Samuel Pearson
email: samueljohnpearson@outlook.com

Marko Pavlovic

Thomas Augé

Valerian Torregrossa

Istvan Szilagyi

Franck D'agosto

Muriel Lansalot

Elodie Bourgeat-Lami

Vanessa Prevot
email: vanessa.prevot@uca.fr

Controlling the Morphology of Film-Forming, Nanocomposite Latexes Containing Layered Double Hydroxide by RAFT-Mediated Emulsion Polymerization

Keywords: Nanocomposites, layered double hydroxides, RAFT polymerization, emulsion polymerization, encapsulation

published or not. The documents may come

Introduction

Nanocomposites can offer dramatically improved properties compared to native polymer matrices. Incorporating a small amount of nanofiller into a polymer can for example impart large gains in hardness, toughness, impermeability, conductivity, and flame retardancy. [START_REF] Hanemann | Polymer-Nanoparticle Composites: From Synthesis to Modern Applications[END_REF][START_REF] Paula | Polymer nanotechnology: Nanocomposites[END_REF] The synthesis of nanocomposite materials is therefore of huge interest academically and industrially, particularly for the coating industry. Central to achieving such property enhancements is high dispersability of nanoparticles within the polymer matrix to ensure a high interfacial surface area, with the ultimate aim of controlling nanofiller orientation in resulting films.

The choice of nanofiller depends on the desired properties of the resulting films, with some inorganic particles simultaneously fulfilling more than one property enhancement. Layered double hydroxides (LDHs) are highly anisotropic, positively-charged inorganic clays which show great promise as nanofillers. [START_REF] Costantino | LDH in physical, chemical, biochemical and life science[END_REF][START_REF] Forano | Layered Double Hydroxides[END_REF] Each layer in a LDH particle comprises of edge-shared octahedra containing a central di-or tri-valent metal (M 2+ or M 3+ ) surrounded by hydroxide ions at the vertices. The presence of M 3+ imparts a net positive charge to each layer, which is compensated by anions occupying the interlamellar spaces (conjointly with water molecules). Particle diameters typically range from tens of nanometers to several microns. A great diversity of LDH compositions with tunable dimensions can be attained by simply varying the nature and the proportions of the metal ions, anions, and synthesis conditions. LDHs have therefore been used for pollutant adsorption, carbon dioxide capture, catalysis, drug and gene delivery, biosensing, and in nanocomposites. [START_REF] Taviot-Guého | Tailoring Hybrid Layered Double Hydroxides for the Development of Innovative Applications[END_REF] As nanofillers, interest in LDHs stems from their significant enhancement of mechanical and thermal properties, flame retardancy, [START_REF] Gao | Flame retardant polymer/layered double hydroxide nanocomposites[END_REF][START_REF] Matusinovic | Fire retardancy and morphology of layered double hydroxide nanocomposites: a review[END_REF] gas impermeability, [START_REF] Tsai | An advanced preparation and characterization of the PET/MgAl-LDH nanocomposites[END_REF] and corrosion resistance. [START_REF] Stimpfling | Phosphate-Based Organic Molecules Interleaved with Layered Double Hydroxide : Unraveling the Roles of Host Cations and the Guest-Inhibiting Effect in Aluminum Corrosion Protection[END_REF][START_REF] Zheludkevich | Active protection coatings with layered double hydroxide nanocontainers of corrosion inhibitor[END_REF] 5 their native layered state, or they can be partially or completely exfoliated to isolate the individual layers and thereby augment the interfacial contact area between inorganic and matrix.

Most LDH/polymer nanocomposites [START_REF] Costa | Layered Double Hydroxide Based Polymer Nanocomposites[END_REF] have been prepared by melt or solvent blending processes using pre-formed polymers, [START_REF] Gao | Flame retardant polymer/layered double hydroxide nanocomposites[END_REF][START_REF] Matusinovic | Fire retardancy and morphology of layered double hydroxide nanocomposites: a review[END_REF][START_REF] Tsai | An advanced preparation and characterization of the PET/MgAl-LDH nanocomposites[END_REF][START_REF] Stimpfling | Phosphate-Based Organic Molecules Interleaved with Layered Double Hydroxide : Unraveling the Roles of Host Cations and the Guest-Inhibiting Effect in Aluminum Corrosion Protection[END_REF][START_REF] Zheludkevich | Active protection coatings with layered double hydroxide nanocontainers of corrosion inhibitor[END_REF][START_REF] Illaik | Unusual Polystyrene Nanocomposite Structure Using Emulsifier-Modified Layered Double Hydroxide as Nanofiller[END_REF][START_REF] Leroux | Polymer Interleaved Layered Double Hydroxide: A New Emerging Class of Nanocomposites[END_REF][START_REF] Roland-Swanson | Polymerization of Sulfopropyl Methacrylate, a Surface Active Monomer, within Layered Double Hydroxide[END_REF] or by in situ polymerization using LDH modified with surfactant, [START_REF] Bao | Preparation and Characterization of Poly(vinyl chloride)/Layered Double Hydroxides Nanocomposite via In Situ Suspension Polymerization[END_REF][START_REF] Ding | Synthesis and characterization of exfoliated polystyrene/ZnAl layered double hydroxide nanocomposite via emulsion polymerization[END_REF][START_REF] Ding | Synthesis and characterization of polystyrene/layered double-hydroxide nanocomposites via in situ emulsion and suspension polymerization[END_REF][START_REF] Chen | In situ synthesis of poly(methyl methacrylate)/MgAl layered double hydroxide nanocomposite with high transparency and enhanced thermal properties[END_REF] initiator, [START_REF] Qiu | Exfoliation of layered double hydroxide in polystyrene by insitu atom transfer radical polymerization using initiator-modified precursor[END_REF]20 monomer, [START_REF] Vaysse | Tentative Mechanisms for Acrylate Intercalation and in Situ Polymerization in Nickel-Based Layered Double Hydroxides[END_REF] or controlling agents. [START_REF] Ding | Homogeneous dispersion and enhanced thermal properties of polystyrene-layered double hydroxide nanocomposites prepared by in situ reversible addition-fragmentation chain transfer (RAFT) polymerization[END_REF] While waterborne processes are preferable for coating applications, the few reports which produced LDH/polymer nanocomposites in aqueous medium by using emulsion [START_REF] Ding | Synthesis and characterization of exfoliated polystyrene/ZnAl layered double hydroxide nanocomposite via emulsion polymerization[END_REF][START_REF] Ding | Synthesis and characterization of polystyrene/layered double-hydroxide nanocomposites via in situ emulsion and suspension polymerization[END_REF][START_REF] Chen | In situ synthesis of poly(methyl methacrylate)/MgAl layered double hydroxide nanocomposite with high transparency and enhanced thermal properties[END_REF]20 or suspension [START_REF] Bao | Preparation and Characterization of Poly(vinyl chloride)/Layered Double Hydroxides Nanocomposite via In Situ Suspension Polymerization[END_REF] polymerization typically required surfactant, which impacts film performance, and were not able to control particle morphology. Recent developments in controlled radical polymerization in aqueous dispersed systems, however, offer promise for overcoming these limitations. [START_REF] Zetterlund | Controlled/Living Radical Polymerization in Dispersed Systems: An Update[END_REF][START_REF] Bourgeat-Lami | Synthesis of Nanocapsules and Polymer/Inorganic Nanoparticles Through Controlled Radical Polymerization At and Near Interfaces in Heterogeneous Media[END_REF] We recently reported the synthesis of LDH/polymer nanocomposite particles [START_REF] Perreira | Nanocomposite latexes containing layered double hydroxides via RAFT-assisted encapsulating emulsion polymerization[END_REF] using a reversible addition-fragmentation chain transfer (RAFT)-mediated emulsion polymerization technique pioneered by Hawkett and co-workers. [START_REF] Nguyen | Pigment encapsulation by emulsion polymerization using macro-RAFT copolymers[END_REF] Coined RAFT-assisted encapsulating emulsion polymerization (REEP), [START_REF] Nguyen | Pigment encapsulation by emulsion polymerization using macro-RAFT copolymers[END_REF] the technique uses short, statistical copolymers synthesized by RAFT polymerization to stabilize nanoparticles in aqueous solution, and provide reactivatable groups from which polymer growth can occur in a subsequent emulsion polymerization step. The first report of nanocomposites made by this approach achieved complete encapsulation of titanium dioxide nanoparticles in a shell polymerized from a methyl methacrylate (MMA)/n-butyl acrylate (BA) 7:3 (mass/mass) feed, [START_REF] Nguyen | Pigment encapsulation by emulsion polymerization using macro-RAFT copolymers[END_REF] and it has since been used to synthesise nanocomposite particles from spherical or near-spherical nanoparticles [START_REF] Nguyen | Pigment encapsulation by emulsion polymerization using macro-RAFT copolymers[END_REF][START_REF] Daigle | A Simple Method for Forming Hybrid Core-Shell Nanoparticles Suspended in Water[END_REF][START_REF] Bourgeat-Lami | Synthesis of Polymer/Silica Hybrid Latexes by Surfactant-Free RAFT-Mediated Emulsion Polymerization[END_REF][START_REF] Li | Polymer-encapsulated γ-Fe2O3 nanoparticles prepared via RAFT-mediated emulsion polymerization[END_REF][START_REF] Nguyen | Monodispersed polymer encapsulated superparamagnetic iron oxide nanoparticles for cell labeling[END_REF][START_REF] Zgheib | Cerium oxide encapsulation by emulsion polymerization using hydrophilic macroRAFT agents[END_REF][START_REF] Das | Synthesis of single-core and multiple-core core-shell nanoparticles by RAFT emulsion polymerization: Lead sulfide-copolymer nanocomposites[END_REF][START_REF] Das | Copolymer nanosphere encapsulated cds quantum dots prepared by RAFT copolymerization: Synthesis, characterization and mechanism of formation[END_REF] and vesicles, [START_REF] Ali | Controlled Synthesis of Polymeric Nanocapsules by RAFT-Based Vesicle Templating[END_REF]35 and from anisotropic nanoparticles including gibbsite, [START_REF] Ali | Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of Anisotropic Composite Latex Particles by RAFT-Based Starved Feed Emulsion Polymerization[END_REF] montmorillonite, [START_REF] Mballa Mballa | Control of the anisotropic morphology of latex nanocomposites containing single montmorillonite clay particles prepared by conventional and reversible additionfragmentation chain transfer based emulsion polymerization[END_REF] carbon nanotubes, [START_REF] Nguyen | Polymer coating of carboxylic acid functionalized multiwalled carbon nanotubes via reversible addition-fragmentation chain transfer mediated emulsion polymerization[END_REF][START_REF] Zhong | A versatile encapsulation method of noncovalently modified carbon nanotubes by RAFT polymerization[END_REF] and graphene oxide. [START_REF] Huynh | Polymer coating of graphene oxide via reversible addition-fragmentation chain transfer mediated emulsion polymerization[END_REF] In our preliminary work on LDH nanocomposites by REEP, the morphology was primarily the so-called "sandwich" morphology, in which each inorganic platelet displays polymer growth on both its faces, while its edges show a strong preference to remain polymer- 6 free. Platelets are therefore "sandwiched" between polymer nodes. We expect this was the thermodynamically preferred configuration given that the polymer shell formed from MA/BA 80:20 (mass/mass) would have been relatively mobile at the polymerization temperature.

Nancomposite morphologies depend on the interplay between thermodynamics and kinetics, [START_REF] Asua | Mapping the Morphology of Polymer-Inorganic Nanocomposites Synthesized by Miniemulsion Polymerization[END_REF] and are hard to predict in REEP systems. In a nanocomposite particle comprising polar inorganic and hydrophobic polymer, a minimization of surface energy would logically place the inorganic at the polymer/water interface. [START_REF] Colver | Multilayered Nanocomposite Polymer Colloids Using Emulsion Polymerization Stabilized by Solid Particles[END_REF][START_REF] Percy | Surface characterization of vinyl polymer-silica colloidal nanocomposites using X-ray photoelectron spectroscopy[END_REF] If the inorganic and polymer surface energies differ less, however, equilibrium structures in which inorganic particles are more enclosed by polymer become envisagable. [START_REF] Mičušík | Morphology of Polymer/Clay Latex Particles Synthesized by Miniemulsion Polymerization: Modeling and Experimental Results[END_REF] For kinetic reasons, non-equilibrium structures may also predominate. [START_REF] Herk | Polymer Encapsulation of Single Clay Platelets by Emulsion Polymerization Approaches, Thermodynamic, and Kinetic Factors[END_REF] Encapsulation of anisotropic particles by REEP has only been reported using high T g monomer compositions, [START_REF] Ali | Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of Anisotropic Composite Latex Particles by RAFT-Based Starved Feed Emulsion Polymerization[END_REF][START_REF] Mballa Mballa | Control of the anisotropic morphology of latex nanocomposites containing single montmorillonite clay particles prepared by conventional and reversible additionfragmentation chain transfer based emulsion polymerization[END_REF][START_REF] Nguyen | Polymer coating of carboxylic acid functionalized multiwalled carbon nanotubes via reversible addition-fragmentation chain transfer mediated emulsion polymerization[END_REF][START_REF] Zhong | A versatile encapsulation method of noncovalently modified carbon nanotubes by RAFT polymerization[END_REF][START_REF] Huynh | Polymer coating of graphene oxide via reversible addition-fragmentation chain transfer mediated emulsion polymerization[END_REF] where the morphology is considered a non-equilibrium state kinetically trapped by the hard polymer shell. [START_REF] Loiko | ATRP mediated encapsulation of Gibbsite: fixation of the morphology using a cross-linker[END_REF] In Ali et al., moving to a lower T g polymer indeed gave armored structures, but the polymer was also more hydrophobic, meaning that higher mobility and higher surface energy may have both contributed to non-encapsulation. [START_REF] Ali | Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of Anisotropic Composite Latex Particles by RAFT-Based Starved Feed Emulsion Polymerization[END_REF] The influence of polymer hydrophobicity and T g , and other factors like nanofiller characteristics and macroRAFT agent nature, are therefore difficult to isolate based on REEP literature.

The Heuts group recently synthesized clay/polymer nanocomposite particles using an atomtransfer radical polymerization (ATRP)-mediated emulsion polymerization technique (which closely resembles the REEP process), raising further questions over the parameters affecting nanocomposite morphology. [START_REF] Loiko | An ATRPbased approach towards water-borne anisotropic polymer-Gibbsite nanocomposites[END_REF] A surprising "muffin-like" morphology, in which the growing polymer formed a truncated sphere on only one face of each inorganic particle, was systematically obtained for monomer feeds of MMA/BA = 10:1 (mass/mass), despite the analogous REEP system giving full encapsulation. [START_REF] Ali | Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of Anisotropic Composite Latex Particles by RAFT-Based Starved Feed Emulsion Polymerization[END_REF] Although the monomer feed produced polymer with T g above the polymerization temperature, it appears that rearrangement to minimize

S u p p o r t i n g I n f o r m a t i o n - F o r R e v i e w O n l y - N o t f o r P u b l i c a t i o n 7
surface energy was somehow possible in the ATRP system. The reason behind this contradiction remains unanswered, although the authors recently reported that feeding a crosslinker prior to the normal monomer feed provided access to the encapsulated morphology in their ATRP system, [START_REF] Loiko | ATRP mediated encapsulation of Gibbsite: fixation of the morphology using a cross-linker[END_REF] suggesting that chain mobility does indeed play some role.

The factors governing nanocomposite morphology are therefore still not completely understood.

In addition, film-forming polymer compositions have received much less attention than non-filmforming compositions, presumably due to the high mobility of the growing polymer shell favouring morphologies other than the preferred encapsulated morphology. Guided by our overall vision of fully controlling nanoparticle orientation in nanocomposite films, the present work explores the effect of macroRAFT agent molar mass, composition, and concentration on the morphology of film-forming LDH nanocomposites synthesized by starved-feed RAFT-mediated emulsion polymerization. We demonstrate the effect of free macroRAFT agent concentration on the prevalence of secondary nucleation, and highlight the role that the reactivatable RAFT function plays in determining the morphology of the resulting nanocomposite particles.

Experimental section

Materials

Unless otherwise specified, all chemicals were reagent grade purchased from Sigma-Aldrich and used as received. Monomers were de-inhibited prior to polymerization by passage through a column of basic alumina. Deuterated NMR solvents (CDCl

Analyses

Nuclear magnetic resonance (NMR) spectroscopy was performed using a Bruker DRX 300S.

Size exclusion chromatography (SEC) was performed using a Viscotek TDA305 equipped with three columns from Malvern Instruments [T6000 M General Mixed Org (300 × 8 mm)] and RI and UV detectors. Measurements were carried out at 40 °C at a flow rate of 1 mL min -1 using toluene as a flow rate marker. Molar mass characteristics of the methylated polymers (see Syntheses section for details) were determined from the RI signals using a calibration curve based on polystyrene (PSt) standards from Polymer Laboratories. X-ray diffraction (XRD) was performed on a PANalyticalX'Pert Pro diffractometer equipped with an X'Celerator Scientific detector and a Cu anticathode (Kα1/Kα2). The diffracted beam was detected over a range of 3-70° (2θ) with a step size of 0.0167° and a counting time of 350 s per step. UV/visible spectroscopy of the supernatant solutions in the macroRAFT adsorption study was conducted with a Shimadzu UV-visible spectrometer in the spectral window from 400 nm to 250 nm. A Nano ZS (Malvern) device was used for dynamic light scattering (DLS) and electrophoretic mobility measurements in accordance with our previously published procedures. [START_REF] Pavlovic | Effect of MacroRAFT Copolymer Adsorption on the Colloidal Stability of Layered Double Hydroxide Nanoparticles[END_REF] Transmission electron microscopy (TEM) was performed at 80 kV with a Philips CM 120 microscope (Centre Technologique des Microstructures (CTµ), platform of the Claude Bernard Lyon 1 University, Villeurbanne, France). Diluted samples (ca. 1 mg mL -1 ) were dropped onto a carbon-coated copper grid and dried in air. CryoTEM observations were carried out on the same instrument. A 6 µL drop of the sample solution (at ca. 20 mg mL -1 ) was placed on a plasma-treated lacy carbon grid, and plunged into liquid ethane using a pneumatic sample preparation apparatus built inhouse (CTµ, Claude Bernard Lyon 1 University). The frozen grids were transferred to a liquidnitrogen-cooled specimen holder and observed at 120 kV.

S u p p o r t i n g I n f o r m a t i o n - F o r R e v i e w O n l y - N o t f o r P u b l i c a t i o n 9 Syntheses Mg 3 Al-LDH
LDH was prepared by flash co-precipitation followed by hydrothermal treatment. [START_REF] Xu | Stable Suspension of Layered Double Hydroxide Nanoparticles in Aqueous Solution[END_REF][START_REF] Xu | Dispersion and Size Control of Layered Double Hydroxide Nanoparticles in Aqueous Solutions[END_REF] Mg(NO 3 ) 2 .6H 2 0 (19.2 g, 75 mmol) and Al(NO 3 ) 3 .9H 2 0 (9.4 g, 25 mmol) were dissolved in water (1.0 L) to give a 0.1 M solution in metal ions. This solution was rapidly added to a vigorouslystirred solution of NaOH (800 mL, 0.25 M) at r.t. The pH value of the resulting suspension was adjusted to 10.0 by adding 0.25 M NaOH (ca. 25 mL) and stirring was continued for 30 min in an open flask. The precipitate was collected by centrifugation (4 krpm, 10 min), and washed twice by redispersing with vigorous shaking in distilled water (2 L) followed by centrifugation. After the final wash, the gel was re-dispersed in water (100 mL) and heated in an autoclave at 150 °C for 4 h. The resulting particles were analysed by DLS and TEM, and an aliquot was dried and analysed by XRD. The sample was stored as a colloidal suspension (ca. 32 g L -1 ) in the fridge.

4-cyano-4-thiothiopropylsulfanyl pentanoic acid (CTPPA)

A synthesis of CTPPA has been described in the literature. 51 We used a procedure adapted from the original RAFT agent synthesis papers [START_REF] Bouhadir | A new practical synthesis of tertiary S-alkyl dithiocarbonates and related derivatives[END_REF][START_REF] Thang | A novel synthesis of functional dithioesters, dithiocarbamates, xanthates and trithiocarbonates[END_REF] which is detailed, with NMR spectra (Fig. S1 and Fig. S2), in the Supporting Information.

MacroRAFT agents

Using P(AA 17.5 -co-BA 17.5 )-R as a representative example, acrylic acid (AA, 2.47 mL, 36.0 mmol) and n-butyl acrylate (BA, 5.16 mL, 36.0 mmol) as monomers, CTPPA (0.400 g, 1.44 mmol) as RAFT agent, ACPA (0.081 g, 0.288 mmol) as initiator, and 1,3,5-trioxane (0.130 g, 1.44 mmol) as internal standard for NMR analysis were dissolved in dioxane (12.0 mL) to give

[monomer]:[RAFT]:[initiator] = 50:1:0.2 and [monomer] = 6.0 M. The solution was cooled to 0 °C and degassed with nitrogen bubbling for 30 minutes, then placed in an oil bath at 60 °C.

Samples were taken periodically using a nitrogen-purged syringe, dissolved in d 6 -DMSO, and analyzed immediately by [START_REF] Hanemann | Polymer-Nanoparticle Composites: From Synthesis to Modern Applications[END_REF] H NMR spectroscopy to determine the conversion by comparing the integral of the 1,3,5-trioxane peak to that of the vinylic protons of the monomers. When the conversion reached 70%, corresponding to a degree of polymerization (DP) of 35, the solution was quenched in an ice bath. The resulting polymer was purified by three precipitations in cold diethyl ether, with redissolution in acetone between each precipitation. The final polymer was dried overnight in a vacuum oven to give the product as a yellow solid, which was analysed by [START_REF] Hanemann | Polymer-Nanoparticle Composites: From Synthesis to Modern Applications[END_REF] H NMR spectroscopy and SEC. Prior to SEC analysis, a sample of polymer (20 mg) was dissolved in THF/H 2 O (90/10 v/v, 10 mL) and methylated using tri(methylsilyl)diazomethane according to literature procedure, [START_REF] Couvreur | First Nitroxide-Mediated Controlled Free-Radical Polymerization of Acrylic Acid[END_REF]

MacroRAFT agent adsorption on LDH

Using P(AA 17.5 -co-BA 17.5 )-R as a representative example, a stock solution of macroRAFT agent was prepared by carefully adding NaOH (1.0 mol L -1 , 2.97 mL) to a dispersion of macroRAFT agent (0.605 g, 1.61 x 10 -4 mol) in water (9.49 mL) until full dissolution and a pH value of 8.0 were achieved. The molar extinction coefficient at 310 nm was then determined by UV-visible spectroscopy from four solutions prepared at concentrations ranging from 1.12 x 10 -5 M to 4.41 x 10 -5 M, giving ε 310nm = 12591 M -1 cm -1 . A series of 8 macroRAFT agent solutions (all 2.21 mL) with concentrations ranging from 0.77 mmol L -1 (2.89 g L -1 ) to 5.44 mmol L -1 (20.51 g L -1 ) were prepared by appropriate dilutions of the stock solution. The solutions were vigorously stirred, and an aliquot (2.79 mL) of LDH stock suspension (35.8 g L -1 , pH value adjusted to 8.0 using 1 M HCl just prior to use) was added to each in one portion. Stirring was continued for 5 min, followed by 5 min of ultrasonic treatment (to redisperse any particles which stuck to the vial wall at the level of the meniscus), before centrifuging at 50 krpm for 60 min. The supernatants were recovered and analyzed by UV-visible spectroscopy (after appropriate dilutions to maintain absorbances below 1.0) to determine the concentration of the P(AA 17.5 -co-BA 17.5 )-R remaining in solution (denoted [macroRAFT] free ) based on the absorbance at 310 nm. The amount of P(AA 17.5co-BA 17.5 )-R adsorbed on the LDH surface (denoted [macroRAFT] ads ) in each sample was calculated by subtracting [macroRAFT] free from the total concentration of the macroRAFT agent present in that solution. Concentrations were determined by weighing all solutions (rather than relying on pipetted volumes) for greater accuracy.

Starved-feed emulsion polymerizations

Using P(AA 17.5 -co-BA 17.5 )-R as a representative example, a stock solution of macroRAFT agent (43.1 mg mL -1 , 11.6 mM, pH = 8.0) at pH value 8.0 was prepared as described in the preceding

S u p p o r t i n g I n f o r m a t i o n - F o r R e v i e w O n l y - N o t f o r P u b l i c a t i o n
12 paragraph. LDH suspension (6.64 mL, 36.1 g L -1 , pH = 8.0) was added in one portion to a stirred aliquot of the macroRAFT agent solution (3.34 mL, 5.94 x 10 -5 mol, 1 eq.) and the mixture was subjected to ultrasound for 10 min. At this point, DLS was performed to confirm that the LDH particle size distribution closely resembled that of the starting LDH suspension. A solution of ACPA (8.33 mg, 2.97 x 10 -5 mol, 0.5 eq.) dissolved in water and adjusted to pH = 8.0 using NaOH (solution volume 0.329 mL) was added, followed by water (3.38 mL) to give

[macroRAFT] = 12.0 g L -1 (3.22 mM), [LDH] = 20.0 g L -1 , and a total volume of 12.0 mL. A portion of the solution (2.0 mL) was removed and dried in a pre-weighed aluminium pan at 50 °C to give the initial solids content by gravimetry. The remaining solution was transferred to a 25 mL round-bottom flask fitted with a magnetic stir bar and a rubber septum, and was degassed by nitrogen bubbling for 15 min. In parallel, the monomer mixture (methyl acrylate (MA)/BA 80/20 mass/mass, ca. 1 mL) was degassed separately for 15 min in a vial capped with a septum. The round bottom flask was accurately weighed, and then placed in an oil bath at 70 °C. Injection of the monomer solution was immediately commenced by taking up the degassed monomer solution in a 1 mL nitrogen-purged syringe fitted with a long needle, inserting the needle through the septum in the macroRAFT/LDH solution such that it was touching the flask wall just above the level of the liquid, quickly placing the syringe in a Perfusor® compact syringe pump, and starting the injection at a rate corresponding to 0.3 g h -1 . After 2.0 h, the monomer injection was ceased, and the reaction mixture was stirred for a further 2 h. Samples (ca. 0.75 mL) were taken every hour using pre-weighed 1 mL syringes, the mass of each removed samples was recorded, and a portion of each sample was analysed by DLS. The remaining portion was dried and weighed to determine conversions gravimetrically. CryoTEM was performed on a portion of the final solution (and in some cases on the intermediate solutions). For SEC analysis, a portion of dried sample (ca. 20 mg) was stirred overnight in THF (5 mL) containing conc. HCl (ca. 2 eq. relative

S u p p o r t i n g I n f o r m a t i o n - F o r R e v i e w O n l y - N o t f o r P u b l i c a t i o n
13 to COOH groups), sonicated for 10 min, filtered using a 0.45 µm PTFE filter, then the filtrate was directly methylated according to the same procedure used for the macroRAFT agent syntheses.

Removal of the RAFT function

Based on literature precedent, 55 P(AA 17.5 -co-BA 17.5 )-R (0.50 g, 1.33 x 10 -4 mol, 1 eq.), AIBN (0.436 g, 2.65 x 10 -3 mol, 20 eq.), and lauroyl peroxide (0.106 g, 2.65 x 10 -4 mol, 2 eq.) were dissolved in dioxane (10.0 mL) in a round-bottom flask fitted with a magnetic stir bar and a septum. The solution was degassed by nitrogen bubbling for 30 min, and the flask was placed in an oil bath at 80 °C. After 4 h, the solution was cooled to room temperature, and the pure polymer was isolated in an identical fashion to that described for the macroRAFT agent syntheses.

Results and discussion

The objective of the present work was to control the morphology of film-forming nanocomposite latexes containing LDH nanoparticles. Full conversion and minimization of secondary nucleation were prerequisites. Our overall approach based on the REEP technique is shown in Scheme 1. It should be noted that since the removal of the RAFT group was also explored, not all systems strictly qualify as REEP processes.

S u p p o r t i n g I n f o r m a t i o n - F o r R e v i e w O n l y - N o t f o r P u b l i c a t i o n 14

MacroRAFT agent and LDH syntheses

The purpose of the macroRAFT agent in REEP systems is purportedly two-fold: to provide a reactivatable group which encourages polymerization to proceed from the particle surface, and to stabilize the resulting composite particles. Statistical copolymers of charged monomers and hydrophobic (non-charged) monomers have proven to be suitable candidates for this purpose. [START_REF] Ali | Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of Anisotropic Composite Latex Particles by RAFT-Based Starved Feed Emulsion Polymerization[END_REF][START_REF] Mballa Mballa | Control of the anisotropic morphology of latex nanocomposites containing single montmorillonite clay particles prepared by conventional and reversible additionfragmentation chain transfer based emulsion polymerization[END_REF] The charged units are responsible for electrostatic adsorption to the charged inorganic surface and colloidal stability of the hydrophobic shell through electrostatic repulsion, and the hydrophobic

S u p p o r t i n g I n f o r m a t i o n - F o r R e v i e w O n l y - N o t f o r P u b l i c a t i o n
15 units encourage hydrophobic interactions between adsorbed chains, monomer, and growing hydrophobic polymer.

The total number and the ratio of hydrophilic to hydrophobic monomer units in the macroRAFT agent have both proven to influence the final latex characteristics. In their encapsulation of gibbsite using a MMA/BA 10:1 (mass/mass) feed, a series of three macroRAFT agents with 10 AA units and either 2.5, 5, or 7.5 BA units was explored by Hawkett and co-workers. [START_REF] Ali | Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of Anisotropic Composite Latex Particles by RAFT-Based Starved Feed Emulsion Polymerization[END_REF] The first gave latex particle agglomeration, the second gave excellent encapsulation with no free latex particle formation, and the third gave encapsulation but with some free particle formation. In explaining the first case, the authors proposed that the hydrophilic chains were more water soluble and less inclined to adsorb on the inorganic surface, giving lower colloidal stability. In the third case, the lower water solubility of this more hydrophobic macroRAFT agent meant that chain extension with the hydrophobic monomer during emulsion polymerization resulted in earlier onset of a self-assembly process in the aqueous phase. It is important to note that not all of the macroRAFT agent in REEP systems is adsorbed on the inorganic surface; some remains free in solution, and can potentially be recruited to the growing composite particles or undergo chain extension in solution to form secondary (nanoparticle-free) particles. This work highlights the fine influence of macroRAFT agent composition on latex properties.

Four macroRAFT agents -P(AA 10 -co-BA 5 )-R, P(AA 23.3 -co-BA 11.7 )-R, P(AA 7.5 -co-BA 7.5 )-R and P(AA 17.5 -co-BA 17.5 )-R -were therefore synthesized in the present work by copolymerizing AA and BA using the RAFT agent CTPPA (Scheme 1). The first and second displayed AA/BA = 2:1 and will be referred to as the "more hydrophilic" pair, and the third and fourth displayed AA/BA = 1:1 to comprise the "more hydrophobic" pair, with the DP either 15 or 35 within each pair. The effect of both hydrophilic/hydrophobic balance and molar mass could therefore be isolated in the subsequent adsorption and emulsion polymerization experiments. Note that all four macroRAFT

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16 agents were completely soluble once the acid groups were deprotonated in the stock solution preparation (at ca. 43 g L -1 ). The RAFT polymerizations to attain these macroRAFT agents were well controlled, giving linear evolution of molar mass with conversion and low dispersities (example for P(AA 10 -co-BA 5 )-R shown in Supporting Information, Fig. S3a). SEC traces for the two DP pairs were almost perfectly overlayed, with the theoretical and measured molar masses and dispersities summarized in Table 1. As previously reported, the monomer units were incorporated at their feed proportions throughout the polymerization, indicating very similar reactivity ratios. [START_REF] Ali | Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of Anisotropic Composite Latex Particles by RAFT-Based Starved Feed Emulsion Polymerization[END_REF] All polymerizations exhibited a significant initialization period, during which RAFT agent is converted almost exclusively to single monomer adducts before any appreciable formation of longer chains. [START_REF] Mcleary | Beyond Inhibition: A 1H NMR Investigation of the Early Kinetics of RAFT-Mediated Polymerization with the Same Initiating and Leaving Groups[END_REF][START_REF] Mcleary | A Mechanistic Interpretation of Initialization Processes in RAFT-Mediated Polymerization[END_REF] Initialization times are known to be more pronounced at low [M]:[RAFT], as was the case here. Adding the monomer to the system 60 min after heating had commenced produced a ca. 60 min delay in the onset of polymerization (Supporting Information, Fig. S3b), confirming that the initialization period was indeed intrinsic to the RAFT mechanism.

The final entry in Table 1 is a RAFT-free version of P(AA 17.5 -co-BA 17.5 )-R, whose synthesis and purpose will be discussed in a later section. Table 1. Charge density and molar mass characteristics (by SEC against PSt standards) of the stabilizing polymers used in this study.

Stabilizing polymer

Negative charge density a (mmol g -1 )

M n,theor The LDH particles with abbreviated composition Mg 3 Al-CO 3 (where M 3+ /M 2+ = 0.33 falls within the range of 0.1 to 0.5 typically used for synthetic LDH), [START_REF] Taviot-Guého | Tailoring Hybrid Layered Double Hydroxides for the Development of Innovative Applications[END_REF] were synthesized using flash precipitation followed by hydrothermal treatment. [START_REF] Xu | Dispersion and Size Control of Layered Double Hydroxide Nanoparticles in Aqueous Solutions[END_REF] Performing the coprecipitation in air produces LDH containing the carbonate anion, which prevents further anionic exchanges in the interlamellar spaces. The resulting particles displayed an intensity average particle size of 108 nm (s.d = 62 nm) by DLS (Supporting Information, Fig. S4a). Powder XRD analysis showed diffraction lines (00l), ( 012), ( 110) and (113) characteristic of LDH (Supporting Information, Fig. S4b), with the 00l diffraction line positions indicating an interlayer distance of 0.79 nm which corresponds to the expected carbonate LDH phase. [START_REF] Perreira | Nanocomposite latexes containing layered double hydroxides via RAFT-assisted encapsulating emulsion polymerization[END_REF]

MacroRAFT agent adsorption onto LDH

The first step in the encapsulation process is the adsorption of macroRAFT agent on the LDH particles. The adsorption capacity of the LDH, and the partitioning of macroRAFT between the inorganic particles and the solution, were therefore of great interest in determining the quantities of macroRAFT agent and LDH required for the subsequent polymerizations.

To study the adsorption behavior, a series of solutions was prepared in which the LDH concentration was held constant at 20 g L -1 and the macroRAFT concentration was varied over the range of ca. 1 to ca. 16 g L -1 . The resulting adsorption isotherms are shown in Fig. 1a.

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The adsorption series exhibit relatively sharp step-like profiles, indicative of strong adsorption, rather than gradual increases in both free and adsorbed polymer that are characteristic of weaker Langmuir-type adsorption behaviour. [START_REF] Bourgeat-Lami | Synthesis of Polymer/Silica Hybrid Latexes by Surfactant-Free RAFT-Mediated Emulsion Polymerization[END_REF] At low macroRAFT concentrations, marked by the nearvertical portions of the curves, almost all macroRAFT agent (>98%) was adsorbed in all four cases (Fig. 1a). The gradient of this portion of the curve reflects the affinity of the polymer for the inorganic surface, [START_REF] Daigle | A Simple Method for Forming Hybrid Core-Shell Nanoparticles Suspended in Water[END_REF] and is particularly high in this case due to the permanent and high charge density on the LDH. As the macroRAFT agent concentration increased, there was a marked difference in the behavior of the macroRAFT agents depending on their AA/BA ratio. P(AA 23.3co-BA 11.7 )-R and P(AA 10 -co-BA 5 )-R both exhibit adsorption capacities of ca. 150 mg g -1 , defined as the intersection of a linear fit of the last 5 data points with the y-axis. Essentially all incremental macroRAFT agent above this concentration remained free in solution. The very close agreement between the two curves shows that the differing molar masses had a negligible effect on adsorption. In contrast, P(AA 17.5 -co-BA 17.5 )-R and P(AA 7.5 -co-BA 7.5 )-R showed almost 2-fold higher adsorption capacities (ca. 300 mg g -1 and ca. 250 mg g -1 respectively), and continually rising adsorption over the tested range. Slightly higher adsorption was expected for the hydrophobic pair due to their lower charge densities (Table 1), but this does not fully account for the ca. 2-fold difference in adsorption. Taken together, these results confirm the role of the hydrophobic BA units in encouraging higher and continually increasing adsorption. [START_REF] Muller | Adsorption of Diblock Copolymers of Poly(ethylene oxide) and Poly(lactide) at Hydrophilic Silica from Aqueous Solution[END_REF] Polyelectrolytes of lower line charge density, such as the BA-rich pair here, have indeed been shown in literature to adsorb in higher amounts due to the decreased electrostatic repulsion between the adsorbed molecules. [START_REF] Szilagyi | Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation[END_REF] Two additional studies with [LDH] = 10 mg L -1 -2000-fold more dilute than in Fig. 1a -were performed to probe the adsorption of the four macroRAFT agents using electrophoresis and DLS (Fig 1b -d). [START_REF] Pavlovic | Effect of MacroRAFT Copolymer Adsorption on the Colloidal Stability of Layered Double Hydroxide Nanoparticles[END_REF] This was intended to give complementary information on the adsorption behavior, For all four macroRAFT agents, the electrophoretic mobilities of the LDH particles began positive, declined through zero (defined as the isoelectric point, IEP) to more negative values before stabilizing at very high macroRAFT agent concentrations (Fig. 1b). The charge inversion originates from hydrophobic interactions between the polyelectrolyte chains, entropy gain due to the release of solvent molecules, and ion-ion correlations. [START_REF] Szilagyi | Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation[END_REF] These measurements revealed the effect of both AA/BA ratio and molar mass on the quantity of macroRAFT agent required to neutralize the LDH surface charge. P(AA 10 -co-BA 5 )-R and P(AA 23.3 -co-BA 11.7 )-R exhibited IEPs of ca. 50 mg g -1 and 60 mg g -1 respectively. In contrast, P(AA 7.5 -co-BA 7.5 )-R and P(AA 17.5 -co-BA 17.5 )-R gave IEPs over 2-fold higher at 110 mg g -1 and 130 mg g -1 respectively, which is not entirely attributable to differing charge densities. The charge density of P(AA 7.5 -co-BA 7.5 )-R was only 31% lower than that of P(AA 10 -co-BA 5 )-R, yet its IEP was over 2-fold higher. The higher molar mass pair showed a comparable difference. When the macroRAFT agents were richer in BA, significantly more polymer was required to neutralize the LDH surface charge than one would expect based on the number of carboxylate groups they contribute. A similar, less pronounced, effect was observed when the molar mass was increased while keeping the AA/BA ratio constant. For both AA/BA ratios, more of the higher molar mass polymer was required to neutralize the LDH surface charge. The effect of both AA/BA ratio and molar mass on IEP is best summarized in Fig. 1c, which shows the number of macroRAFT-contributed carboxylate units present at the IEP for each macroRAFT agent. If the neutralization of LDH surface charge were simply a function of the total carboxylate groups added to the system, then all four macroRAFT agents would fall on a horizontal line. Instead, the two BA-rich macroRAFT agents require 50% more carboxylate groups to reach the IEP compared to their BA-poor counterparts. The two higher molar mass macroRAFT agents require 27% more carboxylate groups to reach the IEP than their lower molar mass counterparts. Lower charge density and higher molar mass are known to favour the formation of loops and tails in adsorbed polyelectrolytes, which explains why the effectiveness of the carboxylate units to neutralize the LDH surface charge was significantly reduced with increasing BA content, and moderately reduced with higher molar mass in the macroRAFT agents used here.

S

S

Beyond the IEPs, electrophoretic mobilities plateau beyond ca. 150 mg g -1 for the AA/BA = 2 pair and at significantly higher concentrations for the AA/BA = 1 pair, which almost perfectly match the plateau values observed in Fig. 1a. The adsorption behavior can therefore be considered relatively independent of the LDH concentration, since this differed 2000-fold between the two experimental procedures.

Finally, we used time-resolved dynamic light scattering (DLS) to measure the rate at which the hydrodynamic radius (R h ) of the LDH particles increased immediately after mixing with macroRAFT agent, which is a measure of aggregation rate. The colloidal stability was expressed in terms of the stability ratio, W, shown in Fig. 1d for all four macroRAFT agents. W is defined as the initial rate of aggregation at the IEP divided by the initial rate of aggregation for the particular sample. The absence of surface charge at the IEP means that the rate of aggregation at the IEP is higher than at any other macroRAFT agent concentration. Stability ratios are therefore greater than or equal to 1, with higher values corresponding to higher stability. [START_REF] Holthoff | Coagulation Rate Measurements of Colloidal Particles by Simultaneous Static and Dynamic Light Scattering[END_REF] The stability ratio trends across the four macroRAFT agents can again be grouped based on the AA/BA ratio. The two macroRAFT agents with AA/BA = 2 show essentially superimposed stability ratio profiles, with stability ratios declining from very high values (at low macroRAFT concentration) to minima of 1 at macroRAFT concentrations which correspond to the IEPs shown in Fig. 1b, before rapidly increasing with further macroRAFT addition. The AA/BA = 1 systems

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22 are also closely aligned, with minima corresponding to the IEPs in Fig. 1b. The stability ratios are interesting in the context of the subsequent emulsion polymerizations. Colloidal stability of the LDH particles prior to commencing the emulsion polymerization is important for ensuring latex stability, and reducing the likelihood of encapsulating many (aggregated) LDH particles within each latex particle. Polymerizations L 4 -L 11 were conducted with a total macroRAFT concentration of 600 mg g -1 , which is significantly higher than the range of data in Fig. 1d and corresponds to high stability ratios and therefore low or not even measurable aggregation rates.

Starved-feed emulsion polymerizations

Starved-feed emulsion polymerization was performed by mixing macroRAFT agent with LDH in a similar fashion to the adsorption experiments, adding initiator, then injecting monomer under starved-feed conditions. Polymerizations were performed on a 10 g scale, with [RAFT]:[initiator] = 2, and targeting solids contents of around 10% (mass/mass). Having determined that the macroRAFT agent concentration strongly influences the colloidal stability of macroRAFTadsorbed LDH, the role of macroRAFT agent concentration was the first parameter explored in the emulsion polymerizations. A summary of all latexes described in this work is presented in Table 2.

Effect of [macroRAFT] free on latex stability and secondary nucleation

The importance of the free macroRAFT agent concentration was tested by using P(AA 17.5 -co-BA 17.5 )-R adsorbed on the LDH at a concentration of 6.5 g L -1 . This LDH concentration was higher than in our previous work to target higher solids contents at a given polymer/LDH ratio while retaining the possibility of producing anisotropic nanocomposite particles at full conversion. [START_REF] Perreira | Nanocomposite latexes containing layered double hydroxides via RAFT-assisted encapsulating emulsion polymerization[END_REF] Three systems were tested containing total macroRAFT concentrations of 4.3 g L -1 , 8.6 g L -1 , and 17.2 g L -1 using a monomer feed of MA/BA 80:20 (mass/mass). At 3 h, the conversions were 84%, 88%, and 91% respectively. Subsequent polymerizations were therefore extended to 4 h to seek full conversion. The morphologies of the resulting latexes are shown in Fig. 2. The stability of L 1 was inferior to that of the other two systems, showing some sedimentation visible to the naked eye. The cryoTEM image in Fig. 2a was taken from the stable portion of the polymerization solution. This image demonstrates that almost every latex particle is associated

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26 with at least one LDH particle, meaning there is very little secondary nucleation. The free macroRAFT concentration of ca. 2.0 g L -1 (estimated from the adsorption isotherm in Fig. 1a)

was not sufficiently high to sustain stability throughout the emulsion polymerization process. The macroRAFT polymer chains which were initially free in solution were most likely adsorbed by the growing latex particles [START_REF] Nguyen | Pigment encapsulation by emulsion polymerization using macro-RAFT copolymers[END_REF] rather than undergoing any free polymer particle formation in the solution. Secondary nucleation was therefore avoided, but at the expense of colloidal stability.

The higher macroRAFT concentration of 8.6 g L -1 in L 2 gave a stable latex, but also encouraged some secondary particle formation, with a significant population of spherical LDH-free polymer particles with diameters around 30 nm evident in the cryoTEM images (Fig. 2b). The

[macroRAFT] free of ca. 6.0 g L -1 in this system promoted chain extension of the dissolved macroRAFT agent and self-assembly of the resulting amphiphilic chains. This chain extension of water-soluble species with hydrophobic monomer, known as polymerization-induced selfassembly (PISA), has become a very popular technique in the last decade for producing polymer nano-objects with various morphologies, [START_REF] Canning | A Critical Appraisal of RAFT-Mediated Polymerization-Induced Self-Assembly[END_REF][START_REF] Jennings | Block copolymer synthesis by controlled/living radical polymerisation in heterogeneous systems[END_REF][START_REF] Lansalot | Polymerization-Induced Self-Assembly: The Contribution of Controlled Radical Polymerization to The Formation of Self-Stabilized Polymer Particles of Various Morphologies[END_REF] but is undesirable in this system because it diverts monomer away from the nanocomposite particles and reduces homogeneity in the final latexes.

At [macroRAFT] total = 17.2 g L -1 in L 3 , secondary nucleation was even more prominent than for L 2 as a result of the higher [macroRAFT] free (ca. 14.5 g L -1 ). Spherical polymer particles were more numerous with lower diameters of ca. 15 nm (Fig. 2c), consistent with a shorter hydrophobic block length formed in the chain extension of the macroRAFT agent in solution.

This lower hydrophobic volume fraction gave higher curvature at the interface of the two blocks, and therefore a lower diameter of self-assembled spheres. [START_REF] Mai | Self-assembly of block copolymers[END_REF] An LDH-free control experiment with similar [macroRAFT] free gave a transparent final solution, with DLS showing a single, narrowly distributed population of particles with intensity average diameter of 16 nm (s.d. 7 nm, Supporting Information, Fig. S4c), which agrees well with the sizes observed in Fig. 2c The systems in Fig. 2, in combination with the adsorption isotherms in Fig. 1a, were instructive for choosing appropriate [macroRAFT] free to ensure latex stability while avoiding secondary nucleation in subsequent encapsulation experiments employing P(AA 17.5 -co-BA 17.5 )-R. The results in Fig. 2 also demonstrate the preferred sandwich morphology adopted here, with polymer growth invariably occurring on both faces of the LDH particles and negligible coverage of the LDH edges. This is presumably the equilibrium morphology, since the MA/BA 80:20

(mass/mass) polymer, with a T g,Fox of -9 °C (Table 2), would have been relatively mobile at the polymerization temperature.

Most nanocomposite latex particles in Fig. 2 contain multiple LDH particles, which indicates a certain extent of particle aggregation. CryoTEM images indicate that the LDH particles are reasonably well dispersed after macroRAFT adsorption and ultrasound treatment (Supporting Information, Fig. S5b), appearing essentially identical to the bare LDH particles (Supporting Information, Fig. S5a), but do show a slight inclination to aggregate rather than being completely dispersed as individual particles. CryoTEM of samples taken every hour throughout a similar polymerization (L 4 , Table 2, performed at higher [LDH] and [macroRAFT] as discussed in the next section) demonstrate that small clusters of LDH particles were already present in the 1 h sample and that aggregation did not noticeably increase throughout the polymerization (Supporting Information, Fig. S5c -e). Indeed, stable latexes were obtained after 4 h with almost negligible shift in the intensity particle size distribution throughout polymerization (DLS, Supporting Information, Fig. S5f). These data combined suggest that the multiple LDH particles observed in some latex particles were most likely present at the very start of the polymerization, due to a slight tendency of LDH to aggregate even when stabilized with macroRAFT agent.

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Daigle et al. used surfactant at a concentration below its critical micelle concentration (CMC) to minimize particle aggregation prior to the encapsulation of various nanoparticles, [START_REF] Daigle | A Simple Method for Forming Hybrid Core-Shell Nanoparticles Suspended in Water[END_REF] but we didn't adopt this approach due to the negative impact of surfactant on film properties. [START_REF] Butler | Effect of surfactant systems on the water sensitivity of latex films[END_REF]68 Influence of monomer feed on nanocomposite morphology

In addition to highlighting some of the important factors affecting the RAFT emulsion process, one of the central aims of this work was to attain control over the morphology of the nanocomposite particles while employing film-forming latex compositions. To further probe the influence of monomer feed on particle morphology, a series of RAFT emulsion polymerizations were conducted changing only this parameter. Targeting higher inorganic loadings by increasing LDH concentration to 20 g L -1 , the P(AA 17.5 -co-BA 17.5 )-R concentration of 12 g L -1 was used in order to give a [macroRAFT] free of ca. 5 g L -1 . This value was intended to give stable latexes while avoiding free particle formation based on the results in Fig. 2. With the MA/BA 80:20

(mass/mass) feed used in the last section clearly showing preference for the sandwich morphology, we sought a monomer feed which might deliver the encapsulated morphology while retaining a film-forming ability under ambient conditions. The monomer feed compositions tested are shown as L 4 -L 7 in Table 2. The monomer feed indeed played an important role in nanocomposite morphology. The sandwich morphology observed in Fig. 2a using MA/BA 80:20 (mass/mass) was again observed here as expected, since the only change was a higher overall concentration of all components. The nonfilm-forming MMA/BA 90:10 (mass/mass) feed gave primarily the encapsulated morphology (Fig. 3b) best observed by conventional TEM (Fig. 3c) which was permitted by the T g well above room temperature (80°C, Table 2). Note that we are defining encapsulation as full coverage of the

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LDH, including the edges, with a polymer shell. This experiment verified the possibility of physically entrapping the encapsulated morphology, as observed for example by Ali et al. using gibbsite particles encapsulated with a MMA/BA 10:1 (mass/mass) shell. [START_REF] Ali | Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of Anisotropic Composite Latex Particles by RAFT-Based Starved Feed Emulsion Polymerization[END_REF] The somewhat uneven polymer shell, displaying bumps and nodes best seen in Fig. 3b, has also been observed in other REEP systems employing a similar MMA/BA feed due to the high T g limiting polymer mobility. [START_REF] Ali | Controlled Synthesis of Polymeric Nanocapsules by RAFT-Based Vesicle Templating[END_REF] A MMA/BA 50:50 (mass/mass) feed, giving T g,Fox = 4°C (Table 2) reverted to the sandwich morphology presumably due to the polymerization temperature exceeding the shell T g and therefore permitting rearrangement (Fig. 3d). Finally, the introduction of the crosslinker ethylene glycol diacrylate (EGDA) to give a feed of MA/BA/EGDA 80:17:3 was intended to keep T g low and thereby allow film formation at room temperature, while chemically restricting the mobility of the growing shell. This indeed encouraged the encapsulated morphology (Fig. 3e), but the latex solution formed a gel-like consistency after standing at room temperature for a few days, presumably due to interparticle crosslinking via residual vinyl groups contributed by the EGDA.

This monomer study therefore confirmed that polymer shells with T g 's lower than the polymerization temperature preferred the sandwich morphology, while the encapsulated morphology could be attained if the growing shell was physically or chemically prevented from rearrangement at the expense of film-forming ability at room temperature. Secondary nucleation was also largely avoided as intended from the initial studies on the effect of macroRAFT agent concentration, and held true for all tested monomer feeds.

Influence of macroRAFT agent hydrophilicity/hydrophobicity and molar mass

The previous section demonstrated that while the encapsulated morphology was accessible by reducing the polymer mobility through using a high T g monomer composition (L ), room temperature film formation would be compromised by the high T g and interparticle crosslinking respectively. The suite of four macroRAFT agents was therefore compared for any effect on nanocomposite morphology using the film-forming MA/BA 80:20 (mass/mass) feed (Table 1, L 4 and L 8 -L 10 ). The mass of macroRAFT agent was fixed at 12.0 g L -1 in each experiment, meaning that molar concentrations varied according to the molar masses of the polymers (Table 1). The concentrations of free macroRAFT agent were approximated from the adsorption isotherms (Fig. 1a) and are included in Table 2. The isotherms plotted on a molar basis are included as Figure S6 (Supporting Information).

P(AA 10 -co-BA 5 )-R, although capable of stabilizing the LDH particles prior to polymerization, invoked rapid latex coagulation within 20 minutes of monomer addition in the emulsion polymerization (L 8 ). Here, [macroRAFT] free was ca. 8.7 g L -1 (5.3 mmol L -1 ). Additional polymerizations were attempted using this macroRAFT agent at different [macroRAFT] free (Supporting Information, Table S1, Entries L S1 -L S3 ), and it was only at the very high

[macroRAFT] free of ca. 22.0 g L -1 (13.3 mmol L -1 ) that stability was attained. CryoTEM of this system, however, showed very thin polymer shells around the LDH particles, and an abundance of secondary particles (Supporting Information, Fig. S7). Returning to reasonable

[macroRAFT] free and switching to a MMA/BA 90:10 (mass/mass) feed again produced rapid coagulation (Table S1 32 that excessively hydrophilic macroRAFT agents depart more easily from the inorganic surface during polymerization and compromise colloidal stability. [START_REF] Nguyen | Pigment encapsulation by emulsion polymerization using macro-RAFT copolymers[END_REF][START_REF] Das | Copolymer nanosphere encapsulated cds quantum dots prepared by RAFT copolymerization: Synthesis, characterization and mechanism of formation[END_REF] The higher charge density and different surface chemistry of LDH compared with gibbsite may have also affected latex stability.

In contrast, P(AA 23.3 -co-BA 11.7 )-R, which shares the same AA/BA ratio as P(AA 10 -co-BA 5 )-R but has a degree of polymerization of 35 rather than 15, gave a stable latex (L 9 ) under the same conditions as L 8 . Here, [macroRAFT] free was ca. 8.9 g L -1 (2.6 mmol L -1 ), and the nanocomposite particles were again of sandwich morphology (Fig. 4a). The macroRAFT length therefore had a significant impact on the latex stability. 2.

As described in the previous section, P(AA 17.5 -co-BA 17.5 )-R gave the sandwich mosphology (L 4 , Fig. 3a). In this L 4 system, [macroRAFT] free was ca. 5.0 g L -1 (1.2 mmol L -1 ). In contrast, P(AA exhibit the encapsulated morphology (Fig. 4b), which is remarkable for a film-forming monomer feed composition. In L 10 , [macroRAFT] free was ca. 5.6 g L -1 (3.1 mmol L -1 ).

Since [RAFT]:[initiator] = 0.5 in all emulsion polymerization systems in the present paper, the initiator concentration in L 10 was more than twice that in L 4 , with all other masses identical. We thought this may have affected the polymerization kinetics and thereby influenced the nanocomposite morphology, since monomer accumulation (i.e. deviation from true starved-feed conditions) has been shown to affect nanocomposite morphology in other starved-feed encapsulation systems. [START_REF] Zhong | A versatile encapsulation method of noncovalently modified carbon nanotubes by RAFT polymerization[END_REF] However, conversion versus time, and the proportion of monomer still unreacted at each time point, were almost identical in the two systems (Supporting Information, Fig. S8a andb). Differing degrees of monomer accumulation are therefore unlikely to account for the distinct morphologies in L 4 and L 10 . Instead, we propose that the higher density of RAFT functions on the LDH in L 10 may have translated to more efficient reversible chain transfer at the particle edges (as well as at the particle faces), favoring complete encapsulation rather than growth solely from the LDH faces. The defined morphologies were not supplemented with statistical analysis due to the difficulty in identifying LDH platelets oriented near-perpendicular to the electron beam, [START_REF] Voorn | Clay Platelets Encapsulated Inside Latex Particles[END_REF] but further cryoTEM images showing the distinct L 4 and L 10 morphologies are provided in Fig. S9 (Supporting Information).

Is the RAFT function important?

The proposed roles of the macroRAFT agent in the REEP process are to provide colloidal stability, and to encourage emulsion polymerization to commence at the surface of the filler material. To the best of our knowledge, however, the role of the RAFT function itself in REEP has not been thoroughly investigated. Furthermore, the Heuts and van [START_REF] Mballa Mballa | Control of the anisotropic morphology of latex nanocomposites containing single montmorillonite clay particles prepared by conventional and reversible additionfragmentation chain transfer based emulsion polymerization[END_REF] and gibbsite nanoparticles [START_REF] Loiko | Encapsulation of unmodified Gibbsite via conventional emulsion polymerisation using charged co-oligomers[END_REF] with a high T g shell by using conventional emulsion polymerization under starved-feed conditions. In the latter report, [START_REF] Loiko | Encapsulation of unmodified Gibbsite via conventional emulsion polymerisation using charged co-oligomers[END_REF] gibbsite particles were stabilized with the same short statistical copolymers used in their ATRPbased nanocomposite work, [START_REF] Loiko | ATRP mediated encapsulation of Gibbsite: fixation of the morphology using a cross-linker[END_REF][START_REF] Loiko | An ATRPbased approach towards water-borne anisotropic polymer-Gibbsite nanocomposites[END_REF] but the stabilizing polymers did not act as initiators in this system since no ATRP catalyst was used. It was sufficient to perform conventional radical polymerization under true starved-feed conditions to achieve full encapsulation. These interesting literature examples show that re-activatable functions are not always essential for successful nanocomposite particle synthesis, and motivated us to explore the contribution of the RAFT function in the present work.

The role of the RAFT function was investigated by removing it from P(AA 17.5 -co-BA 17.5 )-R and using the polymer in a subsequent starved-feed emulsion polymerization (L 11 ) under identical conditions to L 4 . Many methods for end group removal have been developed for RAFT polymers, [START_REF] Jesson | H2O2 Enables Convenient Removal of RAFT End-Groups from Block Copolymer Nano-Objects Prepared via Polymerization-Induced Self-Assembly in Water[END_REF][START_REF] Moad | End-functional polymers, thiocarbonylthio group removal/transformation and reversible addition-fragmentation-chain transfer (RAFT) polymerization[END_REF][START_REF] Willcock | End group removal and modification of RAFT polymers[END_REF][START_REF] Carmean | Catalyst-Free Photoinduced End-Group Removal of Thiocarbonylthio Functionality[END_REF] but one of the most convenient remains the radical approach in which the RAFT function is replaced by an initiator-derived radical by reacting with a large excess of azo initiator. [START_REF] Perrier | Reversible Addition-Fragmentation Chain Transfer Polymerization: End Group Modification for Functionalized Polymers and Chain Transfer Agent Recovery[END_REF] For acrylates, adding 2 eq. of lauryl peroxide in conjunction with the 20 eq. of azo initiator ensures full conversion while avoiding high molar mass coupling products. [START_REF] Chen | Thiocarbonylthio end group removal from RAFTsynthesized polymers by a radical-induced process[END_REF] The primary lauroyl radicals are more reactive than the tertiary azo initiator radicals, and therefore generate RAFT-centred radicals which are more prone to fragment in the forward direction and release the R-group for capping with an initiator-derived radical.

This approach was performed to give the RAFT-free polymer denoted P(AA radical-radical coupling between polymer chains (Fig. 5b). The modified polymer could therefore be considered as the non-reactivatable equivalent of the original macroRAFT agent, allowing the effect of the RAFT function in the encapsulating emulsion polymerization to be isolated. The first notable aspect of L 11 was the formation of a stable latex. The RAFT function, and its proposed role in promoting polymerization from the LDH surface, was therefore not a prerequisite for stable latex formation. The conversion versus time profile also showed no discernible difference to that of the corresponding RAFT system L 4 (Fig. 5c). The morphology, however, was distinctly different to that of the RAFT system. CryoTEM (Fig. 5d) revealed essentially spherical polymer particles, with LDH platelets preferentially located at the particle surface to form an armored-type morphology. Some sandwiches were evident, with the two

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polymer nodes sharing a trapped LDH platelet, but some latex particles displayed a single LDH platelet at their surface or were LDH-free. These features contrast with L 4 , in which every LDH platelet, whether individually dispersed or as part of a small cluster of platelets, displayed polymer growth on both faces, and armored structures were not observed (Fig. 3a). LDH-armored latexes are interesting nanomaterials in their own right, providing access to desirable structures such as honeycomb nanocomposites. [START_REF] Veschambres | Layered double hydroxides: Efficient fillers for waterborne nanocomposite films[END_REF] Comparison of L 4 and L 11 conclusively demonstrates that under the chosen conditions, the Reaction conditions defined in Table 2.

The molar mass at full conversion in L 4 was approximately double that of the P(AA 7.5 -co-BA 7.5 )-R system (L 10 , Fig. 6b and Table 2) as expected from the higher [monomer]:[RAFT] ratio.

Finally, the RAFT-free system demonstrated a molar mass distribution typical of an uncontrolled radical polymerization system, with a distinct minor peak at low molar mass corresponding to the stabilizing polymer which was incapable of chain extension and therefore remained unchanged; its molar mass distribution agrees well with the t = 0 curve of L 4 in Fig. 6b (i.e. that of its parent polymer carrying the RAFT function). The sole role of this polymer as stabilizer was therefore confirmed.

Conclusion

Nanocomposite latexes containing layered double hydroxide (LDH) particles were obtained by using a starved-feed emulsion process employing RAFT polymerization. Unlike most comparable nanoparticle encapsulations reported in literature, film-forming monomer feeds were chosen in this work to allow film formation under ambient conditions, which makes control over the morphology more difficult due to the high mobility of the growing polymer chains at the polymerization temperature. Controlling nanocomposite latex morphology was motivated by a desire to control nanoparticle orientation in resulting films. Here, polymer domains only on the nanoparticle faces is assumed to be energetically preferable to encapsulation, since the low T g polymer should be mobile enough to minimize its surface energy at the polymerization temperature. Changing to a high T g monomer feed, or a feed incorporating a small amount of crosslinker, provided access to the encapsulated morphology but these latexes were no longer film-forming.

Remarkably, the encapsulated morphology could also be achieved with the film-forming MA/BA 80:20 (mass/mass) monomer composition by using the shorter stabilizing polymer P(AA 7.5 -co-BA 7.5 )-R. Attainment of a fully-encapsulating polymer layer was somehow encouraged in this system, which we tentatively ascribe to the higher density of RAFT functions promoting more efficient polymer growth particularly around the particle edges.

A third type of nanocomposite particles, with armored morphology, were produced by removing the RAFT function from the stabilizing polymer P(AA 17.5 -co-BA 17.5 )-R prior to its adsorption on LDH. Inorganic-armored latexes are desirable nanocomposites in their own right, and their attainment in this case unequivocally highlights the importance of the RAFT functions in providing re-activatable sites from which chain extension can proceed to give even polymer growth from both LDH faces in the RAFT-bearing systems.

In addition to accessing three different film-forming nanocomposite morphologies, the effect of molar mass and hydrophilic/hydrophobic balance were isolated in the adsorption of four macroRAFT agents on LDH. Increasing the BA content of the macroRAFT agent at fixed molar mass significantly increased the amount required for LDH charge neutralization (by 50%), and increasing molar mass at a fixed AA/BA ratio had a similar albeit weaker effect (27% increase).

The

Scheme 1 .

 1 Scheme 1. Overall process for the encapsulation of LDH particles in polymer latexes using RAFT-mediated emulsion polymerization; a) Acrylic acid (AA)/n-butyl acrylate (BA), dioxane, 4,4'-azobis(4-cyanopentanoic acid) (ACPA), 60 °C; b) Azobisisobutyronitrile (AIBN), lauroyl peroxide, dioxane, 80 °C, 4 h; c) Water (pH value = 8.0), ACPA, monomer feed (most frequently methyl acrylate (MA)/BA), 70 °C, 4 h.

18 Figure 1

 181 Figure 1. a) Adsorption isotherms for the four macroRAFT agents used in this work; [LDH] = 20 g L -1 , pH value 8.0. The intersections of the grey dotted line (imposed by [macroRAFT] total = 12 mg mL -1 , [LDH] = 20 mg mL -1 ) with each of the four isotherms correspond to the distribution of macroRAFT agent (adsorbed vs. free) in the emulsion polymerization systems L 4 -L 11 (Table 2). The vertical arrows explicitly show the [macroRAFT] free for each. b) Electrophoretic mobility in the presence of different concentrations of the four macroRAFT agents; [LDH] = 10 mg L -1 . c) Relative amount of carboxylate groups required to reach the IEP in b). d) Stability ratio of LDH particles at different macroRAFT agent concentrations. The legend in a) applies to all four plots.

Figure 2 .

 2 Figure 2. CryoTEM images showing final particle morphologies at 3 h in the emulsion polymerizations using [LDH] = 6.5 g L -1 , monomer = MA/BA 80:20 (mass/mass) and P(AA 17.5co-BA 17.5 )-R concentrations of a) 4.3 g L -1 (L 1 ), b) 8.6 g L -1 (L 2 ), and c) 17.2 g L -1 (L 3 ).

Figure 3 .

 3 Figure 3. Monomer feed study showing final particle morphologies using P(AA 17.5 -co-BA 17.5 )-R as macroRAFT agent and monomer feeds of a) L 4 : MA/BA 80:20 (mass/mass); b) and c) L 5 : MMA/BA 90:10 (mass/mass); d) L 6 : MMA/BA 50:50 (mass/mass); and e) L 7 : MA/BA/EGDA 80:17:3 (mass/mass). All are cryoTEM images except c) which is a conventional TEM image. EGDA = ethylene glycol diacrylate.

Figure 4 .

 4 Figure 4. CryoTEM images of the final particle morphologies obtained using a) P(AA 23.3 -co-BA 11.7 )-R (L 9 ), and b) P(AA 7.5 -co-BA 7.5 )-R (L 10 ) with MA/BA 80:20 (mass/mass) monomer feed, conditions defined in Table2.

Figure 5 .

 5 Figure 5. SEC chromatograms of P(AA 17.5 -co-BA 17.5 )-R and P(AA 17.5 -co-BA 17.5 ) prepared at ca. 2 g L -1 after methylation, with a) the UV signal at 310 nm showing essentially complete removal of the RAFT function, while b) the RI signal shows an essentially unchanged molar mass distribution; c) global conversion versus time for the emulsion polymerization using P(AA 17.5 -co-BA 17.5 ) (L 11 ) compared with P(AA 17.5 -co-BA 17.5 )-R (L 4 ); and d) cryoTEM image of the resulting nanocomposite particles displaying the armored morphology (L 11 ).

  reactivatable RAFT function performed a critical role in attaining complete polymer coverage on the LDH faces. SEC of samples taken throughout the L 4 polymerization exhibits an increase in molar mass (vs. PSt standards) with conversion, and molar masses fit reasonably well with theoretical values (Fig.6a), confirming that chain extension indeed occurred in the RAFT systems. Dispersities rose from 1.2 to 1.7 throughout the polymerization, which was expected given that the heterogeneous nature and the high [initiator]:[RAFT] of the system would have diminished the degree of RAFT control.

Figure 6 .

 6 Figure 6. a) Evolution of molar mass (squares) and dispersity (diamonds) with conversion for the emulsion polymerization system using P(AA 17.5 -co-BA 17.5 )-R (L 4 ); b) Corresponding molar mass

For Table of Contents use only Controlling the morphology of film-forming, nanocomposite latexes containing layered double hydroxide by RAFT-mediated emulsion polymerization

  Samuel Pearson, Marko Pavlovic, Thomas Augé, Valerian Torregrossa, Istvan Szilagyi, Franck D'Agosto, Muriel Lansalot, Elodie Bourgeat-Lami, Vanessa Prévot

	S S S S
	u p p u p p u p p u p p
	o r t o r t o r t o r t
	i i i i
	n g n g n g n g
	I I I I
	n f n f n f n f
	o r o r o r o r
	m a t m a t m a t m a t
	i o i o i o i o
	n n n n
	----
	F o r F o r F o r F o r
	R e v i e w R e v i e w R e v i e w R e v i e w
	O O O O
	n l y n l y n l y n l y
	----
	N o t N o t N o t N o t
	f f f f
	o r o r o r o r
	P u P u P u P u
	b l i b l i b l i b l i
	c c c c
	a t i a t i a t i a t i
	o n o n o n o n
	2 3

  When employed in nanocomposites, the LDH particles can be either in

	S
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  3 , d 6 -DMSO) were purchased from Cambridge Isotope Laboratories. Mg(NO 3 ) 2 •6H 2 O (analytical grade, Acros Organics) and Al(NO 3 ) 3 •9H 2 O (analytical grade, Merck) were used for LDH synthesis.

	S
	u p p
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	8

Table 2 .

 2 Latexes produced by the emulsion polymerizations performed in this work. Monomer injection was performed for the first 2 h. b Values just above 100% indicate the accuracy of the conversion calculation method to within a few percentage points. c By SEC against PSt standards. d Theoretical T g for the polymer shell calculated from the monomer feed composition and the T g values of the pure polymers (T g,PMA = 5°C, T g,PMMA = -54°C, T g,PMMA = 106°C)[START_REF] Brandrup | Polymer Handbook[END_REF] using the Fox equation:[START_REF] Fox | Influence of diluent and of copolymer composition on the glass transition temperature of a polymer system[END_REF] 1/T g,Fox = x 1 /T g,1 + x 2 /T g,2 where T g,1 and T g,2 are glass transition temperatures, and x 1 and x 2 are the mass fractions, of components 1 and 2 respectively.

	Label Stabilizing polymer,	[LDH]	[P]	[P] free	Feed	t a	X b	Morphology	M n,theor	M n,SEC c	Đ b	T g,Fox d
		P	(g L -1 )	(g L -1 /	(g L -1 /	(mass/mass)	(h)	(%)		(kg mol -1 )	(kg mol -1 )		(°C)
				mmol L -1 )	mmol L -1 )
	L 1	P ( A A 17.5 -co-BA 17.5 )-R 6.5	4.3 / 1.1	2.0 / 0.5	MA/BA 80:20	3	84	Sandwich	-	-	-	-9
	L 2	P ( A A 17.5 -co-BA 17.5 )-R 6.5	8.6 / 2.3	6.0 / 1.6	MA/BA 80:20	3	88	Sandwich	-	-	-	-9
	L 3	P ( A A 17.5 -co-BA 17.5 )-R 6.5	17.2 / 4.6	14.5 / 3.9	MA/BA 80:20	3	91	Sandwich	-	-	-	-9
	L 4	P ( A A 17.5 -co-BA 17.5 )-R 20	12 / 3.2	5.0 / 1.3	MA/BA 80:20	4	101	Sandwich	25.3	21.5	1.7	-9
	L 5	P ( A A 17.5 -co-BA 17.5 )-R 20	12 / 3.2	5.0 / 1.3	MMA/BA 90:10 4	99	Encapsulated	-	-	-	80
	L 6	P ( A A 17.5 -co-BA 17.5 )-R 20	12 / 3.2	5.0 / 1.3	MMA/BA 50:50 4	97	Sandwich	-	-	-	4
	L 7	P ( A A 17.5 -co-BA 17.5 )-R 20	12 / 3.2	5.0 / 1.3	MA/BA/EGDA	4	95	Encapsulated	-	-	-	-
						80:17:3
	L 8	P ( A A 10 -co-BA 5 )-R	20	12 / 7.3	8.7 / 5.3	MA/BA 80:20	4	-	Unstable	-	-	-	-9
	L 9	P ( A A 23.3 -co-BA 11.7 )-R 20	12 / 3.5	8.9 / 2.6	MA/BA 80:20	4	98	Sandwich	-	-	-	-9
	L 10	P(A A 7.5 -co-BA 7.5 )-R	20	12 / 6.5	5.7 / 3.1	MA/BA 80:20	4	101	Encapsulated	12.2	12.7	1.5	-9
	L 11	P(A A 17.5 -co-BA 17.5 )	20	12 / 3.2	5.0 / 1.3	MA/BA 80:20	4	103	Armored	-	151	3.8	-9

  by DLS at time zero, confirming that the statistical copolymer itself does not self-assemble, and that the formation of the nano-objects occurs through the PISA process.

	particles were detected
	S
	u p p
	o r t
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	a t i
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	. No
	27

  , L S4 , L S5 ). This instability was surprising given that a macroRAFT agent with the same number of AA and BA units, differing only in the nature of the RAFT agent itself, proved the most effective for encapsulating gibbsite in Ali et al.'s investigation of five different AA/BA macroRAFT agents.[START_REF] Ali | Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of Anisotropic Composite Latex Particles by RAFT-Based Starved Feed Emulsion Polymerization[END_REF]

	S
	u p p
	o r t
	i
	n g
	I
	n f
	o r
	m a t
	i o
	n
	-
	F o r
	R e v i e w
	O
	n l y
	-
	N o t
	f
	o r
	P u
	b l i
	c
	a t i However, the symmetrical trithiocarbonate with benzyl termini used by Ali et al. is more o n
	hydrophobic than CTPPA (Scheme 1). The greater hydrophilicity of the P(AA 10 -co-BA 5 )-R used
	in our system may have therefore contributed to the latex instability, with literature indicating

  17.5 -co-BA 17.5 )

	S
	u p p
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	f
	o r
	P u
	b l i
	c (Scheme 1). Successful end group conversion was proven by comparing the RI and UV signals a t i by SEC, with the end group-modified polymer showing a practically complete absence of UV o n
	absorbance at 310 nm normally associated with the RAFT function (Fig. 5a). The IR signal in the
	modified polymer mirrors that of the precursor polymer, indicating that the molar mass

35 distribution did not change to any measurable extent through any undesirable reactions such as

  Three distinct nanocomposite morphologies were successfully attained in this work by modifying the nature of the stabilizing polymer initially adsorbed on the LDH nanoparticles. CryoTEM analysis revealed that LDH nanoparticles stabilized with P(AA 17.5 -co-BA 17.5 )-R (where AA is BA is n-butyl acrylate, and R is the RAFT function) gave the sandwich morphology when a methyl acrylate (MA)/BA 80:20 (mass/mass) feed was used.

	acrylic acid,
	S
	u p p
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	F o r
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	a t i
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	38

  adsorption behavior was combined with observations of latex stability, secondary nucleation, to generate stable emulsion polymerization systems devoid of free polymer particles which reached full conversion at relatively short reaction times. We expect these findings to be broadly applicable to the encapsulation of other nanoparticles.The microstructure and mechanical properties of nanocomposite films made from the three different latex morphologies have been studied in detail and are the subject of a separate publication.

	S and monomer conversion S S S S S
	u p p u p p u p p u p p u p p u p p
	o r t o r t o r t o r t o r t o r t
	i i i i i i
	n g n g n g n g n g n g
	I I I I I I
	n f n f n f n f n f n f
	o r o r o r o r o r o r
	m a t m a t m a t m a t m a t m a t
	i o i o i o i o i o i o
	n n n n n n
	------
	F o r F o r F o r F o r F o r F o r
	R e v i e w R e v i e w R e v i e w R e v i e w R e v i e w R e v i e w
	O O O O O O
	n l y n l y n l y n l y n l y n l y
	------
	N o t N o t N o t N o t N o t N o t
	f f f f f f
	o r o r o r o r o r o r
	P u P u P u P u P u P u
	b l i b l i b l i b l i b l i b l i
	c c c c c c
	a t i a t i a t i a t i a t i a t i
	o n o n o n o n o n o n
	39

Acknowledgements

Financial support from ANR-11-JS08-0013 (PolyHydRAFT) is gratefully acknowledged. The authors would like to thank Ana Maria Cenacchi Perreira and Bastian Ebeling for helpful discussions, and Pierre-Yves Dugas and the Centre Technologique des Microstructures (CTµ), Claude Bernard Lyon 1 University, for cryoTEM training and support.

Supporting Information

Synthesis, and 1 H and [START_REF] Leroux | Polymer Interleaved Layered Double Hydroxide: A New Emerging Class of Nanocomposites[END_REF] C NMR spectra, of 4-cyano-4-thiothiopropylsulfanyl pentanoic acid (CTPPA), SEC traces and conversion versus time for macroRAFT agent synthesis, DLS and XRD of bare LDH particles, cryoTEM and DLS of samples taken throughout the L 4 polymerization, adsorption isotherms on a molar basis, table of additional emulsion polymerizations, cryoTEM of L S3 , and monomer consumption profiles for L 4 and L 10 .


Amélie Jacquot

Marco Sperduti

Margherita Arcangeli

Esteban Buch

Jérôme Dokic

Pascale Piolino

Profiling the musical sublime

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Questionnaire (11 points Likert scales) Analysis:

-one-way ANOVAs (5 tracks) for interesting dimensions (cf. Study 1) Music listening : amelie.jacquot@univ-paris8.fr

Conclusion

-Sublimity in musical context is a multi-component experience -Even if we obtained a significative effect for the dimension « Negative emotion » , the average report for this dimension was lower as compared to the other dimesions. This result could indicate that « negative emotion » is not a relevant dimension to evaluate an experience of the sublime elicited by music.

-The experimental context for musical experience probably minimizes the created sensations as compared to live music (especially for the dimension « being moved »).

-Results of study 1 revealed 2 dimensions linked to the diminished sense of the self; further studies should investigate whether music experience of the sublime impact the body or the cognitive aspects of the Self representation.

[1] Arcangeli, M., Sperduti, M., Dokic, (2018). The Beautiful, the Sublime and the Self. Advances in Experimental Philosophy of Aesthetics, 175.

grandeur, need for accomodation, possible presence of mixed emotions (positive and negative aspects), diminished self, immersion [1] Being moved

Introduction:

  amelie.jacquot@univ-paris8.fr Characteristics of the experience of the sublime Is the music a good elicitor of the sublime? Study 1 * = p< .05, ** = p< .001 Main effect of definition (p < .001) Interaction of definition x factors (p < .01) Study 2 definition of the musical beauty definition of the musical sublime Participants: 177 adults (66 males, age 37.55 ± 16.56) Profiling the musical sublime Jacquot A. 1, 2 *, Sperduti M. 1 *, Arcangeli M. 3, 4 , Buch E. 3 , Dokic J. 3,4 , Piolino P. 1 1 Laboratoire Mémoire, Cerveau et Cognition, URP 7536, Université de Paris, 2 Laboratoire de Psychopathologie et Neuropsychologie , EA 2027, Université Paris 8 , 3 École des Hautes Études en Sciences Sociales, Paris, 4 Institut Jean Nicod, UMR 8129, École Normale Supérieure, Paris, *These two authors contributed equally to the study Questionnaire -50 items (11 points Likert scales) emotional, physical and cognitive sensations Analysis (EFA) on the items (Promax rotation, factor loadings : ≥ 0.5, parallel analysis). -ANOVA 8 Dimensions x 2 definition (sublime vs. beauty) experiences of the sublime and the beauty Participants: 146 adults (31 males, age = 25.34 ± 16.56)


Mariana Vargas

Aurélien Bellet

Pascal Denis

Probabilistic End-to-End Graph-based Semi-Supervised Learning

In this paper we address the problem of graph-based semi-supervised learning in tasks where a graph describing the relationships between data points is not available. We propose a method to jointly learn the graph and the parameters of a semi-supervised model using a probabilistic framework. We empirically show our proposal achieves competitive results in a variety of datasets.

Introduction

Graph-based semi-supervised models are good at leveraging unannotated data when small amounts of labels are available: they take as input a graph whose nodes are data points (both labeled and unlabeled) and edges describe how points are related to each other. There exists a variety of such models which propagate labels based on a smoothness criterion [START_REF] Zhu | Learning from Labeled and Unlabeled Data with Label Propagation[END_REF][START_REF] Zhu | Semi-supervised Learning Using Gaussian Fields and Harmonic Functions[END_REF][START_REF] Zhou | Learning with local and global consistency[END_REF][START_REF] Joachims | Transductive Learning via Spectral Graph Partitioning[END_REF][START_REF] Belkin | Manifold Regularization: A Geometric Framework for Learning from Labeled and Unlabeled Examples[END_REF]. A more recent approach uses Graph Convolutional Networks (GCN) to learn node representations based on all the input data while backpropagating the error on the labeled data [START_REF] Kipf | Semi-Supervised Classification with Graph Convolutional Networks[END_REF]. Unfortunately, in many applications the graph structure is not readily available.

A standard solution is to compute graphs using classical heuristics such as k-nn or -graphs [START_REF] Subramanya | Graph-Based Semi-Supervised Learning[END_REF], but those choices poorly adapt to the underlying data manifold and disregard label information, thus yielding suboptimal results. Dhillon et al. [START_REF] Dhillon | Inference Driven Metric Learning (IDML) for Graph Construction[END_REF] propose a metric learning based framework in which a graph is constructed via a metric that maximizes the confidence of label assignments. Following a different route, Alexandrescu et al. [START_REF] Alexandrescu | Data-Driven Graph Construction for Semi-Supervised Graph-Based Learning in NLP[END_REF] use a supervised model on the labeled subset to transform the data into a new space consisting in soft label predictions where the graph is constructed. These two approaches still rely on the classic heuristics for graph construction, and are not able to learn complex data representations. The recent work of Franceschi et al. [START_REF] Franceschi | Learning Discrete Structures for Graph Neural Networks[END_REF] represents edges as Bernoulli random variables and uses a bilevel programming framework to fit the parameters of the graph and a GCN for semi-supervised classification.

In this paper, we present a probabilistic model to learn the parameters of a semi-supervised classification model and the graph jointly. We model edges as latent variables, and we learn by minimizing a reconstruction error over the predicted labels. Our choice of a probabilistic framework allow us to explicitly define a prior over the graph. This enables the model to account for prior knowledge and provides a principled mechanism to impose specific structures (such as sparsity) upon the graph.

Model

Let us assume a training set of the form D = D L ∪ D U where D L = {(x i , y i )} l i=1 is the set of labeled data points with labels in some discrete set Y, and D U = {x i } l+u i=l+1 the set of unlabeled points. Let X = [x 1 ; . . . ; x l+u ] T be the design matrix and y L = [y 1 , . . . , y l ] the label vector, W ij a random variable representing an edge between x i and x j . Our goal is to find labels {y l+1 , . . . , y l+u }. We start by assuming labels y are generated by a random process that depends on the data X and unknown parameters W that encode how data points are connected. That is, W ij represents an edge between x i and x j . We can describe this process through the conditional probability p θ (y|X, W ) parameterized by θ that gives the likelihood of labels y provided the dataset X and the graph W . In a variational Bayesian context parameters W are latent variables with prior distribution p θ (W ) and approximate posterior q φ (W ij |X, y) parameterized by φ, on which we can make inference.

We therefore aim to find the parameters [θ, φ] so as to maximize the likelihood of the labels while keeping the distribution q φ close to the prior. To do this, we maximize the evidence lower bound (ELBO) given by:

L(θ, φ) = E q φ (W |X,y) [log p θ (y|W, X)] -KL[q φ (W |X, y)||p θ (W )] (1)
This model is similar to a variational autoencoder [START_REF] Diederik | Auto-Encoding Variational Bayes[END_REF] where q φ is the encoder and p θ the decoder. Maximizing this bound is equivalent to minimizing the reconstruction error of the estimations the decoder produces while the encoder remains as close to the prior as possible.

To instantiate our model, we choose p θ (y|W, X) to be a categorical distribution over the labels Y parametrized by a GCN [START_REF] Kipf | Semi-Supervised Classification with Graph Convolutional Networks[END_REF], and q φ (W |X, y) to model edges as a collection of Bernoulli distributions parameterized by a Graph Neural Network (GNN) [START_REF] Scarselli | The Graph Neural Network Model[END_REF]. Each Bernoulli distribution represents the probability of an edge connecting two nodes i and j, hence W ij is a binary variable.

We took inspiration from [START_REF] Kipf | Neural Relational Inference for Interacting Systems[END_REF] to use a message passing Graph Neural Network (GNN) [START_REF] Gilmer | Neural Message Passing for Quantum Chemistry[END_REF] that computes node and edge embeddings as follows:

h (1) i = MLP (1) node (x i ), (2) h (1)
ij = MLP (1) edge ([h (1)
i , h

j ]), (1)
i = MLP (3) h (2)
ij = MLP (2) edge ([h (2) node h (1) i , j∈N (i) h (1) ij , (4) h (2)
i , h

j ]), (2)
and finally g φ (X) = Softmax(h

(2) ij )
. N (i) denotes the neighbors of point i in the GNN (in practice we simply take them to be the closest points to x i , or even all other points). Finally, we define g φ (X) = Softmax(h

(2) ij ).
Here [x i , x j ] denotes concatenation and MLP is short for multilayer perceptron.

We also pick the prior distribution p θ (W ) to be a collection of Bernoulli distributions.

Training details. We train our model by backpropagation. We first run the encoder q φ , which is a distribution taking discrete values. This prevent us from being able to directly backpropagate the error through its reparametrized samples. We then use the concrete distribution [START_REF] Maddison | The Concrete Distribution: A Continuous Relaxation of Discrete Random Variables[END_REF] to get a continuous approximation of q φ and apply the reparametrization trick to compute the gradients. More specifically, we draw samples W as follows: we draw a vector ξ from a Gumbel(0, 1) distribution and then we compute

W ij = Softmax((h (2) ij + ξ)/τ ),
where τ is a parameter controlling how smooth the resulting distribution is (the bigger τ is, the more it will resemble a uniform distribution). To control the variability we take several samples this way, W (1) , . . . , W (r) and feed the decoder to get ŷ(1) , . . . , ŷ(r) where ŷi = GCN (X, W (i) ). We then backpropagate the error with respect to y L through the mean ŷ = 1 r (ŷ (1)

+ • • • + ŷ(r)
). The reconstruction error that corresponds to the first term of Equation 1 is the average cross-entropy over the labeled examples:

L reconstruction = 1 l CrossEnt y L , ŷ D L . ( 6
)
The KL-divergence of q φ that corresponds to the second term of Equation 1, given a Bernoulli prior ρ, is given by:

L KL-divergence = l+u i=1 j:ρij =0 W ij log W ij ρ ij + (1 -W ij ) log 1 -W ij 1 -ρ ij . (7)

Experiments and Results

We carried out experiments to compare our probabilistic approach (denoted by PSSL) with supervised algorithms such as logistic regression (LogReg), support vector machines with a radial kernel (SVM) and feed-forward neural networks (FFNN), 1 and with the state-of-the-art semi-supervised method based on GCNs, which we fed with different types of heuristically computed graphs. We use three strategies for building a graph: k nearest neighbors (knn), radial kernel (RBF), and a random variant of the k-NN graph constructed as follows: denoting by K the regular k-nn graph, an edge e ij between x i and x j is sampled according to a Bernoulli distribution with some high probability α if K ij = 1, or with probability 1 -α otherwise (Sknn). The prior distribution for our model PSSL is constructed in the same way as Sknn. We also specified different sparsity patterns over the prior.

We evaluate the baselines and our method on three datasets: Cora [START_REF] Sen | Collective Classification in Network Data[END_REF], a subset of 20 Newsgroups with three classes and a TFIDF feature space, and Wine, a benchmark dataset available in scikit-learn [START_REF] Pedregosa | Scikit-learn: Machine Learning in Python[END_REF].

We used Adam to optimize our objective function. For all methods we tune the main hyperparameters over five random splits with train, test, and validation sizes as described in Table 1.

Results are shown in Table 2. We can observe that we achieve competitive results in Wine, and outperform the baselines by a considerable margin in 20news3 and Cora.

Conclusion and Discussion

We presented preliminary work on a framework based on autoencoding variational bayes that learns the parameters of a semi-supervised model and the underlying graph structure of the data simultaneously. We empirically showed that our model can achieve considerable gains over different baselines in different semi-supervised datasets.

We plan to run experiments on other semi-supervised datasets, and to compare this method empirically with that of Franceschi et al. [START_REF] Franceschi | Learning Discrete Structures for Graph Neural Networks[END_REF]. We believe our proposal exhibits two advantages. First, we can explicitly specify a prior over the graph, which allow us to bias towards specific structures and sparsity patterns. Second, [START_REF] Franceschi | Learning Discrete Structures for Graph Neural Networks[END_REF] requires two separate validation sets while we require only one: we therefore have access to more training data.

An interesting future research line is to extend this work to an inductive setting in order to be able to elegantly handle unseen test examples.

33rd

  Conference on Neural Information Processing Systems (NeurIPS 2019), Vancouver, Canada.

Table 1 :

 1 Statistics of datasets. Dataset Size Dimension Nb. of classes Train/Val/Test

	Wine	178	13	3	10/20/158
	20news3 2756 229	3	20/40/2696
	Cora	2708 1433	7	140/300/1000

Table 2 :

 2 Mean and standard deviation of test accuracy over five random splits.

		Wine	20news3	Cora
	LogReg	.95 ± .02 .77 ± .01 .53 ± .00
	SVM	.94 ± .03	.76 ± .00 .50 ± .00
	FFNN	.93 ± .01	.77 ± .01 .55 ± .01
	GCN+knn	.95 ± .03 .77 ± .02 .61 ± .01
	GCN+Sknn .93 ± .03	.66 ± .05 .31 ± .00
	GCN+RBF .94 ± .03	.76 ± .01 .51 ± .01
	PSSL	.95 ± .01 .83 ± .01 .65 ± .04

This model is equivalent to a GCN with no graph.