H

 1 Figure 5 1 H NMR spectra in D2O for 8Azo2TAB/NaSal (18.75 mM/15.62 mM, C/4).

  complementary technics that allow to probe different length scales and give an accurate description of the structural organization. VII.1.3.1 Cryo-TEM measurements Cryo-TEM measurements have been performed on 8Azo2TAB/NaSal C/4 in dark and after UV irradiation. After 4 hours the sample was completely fluid and considered to be in its photostationary state. The images are presented in Figure 6.

Figure 6

 6 Figure 6 Cryo-TEM measurements of 8Azo2TAB/NaSal C/4 (18.75 mM/15.62 mM) (a) in dark and (b) after 4 h under UV irradiation.

  Since the volume fraction has been fixed to the theoretical value, we conclude that the two components are fully integrated in the micelles. Water penetration is negligible. The smallest dimension of the aggregate (20 -21 Å) is much smaller than the fully extended size of 8Azo2TAB (around 30 Å) and smaller than what we found in pure 8Azo2TAB micelle (50 mM). Curiously, no difference is observed on the minor radius between dark and UV. The presence of the salicylate in the particle deeply changes the local organization of the surfactant with regard to the classical micelle. VII.1.3.3 Small angle X-ray scattering SANS measurements have been completed by SAXS analyses. The main difference between the two techniques is that the SLD of the particles cannot be considered as uniform with X-rays. As we have done in the previous chapters, we need to introduce a core-shell description. The second difference lies in the illumination setup which is more efficient with the SAXS diffractometer and allows a better control of the UV irradiation.

Figure 10 SAXSFigure 11

 1011 Figure 10 SAXS patterns of 8Azo2TAB/NaSal normalized to the concentration. a) in dark, b) under UV radiation.

Figure 12 Fit

 12 Figure 12 Fit of the SAXS patterns at C/4 (18.75 mM/15.62 mM). a) In the dark state using an infinite core-shell elliptical cylinder. b) Under UV irradiation using a prolate core-shell ellipsoid.

Figure 13

 13 Figure 13 Thermal relaxation by SAXS for 8Azo2TAB/NaSal (18.75 mM/15.62 mM). Position of the maximum as a function of the relaxation time.

Figure 14

 14 Figure 14 Chemical structures of sodium benzenesulfonate (NaBenSulf) (left) and poly(styrene sulfonate) (right).

Figure 15

 15 Figure 15 Photos of 8Azo2TAB/NaBenSulf mixture (75 mM/62.5 mM) in dark state and under UV irradiation.

Figure 16 1 H

 1 Figure 16 1 H NMR spectra in D2O of pure NaBenSulf 15.62 mM, pure 8Azo2TAB 18.75 mM and NaBenSulf/8Azo2TAB mixture (15.62 mM/18.75 mM).

Figure 21 SAXS

 21 Figure 21 SAXS patterns normalized to the concentration, in dark and under UV radiation.

Figure 22 SAXS

 22 Figure 22 SAXS patterns of 8Azo2TAB/NaBenSulf in dark, under UV and blue irradiation. a) concentration C/4 (18.75 mM/15.62 mM); b) concentration C (75 mM/62.5 mM).

Figure 23 Fit

 23 Figure 23 Fit of the SAXS patterns of 8Azo2TAB/NaBenSulf at C/4 (18.75 mM/15.62 mM). a) In the dark state using an infinite core-shell elliptical cylinder. b) Under UV irradiation using a prolate core-shell ellipsoid.

Figure 24 Structural

 24 Figure 24 Structural Reversibility followed by SAXS in 8Azo2TAB/NaBenSulf mixture C (75 mM/62.5 mM)

Figure 25

 25 Figure 25 Thermal relaxation by SAXS. Position of the maximum as a function of the relaxation time. in 8Azo2TAB/NaBenSulf mixture C/4 (18.75 mM/15.62 mM)

d)

  Synthesis of aiphiphilic Azo polyiersAzo moiety can be either incorporated in the main chain of the polymers or grafted as a side chain on the backbone. Various synthesis ways are available depending on the architecture of Azo-polymers.Hoio Azo-polymers and raneoi Azo-copolymers can be prepared directly by radical polymerization. Different examples are given here.Two series of amphiphilic copolymers (Figure1), poly(HPMA-co-MPAP) and poly(HPMA-co-MPAH) were prepared by radical copolymerization using ethanol as solvent and 2,2'-Azobisisobutyronitrile (AIBN) as initiator. N-(2-hydroxypropyl) methacrylamide (HPMA) was used as hydrophilic monomer1 . Azo-monomers were 4-(4-methoxyphenylazo) phenyl methacrylate (MPAP) and 6-[4-(4-methoxyphenylazo) phenoxy] hexyl methacrylate (MPAH). The polymerization lasted 24 h at 70°C and formed copolymers with a fraction of Azo units from 5 to 29 mol.% and 5 to 23 mol.% respectively. Aqueous solutions of these Azo copolymers exhibited the low critical solution temperature (LCST). For both poly(HPMA-co-MPAP) and poly(HPMA-co-MPAH) in the dark state, the LCST decreased with increasing the amount of Azo units in the copolymer. The LCST measured under UV irradiation was 2-4°C higher than the value measured in the dark state.

Figure 1

 1 Figure 1 Structure of copolymers poly(HPMA-co-MPAP) and poly(HPMA-co-MPAH).

Figure 2

 2 Figure 2 Structure of amphiphilic copolymer.

Figure 3

 3 Figure 3 Structure of Azo-containing monomers MEn and MEnME.

Figure 4

 4 Figure 4 Structure of Azo-modified poly(acryclic acid).

Figure 5 a

 5 Figure 5 a). Scheme of the synthesis of poly(acrylic acid) based Azo-polyelectrolytes; b). Scheme of the synthesis of epoxy based Azo-polyelectrolytes.

Figure 6

 6 Figure 6 Synthesis route of block copolymer PEG-b-PAzo.

Figure 7

 7 Figure 7 Synthesis route of diblock copolymer PAzoMA-b-PAA.

Figure 8

 8 Figure 8 Structure of diblock copolymer PNIPAM-b-PAzoM.

  have a suitable terminal group for the azo coupling. One polystyrene block with terminal anilino functionality (PS-N-Ph) was formed by RAFT polymerization in DMF at 60°C for 40 h using the designed agent (CTA-N-Ph). The other block, a diazonium salt mPEG-N 2 + was prepared from aniline functionalized PEG chain (mPEG-NH 2 ).The macromolecular azo coupling reaction between the above two polymer blocks in DMF gives an amphiphilic block Azo-polymer (PEG-b-PS).

Figure 9

 9 Figure 9 Schematic preparation of block copolymers via macromolecular azo-coupling reaction. Adapted from Ref 9 .

Figure 12 SAXS

 12 Figure 12 SAXS profiles of Azo-divalent/PSS (25 mM/100 mM).

Figure 13 SAXS

 13 Figure 13 SAXS profiles of pure Azo-divalent at 25 mM.

Figure 14 SANS

 14 Figure 14 SANS profiles of Azo-divalent/PSS (25 mM/100 mM) in H2O/D2O (φD2O=0.267), only PSS chains are "visible".

Figure 15 SANS

 15 Figure 15 SANS profiles of Azo-divalent/PSS (25 mM/100 mM) in D2O, only Azo-divalent counterions are "visible".

  -Vis. Les isomères trans présentent une bande d'adsorption maximale à 350 nm, tandis que les isomères cis sont caractérisés par deux bandes à 312 nm et 435 nm. Deux points isosbestiques sont présents à 302 nm et à 426 nm. Les isomères cis peuvent rapidement

revenir dans un état trans par irradiation avec de la lumière bleue. Ce processus est parfaitement réversible. La relaxation thermique a été également étudiée. Un temps caractéristique de l'ordre de 85 h pour 4Azo2TAB et de 195 h pour 8Azo2TAB ont été déterminés. La fraction molaire en isomères trans (xtrans) a été estimée par RMN pour les deux Azotensioactifs à 50 mM. xtrans est de 15 % dans l'état stationnaire sous irradiation UV et de 82 % sous irradiation par une lumière bleue.

Chapter I. Polyelectrolytes in aqueous solution

	Nomenclature
	𝑎 Abbreviation	Monomer length	Å
	A AIBN	Absorbance 2,2'-Azobisisobutyronitrile
	𝑐 * 𝑐 𝑝 AMAMPS Polyelectrolytes (PEs) are polymers that own ionizable functional groups. In Critical overlap concentration mol L -1 Monomer concentration mol L -1 Poly(acrylamide-co-sodium-2-acrylamido-2-methylpropane sulfonate
	𝑑 ATRP a polar solvent such as water, PEs can dissociate into charged polyions and Distance m Atomic transfer radical polymerization
	𝐷 Azo counterions. PEs exhibit both polymeric and electrolyte properties, at the origin of Size of the subunit blob Å Azobenzene
	DPn CAC useful and unique applications. As an important element in soft matter, PEs have Degree of polymerization Critical aggregation concentration 𝑒 Electronic charge C 𝐸(𝑅) Free energy J CMC Critical micelle concentration CNC Charge neutralization concentration been the focus of intensive studies both at fundamental and industrial levels. Their
	𝑓 CPP applications are mostly related to flow modification and stability properties of Chemical charge fraction Critical packing parameter
	𝐼 CTAB aqueous solutions and gels (flocculation agents, thickeners, conditioners and Ionic strength mol L -1 Cetyltrimethylammonium bromide
	Ip DMF emulsifiers). They are commonly found in shampoos, soaps, and cosmetic Polydispersity index Dimethylformamide 𝑔 Number of monomers in a subunit blob kB Boltzmann constant J K -1 DMSO Dimethyl sulfoxide LCST Lower critical solution temperature products. Due to their water solubility, they are also investigated for biochemical
	𝜅 𝐷 NaPAA -1 and medical applications. Furthermore, some PEs can be added to food products Debye length Å Sodium polyacrylate
	𝑙 PAA PSS as food coatings and release agents. Distance between two consecutive charges along the polyion Polyacrylic acid Polystyrene sulfonate	Å
	𝑙 𝐵 PDADMAC	Bjerrum length Poly(diallyl-dimethylammonium chloride)	Å
	𝑙 𝑝 PE	Persistence length Polyelectrolyte	Å
	𝑙 𝑝𝑒 PEG	Electrostatic persistence length Poly(ethylene glycol)	Å
	Mn RAFT	Number-average molar mass Reversible addition-fragmentation transfer polymerisation g mol -1
	Mw SANS	Weight-average molar mass Small-angle neutron scattering	g mol -1
	𝑝 SAXS	Conversion rate Small-angle X-ray scattering
	𝑅 SEC SLD	End-to-end distance of a polymer chain Size exclusion chromatography Scattering length density	Å
	𝑇 TGA	Temperature Thermogravimetric analysis	K
	𝑈 𝑖𝑗 THF	Electrostatic interaction energy Tetrahydrofuran	J
	𝑥 𝑡𝑟𝑎𝑛𝑠 UV	Molar fraction of trans-isomer Ultraviolet
	𝑧	Valency of ions
	𝛾	Surface tension	N m -1
	Θ	Theta temperature	K
	𝜖	Dielectric constant
	𝜏 𝑎𝑧𝑜	Molar fraction of azo unit
	𝜉	Correlation length	Å
	UV-Vis spectroscopy will be our main experimental techniques.
			III IV

Table 2

 2 NaOH and dialysis. To be coherent with previous notations, PAA chains obtained from acidolysis of PtBA are noted PAA 54RAFT and PAA 738RAFT thereafter. Unmodified NaPAA chains (NaPAA54RAFT and NaPAA738RAFT) were obtained by dialysis and used as reference in the structural studies (SAXS). Water contents in NaPAA PEs are hygroscopic. The water contents (w/w%) of the PE powders were measured by thermogravimetric analysis (TGA) and are given in Table2. The mass fraction of water was taken into account in the preparation of the solutions. Water contents in NaPAA (NaPAA23Sigma was obtained from the dialysis of a commercial PAA23sigma in presence of excess of NaOH)

	III.1.3.3 Molar mass distribution summary
	SEC plots in Figure 4 demonstrate the narrow distribution of the synthesized

Scheme 7 Acidolysis of PtBA

The polymer was converted to its salt form by neutralization with aquous polymers. The molar masses of the different polymers are summarized in Table

1

.

The DPn was estimated from SEC measurements ( 𝐷𝑃 𝑛 = 𝑀 𝑛,𝑆𝐸𝐶 /𝑀 0 ), where Mn,SEC is the number average molar mass obtained by SEC and M0 is the molar mass of monomer. Figure 4 SEC curves of polymers synthesized by RAFT Mw (g•mol -1 ) Mn (g•mol -1 ) III.1.1 Modification of PAA by azobenzene bearing groups III.1.1.1 Syntheses

Table 3

 3 𝜏 𝑎𝑧𝑜 determined by NMR and UV-Vis

Table 4

 4 Results of azo containing sodium polyacrylates: weight-average molecular weight (Mw), number-average molecular weight (Mn), polydispersity index (Ip = Mw / Mn) and degree of polymerization (DPn), <M> average molar mass for polymers in acid form.

		Mw (g• mol -1 )	Mn (g• mol -1 )	Ip	DPn by SEC	DPn (theoretical)*	𝜏 𝑤𝑎𝑡𝑒𝑟	<M> (g• mol -1 )
	(PAA-Azo0.11)42Sigma	5800	4400	1.3 3	42	/	17.0	104.2
	(PAA-Azo0.15)39Sigma	5700	4500	1.2 6	39	/	15.1	115.9
	(PAA-Azo0.15)54RAFT	7700	6900	1.1 3	56	54	16.2	115.9
	(PAA-Azo0.1)738RAFT	76100	60600	1.2 6	595	738	15.5	104.2

* Theoretical values of unmodified PAA chains: we considered that the grafting of azo group does not change the DPn of PAA synthesized by RAFT.

  The mixture was heated at 50 °C under argon during 24 hrs. K2CO3 solid was removed by filtration. Most of solvent were removed by extraction with EtOAc and water. The residue was precipitated in 300 ml of cold MeOH. The crude product was filtrated and evaporated under vacuum. After a purification by column

	chromatography (SiO2, DCM), the final product (8) is a yellow solid (2.5 g, 86%
	yield). 1 H NMR (400 MHz, CDCl3) δ 7.90 (d, J = 9.0 Hz, 2H, H3-Ar, H5-Ar), 7.80
	(d, J = 8.4 Hz, 2H, H8-Ar, H12-Ar), 7.31 (d, J = 8.5 Hz, 2H, H9-Ar, H11-Ar), 7.02
	(d, J = 9.0 Hz, 2H, H2-Ar, H6-Ar), 4.38 (t, J = 6.4 Hz, 2H, ArO-CH2), 3.68 (t, J =
	6.3 Hz, 2H, ArOCH2-CH2-Br), 2.68 (t, J = 7.6 Hz, 2H, CH2Ar), 1.71 -1.58 (m, 2H,
	CH2-CH2Ar), 1.45 -1.31 (m, 2H, CH2-CH2CH2Ar), 0.94 (t, J = 7.3 Hz, 3H, CH3).
	13 C NMR (100 MHz, CDCl3) δ 160.33 (C1), 151.10 (C7), 147.66 (C10), 146.15
	(C4), 129.21 (C9, C11), 124.75 (C3, C5), 122.73 (C8, C12), 115.03(C2, C6), 68.16
	(ArO-CH2), 35.71 (CH2Ar), 33.62 (CH2-CH2Ar), 28.94 (ArOCH2-CH2), 22.49 (CH2-
	CH2CH2Ar), 14.10 (CH3). HRMS (ESI
	2.68 (t, J = 7.8 Hz, 2H, CH2-Ar), 1.71 -1.58 (m, 2H, CH2-CH2Ar), 1.45 -1.32 (m,
	2H, CH2-CH2CH2Ar), 0.94 (t, J = 7.3 Hz, 3H, CH3). 13 C NMR (100 MHz, CDCl3) δ
	158.04 (C1), 151.08 (C7), 147.45 (C4), 146.08 (C10), 129.21 (C9, C11), 124.94
	(C3, C5), 122.69 (C8, C12), 115.89 (C2, C6), 35.71 (CH2-Ar), 33.62 (CH2-CH2Ar),
	22.49 (CH2-CH2CH2Ar), 14.10 (CH3). HRMS (ESI + ) m/z 255.147731 (calcd for
	C16H17BrN2O: 255.149190). Anal. Found: C, 73.61; H, 7.19; N, 10.77. (calcd for
	C16H17BrN2O: C, 75.56; H, 7.13; N, 11.01).

4-butyl-(4'-(2-bromoethyl)phenyl)azobenzene (C4AzoC2Br) (8): Compound 7 (2.0 g, 7.9 mmol), 1,2-dibromoethane (7.0 ml, 78.6 mmol), K2CO3 (10.3 g, 78.6 mmol) and Bu4NBr (100 mg) were introduced in 20 ml DMF. + ) m/z 361.090751 (calcd for C18H21BrN2O 361.091002). Anal. Found: C, 59.83; H, 5.91; N, 7.67. (calcd for C18H21BrN2O: C, 59.84; H, 5.86; N, 7.75). 2-(4-(4-butylphenyl)diazenylphenoxy)ethyltrimethylammonium bromide (4Azo2TAB)

  CH2), 858 ( CH aromatic). HRMS (ESI + ) m/z 340.239121 (calcd for C 21 H 30 N 3 O + 340.238339). Anal. Found: C, 59.88; H, 7.17; N, 9.94. (calcd for C21H30BrN3O: C, 60.00; H, 7.19; N, 10.00).

	13 C NMR (100 MHz, CDCl3) δ 158.87 (C1),
	150.93 (C7), 148.14 (C10), 146.46 (C4), 129.23 (C9, C11), 124.90 (C3, C5),
	122.82 (C8, C12), 114.90 (C2, C6), 65.18 (ArOCH2-CH2-N(CH3)3), 62.76 (ArO-

CH2), 54.99 (N(CH3)3), 35.70 (CH2Ar), 33.56 (CH2-CH2Ar), 22.47 (CH2-CH2CH2Ar), 14.07 (CH3). ATR-IR (diamond) max: 3009 (C-H aromatic), 2955 (as CH3), 2932 (as CH2), 2870 (s CH3), 2857 (s CH2), 1595 (C=C aromatic), 1485 (N=N), 1244 (aryl-O), 1159 (O-

  C18), 22.82 (C19), 14.26 (C20). HRMS (ESI + ) m/z 311.210913 (calcd for C 20 H 26 N 2 O 311.211790). Anal. Found: C, 77.10; H, 8.55; N, 9.01. (calcd for C20H26N2O C, 77.38; H, 8.44; N, 9.03).

	1-(4-(2-bromoethoxy)phenyl)-2-(4-octylphenyl)diazene (C8AzoC2Br) (11):
	Compound 10 (5.0 g, 16.1 mmol),2-dibromoethane (1.2 ml, 16.1 mmol),
	K2CO3 (1.9 g, 16.1 mmol) and Bu4NBr (10 mg) were introduced in 10 ml DMF. The
	mixture was heated at 50 °C under argon during 26 hrs. K2CO3 solid was removed
	by filtration. Most of solvent were removed by extraction with EtOAc and water.
	Then the residue was precipitated into 300 ml of cold MeOH. The crude product
	was filtrated and evaporated under vacuum. Chromatography (SiO2, DCM)
	afforded (11) as a yellow solid (0.50 g, 74% yield). 1 H NMR (400 MHz, CDCl3) δ
	7.90 (d, J = 9.0 Hz, 2H, H3-Ar, H5-Ar), 7.80 (d, J = 8.4 Hz, 2H, H8-Ar, H12-Ar),
	68 (C4,
	C10), 129.21 (C9, C11), 124.75 (C3, C5), 122.73 (C8, C12), 115.03 (C2, C6),
	68.17 (C21), 36.03 (C13), 32.03 (C18), 31.49 (C14), 29.62 (C15), 29.45 (C16),
	29.41 (C17), 28.93 (C22), 22.82 (C19), 14.26 (C20). HRMS (ESI + ) m/z
	417.150597 (calcd for C22H29BrN2O 417.153602). Anal. Found: C, 63.79; H, 7.16;
	N, 6.76. (calcd for C22H29BrN2O C, 63.31; H, 7.00; N, 6.71).

C NMR (100 MHz, CDCl3) δ 158.02 (C1), 151.08 (C7), 147.46 (C10), 146.13 (C4), 129.20 (C9, C11), 124.94 (C3, C5), 122.70 (C8, C12), 115.88 (C2, C6), 36.02 (C13), 32.03 (C14), 31.49 (C15), 29.62 (C16), 29.45 (C17), 29.41

(7.30 (d, J = 8.4 Hz, 2H, H9-Ar, H11-Ar), 7.02 (d, J = 9.0 Hz, 2H, H2-Ar, H6-Ar), 4.36 (t, J = 6.3 Hz, 2H, H21), 3.68 (t, J = 6.3 Hz, 2H, H22), 2.67 (t, J = 7.8 Hz, 2H, H13), 1.69 -1.61 (m, 2H, H14), 1.39 -1.21 (m, 10H, H15-H19), 0.88 (t, J = 7.1

Hz, 3H, H20).

13

C NMR (101 MHz, CDCl3) δ 160.34 (C1), 151.10 (C7), 147.

Table 1

 1 Irradiance with different light sources.

		Distance between light	Irradiance
		source and sample (cm)	(mW/cm 2 )
	Filtered UV tubes	10	0.7
	UV LED	0.5	1.3
	Blue LED	0.5	7.9
	Setup	Irradiance for UV LEDs (mW/cm 2 )	Irradiance for Blue LEDs (mW/cm 2 )
	SAXS	2.0	7.9
	SANS	1.3	8.1
	UV-Vis/NMR	0.35	3.8

Table 2

 2

Irradiance for different setups. (Measured at a distance of 0.5 cm for SAXS and UV-Vis, 1 cm for SANS).

Table 3 .

 3 For 50 mM 4Azo2TAB solution, 33% of trans isomers are still present after 4 h exposure to UV tube. The trans fraction even increases to 40% after 21 h probably due to lamp heat dissipation problems.

	For the same solution, only 15% of trans isomers are present after 3 h UV LEDs
	irradiation. For a less concentrated solution close to the CMC of cis isomers,
	𝑥 𝑡𝑟𝑎𝑛𝑠 values are lower than those obtained with the 50 mM solution and the
	steady state is reached faster.
	𝑥 trans	UV tubes	UV LED	Blue LED
	4Azo2TAB 50mM in D2O	33 %	15 %	82 %
	4Azo2TAB 8.6 mM in D 2 O	12 %	8 %	82 %

Table 3

 3 𝑥 𝑡𝑟𝑎𝑛𝑠 of 4Azo2TAB determined by NMR.

  Using neutron scattering and isotopic labelling, it is possible to vary the SLD of the water by mixing H2O and D2O. With a deuterated volume fraction 𝜙 𝐷 , the corresponding SLD 𝜌 𝐻 2 𝑂/𝐷 2 𝑂 writes 𝜌 𝐻 2 𝑂/𝐷 2 𝑂 = (1 -𝜙 𝐷 )𝜌 𝐻 2 𝑂 + 𝜙 𝐷 𝜌 𝐷 2 𝑂

	{	𝐾 𝑚 = 𝑎 𝑚 -𝑎 𝑠 𝐾 𝑆𝑢𝑟𝑓 = 𝑎 𝑆𝑢𝑟𝑓 -𝑎 𝑠 𝑣 𝑆𝑢𝑟𝑓 𝑣 𝑚 𝑣 𝑠 = 𝑣 𝑠 = 𝑣 𝑚 𝑁 𝑎 𝑁 𝑎 (𝜌 𝑚 -𝜌 𝑠 ) (𝜌 𝑆𝑢𝑟𝑓 -𝜌 𝑠 ) 𝑣 𝑆𝑢𝑟𝑓	(4)
	Smm, SSurfSurf and SmSurf are the partial structure functions related to the
	monomers (m), the surfactants (Surf) and monomers-surfactants (mSurf). They
	contain all the information on the structural arrangement of monomers, (Smm),
	surfactants (SSurfSurf), as well as their mutual organisation (cross term SmSurf).
	𝐾 𝑚	2 𝑆 𝑚𝑚 + 𝐾 𝑆𝑢𝑟𝑓	2 𝑆 𝑆𝑢𝑟𝑓𝑆𝑢𝑟𝑓 (𝑞) + 2𝐾 𝑚 𝐾 𝑆𝑢𝑟𝑓 𝑆 𝑚𝑆𝑢𝑟𝑓 (𝑞)	(3)
	with

  neutron coherent scattering length of atoms bN are available in many articles or books. The neutron scattering length of a molecule is equal to the sum of the bi over all the atoms in the molecule:

		𝑏 𝑁 𝑡𝑜𝑡𝑎𝑙 (10 -13 𝑐𝑚) = ∑ 𝑏 𝑁𝑖	(8)
	Atoms	M (g mol -1 )	bN (10 -13 cm)	bx (10 -13 cm)
	C	12.011	6.646	16.92
	N	14.0	9.36	19.74
	H	1.008	-3.739	2.82
	D	2.014	6.671	2.82
	O	15.999	5.803	22.56
	S	32.06	2.847	45.12
	Br	79.904	6.795	98.7
	Na	22.99	6.795	31.02

Table 4 Neutron and X-ray scattering lengths of atoms bN and bx

Table 5

 5 Partial molar volume of different chemical groups[10][11][12] .

		3 mol -1 )
	C4H9	81.3
	C8H17	146.1
	C2H4	32.6
	oxo	5.48
	(CH3)3N +	59.6
	azobenzene	106.0

  and on Figure6(for SANS experiments). Calculations are given for the ionized forms only. The contribution of the counterions Na + and Br -

	are negligible for SANS experiments. It is no longer the case for SAXS
	measurements.
		ν	bx	ρx	bN	ρN
		(cm 3 mol -1 )	(10 -13 cm)	(10 +10 cm -2 )	(10 -13 cm)	(10 +10 cm -2 )
	H2O	18.07	28.2	9.4	-1.68	-0.56
	D2O	18.07	28.2	9.4	19.15	6.38
	4Azo2TA +	284.97	518.88	10.97	61.28	1.295
	8Azo2TAB +	349.76	609.12	10.49	57.95	0.998
	PSSH -	114	270.72	14.30	47.25	2.51
	PSSD -	114	270.72	14.30	120.12	6.35
	PAA -	29.1	107.16	22.18	20.33	4.21
	PAA --Azo0.11	48.6	148.73	18.44	-	-
	PAA --Azo0.15	58.1	163.84	17.73	-	-
	Sal -	100.3	203.0	12.19	45.24	2.715
	BenSulf -	109.2	231.2	12.76	41.437	2.286
	Na +	-6.6	28.2	-25.65	3.63	-3.302
	Br -	30.1	101.5	20.30	6.80	1.36

Table 6

 6 Summary of the different compounds involved in this thesis

Table 2

 2 Fits parameters for measurements in dark and under UV irradiation.

Table 1

 1 , with A trans equals to 40 Å 2 while Acis is 102 Å 2 . Another similar Azo-surfactant 4-CMC and area per molecule A values for Azo-surfactants (dark, UV).

	octylazobenzene-40-(oxy-2-hydroxypropyl) trimethylammonium methylsulfate,
	with an additionally OH group near the trimethylammonium polar head, shows an
	area per molecule of 60 Å 2 and 74 Å 2 for the trans and cis isomers respectively 10 .
		CMCtrans (mM)	Atrans (Å 2 )	CMCcis (mM)	Acis (Å 2 )
	4Azo2TAB	2.6	34.5	8.2	50.0
	8Azo2TAB	0.29	33.0	0.82	48.9

Table 2

 2 SLD for SAXS and for SANS for solvents, surfactants and molecular constituents.

	153

Table 3

 3 SANS fit parameters with a one-shell ellipsoid model.

	Core-shell Model	4Azo2TAB	8Azo2TAB
	(oblate)	Dark	UV	Dark	UV
	Minor shell radius (Å)	20	17	28	22
	Major shell radius (Å)	35	23	66	31
	Minor core radius (Å)	5	5	11	13
	Major core radius (Å)	19	18	57	22
	𝜙	0.019	0.020	0.025	0.025
	𝜌 𝑠ℎ𝑒𝑙𝑙 (10 10 cm -2 )	1.93	3.41	3.02	3.42
	𝜌 𝑐𝑜𝑟𝑒 (10 10 cm -2 )	-0.55	-0.55	-0.43	-0.43
	Charge	27	10	110	23
	Monovalent sel (mol L -1 )	0.0020 0.0043 0.0018 0.00037
	Nagg	212	66	796	138

Table 4

 4 SANS fit parameters with a core-shell ellipsoid model.

Table 5

 5 SAXS fit parameters. The differences in the radius between SANS and SAXS under UV can be explained by the different irradiation setup used in two techniques. In SAXS experiments, the sample is irradiated directly by two UV LEDs fixed above and below the capillary (2 mm apart) and measured in absence of external light (in the sample chamber). SANS used a multi-LED setup (ten LEDs arranged circularly, 1 cm apart on one side of the sample). Besides, SANS experiments were performed in ambient luminosity. Consequently, the photostationary state in SAXS experiments contains more cis isomers, leading to larger structural modifications.

	Parameters	4Azo2TAB Dark	UV	8Azo2TAB UV
	Minor shell radius (Å)	24		12	19
	Major shell radius (Å)	35		22	30
	Minor core radius (Å)	5		7	13
	Major core radius (Å)	24		12	20
	𝜙	0.020	0.015	0.025
	𝜌 𝑠ℎ𝑒𝑙𝑙 (10 10 cm -2 )	10.64	11.56	11.38
	𝜌 𝑐𝑜𝑟𝑒 (10 10 cm -2 )	6.89	6.89	7.56
	Charge number	47		17	258
	Monovalent sel (mol L -1 )	0.0045	0.015	0.0078
	Nagg	262		52	124

  The scattered intensity was then measured at different times until it does no longer vary. This final state defines the structural 'equilibrium' state.

	Mixtures of Azo-surfactants and oppositely charged PEs have been
	investigated. Poly(styrene sulfonate) (PSS), a well-known polymer has been
	chosen as a model PE for this study. Deuterated and hydrogenated PSS (chemical
	charge fraction f = 1) have been kindly synthesized by Jean-Philippe Lamps at
	ICS. The synthesis of PSS is presented in Appendix c).
	VI.3.1 Azo-Surfactant/Polyelectrolyte ratios and concentrations
	VI.3.1.1 Phase diagram and reference system
	The choice of the reference Azo-surfactant/PSS system has been guided by
	SAXS measurements on a series of different samples (with different molar ratios,
	concentrations) in dark and under UV irradiation. The equilibrium state is not
	reached instantaneously and requires the diffusion of 'complexes' (containing the
	azo) in solution.

Table 8

 8

	1	-0.06	-0.08
	2	-0.13	-0.16
	3	-0.06	-0.07
	4	-0.06	-0.10
	5	0.29	0.26
	6	0.45	0.43
	7	0.36	0.39
	8	0.26	0.37
	9	0.2	/
	10	0.16	/
	11	0.12	/

Experimental chemical shift displacements Δδ for Azo-surfactant/PSS complexes (Δδ=δcomplexesδpure).

Table 9 and

 9 Table 10. The intermediate contrast D2O = 0.475 ( solvent = 2.74 10 10 cm -2 ) measured with 4Azo2TAB/PSS was not fitted here due to the low signal and the bad statistics.

	for 4Azo2TAB/PSS and

  4 Å -1 .

	Parameters	Surfactant Dark UV	Polyions Dark UV
	Major shell radius (Å)	34	31	32	30
	Minor shell radius (Å)	20	18	23	21
	Major core radius (Å)	19	16	22	23
	Minor core radius (Å)	15	14	12	11
	Distance between pearls (Å)	75	65	65	61
	Pearls number	2	2	2	2
	SLD core (10 10 cm -2 )	-0.43	-0.43	-0.43	-0.43
	SLD shell (10 10 cm -2 )	3.3	3.7	4.2	4.9
	∅	0.025 0.029 0.022 0.022

Table 10

 10

SANS fitting parameters for 8Azo2TAB/PSS.

Table 12

 12 Fit parameters for 4Azo2TAB/PSS and 8Azo2TAB/PSS.

190 Figure 36 SAXS profiles for a) 4Azo2TAB/PSS (50 mM/100 mM) and b) 8Azo2TAB/PSS (50 mM/100 mM) in dark and under UV.

Table 2 Tests

 2

	This composition and concentration define our reference sample. The
	corresponding concentration is noted C hereafter.
	4Azo2TAB (mM) NaSal (mM)	r	texture
	50	100	0.5	precipitate
	50	62.5	0.8	fluid
	50	50	1	fluid
	60	50	1.2	fluid
	75	62.5	1.2	fluid
	Table 1 Tests on 4Azo2TAB/NaSal mixtures, r = [4Azo2TAB]/[NaSal].
	8Azo2TAB (mM) NaSal (mM)	r	texture
	50	100	0.5	fluid
	50	62.5	0.8	viscous
	50	50	1	viscous
	60	100	0.6	viscous
	60	50	1.2	gel
	75	62.5	1.2	gel

on 8Azo2TAB/NaSal mixtures, r = [8Azo2TAB]/[NaSal]

Table 4

 4 Scattering length densities of the different components. The asterisk * means that the value has been calculated according to the volume fraction of each component (taking into account their molar composition in the mixture).

Table 6 SANS

 6 Fit parameters.

 1Main properties of azo containing sodium polyacrylates: weight-average molecular weight (Mw), number-average molecular weight (Mn), polydispersity index (Ip = Mw/Mn) and degree of polymerization (DPn). (* Values determined by NMR kinetic traces for unmodified PAA chains. We considered that the grafting of azo group does not change the DPn of PAA synthesized by RAFT).

V.2 Photoisomerization behaviours

Photoconversion of azo-containing polymers in aqueous solution was investigated by UV-Vis absorption spectroscopy. Several aspects including trans/cis ratio at the equilibrium state, but also, the photoconversion reversibility upon blue light stimulus and the thermal relaxation will be discussed in this chapter.

V.2.1 Trans/Cis ratio

The trans/cis ratio is an important parameter that influence the final organization of photoresponsive systems. This molar fraction can be determined by UV-Visible spectroscopy measurements.

Figure 1 shows the variation of UV-Vis spectra of (PAA-Azo0.11)42Sigma aqueous solution (Azo unit concentration 0.05 mM) under UV irradiation. Trans isomer exhibits a strong absorption band at 338 nm corresponding to a 𝜋 -𝜋 * transition and a weak band at approximately 440 -450 nm which is attributed to an 𝑛 -𝜋 * transition. Under UV irradiation, the maximum absorption at around 340 nm decreases while its position is shifted to 319 nm due to the trans ↔ cis photoisomerization of the Azo groups. Additionally, the absorbance peak centered at 437 nm increases when UV light is applied. Isosbestic points are observed at 300 and 405 nm, which is the signature of the presence of both trans and cis azo groups (two state model): all the UV-Vis spectra can be decomposed as a linear combination of pure trans spectrum and pure cis spectrum. If we go further in the analogy, above a CMC of Azo-surfactants, we have the formation of small pre-micellar aggregates that then grow in size with the concentration. UV irradiation only changes the size and shape of the micelles (and therefore the number of surfactants in the aggregates). This seems again very similar to what we observed. The variation under UV therefore just reflects the partial rearrangement of the micellar core. By the way, under UV, certain polyions are expulsed from the pre-existing aggregates and re-assemble elsewhere.

The physics of the system sounds very similar to that of small surfactants. To go further, it would be necessary to find a model able to fit the data in order to determine important parameters such as the aggregation number and its modification under UV.

For long chains, no CMC*dark or CMC*UV are observed in the investigated concentration range. We always have well defined core-shell particles. The aggregates therefore form at concentration much lower than for short chains. This difference may be related to simple entropic effect.

During this work, we did not find any evidence for a pearl-necklace conformation as it could be expected for hydrophobic PEs. Indeed, the hydrophobicity of the polyions leads to the formation of inter-chains complexes that are out of the scope of the theory. To prevent this aggregation, one possibility would be to work in mixtures of water and organic solvent to slightly improve the quality of the solvent, and work at lower concentrations 11 .

Chapter VI. Complexes of polyelectrolytes and oppositely charged Azo-surfactants

Surfactant/PE complexes have been widely investigated in vast applications areas such as for delivery systems, paints, food preservation, rheological control...

The large variety of PEs and surfactants offers various combinations and many structural organizations exhibiting unique properties in the bulk, at the interface and in the solid state.

Azo-surfactants show interesting structural modifications triggered by UV light stimuli [1][2][3] . The combination of PEs and Azo-surfactants offers new possibilities through the photo-control of the complex under irradiation. Some Azosurfactant/PE systems have already been reported as discussed in Chap II.2.3.

Nevertheless, little work has explored their structural organization in aqueous solution.

This chapter contains two parts. The first part concerns the investigation of pure Azo-surfactants solutions and more specifically, the determination of the CMC, the photoconversion behaviours and the structural description of their micellar aggregates. The objective is to understand the organization of pure Azosurfactant micelles in dark state and the variation upon light irradiation.

The second part of this chapter reports the study of PE/Azo-surfactant complexes. The photoisomerization behaviours (by NMR and UV-Vis spectroscopy) and the structural organization (SAXS and SANS) will be presented.

A pearl-necklace model will be proposed to describe the structure of Azosurfactant/PE complexes.

VI.2.2.2 By NMR

The isomerization behaviour of Azo-surfactants has been investigated by 1 H NMR.

Figure 1 shows the 1 H NMR spectra of 4Azo2TAB (c = 50 mM > CMC) in D2O (steady state) before and after light stimulus (UV and blue). Pure trans isomers (dark state, red curve in Figure 1) show two peaks at 7.62 ppm and at 7.03 ppm corresponding to aromatic protons. The two protons of the CH2 group adjacent to the oxygen (H3) are detected at 4.23 ppm while the signal of the CH2 group adjacent to the ammonium head (H2) is detected at 3.64 ppm. The sharp intense peak located at 3.08 ppm is attributed to the 9 protons of the ammonium head group (H 1 ). Other peaks at 0.5 -2.5 ppm correspond to protons of the hydrophobic chain.

After UV irradiation, isomers are mostly in their cis form (UV, green curve in Figure 1). Main peaks are slightly shifted with regard to the trans form.

Furthermore, they appear to be split because of the presence of small fraction of trans isomers. For example, H 2 observed at 3.64 ppm in dark is now divided into two peaks at 3.84 ppm (trans form) and 3.73 ppm (cis form). When the solution is irradiated with a blue light, most of the cis isomers return to the trans state. We recover the main intense peaks of the pure trans isomer (blue curve in Figure 7).

A small fraction of cis isomers is however still present (small shifted peaks).

The mole fraction of trans isomers at the stationary state can be estimated using the following equation:

where Itrans is the integral of the peak H2 of the trans form and Icis is the integral of the corresponding cis peak.

𝑥 𝑡𝑟𝑎𝑛𝑠 is close to 15% at the stationary state after UV irradiation and returns to 82% after blue light irradiation. A less concentrated solution of 4Azo2TAB (c = 8.6 mM) has also been measured. We cannot reach a state of 100% cis isomers by UV irradiation even for very low concentration. There are still 8% trans isomers left at the stationary state, which is in agreement with the values reported in literature 10,22 . Under blue irradiation, we find the 𝑥 𝑡𝑟𝑎𝑛𝑠 value (82%). These values are very close to that found in UV-Vis measurements on a very dilute sample.

8Azo2TAB at 50 mM shows very similar variation: 15 % of trans isomers are left at the equilibrium state under UV and the ratio of trans isomers increases to 149 82% under blue irradiation.

Figure 7 1 H NMR spectra of 4Azo2TAB 50mM in D2O before (dark) and after light irradiation (UV, blue).

VI.2.3 Structural organization study

The structural organization has been carried out by SAXS and SANS for both 4Azo2TAB and 8Azo2TAB.

VI.2.3.1 Neutron scattering length density (SLD)

SLD calculation for molecules in solution (such as surfactant in water) needs the determination of their partial molar values. They have been evaluated

"theoretically" by the incremental volume method (Table 2).

The SLD can also be obtained by scattering experiments using contrast variation at small enough concentration.

The scattered intensity 𝐼(𝑞) writes Δ𝜌 2 𝜙𝑉𝑃(𝑞)𝑆(𝑞). For very dilute systems, there is no correlation between the particle, 𝑆(𝑞) = 1. For 𝑞 → 0, 𝑃(𝑞 → 0) = 1, 𝐼(𝑞 → 0) = Δ𝜌 2 𝜙𝑉 = (𝜌 -𝜌 𝑠 ) and the SLD of 8Azo2TA + is 1.0 10 10 cm -2 . We have tried to estimate this quantity directly by SANS and contrast variation. Measurements are presented in Figure 8a for different 𝜌 𝑠 values. Our experimental conditions are not optimized since a correlation peak is observed (≈ 0.023 Å -1 ). We nevertheless selected the intensity complex due to the large variation of the SLD along the molecule, and therefore, along the radial direction in the aggregates (Figure 10 c andd). The core-shell structure is certainly a crude approximation, but it allows to get the main geometrical features of the assemblies and to catch the morphological transition under UV irradiation.

VI.2.3.3 SANS results

SANS profiles for Azo-surfactants in D2O are displayed in Figure 11. They have been measured at 50 mM, i.e., at a much higher concentration than the CMC of the surfactants (in between 0.29 to 8.2 mM). The scattered intensity is characteristic of micellar solutions. In between 0.03 and 0.05 Å -1 , we find a broad peak reflecting the correlations between charged particles. Its position q* is directly related to the average distance between the micelles (d = 2/q*).

At higher q values (above 0.2 Å -1 ), a shoulder is roughly visible. It is the signature of the form factor P(q) of the particles. The absence of well-defined characteristic oscillations may result from a polydisperse population but also, from well-defined anisotropic objects (such as ellipsoidal particles).

Under UV exposure, the position of the maximum is displaced to higher q values while the global intensity is reduced. The differences reflect large structural modifications under irradiation. If we omit the variation of the CMC (that also changes the particle concentration in the solution), the average distance between particles under UV is reduced: they are more abundant and therefore, smaller (their number of aggregations is reduced).

major radius of the shell and the core decrease significantly as well. The minor radius of the core, which is related to the length of alkyl tail remain unchanged.

Consequently, the volume of the micelle as well as the aggregation number Nagg falls by 69% for 4Azo2TAB micelles and 83% for 8Azo2TAB micelles.

Structural analysis of 4Azo2TAB and 8Azo2TAB have already been studied in the literature in dark and under UV irradiation. The experimental conditions such as the concentration and the light source are however not always the same. Our SANS study was carried out in a larger q range, thus leading to a better spatial resolution. The geometrical description of trans-4Azo2TAB in the works of Akamatsu et al. 2 cannot be directly compared to our work since they used a prolate model which is definitively not adapted to explain our data in large q range. If we use their model in a limited q range (q < 0. We found a larger micellar size and a distinct oblate shape. The difference in size may be related to our higher concentration. Indeed, in conventional cationic surfactant systems (i.e., CTAB and DTAB), the equatorial radius of micellar aggregates increases with increasing surfactants concentrations 11 .

VI.2.3.4 SAXS results

The structure of micellar aggregates has been investigated by SAXS (Figure 15). Fit results are given in Table 5. SAXS curves are almost similar to SANS data.

They present a clear oscillation at high q, which is characteristic of micellar aggregates, and a broad maximum at low q related to the electrostatic repulsion between aggregates. The main difference is certainly the high intensity of the maximum around 0.2 Å -1 . This is the direct consequence of a very particular radial SLD profile. Upon UV light exposure, the curves decrease significantly in intensity and both the correlation peak and the oscillation of the form factor are shifted to the higher q region suggesting a decrease of the particle size and therefore of PSS lead to a negative chemical shift displacement of the surfactant peaks in the polar head region, and thus to the upfield shift of H 1 -H 3 protons. This is consistent with previous works on classical surfactants/PE systems (without azobenzene) 17- 20 . PSS is therefore embedded between surfactant molecules, interact with the phenyl groups of azobenzene and further influence the chemical environment of the alkyl tail.

The largest magnitude of Δδ is observed in the aromatic region. We can assume that aromatic rings or probably part of the aromatic rings of PSS are in the vicinity of azobenzene moiety. The sign of Δδ depends on the relative orientation of aromatic rings, which may be parallel or perpendicular due to the steric constraint and the stiffness of PSS backbone. This may probably explain why the aromatic protons of Azo unit have different sign of Δδ (negative for H4 and positive for H5 -H7).

Based on 1 H NMR studies, we can therefore conclude that Azosurfactant/PSS complexes adopt a core-shell organization (Scheme 5). The aromatic rings of PSS penetrate deeply in the shell of aggregates. And that the PSS polyions participate to the micellization (co-micellization).

The 8Azo2TAB/PSS shows very similar NMR spectra, which are not presented here. We found in particular that the trans/cis ratio at the photostationary state was 30%. When blue light is applied, the cis peak of H1 is merged with the trans peak. The estimation of the trans fraction is impossible. For the two systems, the intensity increases at low q as the concentration decreases. This is a clear manifestation of a structure factor that varies with the concentration. Curiously, there is no peak in this region. This is certainly due to the release of the counterions (of the surfactants and the polyions) during the complexation process that screens the electrostatic interactions.

At higher q, all the curves are superposed (except maybe for 4Azo2TAB/PSS at C/4 which shows slight variations) and a large oscillation is observed around 0.2 Å -1 . This domain is dominated by the form factor of the aggregates. This maximum is very similar to that found in pure micelle solutions and certainly indicates to the formation of globular particles. We do not observe a classical q -4 variation of the intensity as for spherical uniform objects. The aggregates are probably heterogeneous on a local scale.

Under UV irradiation, the large oscillation is displaced to higher q indicating the size reduction of the aggregates.

From these observations, we can roughly estimate the q value from which the scattered intensity is mainly due to the form factor. This limit depends on the concentration. For the reference concentration C, it is close to 0.05 Å -1 for 4AzoTAB/PSS, and close to 0.04 Å -1 for 8AzoTAB/PSS. Since it is very complicate to find an analytical expression for the intermolecular correlations in our system (S(q)), only a form factor can be fitted to the data in this restricted q range. The fits properly reproduce the data in both contrast conditions, in dark and under UV. The model seems appropriate to describe the structure of the complexes.

In particular, the shoulder observed in the surfactant structure function (below 0.1 Å -1 , measurements in D2O, Figure 33a and Figure 34a) and related to the pearl-pearl correlations is perfectly reproduced and gives accurate value of the average inter-pearl distance. For the 4Azo2TAB/PSS complexes, the radius of the shell (major, minor) are 26 and 18 Å. The inter-pearl distance is 57 Å, which is nearly twice the major shell radius. For the 8Azo2TAB/PSS complexes, the interpearl distance is also twice the major shell radius. The pearls are therefore very close to each other.

When we probe the polyions organization (Figure 33b and Figure 34b), we are also able to reproduce the large oscillation around 0.2 -0.3 Å -1 . This maximum finally comes from a shell type arrangement of the PE around the micellar core.

The geometrical parameters determined for the two contrast conditions are very similar. The PSS does not only decorate pre-existing micelles but is also mixed with the Azo-surfactants in the shell (co-micellization). The minor core and shell radii are very close to the radii observed for pure surfactant micelles. The major core and shell radius are shorter than that for pure surfactant. The global size of the aggregate is therefore smaller than for pure surfactant micelles at the same concentration even with the incorporation of the polyions in the aggregates.

Since the geometrical parameters only slightly vary with the contrast conditions, it is possible to use a single set of parameters to fit the different measurements simultaneously. Since the number of parameters in the model is important, a global fit leads to a more robust description.

We present in Figure 35 the results of the global fits for 4Azo2TAB/PSS. A single set of geometrical parameters for the core radius (minor and major), the shell radius (minor and major) and the inter-pearl distance has been used for the three contrast conditions. We imposed a constant shell thickness as well in the aggregate. Since water may penetrate in the shell, the shell SLD cannot be treated as a general parameter and needs to be fitted independently. This procedure was applied both in the dark state (blue curves) and under UV radiation (red curves).

Fit results are given in Table 11.

A single geometrical description for the three contrast conditions allows to fit correctly the data (the fit can be slightly improved if we relax the constraint on the constant shell thickness). This really proves that the location of both surfactant and PSS are intricately with each other, especially in the shell of the aggregate.

The global fit for 8Azo2TAB/PSS was also tested (without intermediate contrast conditions). The main results are identical, but the fit quality is not as good. To figure-out the location of PSS and the surfactant in the pearls, the SLD of the core and the shell can be compared to that of the molecules or to different constituting molecular groups. The SLD of the core has been fixed to the theoretical value for alkyl groups. The minor core radius is on the order of the alkyl chain length in its fully extended conformation. This validates, the location of the hydrophobic tail in the center of the aggregate. The SLD of the shell lies in between the SLD of PSS and the surfactants (see Figure 10 a). For 4Azo2TAB/PSS, it varies very little with the nature of the solvent (H2O/D2O mixture)

indicating that there is almost no water penetration. The pearls can therefore be described by a dense and compact entity formed by Azo-surfactant and polyions.

This intermediate SLD value then reflects the mixing of PSS and surfactants in the shell (co-micellization).

The number of pearls appear very limited (N = 2). Curiously, they tend to be almost in contact along the chain. We believe that our restricted q range potentially affect these results and cannot exclude a signature of larger assemblies (N > 2)

at smaller q values. Cryo-TEM measurements should be suited to explore larger lengths scales.

Under UV irradiation, we observe a displacement of the typical oscillation in the polyion structure function (H2O/D2O) to larger q values: the size of the aggregate is reduced. This is also visible on the surfactant structure function (D2O)

and more specifically on the shoulder associated to the pearl-pearl correlations. It shifts to higher q under UV indicates that the distance between pearls decreases. gel property (Table 1).

On the other hand, the addition of NaSal to 8Azo2TAB solutions leads to a significant increase of viscosity and a gel behavior (Table 2). The gel texture of 8Azo2TAB/NaSal assembly is more pronounced when increasing the concentration. 8Azo2TAB/NaSal at 75 mM/62.5 mM (r=1. Two distinct experiments (i.e., beam time allocation) have been performed at the LLB on these systems. In the second experiment, we re-measured the C/4 sample under more rigorous conditions to reduce the surrounding light influence.

Only this sample will be fitted below.

The scattering curves normalized to the concentration (of 8Azo2TAB) are presented in Figure 7.

In the dark state (Figure 7 a), the three curves are superposed for q  0.03 Å -

1

. The scattering in this region only reflects the form factor of the particles. Since we have long objects in solution, the large decrease in intensity above 0.03 Å -1 is related to the cross section of particles (and therefore to their radius).

For q  0.03 Å -1 , the intensity does not vary as q -1 (as it should do for a rodlike systems) except for the lowest concentration C/4. We have a clear contribution of a structure factor in this q-range. In the following, only the sample at C/4 will be considered for the fit.

Under UV radiation (Figure 7 b), the curves are close, but not superposed at large q. Two effects may explain these differences. First, the size of the globular micelles varies with the concentration (classical behaviour). Second, the photostationary states are not identical, for physical reasons or due to our irradiation procedure. During the fit, we will only focus on the sample at C/4 for which the difference with the dark state is larger. In Figure 8, we present the variation of the intensity between the dark and the photostationary states (under UV) for two concentrations: C (Figure 8 a) and C/4

(Figure 8 b).

The variation found for the highest concentration C is very modest. Intensity is lowered at low q, while it is shifted to largest q around 0.1 Å -1 .

The lowest concentration is more interesting. As already mentioned, the intensity in dark almost follow the classical q -1 variation characteristic of rod-like particles. This is in accordance with Cryo-TEM measurements. However, a small contribution of the structure factor is still present. We could expect the signature of wormlike micelle at very low q, and the appearance of a Gaussian behaviour below qlP = 1.9 (where lp is the persistence length of the micelles) 4 . However, no significant upturn is present. In the high q regime (above 0.03 Å -1 ), we are sensitive to the cross section of the particles. Since no oscillation (or minimum) is observed for the highest q values, we may have polydisperse radius and an elliptic cross section. Data in the dark state have been fitted with a cylindrical model of elliptic section that seems consistent with these observations.

Under UV irradiation, we note a large decrease at low q. The power low exponent (in the intermediate regime) is changing indicating a modification of the shape of the particles. In the meantime, the intensity around 0.1 Å -1 is clearly shifted indicating a reduction of the typical size of the object. Curiously, we do not find the Guinier regime and a plateau at low q. This is a consequence of the high polydispersity of the objects as demonstrated by Cryo-TEM. Hence the modelling is more complicated. Since small objects are more abundant, we have used a model of uniaxial ellipsoids particles. For the photostationary state (UV), we used the form factor of a prolate ellipsoid. According to Cryo-TEM, globular shapes and elongated systems are present in the solution. It is therefore very hard to find a simple suitable model. In this context, we just tried to reproduce the large q behaviour which certainly contains information about the cross section of the objects (either cylindrical or globular). Since this procedure is not very accurate, we impose the lowest q limit to 0.04 Å -1 . Changing the length of the prolate ellipsoid does not modify the high q behaviour and the final radius. We arbitrary fixed this value to 100 Å. The fitted parameters are the radius of the ellipsoid, the SLD of the particle and the background.

For both models, we fixed the volume fraction to its theoretical value ( = 0.0086) and the SLD of the deuterated water (N D20 = 6.38 10 10 cm -2 ).

The fits reproduce well the experimental measurements without introducing polydispersity in the model. Final parameters are summarized in Table 3.

206 In the elliptic cylinder model, the section has two radii: the major and the minor. The ratio between the major and the minor radius gives the ellipticity. In the prolate ellipsoid, the major axis corresponds to the rotation axis. The cross-section perpendicular to the axis of symmetry is circular. Asterisks correspond to fixed parameters. The parameters for the aggregate under UV irradiation should be taken with care (the system is polydisperse and the fit is restricted on a very particular q range).

Photoconversion: determination of trans/cis ratio

The 1 H spectra of the 8Azo2TAB/NaBenSulf mixture at the reference concentration C (75 mM/62.6 mM) are presented in Figure 17.

Under UV irradiation, two peaks at 3.05 ppm and 2.97 ppm are detected for protons of the ammonium head, corresponding to trans and cis isomers respectively. The trans/cis ratio can be roughly determined from the integrated intensities of these peaks and is equal to 36:64 in the stationary state. When blue light is applied, the cis→ trans transition occurs and the trans/cis ratio reaches 73:27. For the mixture C/4, the 1 H NMR spectra are easier to analyze (Figure 18).

The resonance peaks detected are narrower. Well-separated peaks are observed for the hydrophobic tail protons (Figure 18). The aromatic zone may be too chaotic to extract information, because the peaks for trans and cis isomers coexist with that of NaBenSulf. But for the C2H4 spacer (H2, H3), well distinguished trans and cis peaks are detected. We found a trans/cis ratio of 13:87 at the equilibrium state.

Inversely blue light triggers the cis→trans isomerization, the mixture recovers its initial viscosity. The resonance peaks are broader, but we can still distinguish the trans-peak at 2.93 ppm and the cis-peak at 2.82 ppm for the ammonium head (H1).

The trans/cis ratio turns back to 84:16.

VII.2.2 Structural study

The strategy for the structural investigation of 8Azo2TAB/NaBenSulf systems is identical to that of 8Azo2TAB/NaSal. SAXS and SANS data are very similar with the exception that UV irradiation now leads to a well-defined systems. Hence, we have used the same models (dark state: elliptical cylinder for SANS and elliptical core-shell cylinder for SAXS; under UV radiation: prolate ellipsoid for SANS and core shell prolate ellipsoid for SAXS).

VII.2.2.1 SANS measurements

A single concentration C/4 has been measured by SANS. The scattering curves are presented in Figure 19 (dark, UV). In the dark state, the intensity almost varies as q -1 for very low q. Under UV irradiation, we see the Guinier regime and almost find a plateau below 0.05 Å -1 . The scattered intensity is close to that of a pure form factor, but the presence of a structure factor can still be detected below q  0.03 Å -1 .

215 Figure 19 SANS patterns for 8Azo2TAB/NaBenSulf in dark and under UV irradiation at a concentration C/4 (18.75 mM/15.62 mM). Dash line indicates a q -1 decay.

We have fitted the data with an elliptical cylinder (dark state) and a prolate ellipsoid (UV) models. The volume fraction has been fixed to its theoretical value (0.0087) as well as the SLD of the deuterated water (N D20=6.38 10 10 cm -2 ).

The fits are presented in Figure 20. They can reproduce well the experimental measurements. Final parameters are given in Table 6. Under UV irradiation using a prolate ellipsoid.

In the dark state (wormlike micelles), SANS and SAXS find very similar values for the cross section (major and minor radius of the elliptic cylinder: 33.1 and 20.4

Å; major and minor shell radius of the core-shell elliptic cylinder: 36.9 and 23.4 Å).

The difference of 3-4 Å could reflect be the presence of a counterion layer.

Under UV (globular micelles), SANS and SAXS find identical values for the cross section (major and minor radius of the prolate ellipsoid: 39.7 and 21. 7 SAXS fit parameters. In the elliptic core shell cylinder model, the section has four radii: the major and the minor shells and the major and the minor core. The ellipticity is identical for the core and the shell. In the prolate core shell ellipsoid, the major axis corresponds to the rotation axis. The cross section perpendicular to the axis of symmetry has a circular core-shell geometry. The thickness is held constant. Asterisks correspond to fixed parameters.

The SLD of the core (dark: 7.60 10 10 cm -2 , UV 7.52 10 10 cm -2 ) and the shell (dark: 11.25 10 10 cm -2 , UV 11.3 10 10 cm -2 ) coincide with that of the C8H17 tail (7.56 10 10 cm -2 ) and that of 8Azo2TAB/NaBenSulf (12.0 10 10 cm -2 ).

Furthermore, the minor core radius (dark: 11.3 Å, UV: 13.9 Å) almost represents the size of the tail in its extended conformation (11.6 Å). We definitely validate our previous description. The micellar aggregate has a core mainly composed of the C 8 H 17 tails of the surfactant and a shell made of azo and hydrophilic groups of the surfactant in which NaBenSulf is introduced. This is in and elongated aggregates coexist. In average, we found a prolate ellipsoid with radii of 25 Å and 61 Å. For 8Azo2TAB/NaBenSulf mixture (C/4), we have 87% of cis isomers (trans/cis ratio 13:87), the system is more homogeneous. The globular aggregates have a minor and major radius of 21 -24 Å and 39 -40 Å respectively. 8Azo2TAB/NaBenSulf is certainly the best candidate to elucidate the mechanism of the structural transition.

Since the majority of isomers are in their cis form, and since they are located in the shell of the micelle, one must find structural difference at the local scale in between dark an irradiated states. The thickness of the shell that varies between 12 and 19 Å in the dark state (since we choose a model with constant ellipticity, we have one thickness for minor axis and one for the major) is reduced to 9.7 Å under UV. The difference of about 5 Å (in average) is a direct consequence of the conformational transition between trans to cis conformers.

In the dark state, the fully extended size of the 8Azo2TAB is around 29.2 Å in its fully extended form, or 18.2 Å if we remove the contribution of the alkyl tail. We must therefore introduce the Azo group, the spacer (C2H4) and the hydrophilic headgroup (18.2 Å long) in a shell of thicknesses in between 12 and 19 Å, which appears clearly feasible. Under UV irradiation, the shell thickness is reduced to 10 Å. With additional condition that the polar head lies at the surface of the micelle (in contact with water) and that one benzene of the Azo group is in contact with the micellar core.

The Azo groups are literally sandwiched in the thin shell. The critical packing parameter is naturally increased leading to a morphologic transition.

Salicylate ions are more effective in promoting the micellar growth (leading to more viscous mixtures). This is probably due to the presence of -OH in NaSal molecules that may create hydrogen bonds with surrounding water and stabilize the micellar assembly 5 . Azo-surfactant and oppositely charged PSS interact strongly due to the electrostatic interaction. The 1 H NMR measurements clearly confirm the formation of aggregates and the strong interaction between the aromatic ring of PSS and the shell of the micelles (polar head, Azo unit): polyions participate to the micellization process. The structural organization of Azo-surfactant and PSS in the complex have been studied separately by SANS using contrast variation. We find the existence of globular aggregates (pearls) composed of both surfactants and polyions. These aggregates have a core-shell organization: the core contains only the alkyl chains of surfactant while the shell is composed of Azo moiety, polar head and portions of polyions. The signature of pearl-pearl correlations is observed and is consistent with a pearl necklace organisation. This typical structure is confirmed by Cryo-TEM images. Under irradiation, the structural modification is very modest.

The pearl size is only reduced (in a reversible way) by around 30%.

In this study we definitely demonstrate the existence of a pearl necklace organization in Azo-surfactant/PSS complexes. We also show a small structural photo-sensitivity under UV. This formulation may represent a model for photoresponsive systems. However, the photo-response needs to be improved. The presence of multivalent counterions -in general-can modify the structural organization of PE solutions 1,2 and control the conformation and the compaction of polyions 3 . Thus, Azo-containing counterions can be a new strategy for the photo-control of polymeric systems (in a non-covalent way). This strategy avoids the synthesis of Azo-modified polymers while the Azo fraction can be adjusted easily.

There are few pioneering works about Azo-dications. Zinchenko et al. 4 reported an Azo-containing dication which can match or mismatch the distance of two DNA adjacent phosphates leading to a reversible DNA compaction under light stimulus. A light-induced shape modification has been reported for self-assemblies of poly(diallyldimethylammonium chloride) (PDADMAC) and oppositely charged Azo-dication (Ay38) 5 . PDADMAC/Ay38 forms long flexible cylinders with a large elliptical cross-section in the dark state that turns into a core-shell ellipsoid organization under UV.

We have synthesized an Azo-dication counterion and studied Azodication/PSS solutions by SAXS and SANS. The preliminary results are presented in appendix e). We find that Azo-divalent counterions interact with polyions and re-organize along the polyions in solution. A structural modification is observed under UV irradiation. Data analysis are going on in order to understand their structural organization (form factor and dispersion state), the nature of the interaction and the photoinduced effects.

Calibration Curve

It is found that

Mw

(gmol -1 ) 10 5

Mn

(g mol -1 ) 10 After two days, the crude product was filtrated and evaporated under vacuum.

Chromatography (SiO2, DCM) afforded ( 14) as a yellow solid (0.35 g, 15% yield). Compound 14 (0.2 g, 0.5 mmol) and a solution of trimethylamine in ethanol (1.0 ml, 3 mol L -1 , 2.5 mmol) were dissolved in a mixture of 5 ml THF and 8 ml acetone.

The mixture was stirred at 75 °C under argon for 26 hrs. After cooling to room temperature, the precipitate was filtrated and recrystallized from EtOAc/MeOH (3/1) at 90 °C to afford (15) as a yellow solid (0.24 g, 90% yield). 1 H NMR (400 MHz, D2O) δ 7.80 (dt, J = 9.3, 5.2 Hz, 4H, H3, H3', H5, H5'), 7.17 (dt, J = 9.2, 5.3 Hz, 4H, H2, H2', H6, H6'), 4.59 (s, 4H, H7, H7'), 3.85 (s, 4H, H8, H8'), 3.28 (s, 18H, N(CH3)3). 13

e.2 Photoconversion of pure Azo-divalent ions

The photoconversion behaviour of Azo-divalent ions has been investigated by UV-Vis measurements. The maximum absorption at 361 nm is typical for trans isomers.

Upon UV irradiation, this band decreases progressively and two bands at 317 nm and 450 nm appear for cis isomers.