Synthesis of Polymer/Silica Hybrid Latexes by Surfactant-Free RAFT-Mediated Emulsion PolymerizationE. Bourgeat-Lami,*,† A. J. P. G. Franca,†,‡ T. C. Chaparro,†,‡ R. D. Silva,‡ P.-Y. Dugas,† G. M. Alves,‡

and A. M. Santos*,‡

†Universite de Lyon, Univ. Lyon 1, CPE Lyon, CNRS, UMR 5265,, Laboratoire de Chimie, Catalyse, Polymeres et Procedes (C2P2),LCPP group, 43, Bd. du 11 Novembre 1918, F-69616 Villeurbanne, France‡Laboratory of Polymers, Department of Chemical Engineering, Engineering School of Lorena, University of Sao Paulo, EstradaMunicipal do Campinho, S/N, 12.602-810, Lorena, SP Brazil

*S Supporting Information

ABSTRACT: The reversible addition−fragmentation chaintransfer (RAFT) polymerization technique was used tosynthesize random copolymers of poly(ethylene glycol)methyl ether acrylate) (PEGA) and n-butyl acrylate (BA)and terpolymers of acrylic acid (AA), PEGA and BA with atrithiocarbonate reactive end-group. These macromolecularRAFT agents (macro-RAFTs) were subsequently adsorbed atthe surface of size-monodisperse colloidal silica particles withdiameters varying between 40 and 450 nm. Adsorptionisotherms for both macro-RAFTs could be well fitted to theLangmuir adsorption model, the AA-based macro-RAFT agent showing however a lower maximum adsorption. The adsorbedmacro-RAFT agents were subsequently chain extended with a mixture of methyl methacrylate (MMA) and BA by starved feedemulsion polymerization. Cryo-TEM analysis of the resulting hybrid latexes synthesized in the presence of the P(AA-co-PEGA-co-BA) terpolymers resulted in multipod-like particles while the P(PEGA-co-BA) copolymers showed the formation ofindividually and multiencapsulated silica particles depending on the silica particle size. Decreasing the total silica surface areaavailable by decreasing the silica concentration or by increasing the silica particle size resulted in limited coagulation of the latexparticles due to a less efficient use of the free nonadsorbing macro-RAFT agent. The feeding process also had a strong impact onparticle morphology, and snowman-like particles could be successfully achieved under batch conditions. The use of commercialsilica particles instead of homemade silica led to armored latexes illustrating the determinant role of the surface properties of themacro-RAFT-coated inorganic particles in controlling hybrid particle morphology. At last, core−shell particles with a rigid silicacore and a soft copolymer shell were obtained for the first time by polymerizing a film-forming monomer mixture showing thehigh potential of the P(PEGA-co-BA) macro-RAFT agent for the elaboration of polymer-encapsulated silica particles for coatingapplications.

I. INTRODUCTION

Among the recent progresses in the field of materials science,the creation of organic/inorganic nanocomposite materials canbe cited as one of the most promising developments.1 Thecombination of organic and inorganic components at thenanoscale takes advantage of all the features that these materialscan offer, resulting in a unique composite material withimproved properties. In these composites, each part plays animportant role: while the polymer matrix imparts flexibility andprocessability to the final material, the incorporation of aninorganic component brings enhancements in rigidity andthermal stability, for instance. It is also possible to explore theaddition of nanofillers with special properties to producematerials with specific and outstanding characteristics. More-over, the use of nanosized fillers, instead of conventional micro-and macro-sized particles, causes a considerable increase in theinterfacial area and intensifies the physical interaction betweenthe phases.

Colloidal nanocomposites have emerged as a relatively newcategory of nanocomposites and an alternative to overcomesome of the issues observed in the synthesis of conventionalcomposites (such as for instance the melt processingirreproducibility).2 Different techniques can be used in thepreparation of these new materials, from heterocoagulation tolayer-by-layer assembly, but in recent years special attention hasbeen dedicated to in situ polymerization.3 In this technique,monomers are polymerized in the presence of inorganiccolloidal particles. Emulsion polymerization, a widely usedfree radical polymerization process, can be considered one ofthe most interesting methods to synthesize colloidal nano-composites via in situ polymerization. To circumvent the lack ofcompatibility between the organic and inorganic parts, various

Received: April 8, 2016Revised: May 31, 2016

Article

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© XXXX American Chemical Society A DOI: 10.1021/acs.macromol.6b00737Macromolecules XXXX, XXX, XXX−XXX

synthetic methods have been developed in the literature. Thesemethods usually involve the use of auxiliary molecules, such assilane coupling agents, comonomers, or initiators, which aregrafted or adsorbed onto the inorganic surface.4 However, inmost of the strategies reported, the use of surfactants isrequired to provide colloidal stability to the hybrid particlesalthough it is known that surfactant molecules can migrate inthe final materials and adversely affect film formation andproperties. To overcome this issue, new approaches forobtaining composite latexes with different particle morpholo-gies in the absence of molecular surfactants have been recentlyreported.5 These approaches take advantage of the recentdevelopments in the field of controlled radical polymerization(CRP) techniques in aqueous media, and the so-calledpolymerization-induced self-assembly (PISA) of amphiphilicblock copolymers. These new CRP-based methods rely on theuse of macroalkoxyamine initiators,6 or more commonly,macromolecular RAFT agents (macro-RAFT), which canadsorb on the inorganic particles and lead the emulsionpolymerization to occur at their surface. In short, the methoduses living amphipathic random copolymers with a RAFTfunctionality on one extremity, which is able to be reactivatedfor the polymerization of hydrophobic monomers. In addition,the relatively high hydrophilicity of the macro-RAFT agentprovides stability to the formed objects in water dispersion.Nguyen et al.7 were the first to apply this strategy for theencapsulation of alumina- and zirconia-coated titanium dioxidepigments using amphipathic copolymers composed of acrylicacid (AA) and n-butyl acrylate (BA) units under starved-feedconditions. Since then, many works have been focused on usingsimilar approaches to generate a variety of hybrid morphologiesusing different types of inorganic particles8 with different shapesand surface chemistries, such as cerium oxide,9 cadmium10 andlead11 sulfide quantum dots, graphene oxide,12 gibbsite,13 andMMT14 platelets and carbon nanotubes.15 Here we demon-strate that this macro-RAFT agent strategy can be easilyimplemented to the synthesis of polymer/silica hybrid particles.The advantages of using silica as inorganic fillers to producepolymer-based nanocomposites are well-known and include,among others, the ability of these particles to improve thephysical properties (such as mechanical and thermal) of thepolymer matrix.16

In this work, the surfactant-free synthesis of silica−polymercomposite particles by RAFT-mediated emulsion polymer-ization is reported. The strategy used does not involve thechemical modification of silica or the addition of anysurfactants: we rely, instead, on the use of amphipathicPEGA-based macro-RAFT agents to adsorb on the surface ofsilica, induce the growth of the polymeric material from theinorganic surface by chain extension in emulsion polymer-ization and provide stability to the resulting hybrid colloids.The effect of some parameters, such as the nature of the macro-RAFT agent and the silica particle size, as well as the processtype (batch versus semibatch), on particle size and morphologywas studied, yielding a variety of different hybrid structures,from snowman to multipod-like and individually or multi-encapsulated silica particles as characterized by cryogenic-transmission electron microscopy (cryo-TEM). Althoughpolymer/silica composites have been the most commonlyreported organic/inorganic nanocomposites in the literature,6,17

as far as we know, this is the first work to describe the synthesisof silica-based nanocomposite latexes by macro-RAFT-medi-ated emulsion polymerization.

II. EXPERIMENTAL SECTIONII.1. Materials. Tetraethyl orthosilicate (TEOS, ≥ 99%, Sigma-

Aldrich), L-arginine (≥98,5%, Sigma-Aldrich), ethanol (EtOH, 99.5%,LabSynth), ammonium hydroxide (27%, LabSynth), 4−4′-azobis-4-cyanovaleric acid (ACPA, ≥ 98%, Sigma-Aldrich), n-hexane (95%Vetec), 1,4-dioxane (99%, Vetec), ethyl ether (99,5%, Vetec),poly(ethylene glycol) methyl ether acrylate (PEGA, Mn = 480 gmol−1, Sigma-Aldrich), and acrylic acid (AA, 99%, Sigma-Aldrich)were used as received. Deuterated chloroform (CDCl3, 100%, Aldrich)and deuterated dimethyl sulfoxide (DMSO-d6, Aldrich, 100%) wereused as solvents for proton nuclear magnetic resonance analysis (1HNMR). Tetrahydrofuran (THF, 99.9%, HPLC grade, Aldrich) wasfiltered and degassed in an ultrasound bath before use as solvent in sizeexclusion chromatography (SEC). 1,3,5-Trioxane (99%, Aldrich) wasused as internal standard to determine monomer conversion by 1HNMR during macro-RAFT synthesis. Trimethylsilyl diazomethane (2M solution in diethyl ether, Aldrich) was used to methylate the acrylicacid units present in the structure of the macro-RAFT agents beforeSEC analysis. The monomers, methyl methacrylate (MMA, 99.5%)and n-butyl acrylate (BA, 99.5%), kindly supplied by BASF (Brazil),were purified by distillation at reduced pressure. 4-Cyano-4-(propylsulfanyltiocarbonyl) sulfanyl pentanoic acid (CTPPA) wassynthesized by reaction of ACPA with bis(propylsulfanylthiocarbonyl)disulfide according to the literature.18 The commercial silica sol(Klebosol 30N12) was kindly supplied by Clariant (France). All thewater used in the experiments was deionized (Purelab Classic UV,ElgaLabWater).

II.2. Characterizations. The composition of the macro-RAFTmolecules was determined by 1H NMR spectroscopy (Bruker DRX300) by measuring the vinyl proton integrals of the monomers (threeCH2 protons at δ = 6.27 ppm, δ = 6.05 ppm and δ = 5.59 ppm forPEGA, δ = 6.27 ppm, δ = 6.05 ppm and δ = 5.59 ppm for BA and δ =6.50 ppm, δ = 6.22 ppm and δ = 5.75 ppm for AA) using 1,3,5-trioxaneas an internal reference.19 Size exclusion chromatography (SEC)analyses were performed in THF at 40 °C. The flow rate of the mobilephase was 1 mL min−1, and toluene was used as a flow rate marker. Allpolymers were injected at a concentration of 3 mg mL−1 after filtrationthrough a 0.45 μm pore-size membrane. The separation was carriedout using three Polymer Laboratories columns [3 × PLgel 5 μm MixedC (300 × 7.5 mm)] and a guard column (PL gel 5 μm). The averagemolar masses and molar mass distributions were calculated with acalibration curve based on PMMA standards. Dynamic light scattering(DLS, Malvern Zetasizer Nano ZS) was used to measure the particlesize (average hydrodynamic diameter, Zav) and the dispersity of thesamples (poly valuethe higher this value, the broader the sizedistribution). The silica particles were imaged by transmission electronmicroscopy at the Centre Technologique des Microstructures,platform of the Universite Claude Bernard, Lyon 1, France. A dropof the diluted silica sol was deposited on a carbon/Formvar-coatedcopper grid and the water was allowed to evaporate. The analysis wascarried out at room temperature with a Philips CM120 microscopeoperating at an accelerating voltage of 80 kV. To prevent particlesdeformation (for soft polymer compositions) and/or the deleteriouseffect of radiation (PMMA is known to be particularly sensitive toelectron damage), and allow reliable determination of particles sizeand morphology, the composite latex particles were characterized bycryogenic-transmission electron microscopy (cryo-TEM). Following amethod described elsewhere,20 the diluted samples were dropped onto300 Mesh holey carbon films (Quantifoil R2/1) and quench-frozen inliquid ethane using a cryo-plunge workstation (made at LPS Orsay).The specimens were then mounted on a precooled Gatan 626specimen holder, transferred in the microscope (Phillips CM120) andobserved at an accelerating voltage of 120 kV. The number-average(Dn) and the weight-average (Dw) diameters of the silica or compositelatex particles were determined directly on the TEM or cryo-TEM

micrographs according to = ∑∑Dn

n Dni i

iand = ∑

∑Dw

n D

n Di i

i i

4

3 , where ni is the

number of particles with diameter Di. In the case of silica, a minimumof 200 particles was counted for each batch. The number of silica and

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B

composite particles, (Np (L−1)), was then determined from the particle

diameter determined by TEM or cryo-TEM according to the followingequation:

ρ π=−N L

C

D( )

6 10pComposite silica

Composite silica

Composite silica Composite silica( )

1 ( )21

( ) ( )3

(1)

where CComposite(silica) (g L−1), ρComposite(silica) (g cm

−3) and DComposite(silica)are respectively the concentration, the density, and the cryo-TEM(respectively TEM) diameter of the composite and silica particles.Knowing the average number of silica bead per composite particle (asdirectly determined on the TEM images), R = Np silica/Np composite, onecan estimate the polymer concentration for which the number ofcomposite particles is equal to Np silica/R. This polymer concentrationcorresponds to the shell polymer concentration. The fraction of freepolymer is then determined as follows:

=−

×free polymer (%)[total polymer] [shell polymer]

[total polymer]100

(2)

with [total polymer] and [shell polymer] expressed in g L−1

II.3. Synthesis of Silica Particles. Silica particles with diametersranging from 40 to 450 nm and narrow size distributions weresynthesized using two different procedures. First, ultrafine, highlymonodisperse silica particles were synthesized using the two-phaseprocess of Hartlen et al.21 Following this process, an immiscibleorganic top layer of TEOS was left diffusing into an aqueous solution

of amino acid catalyst, leading to a slow increase in the solutionsupersaturation and subsequent formation of tiny silica particles with avery narrow size distribution.22 In contrast with previous reports, noinert oil (i.e., octane or cyclohexane) was employed in the presentwork. In a typical procedure, L-arginine (26.7 mg, 6 mmol L−1) wasfirst diluted in deionized water (25 g), charged into the reactor, andkept stirring for several minutes with a stirring bar. Once the solutiontemperature reached 60 °C, 4.0 mL of TEOS were carefullyintroduced in the reactor to form a top layer. The stirring rate wasfixed at 250 rpm so that the water phase could be well mixed and thetop organic layer could be maintained almost undisturbed. Themixture was stirred at 60 °C for 72 h to form 41 nm diameter silicaparticles as determined by transmission electron microscopy (TEM)analysis (S2, Table 1). These silica particles were then used as seeds togenerate larger monodisperse particles according to the so-called seed-regrowth process.21−23 In a typical regrowth reaction, a certain amountof the silica seed was gradually dispersed in an ethanol solutioncontaining water and ammonia (0.6 M). Upon uniform mixing, acertain amount of TEOS (Table 1) was added with the help of asyringe pump (rate =3.0 mL h−1, B. Braun Perfusor Compact). Themixture was stirred at room temperature for 24 h to obtain 101, 254,and 432 nm diameter silica particles as determined by TEM (S3, S4and S5 respectively, Table 1).

II.4. Synthesis of Macro-RAFT Agents. Two types of macro-RAFT agents were synthesized in this work: an amphipathic randomterpolymer of AA, BA and PEGA and a random copolymer of BA and

Table 1. Experimental Conditions and Main Characteristics of the Silica Particles used in this Work.a

samplename

syntheticprocedure catalyst

[catalyst](mol L‑1)

[TEOS](mol L‑1)

solids content(%) pHd

Zav. (nm)(DLS)

polyvalue

Dn (nm)(TEM)

Dw/Dn(TEM)

S1b Klebosol30N12

− − − 19.9 8.7 32 0.45 − −

S2 hartlen L-arginine 6.0 × 10−3 0.72 2.6 7.1 52 0.04 41 1.012S3c seed regrowth NH3 0.6 0.30 1.3 8.7 116 0.09 101 1.002S4c seed regrowth NH3 0.6 0.43 2.3 8.6 290 0.01 254 1.002S5c seed regrowth NH3 0.6 0.40 2.4 8.8 553 0.14 432 1.002

aAll suspensions were dialyzed against water before use in order to remove any nonreacted reagents. bAqueous commercial silica sols. cSeedconcentrations equal to 0.0129, 0.0013, and 0.0003 mol L−1 for S3, S4, and S5, respectively. dpH determined after dialysis.

Scheme 1. Chemical Structures of the Chain Transfer Agent, the Comonomers, and the Corresponding Macro-RAFT Agentsused in this Work

Table 2. Experimental Conditions and main Characteristics of the Macro-RAFT Agents Synthesized in this Work

macro-RAFT targeted compositionCTPPA(mM)

ACPA(mM)

AA(mM)

PEGA(mM)

BA(mM)

Mn,theo(g mol‑1)

XM(%)

Mn, SEC(g mol‑1) Mw/Mn

A1 P(AA5-co-PEGA5-co-BA5)-CTPPA 4.1 0.41 20.4 20.4 20.4 3100 87 3120 1.19A2 P(PEGA7-co-BA4)-CTPPA 3.8 0.38 − 26.5 15.1 3040 79 2900 1.12

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PEGA. Both polymers were synthesized by RAFT polymerization insolution using CTPPA as a chain transfer agent (Scheme 1).The macro-RAFT agents were synthesized as follows. In a two-

necked round-bottom flask equipped with a condenser, properamounts of CTPPA, monomers, and ACPA (Table 2), and 30.4 mgof 1,3,5-trioxane were dissolved in a calculated amount of 1,4-dioxaneso the final concentration of macro-RAFT agent in solvent would notexceed 25 wt %. After deoxygenation by nitrogen bubbling for 30 min,the resulting mixture was immersed in an oil bath thermostated at 80°C for 7 h. Samples were taken for kinetics studies by 1H NMRanalysis. Finally, the obtained polymers were purified by precipitationin n-hexane for three times and characterized by 1H NMR and SEC:P(AA4-co-PEGA4-co-BA4)-CTPPA, Mn = 3120 g mol−1, Đ = 1.19 (A1)and P(PEGA6-co-BA4)-CTPPA, Mn = 2900 g mol−1, Đ = 1.12 (A2)(the SEC traces are shown in the Supporting Information). Their maincharacteristics are shown in Table 2.II.5. Macro-RAFT Agents Adsorption on the Silica Surface. A

macro-RAFT stock solution (40 g L−1) and a silica stock dispersion(10 g L−1) were made and used to prepare 5 g L−1 g silica dispersionswith a concentration range of macro-RAFT from 1.0 to 20.0 g L−1.The dispersions were stirred for 12 h and further ultracentrifuged at60000 rpm for 1 h (Thermo Scientific, Sorvall MTX 150 model micro-ultracentrifuge). The equilibrium macro-RAFT concentration in thesupernatant was determined by UV−visible spectroscopy. Theadsorbed amount, Qe (mg g

−1), was then determined by the differencebetween the initial and the equilibrium concentration according to

=−

×−QC C V

m(mg g )

( )1000e

o e1(3)

where C0 (g L−1) is the initial macro-RAFT concentration, Ce (g L

−1)is the macro-RAFT equilibrium concentration in the supernatant, V(L) is the volume of solution, and m (g) is the mass of silica.II.6. Surfactant-Free Emulsion Polymerization of MMA and

BA in the Presence of Silica Particles. Hybrid latex particles weresynthesized by starved feed emulsion polymerization in a 50 mL three-necked round-bottom flask equipped with a condenser. In a typicalexperiment (EP04, Table 3), 0.2 g of macro-RAFT agent (A2) wasfirst dissolved in 8.6 g of water with magnetic stirring. Then, 8.8 g ofsilica sol (S3, 1.3 wt %) was slowly dropped in the macro-RAFT agentsolution maintaining the stirring throughout (final macro-RAFTconcentration = 3.3 mM). The dispersion was stirred for 1 h, thenit was added to the flask a solution prepared with 0.0060 g of ACPAand 3.6 g of water (macro-RAFT:ACPA = 3:1, molar ratio) and 0.1 gof a previously prepared monomer mixture (mass ratio MMA:BA =90:10). The content was flushed with nitrogen for 30 min. The flaskwas immersed in an oil bath thermostated at 80 °C. Then 2.1 g of apreviously deoxygenated monomer mixture was fed in the flask at arate of 0.67 mL h−1 over 4 h using a syringe pump (B. Braun PerfusorCompact) to target a dispersion with a polymer content of 10.5 and

5.2 wt % of silica based on the polymer. Samples were taken in a 1-hinterval during 5 h of reaction for determination of the monomer topolymer conversion by gravimetric analysis and the particle size. Asimilar procedure was used for the batch experiment except that themonomer mixture was introduced as a shot together with the initiatorsolution.

III. RESULTS AND DISCUSSION

III.1. Synthesis of Silica Particles. Silica is an amorphousoxide-based nanostructured material that can be easily modifiedeither by covalent conjugation or simply by physical adsorption.Silica is an ideal particle to use in model systems, presentinggreat colloidal stability and processability, and is widelyemployed in many industrial areas such as paints, drug deliveryand composite materials.17 Commercial silica nanoparticlespresent diameters in the range of 10−80 nm, but larger spherescan be prepared by the Stober process,24 via the hydrolysis andcondensation of TEOS in a mixture that includes water,alcohol, and ammonia as base catalyst. Even though the Stoberprocess is the most widely used method to prepare colloidalsilica nanoparticles due to a variety of advantages, the control ofparticle size and particle size distribution is still challengingthrough this method. Many studies have been dedicated toovercome this issue, for example, the “seed regrowth”approach,22,23a in which presynthesized silica particles areused as seeds to be further grown by the addition of the silicaprecursor (e.g., TEOS); the amino acid catalyzed,25 that usesamino acids instead of base; and the regrowth technique with L-arginine as a base catalyst reported by Hartlen et al.21 While thefirst one is more suitable for producing particles above 50 nmand the second is restricted to low solid contents (typicallybelow 5 wt %), the method proposed by Hartlen is gainingincreasing attention since it produces particles from 15 nm tomore than 200 nm with low polydispersity. Although, it isconsidered a simple and efficient waterborne route for theobtainment of silica seeds, this method is however not suitableto regrow particles, and leads to poorly controlled particles insize or shape.Herein, four batches of size-monodisperse silica particles with

diameters in the range 40−450 nm were prepared in two steps.First, ultrafine, highly monodisperse silica particles weresynthesized using the Hartlen process, and these particleswere subsequently used as seeds to generate bigger particlesusing ammonia as base catalyst. The size of the silica seed wascontrolled by the amount of TEOS introduced in the Hartlen

Table 3. Experimental Conditions and Characteristics of Silica/P(MMA-co-BA) Hybrid Latexes Synthesized by Surfactant-FreeRAFT-Mediated Emulsion Copolymerization of MMA and BA Using P(AA4-co-PEGA4-co-BA4)-CTPPA (A1) and P(PEGA6-co-BA4)-CTPPA (A2) as Macro-RAFT Agents

entry silicaDn silica

(TEM, nm)[silica](g L‑1) macro- RAFT

MMA:BA(wt:wt)

operationmode

convn(%)

Zav latexa

(DLS, nm)polyvalue

Dn compositeb

(TEM, nm) Np silica/Np composite

freepolymc

(%)

EP01 S3 101 5.5 A1 10:1 semibatch 79 114 0.23 − − −EP02 S3 101 5.5 A2 10:1 semibatch 75 183 0.23 266 1 39.7EP03 S3 101 5.5 A2 10:1 batch 89 209 0.01 − − −EP04 S2 41 5.5 A2 10:1 semibatch 81 185 0.08 161 2.49 18.4EP05 S4 254 5.5 A2 10:1 semibatch 84 140 0.41 508 1 78.2EP06 S5 432 5.5 A2 10:1 semibatch 77 178 0.20 − − −EP07 S2 41 0.37 A2 10:1 semibatch 78 571 0.06 − − −EP08 S1 32a 5.5 A2 10:1 semibatch 82 165 0.05 − − −EP09 S1 32a 0.09 A2 10:1 semibatch 72 559 0.29 − − −EP10 S3 101 5.5 A2 1:1 semibatch 68 157 0.27 212 1 68.2

aDetermined by dynamic light scattering. bDetermined by cryo-TEM. cCalculated using eq 2. [MMA + BA] = 105 g L−1

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process (S2, Table 1) while varying the seed concentration inthe regrowth experiments enabled to get particles in the desiredsize range with polydispersity indexes as low as 1.002 (S3, S4,and S5 in Table 1). The corresponding TEM images are shownin the Supporting Information (Figure S1). In addition, acommercial Klebosol silica sol (Klebosol 30N12, Zav = 32 nm,S1 in Table 1) was used to complete the series and study theeffect of the nature of the silica particles.III.2. Macro-RAFT Adsorption on the Silica Surface. In

order to understand the mechanism of hybrid particleformation, the adsorption of the macro-RAFT agents wasstudied using the 101 nm silica particles at pH 8.7 (S3, Table1). The adsorption isotherms of both macro-RAFT agentsexhibit a L-shape, characteristic of a Langmuir-type ofadsorption (Figure 1). The adsorbed amount of macro-RAFT

agent increases with the increase in the copolymer concen-tration until a plateau, which corresponds to the maximaladsorption capacity, is reached.26 For P(PEGA6-co-BA4)-CTPPA, a greater mass of copolymer is needed to completelycover the silica surface, since the amount of macro-RAFTadsorbed at the plateau was around 189 mg g−1 (i.e., 6.4 mgm−2) while the amount of P(AA4-co-PEGA4-co-BA4)-CTPPAadsorbed at the plateau was about 69 mg g−1 (i.e., 2.3 mg m−2).P(PEGA6-co-BA4)-CTPPA also displayed a stronger affinity forthe silica surface as indicated by a higher initial slope. TheLangmuir model was fitted to the adsorption isotherms inFigure 1 (dotted lines) and the corresponding adsorptionparameters are given in the Supporting Information (Figure S3and Table S1). The model could effectively fit the adsorptionisotherm of P(PEGA6-co-BA4)-CTPPA on silica but the fit wasless adequate for P(AA4-co-PEGA4-co-BA4)-CTPPA suggestinga more complex adsorption mechanism. The equilibriumconstant KL is much lower for the P(AA4-co-PEGA4-co-BA4)-CTPPA macro-RAFT agent indicating lower binding strengthof the AA-containing terpolymer to the silica surface inagreement with the shape of the curve and previous literaturereports.27

It is well-known that poly(ethylene oxide) adsorbs onto silicaparticles by hydrogen bonds between ether oxygen atoms ofPEO and silanol groups on the silica surface.28 It has beenreported that polymers bearing PEO side chains, known asbrush-type or comb-like polymers, adsorb more than theanalogous linear PEO polymers.6,29 The maximum adsorptioncapacity of the P(AA4-co-PEGA4-co-BA4)-CTPPA macro-RAFTcopolymer: 60.4 mg g−1 (i.e., 2 mg m−2) is almost two times

lower than the maximum adsorption capacity reported by Qiaoet al. for similar brush-type copolymers.6 The lower adsorptionof P(AA4-co-PEGA4-co-BA4)-CTPPA compared to the literatureand to P(PEGA6-co-BA4)-CTPPA is probably due to thepresence of AA units. At pH 8.7, both the silica surface and theAA units are negatively charged generating electrostaticrepulsion that hinders the adsorption.27 In contrast, the highadsorption of P(PEGA6-co-BA4)-CTPPA may be attributed tothe presence of hydrophobic BA units in the structure whichwould adsorb on the silica surface in order to minimize theinteraction between the BA units and the aqueous medium thusincreasing the driving force for adsorption.30

III.3. Synthesis of Silica/Polymer Hybrid Particles.Effect of the Nature of the Macro-RAFT Agent. Previousstudies in the literature showed that the composition of themacro-RAFT agent had a determinant impact on the stabilityand morphology of the resulting hybrid particles.9a In order toassess the effect of the macro-RAFT composition on particlemorphology, AA units were incorporated in the copolymerchains to tune their hydrophobic/hydrophilic balance. Whilethe hybrid latex synthesized with P(AA4-co-PEGA4-co-BA4)-CTPPA (EP01, Table 3) comprises multipod-like silica/polymer particles (Figure 2a), the hybrid latex synthesized

with P(PEGA6-co-BA4)-CTPPA (EP02, Table 3) comprisespolymer-encapsulated silica nanoparticles (Figure 2b). This canbe attributed to the fact that this macro-RAFT is morehydrophobic and hence displayed a higher affinity for the silicasurface allowing a greater amount to be adsorbed on theinorganic particles, while simultaneously rendering theinorganic surface hydrophobic enough to efficiently shift thepolymerization locus from the water phase to the surface of thesilica particles. In both cases, free polymer particles were seen inthe water phase. As in both cases there was free macro-RAFTagent in water since the beginning of the polymerization, thesenonadsorbed chains may have undergone chain extensionleading to self-assembling and further particle formationaccording to the PISA mechanism.31 The free polymer particlesare however more abundant in the latex synthesized withP(AA4-co-PEGA4-co-BA4)-CTPPA than when the AA-freemacro-RAFT agent was used. This result is in agreementwith the adsorption isotherms described above, and can beattributed to the fact that P(AA4-co-PEGA4-co-BA4)-CTPPAdisplayed a lower affinity for the silica surface, and is also more

Figure 1. Adsorption isotherms of P(PEGA6-co-BA4)-CTPPA andP(AA4-co-PEGA4-co-BA4)-CTPPA on 101 nm silica particles at pH 8.7with Langmuir fits (dashed lines).

Figure 2. Cryo-TEM images showing the effect of the composition ofthe macro-RAFT agent on the particle morphology in the semibatchemulsion polymerization of MMA:BA (90:10 wt %) at 10.5 wt %solids content, using 5.5 g L−1 of 101 nm silica and 3.3 mM of macro-RAFT agents. (a) P(AA4-co-PEGA4-co-BA4)-CTPPA (EP01) and (b)P(PEGA6-co-BA4)-CTPPA (EP02).

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hydrophilic, which would both contribute to the formation of ahigher proportion of self-stabilized free latex particles.Effect of Process Type. The feasibility of encapsulating silica

by RAFT-mediated emulsion polymerization performed inbatch mode was checked. A stable latex could be synthesized inbatch mode (EP03, Table 3) under similar conditions ofexperiment EP02 (Figure 2b and Figure 3c), however the cryo-TEM and TEM images of the latex EP03 show thatencapsulation was not achieved (Figures 3, parts a and b).Snowman-like hybrid particles were identified instead, as well asa few free silica beads and a large number of free polymerparticles indicating a decrease in polymer−silica interaction anda predominance of polymer particle formation in the waterphase.This change in morphology was probably caused by the

desorption of macro-RAFT agent from the silica surface due topartitioning of macro-RAFT agent between the inorganicsurface, water and monomer droplets. Thus, the amount ofmacro-RAFT agent adsorbed on the silica surface was notenough to guarantee the complete encapsulation of silica

particles. Besides, MMA-rich systems are prone to homoge-neous nucleation which may have also favored the formation offree polymer particles.32 In addition, in batch mode, the highconcentration of monomer in the growing latex particlespromotes a plasticizing effect, which lowers the glass transitiontemperature (Tg) of the polymer facilitating the expelling of thehydrophilic inorganic particles from the polymer phase.9a,13,33

Indeed, several reports have shown the importance of the effectof the Tg of the polymer shell on particle morphology.5a,b

Although encapsulation of silica particles was not achieved, thisresult suggests however that RAFT-mediated emulsionpolymerization performed in batch mode can be explored asa potential tool for the synthesis of snowman-like particles.

Effect of the Silica Particle Size. The effect of the size of thesilica particles on the composite morphology was evaluated.Hybrid latexes were synthesized with 3.3 mmol L−1 ofP(PEGA6-co-BA4)-CTPPA and a fixed silica concentration of5.5 g L−1 using particles with 41, 101, 254, and 432 nm (EP04,EP02, EP05, and EP06, respectively, Table 3). The cryo-TEManalysis of the hybrid latex synthesized with 41 nm silica

Figure 3. (a, c) Cryo-TEM and (b) TEM images showing the effect of the process type on the particle morphology in the RAFT-mediated emulsionpolymerization of MMA:BA (90:10 wt %) at 10 wt % solids content, using 5.5 g L−1 of 101 nm silica particles and 3.3 mM of P(PEGA7-co-BA4)-CTPPA macro-RAFT agent in (a, b) batch (EP03) and (c) semibatch (EP02). Scale bar: 200 nm.

Figure 4. Cryo-TEM images showing the effect of the silica nanoparticles size on the morphology of the hybrid particles synthesized by RAFT-mediated emulsion polymerization of MMA:BA (90:10 by weight) at 10 wt % solids content, using 3.3 mM of P(PEGA6-co-BA4)-CTPPA macro-RAFT agent and 5.5 g L−1 of (a, b) 41 nm (EP04), (c) 101 nm (EP02), (d, e) 254 nm (EP05), and (f) 432 nm (EP06) silica particles. Scale bar: 200nm.

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particles (Figure 4, parts a and b) revealed the presence ofcore−shell particles (constituted of only one encapsulated silicabead) and multiencapsulated silica particles constituted of two,three, four, or more than four encapsulated silica beads. Das etal.11 also observed a similar multiencapsulation of quantumdots nanoparticles and attributed this morphology to limitedcoagulation. According to these authors, individual encapsula-tion of inorganic particles was achieved in the first stages ofpolymerization. However, as the polymerization continued, thesingle encapsulated nanoparticles coagulated leading to multi-encapsulated particles. The fact that we observed both mono-and multiencapsulated silica particles in a same batch is in favorof such a scenario.As the silica surface area was reduced by increasing the silica

particle size to 101 nm, the morphology of the hybrid latexparticles became more uniform as almost only single-core−shellparticles were observed with a very few number of double-core−shell particles and secondary-nucleated latex particles(EP02, Figure 4c). The use of 254 nm silica particles resultedexclusively in single-core−shell nanocomposite particles, with agreat number of free latex particles, as observed by cryo-TEM(EP05, Figure 4, parts d and e). The fraction of secondarynucleated polymer particles was quantitatively determined asdescribed in the Experimental Section, and was found toincrease from 18.4 to 78.2 wt % with increasing the silicaparticle size from 41 to 254 nm in agreement with the TEMobservations (Table 3). When the 432 nm silica was used, asignificantly different morphology was observed. The polymershell formed around the silica particles presented a protrusionon one side of the particles formed by accumulated smallerlatex particles (EP06, Figure 4f). This is to our knowledge thefirst time that such a morphology is reported in the literature.We suspect that latex agglomeration at the composite particlessurface is due to a lack of stability. As a matter of fact, a rapidcalculation shows that the amount of free macro-RAFT agentincreases with increasing the silica particle size (for fixed macro-RAFT and silica concentrations). Because of its hydrophobiccharacter, the excess macro-RAFT can partition between themonomer and water phases,34 which decreases the amount ofmacro-RAFT that can effectively participate to latex stabiliza-tion resulting in latex particles aggregation.35 Indeed, a controlexperiment performed under the same conditions in theabsence of silica beads showed the formation of 410 nmdiameter latex particles (as determined by DLS), illustrating thepoor stabilizing efficiency of the P(PEGA6-co-BA4)-CTPPA

macro-RAFT agent under batch conditions, in agreement withthe above-mentioned hypothesis.To further demonstrate the importance of excess macro-

RAFT on particle morphology, we performed another experi-ment in which the concentration of the 41 nm silica particleswas decreased to 0.37 g L−1 (EP07, Table 3) so as to totalize anoverall surface area equivalent to that of the 432 nm silicaparticles used in experiment EP06. The amount of free macro-RAFT was thus roughly the same in EP06 and EP07. The TEMimage of sample EP07 (Figure 5b) suggests multi encapsulationof the silica particles while cryo-TEM showed the formation oflarge particles with a rough surface. Their diameter (around 500nm) is in agreement with the DLS particle size.The morphology of these particles is remindful of the

polymer protrusion observed in experiment EP06, which wasperformed using 432 nm silica particles. Thus, although ahigher concentration of macro-RAFT agent in relation to theconcentration of silica was used in experiment EP07, it was notpossible to monoencapsulate the small silica particles, whichinstead aggregated into larger particles as a consequence of alack of stabilization due to partitioning of the excess macro-RAFT between the monomer and aqueous phases. Such apartitioning would decrease the amount of macro-RAFTeffectively available for stabilization of the growing latexparticles resulting in limited aggregation of the nucleatedparticles. This result clearly illustrates the effect of the macro-RAFT/inorganic particles complex on controlling not only theparticle morphology but also the stability of the final latexsuspension. The interest in using inorganic particles to supportthe controlling agent with regards to latex colloidal stability andliving character of the polymerization has already been reportedby Guimaraes et al.36 during the synthesis of clay-armoredlatexes by RAFT-mediated emulsion polymerization.

Effect of the Nature of the Silica Particles. In order to studythe effect of the nature of the silica particles, a commercial silicasol (Klebosol 30N12, Zav = 32 nm, Table 1) was used for thesynthesis of hybrid latexes. The TEM and cryo-TEM images ofthe latex samples synthesized with 0.09 g L−1 and 5.5 g L−1 of32 nm silica are displayed in Figure 6. For the higher silicaconcentration (EP08, Table 3), the morphology of the hybridparticles suggests that silica is located at the surface of theparticles (Figure 6a). Their diameter determined by DLS (165nm) is slightly smaller than their cryo-TEM diameter, whichsuggests the presence of free polymer particles as confirmed byTEM analysis. However, no free silica particles were identifiedindicating that all silica particles were incorporated in the

Figure 5. (a) Cryo-TEM and (b) TEM images showing the effect of decreasing the silica concentration on the morphology of silica/P(MMA-co-BA)hybrid particles synthesized using 41 nm silica particles (EP07) for a fixed P(PEGA6-co-BA4)-CTPPA concentration of 3.3 mM. Note that thenumber of silica particles was maintained the same as for run EP02, which was performed with 101 nm silica particles. The arrow points to theencapsulated silica particles. Scale bar: (a) 200 nm and (b) 500 nm.

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polymer matrix. Decreasing the silica concentration by a factor10 again resulted in the formation of cauliflower-like compositeparticles with a hydrodynamic diameter of 559 nm (EP09,Table 3), which agreed well with the cryo-TEM observation(Figure 6b).As before, this unusual morphology likely originates from the

limited coagulation of smaller particles formed in the earlystages of polymerization due to a lack of stabilization. Incontrast, EP08 performed at a higher silica concentrationshowed that immobilization of the macro-RAFT agent on thesilica surface minimized its partitioning between the monomerdroplets and the water phase, and significantly improved thelatex colloidal stability resulting in smaller particles with anarmored morphology. The armored structure suggests adifferent adsorption mechanism of the macro-RAFT agent onthe silica surface likely due to different surface chemistries, andhighlights the importance of the surface properties of themacro-RAFT-coated inorganic particles in determining hybridparticle morphology.III.4. Film-Forming Formulation. Several studies employ-

ing amphipathic macro-RAFT agents to encapsulate inorganicparticles showed that the composition of the monomer mixtureplayed a key role in particle morphology. While great successwas obtained for monomer compositions whose Tgs of thecorresponding polymers were above the reaction temperature,achieving the targeted encapsulated morphology proved to bemore challenging when the Tg of the polymer shell wasdecreased in order to enable film-formation at room temper-ature. In this section, a film-forming latex was synthesized witha higher BA content in the comonomer mixture (MMA:BA =1:1 by weight) (EP10, Table 3). This monomer compositionwas selected to target a polymer with a predicted Tg belowroom temperature to allow film formation under ambientconditions. The cryo-TEM images of the hybrid latex showedthat encapsulation of silica particles was unexpectedly achieved(Figure 7). Some hybrid particles containing more than onesilica bead were also identified and may have been formed uponcoagulation of single encapsulated silica particles. Interestingly,the shell thickness was smaller than for the experimentperformed under the same conditions using a 90:10 (wt:wt)MMA/BA mixture (EP02, Table 3), resulting in an increase ofthe proportion of free polymer from 39.7 to 68.2 wt % (Table3) as determined using eq 2. The TEM images confirmed theformation of a greater number of free polymer particles in EP10than in EP02, supporting the quantitative analysis.

To our knowledge, this is the first example of film-forminghybrid latex with a core−shell morphology synthesized byRAFT-mediated emulsion polymerization. Indeed, in theprevious literature examples,9a,13 a decrease in Tg led to a lossof control of the encapsulated morphology, the higher chainmobility of the polymer shell allowing migration of thehydrophilic inorganic particles to the polymer−water interfaceto minimize interfacial energy. Snowman-like or dumbbellmorphologies, which would indicate a possible migration ofsilica to the polymer−water interface, were not observed in thecryo-TEM images of sample EP10. Although the Tg of thepolymer shell was lower than the reaction temperature, apolymer shell could be formed around the silica particlesresulting in transparent nanocomposite polymer films uponwater evaporation of the latex suspension.

IV. CONCLUSIONSWe have investigated the unprecedented synthesis of silica/poly(MMA-co-BA) hybrid latexes by surfactant-free emulsionpolymerization mediated by poly(ethylene oxide)-containingmacro-RAFT agents. Composite colloidal particles withdifferent morphologies were achieved depending on the natureof the macro-RAFT agent, the process type, the silica particlesize or concentration and the nature of the silica surface. Thesemibatch emulsion polymerization of a comonomer mixturewith MMA:BA = 10:1 (wt:wt) mediated by P(AA4-co-PEGA4-co-BA4)-CTPPA allowed the preparation of multipod-likecomposite particles. This morphology was ascribed to thepresence of AA units in this macro-RAFT agent decreasing itsaffinity for the silica surface and lowering its adsorption incomparison to the adsorption behavior of P(PEGA6-co-BA4)-CTPPA, which successfully mediated the encapsulation of size-monodisperse colloidal silica particles with diameters in therange 40−450 nm. As the silica particle size increased (and thetotal silica surface area decreased), the morphology of thecomposite particles evolved from multiencapsulated silicaparticles to individually encapsulated silica particles andpolymer-protruded encapsulated particles. Decreasing thetotal silica surface area available affected the partitioning ofthe macro-RAFT agent between the monomer and the aqueous

Figure 6. Cryo-TEM images showing the effect of the nature of thesilica particles on the morphology of silica/P(MMA-co-BA) hybridparticles, obtained using the Klebosol 30N12 commercial silica (S1 inTable 1) for two different concentrations: (a) 5.5 g L−1 (EP08) and(b) 0.09 g L−1 (EP09). [P(PEGA6-co-BA4)-CTPPA] = 3.3 mM.

Figure 7. Cryo-TEM images showing the effect of the comonomermixture composition on the morphology of silica/P(MMA-co-BA)hybrid particles, using 101 nm silica particles (EP10) and MMA:BA(1:1 by weight) as monomer mixture. [P(PEGA6-co-BA4)-CTPPA] =3.3 mM.

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phase, which influenced the stability of the growing latexparticles as less macro-RAFT agent effectively participated inlatex stabilization, promoting aggregation. When RAFT-mediated emulsion polymerization was conducted in batchmode, snowman-like composite particles could be successfullyachieved. The surface properties of the colloidal silica alsoplayed a decisive role on the morphology of the hybrid latexesas the use of a commercial silica led to the formation of silica-armored composite particles. In addition, we also reported forthe first time the RAFT-mediated synthesis of silica-containingfilm-forming hybrid latexes with core−shell morphology, whichallowed the preparation of transparent nanocomposite films bywater evaporation. This study not only brought new insightsinto the parameters influencing the morphology of hybridlatexes prepared by RAFT-mediated emulsion polymerizationbut also opened a new avenue for the preparation of polymernanocomposite films with enhanced properties without thedeleterious effects that surfactants can bring in the resultingmaterials.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.macro-mol.6b00737.

TEM images of the synthesized silica particles listed inTable 1, evolution of the SEC traces of the P(AA4-co-PEGA4-co-BA4)-CTPPA and P(PEGA6-co-BA4)-CTPPAmacro-RAFT agents with conversion, the linearizedLangmuir isotherms and the adsorption parameters ofthe Langmuir model for the adsorption of macro-RAFTsonto 101 nm silica particles in water, and the kineticscurves for the synthesis of polymer/silica hybrid particleswith 432 nm silica (instantaneous and overall con-versions as a function of time) (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: bourgeat@lccp.cpe.fr (E.B.-L.).*E-mail: amsantos@usp.br (A.M.S.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank the FAPESP (Brazil) and CNRS (France)funding agencies for financial support. A.J.P.G.F. thanks theC2P2 Laboratory for providing support during his under-graduate internship.

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