J. Nanosci. Nanotechnol. 2006, 6, 432-444.pdf

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Copyright 2006 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 6, 432444, 2006

Nucleation of Polystyrene Latex Particles

in the Presence of -Methacryloxypropyl-trimethoxysilane: Functionalized Silica Particles

Elodie Bourgeat-Lami,1 Mickaelle Insulaire,1 Stphane Reculusa,2 Adeline Perro,2

Serge Ravaine,2 and Etienne Duguet3

1Laboratoire de Chimie et Procds de Polymrisation CNRS-CPE Lyon, Btiment 308 F,

43, boulevard du 11 novembre 1918, BP 2077, 69616 Villeurbanne Cedex, France2Centre de Recherche Paul Pascal CNRS, 115, Avenue du Dr Schweitzer, 33600 Pessac, France

3Institut de Chimie de la Matire Condense de Bordeaux CNRS,

87, Avenue du Dr Schweitzer, 33608 Pessac Cedex, France

Silica/polystyrene nanocomposite particles with different morphologies were synthesized through

emulsion polymerization of styrene in the presence of silica particles previously modified by

-methacryloxypropyltrimethoxysilane (MPS). Grafting of the silane molecule was performed by

direct addition of MPS to the aqueous silica suspension in the presence of an anionic surfactant

under basic conditions. The MPS grafting density on the silica surface was determined using the

depletion method and plotted against the initial MPS concentration. The influence of the MPS

grafting density, the silica particles size and concentration and the nature of the surfactant on

the polymerization kinetics and the particles morphology was investigated. When the polymeriza-

tion was performed in the presence of an anionic surfactant, transmission electron microscopy

images showed the formation of polymer spheres around silica for MPS grafting densities lower than

typically 1 mole m2 while the conversion versus time curves indicated a strong acceleration effectunder such conditions. In contrast, polymerizations performed in the presence of a larger amount of

MPS moieties or in the presence of a non ionic emulsifier resulted in the formation of excentered

core-shell morphologies and lower polymerization rates. The paper identifies the parameters that

allow to control particles morphology and polymerization kinetics and describes the mechanism of

formation of the nanocomposite colloids.

Keywords: Silica, -Methacryloxypropyltrimethoxysilane, Nanocomposite Particles, Polystyrene,Nucleation, Emulsion Polymerization, Particles Morphology.

1. INTRODUCTION

In the last few decades, the interest for hybrid particles thatcombine organic and inorganic parts has increased consid-erably due to the potential benefits of these nanoobjects inmultiple areas of material science.16

For example, Furusawa et al.7 described the synthesisof silica/polystyrene core-shell particles in the presenceof hydropropyl cellulose (HPC) used as polymeric binder/stabilizer. HPC provided hydrophobic loci that promotedefficient capture of the growing polymer chains. When theemulsion polymerization reaction was performed in the

presence of an anionic emulsifier at concentrations higherthan its critical micellar concentration (cmc), particles with

Author to whom correspondence should be addressed.

a raspberry shape were obtained illustrating the determi-

nant role of interfaces in the control of particles morphol-

ogy. More recently, Armes et al.810 synthesized polymer/

silica nanocomposite particles through soapless emulsion

copolymerization of styrene and acrylic monomers using

4-vinyl pyridine (4-VP) as a basic comonomer. The pres-

ence of the 4-VP comonomer ensured strong interaction

of the vinyl polymers with the acidic silica surface result-

ing in the formation of nanocomposite colloids with a

currant-bun morphology characterized by silica beads

assembled into colloidal aggregates cemented together by

the polymer synthesized during the emulsion polymeriza-tion reaction. The small silica particles emerging from

the composite surface were shown to participate to the

colloidal stability of the particles. Following a related

432 J. Nanosci. Nanotechnol. 2006, Vol. 6, No. 2 1533-4880/2006/6/432/013 doi:10.1166/jnn.2006.021


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Bourgeat-Lami et al. Nucleation of Polystyrene Latexes on Functionalized Silica Particles

strategy, Luna-Xavier et al.1113 described the prepara-

tion of poly(methyl methacrylate)/silica composite parti-cles using an electrostatically-adsorbed cationic initiator to

promote polymerization at the surface of an anionic sil-ica sol, while Reculusa et al.1415 recently demonstrated

that the adsorption of a monomethylether mono methyl-methacrylate poly(ethylene oxide) macromonomer allowed

to control the assembly of polystyrene latex particles on thesurface of silica particles through an in situnucleation and

growth process. Either raspberry-like14 or dumbbell-like15

morphologies were obtained with this technique depending

on the size and concentration of the silica seed particles.In the 1990s, Espiard et al.1618 reported results on the

encapsulation of silica particles through emulsion poly-

merization using -methacryloxypropyl trimethoxysilane(MPS) as capping agent. The role of MPS is to provide

reactive double bonds for covalent attachment of the grow-ing polymer chains on the silica surface. Owing to the

presence of the grafted polymer, the inorganic surfaceturned out to be more hydrophobic and presented loci

for the adsorption of monomer molecules and capture ofoligoradicals, which process enabled the formation of a

polymer shell surrounding the silica particles. Followingthis direction, Bourgeat-Lami and Lang1923 described in a

series of articles the encapsulation of MPS-grafted silicabeads using dispersion polymerization of styrene in aque-

ous alcoholic solutions. The authors first demonstrated thatthe use of MPS was a prerequisite condition for encap-

sulation to succeed.19 In the following articles,2022 they

have investigated in details the influence of the size andconcentration of the silica beads on the shape and compo-sition of the composite particles for a variety of monomers.The average number of silica beads per polymer particle

was statistically determined from the transmission elec-tron microscopy (TEM) images, and was found to vary

from one to several thousands with increasing the num-ber of silica beads in the suspension medium. While the

particles containing only one silica bead had an irregularcontour, those loaded with a larger amount of silica

displayed a spherical shape. Regular core-shell morpholo-

gies were also obtained when using a poly(styrene-b-ethylene oxide) copolymer as steric stabilizer.23 In a recentwork, Reculusa et al. showed that daisy-shaped and multi-

pod silica/polystyrene colloids were produced in emul-sion polymerization when the grafting density of the MPS

silane molecule was sufficiently low to promote forma-tion of polymer nodules on the silica surface.25 Origi-

nal morphologies were also achieved when the emulsionpolymerization reaction was performed in the presence of

methacryloxy propyl silane-functionalized clay colloids.26

Nanocomposite particles with an inverted core-shell

morphology (the clay platelets forming the shell of theparticles) were successfully produced by this approach.

Actually, a rapid survey of the literature on the use of

MPS in encapsulation reactions shows that only a few

academic reports have been devoted to this system.2731

Moreover, very few efforts have been done up to now todescribe the mechanism of formation of the nanocompos-ite particles. As a matter of fact, a better understanding isneeded of where is nucleation occurring and how are theseparticles formed. In addition, no data have been providedso far on the kinetic aspects of the polymerization although

previous works in the literature have demonstrated thatpolymerizations performed in the presence of mineral par-ticles may exhibit particular kinetic behaviors.3234

The aim of this paper is to give insights into themechanism of formation of polymer/silica nanocompositeparticles through emulsion polymerization using MPS ascapping agent. Following a previous work,35 the graftingof MPS was performed directly in water and the emulsionpolymerization reaction was carried out in the presenceof either an anionic or a non-ionic surfactant. In order toget a real control of the encapsulation process, we varied

the number of inorganic seed particles by changing theirsize or their mass concentration. This work mainly focuseson the effect of the MPS grafting density, the silica par-ticles size and concentration, the amount and nature ofthe surfactant on the polymerization kinetics and particlesmorphology.

2. EXPERIMENTAL DETAILS

2.1. Materials

Two kinds of silica suspensions were used: a colloidal

silica sol produced through the Stber process,36 and aconcentrated aqueous suspension of commercial silica par-ticles (Klebosol 30N50, Clariant) with an average diameterof 68 nm (as determined by dynamic light scattering) and32 wt% solid content. The reagents for synthesis of theStber silica sol: absolute ethanol (Carlo Erba), ammo-nium hydroxide (25%, Laurylab) and tetraethyl orthosil-icate (TEOS, Fluka) were of analytical grade and usedas supplied. Methacryloxy propyl trimethoxysilane (MPS,Aldrich) was used as received. The monomer, styrene(Aldrich) and the initiator, potassium persulfate (KPS,

MW = 27033 g mol1, Acros Organics), were used with-out further purification. The anionic surfactant, sodiumdodecyl sulfate (SDS, MW = 28838 g mol1, AcrosOrganics) and the nonylphenol poly(oxyethylenic) non-ionic surfactant (Remcopal NP30, MW =1540 g mol

1,a gift from CECA S.A, Paris) were of analytical gradeand used as received. Deionized water was purified by aMilli-Q Academic system (Millipore Cooperation).

2.2. Synthetic Procedures

The suspension of silica particles in a mixture of ethanoland water was prepared according to the procedure ofStber et al.36 Typically, ethanol (665 g), water (89.7 g)and concentrated ammonia (23.4 g) were introduced in a

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Nucleation of Polystyrene Latexes on Functionalized Silica Particles Bourgeat-Lami et al.

reaction flask. Then, TEOS (41.8 g) was added quicklyand the mixture was stirred at ambient temperature forseveral days. The alcoholic suspension was then cen-trifuged (Beckman Coulter, Allegra 64R centrifuge) at7,000 rpm for 10 minutes and redispersed into water withthe help of a laboratory ultrasonic bath (Branson ultrasoniccleaner 5510).

The commercial aqueous Klebosol suspension wasdiluted in absolute ethanol and centrifuged at 20,000 rpmfor 10 minutes (Beckman Coulter, Allegra 64R centrifuge)in order to remove the small particles which are present inthe original suspension.

Grafting of the silane coupling agent was performed byaddition of the coupling agent to the aqueous silica solcontaining 1 g L1 of the anionic surfactant. The mixtureswere prepared by introducing known amounts of MPS into100 mL of a 10 g L1 stock silica suspension. The dis-persions were stirred magnetically at ambient temperature

and allowed to equilibrate for 19 hours. They were nextcentrifuged and the supernatant solutions were analysedby UV spectroscopy. The amount of grafted MPS wasdetermined by difference between the total amount andthe free amount of MPS using a predetermined calibrationcurve established on silica-free suspensions of identicalcomposition.

Emulsion polymerization reactions were carried out inbatch at 70 C for up to 24 hours under nitrogen atmo-sphere. The 300 mL glass reactor fitted with a condenserwas charged with an aqueous dispersion containing knownconcentrations of grafted silica and surfactant. Degassingwas carried out for 15 minutes under gentle stirring beforeincreasing the temperature up to 70 C. Then, the initiatordissolved in 10 mL of de-ionized water and the monomerwere added at once to start polymerization. Samples werewithdrawn from the polymerization mixture at regularintervals of time and cooled down to stop polymerizationbefore analysis.

All the latexes produced were coded according to thefollowing nomenclature: A(N)X-K(S)Y-Z where A and Nstand for Anionic or Non ionic surfactants, respectively, Xis the surfactant concentration in g L1, K and S stand for

Klebosol and Stber silicas, respectively, Y (g L1) is thesilica concentration and Z is the initial MPS concentration(mol m2). For example, A1-K10-10 designates a poly-merization performed in the presence of 1 g L1 of SDSand 10 g L1 of the Klebosol silica particles grafted with10mol m2 of MPS while N3 is an emulsion polymer-ization of styrene carried out in the presence of 3 g L1

of the non ionic surfactant in the absence of silica. Thestyrene and initiator concentrations were kept constant allalong this work.

2.3. Characterizations

UV analysis was performed using an UV/VIS spectropho-tometer and quartz cells. The measurements were carried

out at the wavelength of 205 nm using predetermined cal-ibration curves. The MPS grafting density, QMPS (mol m2), was determined by titration of the supernatant solu-tions using the depletion method according to:

QMPSmolm2 =C0 CeV

Mw MSspec106 (1)

where C0 (g L

1) is the initial MPS concentration, Ce(g L1) is the MPS equilibrium concentration determinedby UV analysis, V (l) designates the volume of solution,M (g) is the mass of silica, Mw (g mol

1) is the molarmass of MPS and Sspec. (m

2 g1 is the specific surface

area of silica.Sspec. was calculated using Eq. (2) assuming that the

particles are non porous and spherical.

Sspecm2 g1 =

6000

Dn (2)

where (g cm3

) is the density of silica and Dn (nm) isits number average particle diameter.The grafting yield, which corresponds to the fraction of

MPS effectively attached to the silica surface, was deter-mined using the following formula:

Grafting yield% = 100QMPS/MPS (3)

where QMPS (mol m2) is the MPS grafting density

determined by UV analysis and [MPS] (mol m2) des-ignates the initial concentration of the silane molecule perunit surface of silica.

The monomer to polymer conversions were determinedgravimetrically. Typically, 37 g of the latex suspensionwas placed in an aluminium dish and dried to constantweight at 80 C. The diameter, Dp, of the polymer latexparticles synthesized in the absence of silica was deter-mined by dynamic light scattering (DLS) using a MalvernAutosizer Lo-C instrument with a detection angle of 90.

The morphology of the nanocomposite particles wascharacterized by transmission electron microscopy (TEM).Specimens for TEM were prepared by evaporating onedrop of dilute latex (103/106 g cm3) on a 200 mesh

formvar-coated copper electron microscope grid. The gridswere placed in the vacuum chamber of a Philips CM120electron microscope operating at 80 kV and observedunder low illumination dose.

The diameters of the polymer particles were measureddirectly from the electron micrographs. A minimum of 100particles were counted for each batch. The number aver-age diameter, Dn, was calculated using Eq. (4) where nidesignates the number of particles of diameter Di.

Dn =

niDini

(4)

The weight average diameter, Dw, was calculated from:

Dw =

niDi

4

niDi

3 (5)

434 J. Nanosci. Nanotechnol. 6, 432444, 2006


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and the polydispersity index (PDI) was given by:

PDI =DwDn

(6)

The particle number per unit volume of water (Np/L) wascalculated by the following equation:

NpL = M//6Dn

3V1019 (7)

where M(g) is the total mass of solid, (g cm3) is thedensity of the particles, Dn (nm) is the number averageparticles diameter determined from the TEM micrographs,andV(in liter) is the total volume of water.

3. RESULTS

3.1. Characterization of the Silica Sols

Figure 1 shows TEM micrographs of the Stber andKlebosol silica suspensions, respectively. Both sets of par-ticles exhibit a spherical shape and a low size distribution.The number and weight average diameters are indi-cated in Table I together with the polydispersity index

(a)

(b)

Fig. 1. TEM micrographs of: (a) The commercial fractionated Klebosol

30N50 silica sol and (b) the Stber silica.

Table I. Number average diameters, weight average diameters and

polydispersity indexes of the silica sols used in the polymerization

experiments.

Klebosol 30N50 Stber silica

Dn 830 2460

Dw 834 2476

Dw/Dn 1005 1006

(PDI=Dw/Dn). PDI values lower than typically 1.05 indi-cate that the particles are monodisperse.

3.2. Grafting of the MPS Coupling Agent

A lot of work has been done in the literature on the graft-ing of organoalkoxysilanes onto several inorganic surfacesfor applications in chromatography, catalysis, etc. Clas-sically, the grafting is performed in dried solvents andthe water for hydrolysis is that contained on the mineral

surface.17 The water may also be introduced in the reactionmedium and removed further by distillation to promotecondensation reactions as reported for instance by Philipseet al.37 Silane molecules can also be conveniently graftedin mixtures of ethanol and water by direct addition of thesilane molecule to the alcosol as reported in previous stud-ies from our group.1925 In the present work, the graftingwas done directly by simply adding the coupling agent toa dilute aqueous suspension of the silica beads in the pres-ence of surfactant. The role of the surfactant is to helpdisperse the MPS molecules in water and also to reduce

the size of the polycondensates that are susceptible to beformed in the surrounding aqueous solution.38

Because of the large surface area which commonlyoccurs in colloidal systems, the amount of adsorption canoften be determined directly from the depletion of theadjoining liquid phase, especially if strong interactions areinvolved in the process. This technique is well developedin the literature for the determination of adsorbed surfac-tant concentrations and allows the establishment of adsorp-tion isotherms. We decided to extrapolate this methodto the quantitative determination of the amount of MPS

on the silica surface without distinction on whether thesilane is simply adsorbed (as in surfactant systems) or trulygrafted. For simplicity, the MPS surface concentration willbe defined hereafter as the grafting density. The advan-tage of this technique compared to conventional titrationmethods such as the estimation of the carbon content, isthe sensibility of UV analysis which allows accurate deter-mination of very low residual concentrations in water solu-tion. The technique thus appears particularly well suitedfor low grafting densities determination which is actuallyone of the objectives of this work.

Figure 2 shows the evolution of the MPS grafting den-sity and the MPS grafting yield as a function of theinitial MPS concentration. The grafting density increaseswith increasing the silane content and reaches a plateau at



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0

0.5

1

1.5

2

2.5

3

0 5 10 15 20

0

10

20

30

40

50

60

70

80

90

100

[MPS] (moles/m)

Grafting yield (%)Grafting density (moles/m)

Fig. 2. MPS grafting density and grafting yield as a function of the

initial MPS concentration. The grafting was performed by direct addition

of MPS to a 10 g L1 aqueous silica particles suspension containing

1 g L1 SDS. The reaction was carried out at pH =93 for 19 hours

under magnetic stirring.

high concentrations while, concurrently, the grafting yielddecreases. It is worth noting that although the grafting was

performed under very different conditions, a similar curve

was reported by Bourgeat-Lami for the grafting of aerosilsilica with MPS in toluene.17

3.3. Polymerizations Performed in the Absenceof Silica Particles

Polystyrene latexes were prepared in batch through emul-sion polymerization under conventional conditions using

either a non ionic or an anionic surfactant. The reactor was

charged with the surfactant, styrene monomer, KPS, andwater in the quantities shown in Table II.

Monodisperse spherical PS particles were produced

whatever the experimental conditions. The particles diam-eter and particles number determined by DLS are reported

in Table III together with the monomer conversion. The

polymer particle number is also plotted in Figure 3 as a

function of the surfactant concentration.The curve of Figure 3 exhibits a sigmoidal shape with

three distinct slopes (1.5, 4.3, and 0.5, respectively).

Table II. Recipes for studying the emulsion polymerization of styrene with various surfactant concentrations.

Latex code

Ingredient A0.5 A1 A1.5 A2 A3 A8 N3

Water (g) 100 100 100 100 100 100 100

SDS (g) 005 01 015 02 03 08 /

NP30 (g) / / / / / / 03

Styrene (g) 10 10 10 10 10 10 10KPS (g) 005 005 005 005 005 005 005

[Surfactant] (mM)a 173 347 520 693 1040 2774 195

aBased on water. Temperature = 70 C.

Similar curves have been reported in the literature forthe emulsion polymerization of butyl acrylate using SDSas surfactant39 or for styrene emulsion polymerization withaerosol MA as surfactant.40 Considering that the cmc ofSDS in electrolyte free aqueous solution is 2 g L1 (i.e.,6.93 mM), the different parts of the curves correspondto the following regimes: surfactant concentration below,

around or above the cmc and can be explained as follows.At low surfactant concentrations, homogeneous nucleation

predominates and the overall particle number is almostindependent on the SDS concentration. As surfactant con-centration increases, there is a rise in particle number atSDS concentration close to the cmc. This clearly illus-trates that micelles are the main locus of particle formationas suggested by the Smith-Ewart micellar theory.41 Wellabove the cmc, micellar nucleation dominates and the par-ticle number follows the [S]3/5 dependence on surfactantconcentration predicted by Smith-Ewart.

3.4. Polymerizations Performed in the Presence of

MPS-Functionalized Silica Particles

(a) Effect of the MPS Grafting Density on Particles Mor-

phology.As shown in previous works, grafting of the MPScoupling agent is a prerequisite condition for success-ful formation of nanocomposite particles surrounded bypolymer.19 In order to investigate this parameter in moredepth, a set of experiments was performed using SDS assurfactant and various MPS concentrations according to

the recipes given in Table IV. The actual amount of MPSpresent on the silica surface was determined from the UVisotherm shown in Figure 2 and is reported in Table IV. Allthe polymerization were conducted under the same condi-tions, that is to say with 100 g L1 of styrene, 0.5 wt% ofKPS with respect to styrene, 1 g L1 of SDS and 10 g L1

of silica.Figure 4 shows the conversion versus time curves for

polymerizations performed in the presence of silica parti-

cles containing an increasing amount of double bonds ontheir surface. While the polymerization carried out with

un-grafted silica particles exhibits the same kinetic behav-ior as that of the reference experiment without silica (A1),all the other experiments showed a higher polymerization



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Table III. Monomer conversion, particle size and particle number for a series of polymer latexes produced by emulsion

polymerization of styrene at 70 C using various concentrations of SDS and NP30 surfactants.

Latex code

A0.5 A1 A1.5 A2 A3 A8 N3

Conversion (%) 87 97 100 100 100 100 90

Dp(nm) a 272 199 157 99 58 50 202

Np/Lb 821015 231016 491016 21017 981017 161018 211016

a Determined by DLS.b Determined from Eq. (7) by replacing Dn by Dp .

rate. It can be noticed that the lower the MPS graftingdensity, the higher the polymerization rate.

Figure 5 shows TEM micrographs of PS/silica compos-ite particles produced for increasing MPS grafting densi-ties. For the sake of clarity, micrographs were recordedat low and high magnification to better visualize the par-ticles shape. It can be seen from Figure 5 that only thepolymerization performed in the presence of un-graftedsilica particles gave rise to separate populations of silicabeads and polymer latexes with no apparent interactionsbetween them. For all the other experiments, the latexspheres showed more or less affinity for the silica surface.

For very low MPS grafting densities (typically below0.5 moles m2), irregular daisy-shaped morphologieswere produced with only a few colloids surrounding thesilica spheres in an irregular fashion. It is worth noticingalso the large size distribution of the polymer particles.When the MPS grafting density increases and reaches avalue close to e.g., 1 mole m2, most of the polymer

latexes are located around the silica particles in moreregular flower-like morphologies, each particles being sur-rounded in average by eight polystyrene petals as deter-mined by the ratio of the number of polymer particles tothe number of silica particles. With increasing further theMPS grafting density, core-shell particles were obtained asshown in Figure 5d.

Table V summarizes the particle size and size distribu-tion of the polystyrene latexes directly determined fromthe TEM micrographs. It can be seen that the diameter ofthe polystyrene latex particles decreases with increasing

1.00E+15

1.00E+16

1.00E+17

1.00E+18

1.00E+19

0.001 0.01 0.1

Slope=4.3

Np/L

Slope=1.5

Slope=0.5CMC

[SDS] (mol.L1

)

Fig. 3. Particle number as a function of surfactant concentration for

styrene emulsion polymerization at 70 C with SDS as surfactant and

1.85 mM K2S2O8 initiator.

Table IV. Recipes for emulsion polymerizations of styrene in the

presence of MPS-grafted silica particles and a constant surfactant

concentration.

Latex code [MPS] (moles/m2) QMPS (moles/m2)

A1-K10-0 0 0

A1-K10-0.1 01 01

A1-K10-0.2 02 02

A1-K10-0.5 05 035

A1-K10-1 1 065A1-K10-2 2 095

A1-K10-10 10 18

[SiO2] = 10 g L1, [SDS] = 1 g L1 , [Styrene] = 100 g L1 , KPS = 0.5 wt%

relative to styrene.

the MPS grafting density and then increases for larger

MPS concentrations to reach a value which is close to thatmeasured in the absence of silica particles. The evolution

of the polystyrene particles diameter as a function of theMPS grafting density is shown in Figure 6.

(b)Effect of the Silica Concentration. A series of experi-ments was performed with increasing silica concentrations

and under conditions giving rise to the formation offlowers in order to give insights into the mechanism of

formation of this original morphology. The size of thelatexes produced was again directly determined from the

TEM micrographs. Table VI shows the number averageparticle diameter calculated by measuring the diameter of

0

20

40

60

80

100

0 1 2 3 6 8 10

Time (hours)

Conversion(%)

Un-grafted silica

QMPS=1.9 mol/m2

QMPS=0.95 mol/m2

QMPS=0.35 mol/m2

4 5 7 9

Fig. 4. Conversion versus time curves for polymerization of styrene

performed in the presence of silica particles with increasing amounts of

MPS on their surface. QMPS indicates the actual MPS grafting density as

determined by UV analysis using the depletion method.



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Silica

Silica

Silica

Silica

Polystyrene

Polystyrene

Polystyrene

Polystyrene

(a)

(b)

(c)

(d)

Fig. 5. TEM images of silica/polystyrene composite particles obtained

for different MPS grafting densities. (a) Without MPS, (b) QMPS =

02 mol m2, (c) QMPS = 095 mol m2, and (d) QMPS =

19 mol m2. The images were recorded at low (left) and high magni-

fications (right). Scale bars: 500 nm (left) and 100 nm (right).

more than one hundred particles as a function of the silicaparticles concentration.

It can be seen in Table VI that the diameter of thepolystyrene latexes decreases with increasing the silicaconcentration. The overall number of particles thus

increases.

Table V. Number average particle diameters, weight average particle

diameters and polydispersity indexes of polystyrene latex particles syn-

thesized in the presence of MPS-grafted silica particles with increasing

MPS grafting densities.

QMPS Conversion

Latex code (mol m2) (%) Dn Dw Dw/Dn

A1-K10-0 0 100 1972 2011 1.02

A1-K10-0.1 01 96 1295 1434 1.11

A1-K10-0.2 02 95 1139 1283 1.13

A1-K10-0.5 035 86 928 1136 1.22

A1-K10-1 065 962 1105 1226 1.11A1-K10-2 095 996 1130 1169 1.03

A1-K10-10 19 985 1905 1943 1.02

[SiO2] =10 g L1 , [SDS] =1 g L1.

0

50

100

150

200

250

0 0.5 1 2

QMPS(mol.m2)

Dn(nm)

1.5

Fig. 6. Polystyrene number average particles diameter as a function of

the MPS grafting density.

Table VI. Number average particle diameters, weight average particle

diameters and polydispersity indexes of polystyrene latex particles pro-

duced in the presence of increasing concentrations of MPS-grafted silica

beads.

Latex code [SiO2](g L1) Dn Dw Dw/Dn

A1-K10-2 10 1130 1169 103

A1-K50-2 50 604 713 118

A1-K100-2 100 486 638 131

QMPS = 095mol m2. [SDS] =1 g L1 .

0

20

40

60

80

100

0 3 8 10

Time (hours)

Conversion(%)

Without silica

[SiO2]=5gL1

[SiO2]=10gL1

[SiO2]=50gL1

1 2 7654 9

Fig. 7. Conversion versus time curves for emulsion polymerizationsof styrene performed at 70 C in the absence of silica or with MPS-

grafted silica of increasing concentrations. [Styrene] = 100 g L1,

[KPS] =05 wt% relative to styrene, [SDS] =1 g L1, and QMPS =

095 mol m2.

Table VII. Experimental conditions, number average particle diame-

ters, weight average particle diameters and polydispersity indexes for

emulsion polymerization of styrene in the presence of various types of

silica particles.

Latex code Silica Dn Dw Dw/Dn

A1-K10-2 Klebosol 113.0 116.9 1.03

A1-S10-2 Stber 146.9 156.1 1.06

[SiO2] =10 g L1 , [SDS] = 1 g L1.



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(b)

(a)

200 nm

1 m

100 nm

200 nm

Fig. 8. Effect of the silica particles size on the morphology of the

silica/polystyrene composite particles. (a) Klebosol 30N50 (diameter =

83 nm) and (b) Stber silica (diameter =267 nm). QMPS

=095 mol

m2. [SiO2 = 10 g L1 and [SDS] =1 g L1.

Figure 7 shows the corresponding conversion versus timecurves. We observe that the polymerization rate increaseswith increasing the silica concentration in agreement withthe increase in the overall particle number.

(c) Effect of the silica particle size. In this series ofexperiments, we varied the diameter of the silica parti-cles while keeping a constant silica concentration. To dothis, a silica suspension in alcohol was synthesized accord-ing to the so-called Stber method and next suspended

into water by successive centrifugation/redispersion cycles.It was then grafted as mentioned above for the com-mercial aqueous silica suspension. The final characteris-tics of the composite latex suspensions are indicated inTable VII together with the experimental conditions usedfor polymerization. A flower-like morphology is achieved

(a)

(b)

Fig. 9. Effect of the surfactant concentration on the morphology of

the silica/polystyrene composite particles. (a) SDS = 1 g L1 and

(b) [SDS] = 3 g L1. QMPS =095 mol m2. Scale bars: 400 nm.

Table VIII. Experimental conditions, number average particle diam-

eters, weight average particle diameters and polydispersity indexes of

emulsion polymerizations of styrene carried out in the presence of

increasing surfactant concentrations and various silica concentrations.

Latex code [SiO2] (g L1) SDS (g L1) Dn Dw Dw/Dn

A1-K10-2 10 1 1130 1169 1.03

A2-K10-2 10 2 833 866 1.04

A3-K10-2 10 3 593 626 1.05

A2-K70-2 70 2 512 634 1.23

again for the large silica particles although the diametersof the polymer spheres are bigger than when smaller silicaparticles are used as seeds (see Fig. 8 and Table VII).

(d) Effect of the surfactant concentration. As shown inthe previous section, the surfactant plays a key role inemulsion polymerization by determining the final particlenumber. Experiments were thus conducted under varioussurfactant concentration in order to clarify the role of the

surfactant in our system. As can be seen in Figure 9,increasing the surfactant concentration for a constant sil-ica particle concentration resulted in the formation of freelatex particles that were grown separately from the silicasurface. As will be discussed later, these particles presum-ably result from the presence of micelles that may competefor capture of the growing radicals. However, increasingfurther the silica concentration enabled efficient captureof all the growing polymer even for concentrations abovethe cmc illustrating the determinant role of the seed sur-face in the nucleation mechanism. Table VIII shows thediameters and the polydispersity indexes of the resultinglatexes together with the experimental conditions used inthese experiments. It is worth noticing that the higher thesilica concentration, the higher the proportion of polymerthat is located on its surface.

(e) Effect of the Nature of Surfactant. By changing thenature of the surfactant, we expect again to significantly

(a)

(b)

Fig. 10. Effect of the nature of the surfactant on the morphology of the

silica/polystyrene composite particles. (a) SDS (1 g L1) and (b) NP30(3 g L1). QMPS =095 mol m

2.



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Table IX. Experimental conditions, number average particle diameters,

weight average particle diameters and polydispersity indexes of emulsion

polymerizations of styrene carried out in the presence of SDS or NP 30and various Klebosol silica concentrations.

[SiO2] SDS NP30Latex code (gL1) (g L1) (g L1) Dn Dw Dw/Dn

A1-K10-2 10 1 / 113.0 116.9 1.03

N3-K10-2* 10 / 3 / / /

N3-K2-2 2 / 3 295.5 299.4 1.01

QMPS = 08 mol m2 . *Flocculation after 1 hour of reaction (e.g., 10%

conversion).

influence the nucleation mechanism and generate differentparticles morphologies. Figure 10 shows the TEM imagesof the composite particles obtained using either SDS orNP30 as surfactants with silica concentrations of 10 g L

1

and 2 g L1, respectively (see Table IX). While SDS ledto the formation of flowers, the polymerization carried outwith the non ionic surfactant gave encapsulated silicas.

It is worthwhile to notice that the core-shell particles ofFigure 10b have a lower size distribution than the poly-mer latexes of Figure 10a. In addition, it must be empha-sized that the polymerization carried out in the presence of10 g L1 of silica and 3 g L1 of NP30 gave an unstablelatex after around 1 hour of reaction. This is presumably tobe related to the specific nucleation mechanism involvedin this system as will be discussed later.

4. DISCUSSION

4.1. Grafting of MPS onto Silica

from Aqueous Suspensions

It is generally admitted from previous studies in the liter-ature that under dry conditions, the grafting of organosi-lanes to mineral oxide surfaces requires high temperatureconditions (e.g., curing under vacuum or solvent refluxing)to promote condensation.

In the presence of water, the grafting involves a sig-nificantly different mechanism. The alkoxy groups ofthe organosilane first quickly hydrolyze into silane tri-

ols that can undergo self condensation.4243 Therefore,it is assumed that aqueous silylation techniques lead tothe formation of localized polysiloxane networks on thenanoparticles surface instead of a monolayer coverage.44

However, despite the fact that the grafting was performedin an aqueous environment, our results show that the max-imum surface coverage at saturation determined by UVanalysis (e.g., 2 mol m2) is of the same order of mag-nitude as the value commonly accepted for a monomolec-ular coverage (e.g., around 2.3 mol m2).17 It mustbe recalled nevertheless that as the organo alkoxysilane

is initially insoluble in water, a surfactant was intro-duced in the suspension medium for effective dispersionof the MPS molecule. Indeed, in the absence of surfactant,large silsesquioxane particles are formed that cannot be

separated from the silica surface. Silsesquioxane colloidsformation has already been reported to occur under similarconditions.38 Therefore, the surfactant is absolutely neces-sary for an accurate determination of the grafting densityas the presence of these silsesquioxane particles rendersimpossible any quantitative treatment. The low MPS graft-ing density observed under our experimental conditions

is thus presumably due to the presence of the surfactantwhich may significantly influence the grafting reaction andthe MPS surface density. At last, it is worth pointing outthat the lower the MPS concentration, the higher the graft-ing efficiency in agreement with previous studies on simi-lar systems.174546

4.2. Nanocomposite Particles Formation

A number of mechanisms have been proposed in the liter-ature for latex particle formation in emulsion polymeriza-tion systems.47 It is commonly accepted that for monomersof low water solubility like styrene and for polymeriza-tions performed well above the cmc of the soap, nucleationmainly occurs in surfactant micelles (so-called Smith-Ewart micellar nucleation theory), while below the cmcor for monomers of high water solubility, homogeneousnucleation is predominant (so-called HUFT theory basedon the assumptions of Fitch and Tsai and those of Hansenand Ugelstad). The HUFT theory assumes that polymerchains grow until they reach a critical degree of polymer-ization, z, for which they undergo self-nucleation.4849 Thefreshly nucleated polymer particles are not stable unless

they are fully covered by surfactant and will undergo lim-ited coagulation (limited because the coagulation stopswhen the surface charge and potential has reached a levelwhere the particles are stable). In the following, the termprimary particle will be used to designate a precursor par-ticle formed by micellar entry or homogeneous nucleationand resulting from the growth of a single (or at most, afew) polymer chains.

Determining which nucleation mechanism predominatesin emulsion polymerization is not straightforward. How-ever, compared to conventional emulsion recipes, the

present system appears to be far more complicated. There-fore, a fulfilled description of the mechanism of nucleationwill not be attempted here and only a few directions willbe given in order to account for nanocomposite particlesformation and polymerization kinetics.

When bare silica particles are engaged in the poly-merization reaction, polymer spheres are grown separatelyfrom the silica surface while the conversion versus timecurve is identical to that obtained without silica. Indeed, itis well known that the encapsulation of inorganic particlesby polymers necessitates a preliminary surface modifi-

cation by molecular agents able to promote favorableinteractions.45 Similar conclusions were already pointedout by Bourgeat-Lami and Lang19 and Reculusa et al.24 onsimilar systems.



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Aqueous phase

propagation

Z-mer

Above cmc

Below cmc

High MPS

grafting densityLow MPS

grafting density

Radical

capture

Aqueous phase

termination

H2C=C-C-O(CH2)3-Si

OO

O

OCH3

OH2C=C-C-O(CH2)3-Si

O O

CH3

O Propagation

+

Coagulation

Micellar entry

Surface

nucleation

SO4 + nM

Fig. 11. Schematic illustration of the main features taking place during

the formation of the silica/polystyrene nanocomposite particles.

What is more interesting in this work is the mech-anism of nanocomposite particles formation when MPSunits are present on the silica surface. This system obvi-ously presents major differences in comparison to con-ventional emulsion polymerization concerning in particularthe nucleation step which can not be neglected contraryto conventional seeded emulsion polymerization in whichthe final particle number is determined by the number ofseed particles, thus avoiding the complexities of nucleation(provided, of course, that most of the surfactant is locatedon the seed surface and that there is not enough emulsifierin the water phase to form micelles).

By analogy with classical emulsion polymerization sys-tems, nanocomposite particles formation can be describedas follows (Fig. 11). The persulfate initiator starts todecompose in the water phase giving rise to the formationof radicals. These radicals will propagate with aqueousphase monomers until they undergo one of the followingfates: (i) Aqueous phase termination or (ii) entry into amicelle or precipitation (depending on the surfactant con-centration), creating somehow a new particle. Aqueous-phase oligomers of all degree of polymerization can alsoundergo frequent collision with the surface of the silica

seed particles, and have therefore a high probability tocopolymerize with the double bonds of silica, thus gen-erating chemisorbed polymer chains in the early stagesof polymerization. These discrete loci of adsorption arepreferred to adsorb further oligomers or radicals comparedwith the bare seed surface. As a result, these discrete lociof adsorption become discrete loci of polymerization. Pro-vided that new polymer particles formation is not pro-moted, polymerization will exclusively take place at thesilica surface.

Let us now envisage what are the main factors that

will govern radical capture and (re)nucleation. First, itseems obvious that as the MPS grafting density increases,the probability of radical capture also increases so thatpolymerization on the seed surface is promoted under such

conditions. In addition, if the MPS grafting density is suf-

ficiently high to account for the formation of a large num-ber of primary particles, the formation of new islands at

the silica surface increases too and an homogeneous shellis likely to be formed around silica. The shell may resultfrom the collapsing of the growing polymer chains on

the functionalized silica surface or from the coalescence

of freshly nucleated neighboring primary particles, thislast issue being promoted by the close proximity of theseprecursor particles and the correspondingly low surface

energy. For a too low MPS concentration, in contrast, thepolymer chains form segregated domains around the silica

particles as the high interfacial energy (due to the presenceof unreacted silanol groups), does not promote spreading

of the polymer chains on the surface nor interparticles coa-lescence. A schematic picture of nanocomposite particles

formation and growth is shown in Figure 11.When considering the particle size or particle number

variation versus the MPS grafting density (Fig. 6), someinteresting features were additionally evidenced that could

help us support or refute the above mechanism. The sys-tem obviously shows some variation in particles nucleation

as the overall number of polymer spheres is not constantin the different experiments that we have presented. The

polymerization kinetics is also strongly affected as it isknown that the rate of an emulsion polymerization is pro-

portional to the particle number. Therefore, the lower theparticles size, the higher the polymerization rate. When

the silica surface contains a large amount of double bonds,

the final particle size (and therefore the final particle num-ber) is very close to that obtained in the absence of silica

particles. The silica seed particles seem to have no influ-ence on particles nucleation in this case. This may be due

to the fact that for high MPS grafting densities, the sil-ica seed particles are converted into a polymer latex in

the early stages of polymerization and since their numberis identical to that of the polymer particles formed in the

absence of silica, there are no new particles created norcoagulation occurring under such conditions. For interme-

diate MPS concentrations, in contrast, latex spheres can

be clearly identified on the seed surface. As they are alllocated around silica, and since there is in average 8 poly-mer petals per silica sphere, their size is much smaller than

the one observed for unseeded experiments under identicalconditions. These polymer latexes thus display a surface

area 5 times higher than that of the reference experimentwithout silica. This result suggests that the silica particles

are able to stabilize the growing polymer spheres and gen-erate a greater latex particle number than when SDS is

used as surfactant alone. The number of polymer spheresaround each silica particle depends not only on the MPS

grafting density but also on geometric considerations, asincreasing the silica particle size allows to introduce alarger number of polymer spheres around each individual

silica particle (see Fig. 8b). The silica seed particle ability



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to stabilize the growing latex sphere is not very surpris-ing if one considers that the silica surface is negatively

charged and can thus ensure electrostatic stabilization ofthe latex spheres nucleated on top of it. This mechanism

is expected to be less effective however when the polymercompletely recovers the seed surface, as it is the case for

high MPS grafting densities. In this situation, the over-

all particle number (and therefore the total surface area),decreases to reach the value given by the reference exper-iment without silica. More surprising however is the fact

that polymer spheres of small dimensions are formed forvery low MPS grafting densities even though they are not

all surrounding silica, most of them being present as freepolymer latexes with no particular affinity for the min-

eral surface. As before for higher MPS grafting densities,these particles have been presumably nucleated on the sil-

ica surface in the earlier stages of polymerization. How-ever, contrary to the previous situation, they might have

desorbed later from the surface. Indeed, it is known thatsiloxane bonds can form and break reversibly depending

on the experimental conditions and more precisely on pH.This scheme would be promoted by the nature of the inter-

facial layer, the MPS units being irregularly distributedon the surface in the form of oligomeric regions whichwould be highly sensitive to hydrolysis. It is clear that it

is not possible to distinguish between a regular polysilox-ane film from islands of poly(MPS) by the techniques

described in this paper. However, the relatively low MPSgrafting density determined experimentally strongly sup-

ports the hypothesis of a patchy-like configuration of thegrafted MPS molecules. It is noteworthy to mention that

the formation of siloxane nanosized clusters has alreadybeen pointed out by several authors for the grafting of

MPS on silica surfaces.5051 To determine whether the latexspheres were first produced in solution (according to con-

ventional nucleation mechanisms) and adsorbed further onthe seed surface or if they were directly nucleated on the

seed surface is not straightforward. However, it is likelythat primary particles may be present since the beginning

of the reaction in both places and continue to grow until

they become mature and identifiable particles. It shouldbe noticed again that the formation of small particles forlow MPS grafting densities is supported by the kinetics

data which indicate that the polymerization rate increaseswith decreasing the amount of MPS on the silica surface

(see Fig. 4).The stabilizing role of the silica particles is clearly

illustrated in Figure 12 which shows the evolution of thepolymer particle number as a function of the number ofsilica particles. While for low silica particles number, the

number of polymer latexes is close to that of the refer-

ence experiments, this number increases with increasingthe number of seed particles and reaches a value indepen-dent on the surfactant concentration. Although we would

expect that the contribution of the silica surface charges to

1E+16

1E+17

1E+18

1E+19

1E+14 1E+15 1E+16 1E+17 1E+18

Nplatex/L

[SDS]=2g/L

Np SiO2/L

[SDS]=1g/L

Fig. 12. Evolution of the number of polymer particles as a function

of the silica particle number for a series of emulsion polymerization of

styrene performed in the presence of various types and concentration of

silica particles. QMPS =095 mol m2.

the stability of the particles would be much less importantwhen the contribution of micellar nucleation is not negli-

gible, (especially in the batch reactions where the emulsi-fier concentration is close or largely above the cmc), thecurves of Figure 12 indicate on the contrary that even forpolymerizations performed above the cmc, the number ofpolymer particles is mainly controlled by the number ofsilica particles introduced in the suspension medium.

The results of the influence of the silica concentrationon particles morphology are also quite instructive. It seemsobvious from our data that the MPS grafting density isnot the only parameter that promotes capture of the grow-ing radicals. Indeed, increasing the silica concentration

enabled the exclusive formation of polymer spheres aroundsilica whereas renucleation occurred when the seed con-centration was too low. In other words, the larger the silicaparticles number, the higher the amount of polymer ontheir surface and the smaller the diameter of the nucle-ated latex particles (compare for instance A2-K10-2 andA2-K70-2 in Table VIII). Secondary nucleation in con-ventional emulsion polymerization is most often relatedto insufficient rates of capture. We can thus assume thatirrespective of the mechanism of particle formation, renu-cleation will occur as long as there is not a sufficient num-

ber of particles of sufficient size to capture all z-mers.Increasing the silica seed concentration allows creating alarger polymer surface area (as the growing latexes have asmaller size) which enables to avoid renucleation as moresoap is adsorbed on this freshly created surface and less isavailable in the water phase to stabilize new particles.

Also of major interest was the investigation of the effectof the nature of the surfactant on the particles morphology.While the polymerizations performed in the presenceof SDS and moderate MPS grafting densities alwaysafforded flower-like morphologies, those performed with

NP30 gave rise to a core-shell morphology. This is obvi-ously to be related to changes in interfacial properties(and thus in local monomer concentrations) due to thepresence of adsorbed surfactant molecules. Indeed, it is



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well known that non ionic emulsifiers with ethylene oxideunits strongly adsorb onto silica in aqueous solution.52 Theadsorbed surfactant molecules form a bilayer on the sur-face which interfacial region is able to solubilize monomermolecules as micelles do in conventional emulsionpolymerization (so-called admicellization/adsolubilizationbehavior). The capture of the growing radical by this inter-

facial region is thus promoted giving rise to polymer shellformation even for very low MPS grafting densities.

In summary, all the experimental data presented in thisarticle indicate that MPS-functionalized silica particlesallows to nucleate polymer latex spheres on their sur-face giving rise to the formation of original morphologies.The diameter and therefore the number of polymer latexesformed around silica, depend on the MPS grafting density,the seed particle number and the surfactant concentration.

The question of whether or not a primary particle nucle-ated on the surface of silica particles of very low MPS

grafting densities can desorb and end up in the aqueousphase is hard to answer. However, the very high poly-merization rate observed under such conditions and thecorresponding low polymer particles size support thisassumption.

5. CONCLUSION

Silica/polystyrene nanocomposite latexes were synthesizedthrough emulsion polymerization under batch conditions.By varying the surface density of the reactive double bonds

or the nature of the surfactant, either flower-like or core-shell morphologies were obtained. All the results reportedin this article support the idea that the nucleation is takingplace through the capture of the growing radicals by thesilica particles. As the latter carry negative charges on theirsurface, they are able to stabilize the growing latex sphereswhich number (and therefore overall surface area) is muchlarger than in the absence of silica. As the rate in emulsionpolymerization is proportional to the particle number, thekinetics of polymerization was strongly accelerated underthese conditions.

Acknowledgments: The authors thank Christian Novatand Nicolas Tissier (LCPP, Villeurbanne) for their greathelp in TEM analysis.

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Received: 13 April 2005. Revised/Accepted: 12 July 2005.
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J. Nanosci. Nanotechnol. 2006, 6, 432-444.pdf

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