T T H Luu

Z Jia

A Kanaev

L Museur
email: luc.museur@univ-paris13.fr

Photopolymerization of TiO 2 -based hybrid materials: Effect of nanoparticles loading and photosensitive 1D microstructures fabrication

Keywords: organic-inorganic hybrids, composites, polymer, photopolymerization, TiO 2 nanoparticles, microstructure, photosensitivity

Hybrid materials, based on inorganic nanoblocks incorporated into polymer networks, are an important class of functional materials. Although the kinetic of the free-radical polymerization of neat polymers has been studied for many years, both theoretically and experimentally, studies devoted to the polymerization kinetics of nanocomposite materials are rare. In this study, photosensitive hybrids formed using poly(2-hydroxyethyl methacrylate) (pHEMA) and titanium dioxide (TiO 2 ) nanoparticles functionalized with polymerizable ligands were considered. Their bulk UV-induced free-radical photopolymerization was investigated to evaluate the potential of pHEMA/TiO 2 hybrids as a material of choice for the fabrication of micro-optical devices. The influence of the nanoparticle concentration and UV intensity on the polymerization kinetics, probed in situ by Raman spectroscopy, is discussed. A decrease in the initiation rate was observed, which was attributed to the UV absorption of TiO 2 nanoparticles. This leads to a decrease in the propagation rate with an increase in nanoparticle concentration. Furthermore, a decrease in the maximum C=C conversion yield was observed when the nanoparticle concentration was increased. This was attributed to the formation of microgel regions around the nanoparticles, which prevented the polymer chains from bonding with surface ligands. Despite the lower conversion yield, we demonstrated the fabrication of one dimension (1D) photoactive microstructures in pHEMA/TiO 2 hybrids. The photochromic efficiency of the obtained structure was evaluated based on the reduction of silver (Ag) ions under UV irradiation. The results show that pHEMA/TiO 2 hybrids can be considered as a prospective material for the realization of photosensitive microelements.

INTRODUCTION

The broad interest in organic-inorganic hybrid materials over the past two decades has been motivated by the possibility of combining the useful properties of their two constituent components [START_REF] Kickelbick | Hybrid Materials : Synthesis, characterization and apllications[END_REF][START_REF] Sanchez | Applications of advanced hybrid organic-inorganic nanomaterials: from laboratory to market[END_REF]. Many studies have been conducted to control the synthesis of these nanocomposite materials and understand their physical properties [START_REF] Nicole | Hybrid materials science: a promised land for the integrative design of multifunctional materials[END_REF]. Soft chemistry is one of the most widely used approaches for preparing and incorporating inorganic building blocks into organic polymer matrices [START_REF] Sanchez | Chimie douce": A land of opportunities for the designed construction of functional inorganic and hybrid organic-inorganic nanomaterials[END_REF]. The mechanical, thermal, and physical properties of the obtained nanocomposites were improved compared with those of the neat polymer. These materials can also acquire new properties directly related to the nature of the interactions at the interface between organic and inorganic components [START_REF] Díez-Pascual | Nanoparticle Reinforced Polymers[END_REF]. Currently, these hybrids are used in a wide range of fields including optics, electronics, optoelectronics, protective coatings, sensors, and solar cells, etc. [START_REF] Sanchez | Applications of advanced hybrid organic-inorganic nanomaterials: from laboratory to market[END_REF][START_REF] Sanchez | Optical Properties of Functional Hybrid Organic-Inorganic Nanocomposites[END_REF]. Hybrids based on silica, titania, and zirconia have attracted particular interest because of their high refractive index and biocompatibility [START_REF] Ovsianikov | Two-photon polymerization of hybrid sol-gel materials for photonics applications[END_REF][START_REF] Malinauskas | Ultrafast laser nanostructuring of photopolymers: A decade of advances[END_REF][START_REF] Sakellari | Two-photon polymerization of titanium-containing sol-gel composites for three-dimensional structure fabrication[END_REF]. Despite evident achievements, in situ polymerization in the presence of functionalized inorganic blocks such as nanoparticles (NPs) has not yet been well documented and understood. Although the kinetics of the free radical polymerization (FRP) of neat polymers have been studied for many years, both theoretically and experimentally [START_REF] Andrzejewska | Chapter 2 -Free Radical Photopolymerization of Multifunctional Monomers[END_REF][START_REF] Achilias | A Review of Modeling of Diffusion Controlled Polymerization Reactions[END_REF], only few studies have been devoted to the polymerization kinetics of polymers combined with nanoparticles. It appears that the presence of inorganic nanoparticles significantly modifies the FRP kinetics. Compared to a solution of neat monomers, acceleration or retardation of polymerization reactions has been reported depending on the nature of the ligands functionalized on the nanoparticle surface [START_REF] Cho | The effects of silica nanoparticles on the photocuring behaviors of UV-curable polyester acrylate-based coating systems[END_REF][START_REF] Cho | Photocuring kinetics of UV-initiated free-radical photopolymerizations with and without silica nanoparticles[END_REF][START_REF] Kaddami | Influence of the initiation rate on the polymerization kinetics of hydroxy ethyl methacrylate (HEMA) filled with HEMAgrafted silica preformed nanoparticles[END_REF][START_REF] Kaddami | Silica-filled poly(HEMA) from hema/grafted SiO2 nanoparticles: Polymerization kinetics and rheological changes[END_REF][START_REF] Achilias | Polymerization kinetics of poly (2-Hydroxyethyl methacrylate) hydrogels and Nanocomposite materials[END_REF][START_REF] Lin | Effects of Surface Groups on SiO2 Nanoparticles on in Situ Solution Polymerization: Kinetics and Mechanism[END_REF][START_REF] Demir | PMMA/Zinc Oxide Nanocomposites Prepared by In-Situ Bulk Polymerization[END_REF][START_REF] Michailidis | An experimental and theoretical study of butyl methacrylate in situ radical polymerization kinetics in the presence of graphene oxide nanoadditive[END_REF].

Moreover, the influence of NPs on initiator decomposition and chain transfer reactions to the monomer must be considered Due to a large diversity of side reactions that can be possibly induced in presence of nanoparticles, theoretical models considering their influence on polymerization kinetics are very rare. Verros et al. proposed a model based on mass balance equations to describe the radical polymerization of methyl methacrylate (MMA) in presence of different kinds of non polymerizable nano-additives [START_REF] Verros | Toward the development of a mathematical model for the bulk in situ radical polymerization of methyl methacrylate in the presence of nano-additives[END_REF]. In this work, diffusional limitations on initiation, propagation and termination reactions have been taken into account using appropriate equations based on free volume theory. In addition to the possible effect of nano-additives on both free volume fraction and glass transition temperature of the reaction mixture, the authors underlined the role of nanoadditives as radical scavengers, causing deactivation of macroradicals and primary initiator radicals.

Hybrid materials, which combine mechanical and optical properties of their organic and inorganic components, have found numerous applications in optics and photonics [START_REF] Sanchez | Optical Properties of Functional Hybrid Organic-Inorganic Nanocomposites[END_REF][START_REF] Parola | Optical properties of hybrid organic-inorganic materials and their applications[END_REF][START_REF] Lebeau | Hybrid materials for optics and photonics[END_REF]. We have previously reported the preparation of hybrids containing preformed monodispersed titanium oxoalkoxy (TOA) nanoparticles embedded in organic poly(2-hydroxethyl methacrylate) (pHEMA) matrix [START_REF] Gorbovyi | Novel nanostructured pHEMA-TiO2 hybrid materials with efficient light-induced charge separation[END_REF][START_REF] Gorbovyi | Formation of gel of preformed size-selected titanium-oxo-alkoxy nanoparticles: towards organicinorganic hybrid material with efficient interfacial electron transfer[END_REF][START_REF] Gorbovyi | Alkoxysilane effect in hybrid material: A comparison of pHEMA-TiO2 and pMAPTMS-TiO2 nanoparticulate hybrids[END_REF][START_REF] Museur | Luminescence properties of pHEMA-TiO2 gels based hybrids materials[END_REF][START_REF] Evlyukhin | Synthesis of organic-inorganic hybrids via a high-pressure-ramp process: the effect of inorganic nanoparticle loading on structural and photochromic properties[END_REF][START_REF] Evlyukhin | Laser Assisted High Pressure Induced Polymerization of 2-(hydroxyethyl) Methacrylate[END_REF]. In situ polymerization was induced either by the thermal activation of an initiator [START_REF] Gorbovyi | Novel nanostructured pHEMA-TiO2 hybrid materials with efficient light-induced charge separation[END_REF][START_REF] Gorbovyi | Formation of gel of preformed size-selected titanium-oxo-alkoxy nanoparticles: towards organicinorganic hybrid material with efficient interfacial electron transfer[END_REF][START_REF] Gorbovyi | Alkoxysilane effect in hybrid material: A comparison of pHEMA-TiO2 and pMAPTMS-TiO2 nanoparticulate hybrids[END_REF], or by the use of high pressures in the absence of an initiator [START_REF] Evlyukhin | Synthesis of organic-inorganic hybrids via a high-pressure-ramp process: the effect of inorganic nanoparticle loading on structural and photochromic properties[END_REF][START_REF] Evlyukhin | Laser Assisted High Pressure Induced Polymerization of 2-(hydroxyethyl) Methacrylate[END_REF][START_REF] Evlyukhin | A New Route for High-Purity Organic Materials: High-Pressure-Ramp-Induced Ultrafast Polymerization of 2-(Hydroxyethyl)Methacrylate[END_REF][START_REF] Jia | Polymerization initiation of pure 2-hydroxyethylmethacrylate under shock wave compression[END_REF]. In addition to the increase in refractive index compared to neat pHEMA, these hybrids have also shown reversible photochromic [START_REF] Gorbovyi | Novel nanostructured pHEMA-TiO2 hybrid materials with efficient light-induced charge separation[END_REF][START_REF] Kuznetsov | Laser-induced photopatterning of organic-inorganic TiO 2based hybrid materials with tunable interfacial electron transfer[END_REF] and photorefractive properties [START_REF] Uklein | Photo-induced refraction of nanoparticulate organic-inorganic TiO2-pHEMA hybrids[END_REF], making them good candidates for applications in integrated optics. Some of these applications, such as fabrication of micro-optic element like gratings, couplers, waveguides or 3D photonic structures, require precise shaping of the material on micro-or sub-micrometric scales [START_REF] Lebeau | Hybrid materials for optics and photonics[END_REF]. Free radical photopolymerization, triggered by absorption of one or two photons, became then a method of choice because it allows the spatial control of polymerization reactions on the sub-micrometric scale [START_REF] Farsari | Materials processing: Two-photon fabrication[END_REF][START_REF] Do | Submicrometer 3D structures fabrication enabled by one-photon absorption direct laser writing[END_REF]. However, in order to ensure a good mechanical stability of the obtained structures, only multifunctional monomers, leading to an important crosslinking between polymeric chains, are usually considered in this connection. This issue is even more critical in the case of hybrid materials, in which inorganic component strongly affects polymerization of the organic phase. Therefore, a detailed understanding of the effects of titania nanoparticles on the reactions and processes involved in photopolymerization of HEMA is required before considering the realization of micro-optical components based on pHEMA/TiO 2 nanoparticulate hybrids.

In this study, we analyzed the effect of TiO 2 nanoparticles concentration and light intensity on the bulk free-radical photopolymerization of HEMA/TiO 2 hybrid solutions under UV irradiation. The polymerization reactions were probed in situ by Raman spectroscopy. Conversely, to the most commonly used experimental methods, based on differential scanning calorimetry (DSC) or dilatometry, which are indirect and can have response times too long to properly monitor in situ fast polymerization reactions, spectroscopic approaches, such as Raman spectroscopies, allow the evolution of the reaction to be monitored by direct observation of the disappearance of the C=C double bonds in monomers. Polymerization reactions occurring in a fraction of a second can thus be studied. With this approach, influence of TiO 2 nanoparticles insertion can be clarified not only on the final conversion yield of HEMA monomers but also on the polymerization kinetics of the hybrid solutions. To test the potential of pHEMA/TiO 2 hybrids in the fabrication of microptical components, 1D micrometric structures were realized using interference lithography and their photoactivity was demonstrated.

Experimental section

Materials

Liquid HEMA (purity >99%) was purchased from Sigma-Aldrich. Before use, oxygen was removed by repeated freeze-thaw cycles. The liquid HEMA was frozen using liquid nitrogen and pumped under vacuum for a few minutes until freezing. Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO, 97%) was purchased from Sigma-Aldrich and used as a photoinitiator without further purification. It was mixed with HEMA at concentration 0.5 wt% under a controlled atmosphere.

For the fabrication of 1D microstructures, 3-(acryloyloxy)-2-hydroxypropyl methacrylate (AHPMA), purchased from Sigma Aldrich, was used as a crosslinker. The mixture was then mixed with the HEMA-TiO 2 hybrid solution, without further purification. Acetyl acetone, used as a development agent for 1D microstructures, was purchased from Sigma-Aldrich.

Hybrids synthesis

The pHEMA-TiO 2 hybrids were prepared as previously described by Gorbovyi et al. [START_REF] Gorbovyi | Novel nanostructured pHEMA-TiO2 hybrid materials with efficient light-induced charge separation[END_REF]. Briefly, size-selected titanium-oxo-alkoxo (TOA) nanoparticles with a diameter of 5 nm were generated in a thermostatic sol-gel reactor with rapid micromixing using a titanium tetraisopropoxyde (TTIP)

precursor (Acros Organics, 98% purity) and distilled water in a 2-propanol (Acros Organics, 99.5% purity) solution and a hydrolysis ratio H=2.0 at 20.0 °C. Through a one-step hydrolysis-condensation transformation, the underlying sol-gel chemistry results in well-defined titanium-oxo-alkoxo TiO 1.

In the final step, the hybrid solutions were placed under a controlled atmosphere between two glass plates separated by 250 µm using a 1 × 1 cm adhesive gene frame (Thermo Scientific). This procedure prevented continuous diffusion of atmospheric oxygen through the sample during the laser irradiation experiments. A high-repetition-rate (10 kHz) UV laser delivering pulses of 7 ns at 355 nm and mean power of 10 mW (CNI MPL-F-355) was used to photopolymerize the hybrid solutions.

The laser beam was expanded to irradiate the entire surface of the sample and its power was controlled using a set of polarizers (Figure S1.a). Thin films were prepared for the fabrication of 1D microstructures. An HEMA-TiO 2 hybrid solution containing a crosslinker (AHPMA) was deposited on the surface of a clean glass plate. Thin films of 10 µm thickness were then prepared using a spin coater device at a speed of 1000 rpm for 20s. The entire process was performed under yellow light. After preparation, the thin films were stored in a black box to avoid any interaction with external light.

Sample name

Volume of nanoparticles

Experimental methods.

The kinetics of photopolymerization under UV irradiation was studied by Raman spectroscopy.

All Raman spectra of the samples were measured at room temperature in a backscattering configuration using an HR800 spectrometer equipped with a Peltier-cooled CCD detector (Horiba Jobin Yvon) with spectral and spatial resolutions of 0.25 cm -1 and 5 µm, respectively. In-situ Raman measurements were performed by focusing the probe beam at 640 nm in the middle of the sample.

The FRP process was monitored by a decrease in the intensity of the C=C bond stretching band at 1640 cm -1 of the HEMA monomer, which was simultaneously accompanied by an increase in the intensity of the C-CH 2 band (1455 cm -1 ). The spectra were normalized to the intensity of the C=O band at 1730 cm -1 , which is not involved in the polymerization process (Figure S1.b). After fitting the Raman spectra with a multi-peak function, the conversion yield (CY) of the monomer was calculated as [START_REF] Evlyukhin | Laser Assisted High Pressure Induced Polymerization of 2-(hydroxyethyl) Methacrylate[END_REF] (

) ( ) 0 / ( ) 1 / C C C O t C C C O t I I CY t I I = = = = = = - (1)
where I C=C and I C=O are the areas of ν(C=C) and ν(C=O) Raman bands before irradiation (t=0) and at irradiation time t. To calculate the rate of polymerization, CY(t) was first smoothed using the rlowess algorithm. The normalized rate of polymerization was then obtained directly from the smoothed conversion yield, as follows:

p d CY R dt = (2)
To monitor the FRP reaction in real time, the Raman spectra were recorded every 0.5s during the total process duration of typically 500 s. Each series of measurements was repeated at least four times to ensure good reproducibility. The data presented in this article are based on the average of these measurements.

To realize 1D microstructures, we used a simple direct laser writing approach, in which the original hybrid solutions were exposed to interference patterns formed by the superposition of two UV beams. The aforementioned UV laser (CNI MPL-F-355) was used for this purpose. The main laser beam, with a diameter of 5 mm, was divided into two subbeams using a beam splitter. The optical paths of both beams were adjusted using mirrors to ensure the same path length through the optical system before the beams interfered with the sample surface.

RESULTS AND DISCUSSION

Under UV irradiation, BAPO dissociates to form phosphinoyl and benzoyl primary radicals, which react with monomers to initiate polymerization. Figures 1 and2 show the conversion yields and polymerization rates of pristine HEMA and hybrids with different NP concentrations under UV irradiation at intensities of 1 and 0.1 mW/cm 2 . In the case of HEMA, the maximum conversion yield (approximately 80%) did not depend on UV intensity. In addition, we clearly observed an auto-acceleration of the reaction characterized by an increase of the rate of polymerization (R p ), which is a typical feature of FRP process [START_REF] Odian | Principles of polymerization[END_REF]. A large number of more or less sophisticated models describing this effect can be find in literature (see for example recent reviews [START_REF] Achilias | A Review of Modeling of Diffusion Controlled Polymerization Reactions[END_REF][START_REF] Lang | A Review on Modeling Cure Kinetics and Mechanisms of Photopolymerization[END_REF]). These works attribute the auto-acceleration effect to the gradual diffusion-control of propagation and termination reactions as the polymerization reaction proceeds and the reaction mixture becomes increasingly dense. Indeed, in the early process stage, the rates of initiation (R i ) and termination (R T ) reactions are equal, resulting in a constant rate of propagation (R p ). As polymerization progresses, the viscosity increases because of the higher concentration of macromolecules. The movement of macroradicals to meet and react together is significantly impeded. As a result, the rate of termination decreases and the concentration of macroradicals increases, leading to an increase in the propagation rate up to the maximum value R p,max . At higher conversion yields, progressive vitrification of the medium occurs, reducing the monomer mobility, leading to a decrease in the polymerization rate, and eventually to the stop of the reaction. A higher UV intensity has the effect of an apparent acceleration of the process, in the sense that the maximum conversion yield is reached earlier. The effect of UV light intensity on the photopolymerization kinetics of HEMA has recently been investigated in more detail [START_REF] Luu | Effect of Light Intensity on the Free-Radical Photopolymerization Kinetics of 2-Hydroxyethyl Methacrylate: Experiments and Simulations[END_REF]. The kinetics of the photopolymerization reaction changed both when titania nanoparticles were mixed with HEMA and UV light intensity was increased. Regarding first the effects of UV intensity. As in the case of pure HEMA a higher UV intensity has led to an acceleration of the polymerization process. However, contrary to what has been observed in neat HEMA, a higher UV intensity, and consequently a higher rate of initiation, results in a higher maximum conversion yield at the end of the reaction. Thus, a minimum of 22% was obtained for sample X20 irradiated with 0.1 mW/cm 2 , while with an intensity of 1 mW/cm 2 about 30% of the monomers are converted (Figure 1). In fact, the formation of a polymer network is accompanied by volume shrinkage, which rate is much slower than that of the propagation reaction. The temporary excess of free volume thus induced, maintains the mobility of monomers and radicals, which promotes their reactions leading to higher conversion yields than those obtained under equilibrium conditions.

The effects of nanoparticles contents on polymerization kinetics are manifold. First of all, as can be seen of Figure 1, the higher the titania concentration, the lower is the maximum conversion yield.

Moreover, the autoacceleration effect seems to be less pronounced when the titanium oxide concentration increases, until it no longer occurs in the X20 sample. This last point is more clearly evidenced by the evolution of propagation rates, as shown in Figure 2. Indeed, the maximum rate faded, and eventually disappeared, when the titania concentration increased. A third consequence of nanoparticle loading can be observed. Indeed, the presence of NPs appeared to slow the reaction, since the maximum propagation rate was observed later. This is particularly visible at low UV intensities (Figure 2). As mentioned above, NPs functionalized with polymerizable OEMA ligands are expected to copolymerize with free HEMA monomers. Radical polymerization between the OEMA ligands is also expected to occur on the nanoparticle surface. The observed slowing of the reaction as the nanoparticle concentration increases is therefore surprising, since the addition of multifunctional monomers to a solution of monofunctional monomers is expected to advance the gelation process [START_REF] Andrzejewska | Chapter 2 -Free Radical Photopolymerization of Multifunctional Monomers[END_REF][START_REF] Andrzejewska | Photopolymerization kinetics of multifunctional monomers[END_REF]. Actually, it appears that the functionalized titania NP's affect both the initiation and propagation rates. The reasons for these changes are discussed below.

We first analyse the effect of nanoparticles concentration on the initiation rate and show that the observed decrease is not related to the deactivation of macro or primary radicals initiator as usually proposed in literature. At the very beginning of the polymerization process, steady-state conditions are fulfilled, and the propagation reaction rate can be expressed as

0.5 0 /2 0 (1 ) with 2 [ ] 2 i p i PI L t R dCY k CY R f PI I dt k σ   = - =     (1)
where f is the initiator efficiency,

18 2
2.9 10

PI cm σ - =
is the photoinitiator absorption cross section, and I L/2 is the incident UV light intensity at the point where Raman analysis is performed. Assuming that the photoinitiator concentration [PI] and the initial rate constants of propagation (k p0 ) and termination (k t0 ) do not vary, the integration of Equation ( 1) yields ( )

0.5 0 0 ln 1 with 2 i eff eff p t R CY k t k k k   - - = =     (2)
The linear fit of ( )

ln
k k R R = =
, as indicated by the dotted line in Figure 3. a). Because it is unlikely that k p0 and k t0 change in the presence of TiO 2, these experiments reflect a decrease in the initiation rate with an increase in titania NP concentration.

The reason of this decrease is the following. In the synthesis of nanocomposites based on the in situ polymerization of acrylates or methacrylate monomers, the reduction in the initiation rate with inorganic content is usually attributed to the influence on primary radicals of the hydroxyl groups present on the NP surface [START_REF] Lin | Effects of Surface Groups on SiO2 Nanoparticles on in Situ Solution Polymerization: Kinetics and Mechanism[END_REF][START_REF] Demir | PMMA/Zinc Oxide Nanocomposites Prepared by In-Situ Bulk Polymerization[END_REF][START_REF] Michailidis | An experimental and theoretical study of butyl methacrylate in situ radical polymerization kinetics in the presence of graphene oxide nanoadditive[END_REF][START_REF] Tsagkalias | Role of Functional Groups in the Monomer Molecule on the Radical Polymerization in the Presence of Graphene Oxide. Polymerization of Hydroxyethyl Acrylate under Isothermal and Non-Isothermal Conditions[END_REF]. It has been suggested that primary radicals undergo chain transfer by H abstraction to the hydroxyl groups, thus forming oxygen radicals on the surface. These newly formed radicals can then scavenge another radical and/or be involved in termination reactions, which lead to a reduction in radical efficiency and result in a slowdown of the reaction.

However, this mechanism may be not effective in the present experiment. Indeed, the observed decrease in the initiation rate seems to be mainly related to the UV absorption of the titania nanoparticles.

10

TiO cm σ -= [START_REF] Gorbovyi | Novel nanostructured pHEMA-TiO2 hybrid materials with efficient light-induced charge separation[END_REF] is the absorption cross-section of TiO 2 at 355 nm. The calculated ratio, represented by the dashed line in Figure 3.b), is in good agreement with the measurements. Thus, OEMA ligands grafted onto the titania surface probably prevent the interaction of primary radicals with surface hydroxyl groups. This interpretation is in agreement with results recently reported by Lin et al. who analysed the FRP of MMA in presence of SiO 2 nanoparticles [START_REF] Lin | Effects of Surface Groups on SiO2 Nanoparticles on in Situ Solution Polymerization: Kinetics and Mechanism[END_REF]. They observed that when the SiO2 surface is grafted with ligands bearing reactive double bonds, the grafted polymer layer generated in situ, effectively prevents initiator molecules and free radicals from being wasted by reacting with the surface hydroxyl groups. Finally, we can consider that the primary radicals have a constant initiator efficiency, which can be estimated from the k eff values of the HEMA solution.

Using the rate constants

=

found in the literature [START_REF] Goodner | Method for Determining the Kinetic Parameters in Diffusion-Controlled Free-Radical Homopolymerizations[END_REF], we obtained an average value of f = 0.4, which is consistent with the reported value of BAPO quantum yield of photodissociation under UV irradiation φ = 0.6 [START_REF] Jockusch | A steady-state and picosecond pump-probe investigation of the photophysics of an acyl and a bis (acyl) phosphine oxide[END_REF].

We now discuss the influence of titania NP on the auto-acceleration process of polymerization reaction. In their analysis of in situ bulk polymerization of MMA in presence of zinc oxide (ZnO) nanoparticles, Demir et al. [START_REF] Demir | PMMA/Zinc Oxide Nanocomposites Prepared by In-Situ Bulk Polymerization[END_REF] observed the suppression of the auto-acceleration effect when ZnO NP are added to MMA solution. They assigned this effect to a degenerative chain-transfer reaction of macroradicals induced by hydroxyl groups present on nanoparticles surface. Since in our system, OEMA ligands grafted on TiO 2 nanoparticles appear to prevent interactions with surface hydroxyl groups, we do not consider this interpretation. Actually, at long processing times, steadystate conditions could no longer be verified. However, the propagation rate is still directly related to the concentration of radical polymer chains. The observed autoacceleration (Figure .2) was due to an increase in the concentration of these macroradicals as a result of the progressive hindrance of their translational movement when the polymer network grew. The disappearance of the autoacceleration effect when the TiO 2 NPs concentration was increased may be related to the heterogeneity of the reaction medium. Indeed, hybrid polymerization involves two distinct populations of radicals with strongly different reactivity: on the one hand, free radical polymer chains in the bulk, which polymerize as in pure HEMA, and on the other hand, radicals localized on OEMA groups grafted on titania nanoparticles. These latter are immobilized on the nanoparticle surface and, therefore, have a lower reactivity than free macroradicals. Under these conditions, the propagation rate can be written as [START_REF] Kaddami | Silica-filled poly(HEMA) from hema/grafted SiO2 nanoparticles: Polymerization kinetics and rheological changes[END_REF] [ ] [ ]

with p Hybrid p HEMA p OEMA p OEMA p HEMA HEMA OEMA R k M R k M R k k • • - - - - -     = +     
where p HEMA k - and p OEMA k - are the propagation rate constants of the radicals localized on the free HEMA polymer chains and OEMA ligands, respectively, and

HEMA R •     and OEMA R •    
corresponding radicals concentrations. Thus, the rate of polymerization in the hybrids was always lower than that of pure HEMA. Moreover, only free macroradicals, whose movement is significantly affected by the growth of the polymer structure, are involved in autoacceleration phenomena. Thus, as their concentration decreased with increasing titanium content, auto-acceleration was less pronounced, and eventually the polymerization rate became constant, as shown in Figure 2, for the samples with the highest titania concentration.

The third observed effect of the nanoparticle loading was a reduction in the maximum conversion yield measured at the end of the polymerization process (Figure 1). Achilias et al. [START_REF] Achilias | Polymerization kinetics of poly (2-Hydroxyethyl methacrylate) hydrogels and Nanocomposite materials[END_REF] studied the in situ bulk polymerization of HEMA in the presence of different types of nano-additives.

They observed a reduction in the maximum conversion yield when the surface groups of the nanoadditives promoted the formation of hydrogen bonds with the monomers. In the present study, the polymerizable nature of the ligands influenced the final conversion yield. Indeed, along the reaction, the grafted nanoparticles were incorporated into the growing polymer chains as particular units containing a large number of pendant double bonds. Propagation proceeds further through the addition of monomers from a radical site on the OEMA ligand. This process can lead to cross-linking if another nanoparticle is incorporated in the growing polymer chain or to a type of cyclization reaction when one of the remaining pendant double bonds of the same nanoparticle reacts with the propagating macroradical. If realized, this can result in the formation of microgel regions, inside which many unreacted pendant double bonds and monomers are entrapped. The reactivity of these species then decreases because their access by radicals is gradually prevented by the development of a polymer network. Because of this shielding effect, as well as the drop in radical mobility in the course of the reactions, many of these unreacted double bonds will never react, thus limiting the final conversion yield with respect to HEMA polymerization. This effect was more pronounced when the titania nanoparticle concentration increased.

It is worth noting that spatial inhomogeneities introduced by the formation of microgel regions may be responsible for the reduction in the mechanical strength of hybrids with high nanoparticle content. Indeed, hybrids with titania concentrations higher than 4M are brittle and can be easily reduced to powder when pressed between two fingers. The use of high pressure allows to overcome this limitation and to synthetize hybrids with titania concentrations as high as 12M [START_REF] Evlyukhin | Synthesis of organic-inorganic hybrids via a high-pressure-ramp process: the effect of inorganic nanoparticle loading on structural and photochromic properties[END_REF].

The results presented above indicate that increasing the concentration of TiO 2 nanoparticles decreases the maximum conversion rate of the C=C double bonds, which affects the mechanical stability of the hybrid material. On the other hand, the refraction and photochromic response of the material are enhanced with an increase in the nanoparticle concentration [START_REF] Gorbovyi | Novel nanostructured pHEMA-TiO2 hybrid materials with efficient light-induced charge separation[END_REF][START_REF] Uklein | Photo-induced refraction of nanoparticulate organic-inorganic TiO2-pHEMA hybrids[END_REF]. For these hybrids to be used in the fabrication of optical microelements, the polymerization effectiveness and photoactivity of the realized microstructures must be tested. This issue was addressed in the last part of this study. To fabricate the microstructures, the original hybrid solutions were exposed to the interference pattern formed by the superposition of two UV laser beams. In neat pHEMA, entanglement of the polymer chains and hydrogen bonds plays an important role in the cohesion of the polymer network. Indeed, all OH groups on the pHEMA side chain are thought to be involved in OH…OH or OH…O=C type of hydrogen bonds [START_REF] Morita | Hydrogen-bonds structure in poly(2-hydroxyethyl methacrylate) studied by temperature-dependent infrared spectroscopy[END_REF][START_REF] Morita | Hydrogen-bond structures in poly(2hydroxyethyl methacrylate): Infrared spectroscopy and quantum chemical calculations with model compounds[END_REF]. However, the addition of nano additives can strongly affect the formation of H-bonds [START_REF] Achilias | Polymerization kinetics of poly (2-Hydroxyethyl methacrylate) hydrogels and Nanocomposite materials[END_REF], which reduce the stability of the polymer network, particularly when the size of the polymerized volume is reduced. Our results show that the addition of a cross-linking agent is necessary to achieve the formation of stable solid microstructures in pHEMA/TiO 2 hybrids. This could be directly related to the formation of the microgel regions discussed above, and indicates that if functionalized TiO 2 nanoparticles act as cross-linkers, they seem to only contribute to the formation of very loose networks. Finally, 1D stable structures were obtained by adding AHPMA (38 vol%) as a crosslinker agent to the HEMA-TiO 2 hybrid solutions.

Figure 4 shows the structure of the X10 hybrids after irradiation at a UV intensity of 7 mW/cm2 for 7 s. A parallel linear stripe relief of ∼2 µm in height and regularly spaced by 10 µm, covering a surface area of a few mm 2 , was observed after the development procedure. Finally, we verified that the development procedure of former 1D microstructures did not modify the hybrid composition by removing the titanium dioxide nanoparticles, and preserve the photosensitivity of the hybrids. The photoactivity of pHEMA/TiO 2 hybrids is related to an efficient separation and trapping of photoinduced charges at the organic/inorganic interface under UV irradiation [START_REF] Gorbovyi | Novel nanostructured pHEMA-TiO2 hybrid materials with efficient light-induced charge separation[END_REF][START_REF] Museur | Luminescence properties of pHEMA-TiO2 gels based hybrids materials[END_REF]. As a consequence of the trapping of photoinduced electrons as small polarons Ti 3+ , the photocatalytic reduction of Ag + ions on the surface of titanium oxo-alkoxy nanoparticles has been previously reported [START_REF] Jia | Growth of Silver Nanoclusters on Monolayer Nanoparticulate Titaniumoxo-alkoxy Coatings[END_REF]. This effect was used to evaluate the photosensitivity of the fabricated microstructures. Following the procedure proposed in [START_REF] Jia | Growth of Silver Nanoclusters on Monolayer Nanoparticulate Titaniumoxo-alkoxy Coatings[END_REF], the X10 hybrid microstructure was immersed in an aqueous solution of AgNO 3 and illuminated at a UV intensity of 38 mW/cm 2 for 300 s.

Energy dispersive X-ray spectroscopy (EDX) analysis was performed to confirm the formation of Ag particles by the reduction of Ag + ions on the surface (Figure 5). While only the lines assigned to C (0.277 keV), O (0.523 keV), and Ti (0.452 and 4.508 keV) were observed in the fresh sample, the presence of Ag (2.984 keV) was observed after UV light illumination. More evidence is presented in the inset of Figure 5, which shows an SEM image of the surface of the UV-light-treated sample. The observed nanoparticles covering the surface were assigned to reduced silver. The optical absorption measurements shown in Figure 6 support this conclusion. The higher absorption observed on the sample after treatment can be assigned to the Ag plasmon absorption of core-shell Ag&TiO 2 nanoparticles with a small Ag shell thickness of D sh ≤0.5 nm around titania nanoparticles of 5 nm in diameter (see inset in Figure 6). This is an expected result considering the short deposition time [START_REF] Jia | Growth of Silver Nanoclusters on Monolayer Nanoparticulate Titaniumoxo-alkoxy Coatings[END_REF] and easy penetration of silver ions into the polymer-like material.

These results show that pHEMA/TIO 2 hybrids can be considered as materials of interest for the realization of photosensitive microstructures. Optimization of the hybrid solution composition is in progress by our group with the aim of increasing the TiO 2 nanoparticle concentration without losing mechanical stability and realizing photoactive and high-refractive-index structures with submicronic spatial resolution.

CONCLUSION AND PROSPECTS

In this study, we investigated the bulk free-radical UV photopolymerization of original hybrid materials formed from two components: organic HEMA and inorganic TiO 2 nanoparticles functionalized with polymerizable ligands. The influence of the nanoparticle concentration on the kinetics of the polymerization reaction was studied using Raman spectroscopy. We showed that a decrease in the initiation rate resulted from the absorption of UV photons by the nanoparticles, and not from the loss of efficiency of primary radicals due to chemical reactions with the surface ligands.

This effect explains the pronounced decrease in propagation rate with increasing nanoparticle concentration. Furthermore, we assigned a decrease in the conversion yield of the C=C double bond with an increase in TiO 2 loading to the formation of microgel regions around the nanoparticles, which prevented polymerization of the surface ligands.

The fabrication of 1D microstructures was demonstrated using direct UV laser writing. The photochromic efficiency of the obtained structure was evaluated based on the reduction of Ag + ions under UV irradiation. Our results show that pHEMA/TiO 2 hybrids can be considered as prospective key materials for the realization of photosensitive microoptical elements.

The results of this study contribute to the development and applications of organic-inorganic hybrid nanocomposites obtained by in situ free radical polymerization. Moreover, they may also provide an experimental basis for further development of theoretical models devoted to the polymerization of organic/inorganic hybrid materials. Indeed, since side reactions induced the presence of functionalized TiO 2 nanoparticles in an HEMA solution are less numerous than in other hybrids systems involving nanoadditives, these hybrid solutions may serve as a model system to address these issues.

Figure 1 :

 1 Figure 1: Conversion yield of C=C double bond measured in HEMA and hybrids under UV irradiation with 1 mW/cm 2 (--) and 0.1 mW/cm 2 () UV-laser intensities.

Figure 2 :

 2 Figure 2: Propagation rate in HEMA and hybrids under UV irradiation with 1 mW/cm 2 (--) and 0.1 mW/cm 2 () UV-laser intensities.

Figure 3 .I

 3 b) shows the evolution of the effective rate constant ratio, HEMA , for different samples at two UV laser intensities. This ratio can be easily calculated by considering that the UV intensity in the middle of the sample is expressed as is the incident UV light intensity, and 2 20 2

Figure 3 : 2 ,k

 32 Figure 3 : (a) Overall rate constant k eff measured for different concentrations of titania nanoparticles in hybrids and UV intensities 0.1 mW/cm 2 (■) and 1 mW/cm 2 (□). The dashed line represents

Figure 4 :

 4 Figure 4 : 1D structures of X10 hybrids obtained under UV-laser irradiation (intensity 7 mW/cm

Figure 5 :Figure 6 :

 56 Figure 5: Energy dispersive X-ray measurement before (a) and after (b) Ag + ions reduction on microstructured X10 hybrid sample. SEM image of sample surface covered with Ag nanoparticles is shown in insert

Table 1 :

 1 pHEMA-TiO 2 hybrid samples composition (see text for details)

, irradiation time 7s).

ACKNOLEDGMENTS

ETHIC STATEMENT

The authors declare they have no conflict of interest.