Journal of Colloid and Interface Science 310 (2007) 112–120www.elsevier.com/locate/jcis

Synthesis of polystyrene/silica composite particles by soap-free emulsionpolymerization using positively charged colloidal silica

Jeongwoo Lee a, Chang Kook Hong b, Soonja Choe a, Sang Eun Shim a,∗

a Department of Chemical Engineering, Inha University, 253 Yonghyundong, Namgu, Incheon 402-751, South Koreab Digital Printing Division, Samsung Electronics Co., Ltd., 416 Maetandong, Yeongtongu, Suwon, Gyeonggido 443-742, South Korea

Received 15 July 2006; accepted 6 November 2006

Available online 15 February 2007

Abstract

Core-polystyrene/shell-silica nanocomposite particles are synthesized by simple soap-free emulsion polymerization employing positivelycharged silica sol. The polymerization is initiated with conventional anionic KPS. It is found that the silica sol should be added after the ini-tiation and nucleation of the soap-free emulsion polymerization in order to obtain colloidally stable composite particles. The excess amount ofsilica promotes the formation of secondary particles, giving a broader particle size distribution. Above 10 wt% silica relative to styrene, the disper-sion stability of the composite particles in water is significantly improved, showing a reduced sedimentation rate. The silica particles of averagediameter 25 nm form almost a monolayer shell of approximately 30 nm thickness on the polystyrene particles. The incorporation of silica particlesalso gives rise to the enhancement of thermal stability when the silica concentration exceeds 10 wt% due to the strong interaction between silicaand polystyrene molecules.© 2006 Elsevier Inc. All rights reserved.

Keywords: Polystyrene; Silica; Nanocomposite; Soap-free emulsion polymerization; Core–shell

1. Introduction

Recently, the synthesis of polymer/inorganic composite par-ticles has received much attention because such hybrid par-ticles can possess combined properties of both incorporatedinorganic materials and base polymers. A wide variety of col-loidal inorganic materials has been used for polymer-based hy-brid colloidal particles, including titanium dioxide [1], copperoxide [2], magnetic oxide [3], aluminum hydroxide [4], alu-mina [5], clay [6], and carbon black [7]. Among them, sil-ica is the most studied model system. Silica is manipulatedto form a shell or core layer for a desired purpose. Silica-containing hybrid polymer particles could embody various col-ors and have excellent physical reinforcement and thermal re-sistance by means of silica. Therefore they have been appliedto thermal insulation, bioactive supports, paints, drug delivery,and composite materials [8–10]. These polymer/silica compos-ite particles are generally produced by incorporation of col-

* Corresponding author. Fax: +82 32 872 0959.E-mail address: seshim@inha.ac.kr (S.E. Shim).

0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2006.11.008

loidal silica at the beginning of heterogeneous polymerizations[11–16] such as emulsion, dispersion, and suspension polymer-ization.

The synthesis of well-developed silica and polymer hybridparticles is attributed to the following causes: (1) compatibilitybetween silica and polymer, (2) an electrostatic attraction, and(3) an acid–base interaction. To improve the encapsulation effi-ciency, i.e., the compatibility, the surface of silica particles is of-ten modified using silane coupling agents [17,18]. On the otherhand, utilizing the negative surface charge of silica particles,a positive surface charge is endowed to the polymer particles tomaximize an electrostatic attraction between silica nanoparti-cles and polymer particles. A cationic initiator, azobisisobutyr-amidine dihydrochloride (AIBA), has been successfully usedfor the capsulation of negatively charged silica particles bydispersion [13,19] or emulsion polymerization [10]. When aneutral initiator, 2,2′-azobisisobutyronitrile (AIBN), is used in-stead, only a small amount of silica (negatively charged), ca.1 wt% relative to polymer, is adsorbed. The use of an initiatoroppositely charged to silica enhances the adsorption efficiencyup to 40 wt% [13].

J. Lee et al. / Journal of Colloid and Interface Science 310 (2007) 112–120 113

Auxiliary comonomers such as 4-vinylpyridine (4VP) [12,20] and 1-vinylimidazole (1-VID) [21,22] have been known toform a strong acid–base interaction with acidic (due to hydroxylgroups) silica particles.

Soap-free emulsion polymerization is frequently used tosynthesize such inorganic/polymer hybrid particles, since it isadvantageous in terms of no contaminants being involved andbeing environmentally benign. The typical formulation of soap-free emulsion polymerization simply consists of monomer andwater-soluble initiators, where inexpensive anionic initiatorssuch as potassium persulfate (KPS) and ammonium persulfate(APS) are typically used. However, a cationic initiator, AIBA,should be used to prepare silica/polymer composite particles,since silica is normally negatively charged. In the early stage ofsoap-free emulsion polymerization, charged growing particlesappear by aggregation of the surface-active oligoradicals, andthey are self-stabilized by an electrostatic mechanism [23–25].The surface charge of particles originates from the initiator de-composition [26].

The motivation of the current research was to use the in-expensive anionic initiator KPS to prepare silica/polystyrenenanocomposite particles by soap-free emulsion polymerization.Therefore, a silica sol with a positive surface charge was cho-sen. The silica sol dispersed in water (25 wt%) has a zeta po-tential of +8.2 mV, an average particle size of 25 nm, and aspecific surface area of 150 m2/g. The polymerization char-acteristics, ultimate particle morphology, aqueous dispersionstability of composite particles, and size and size distributionare investigated by varying the silica addition time and silicaconcentration.

2. Experimental

2.1. Materials

Styrene (99%, Kanto Chemical, Japan) was purified usingan inhibitor removal column (Aldrich, USA) and stored at−5 ◦C prior to use. Potassium persulfate (KPS, Aldrich, USA)was used as an initiator without further purification. Positivelycharged colloidal silica (NANOS-AS30) having a zeta potentialof +8.2 mV, an average diameter of 25 nm, and a specific sur-face area of 150 m2/g was donated by ABCNanoTech Co., Ko-rea and used without any surface modification. The as-receivedsilica sol dispersed in water (25 wt%) had a pH of 4.0.

Fig. 1. Mechanism of particle formation and growth of polystyrene/silica com-posite particles.

2.2. Polymerization

The soap-free emulsion polymerization of styrene with sil-ica sol was conducted in doubly distilled water with KPS as an

Table 1Average diameter and coefficient of variation of size of polystyrene/silica composite particles in each condition

Run Concentrationof silica (wt%)

Addition timeof silica (min)

Averagediameter (µm)

Coefficient ofvariation (%)

Conversion(%)

Remarks

1 12.5 0 5.00 138.0 n/a Collapsed, bimodal2 12.5 10 0.64 75.3 86.3 Stable, bimodal3 12.5 20 0.71 43.7 84.6 Stable, bimodal4 12.5 30 1.01 36.4 85.1 Stable5 12.5 60 0.77 66.0 84.3 Stable6 0 n/a 0.55 13 93.2 Stable7 7.5 30 0.70 32.3 63.8 Stable, bimodal8 10 30 0.55 23.2 70.4 Stable9 15 30 0.55 61.6 88.2 Stable, bimodal

114 J. Lee et al. / Journal of Colloid and Interface Science 310 (2007) 112–120

Fig. 2. SEM microphotographs of polystyrene/silica composite particles with different addition times of silica 12.5 wt% relative to styrene: (a) 0, (b) 10, (c) 20,(d) 30, and (e) 60 min.

initiator. The polymerization was carried out in a 500-ml three-neck round-bottom flask with stirring of 300 rpm under nitro-gen at 70 ◦C. A quantity of 200 g distilled water was pouredinto the flask and 20 g styrene, 10 wt% relative to the medium,was charged. When the temperature of reactants in the reactorreached 70 ◦C, the aqueous solution of KPS was added to thereaction system. The amount of KPS was 0.1 g, 0.5 wt% rela-tive to styrene. After initiation of the polymerization, the silicawas injected to the reaction system. For comparison, a certainamount of silica was added before the initiation (run 1 in Ta-ble 1). The net amount of silica was varied from 7.5 (1.5 g)to 15 (3 g) wt% relative to styrene and the injection timesof silica were 5, 10, 30, and 60 min after the initiation. Therecipe and the parameters are given in Table 1. After comple-tion of the polymerization, the colloidal solution was rinsed offwith distilled water and methanol in order to remove unreactedspecies.

2.3. Characterization

A Hitachi SEM (scanning electron microscope) S-4300 anda Philips TEM (transmission electron microscope) CM200were used to observe the morphology of polystyrene/silicacomposite particles. The average particle diameter and the co-efficient of variation (Cv) of particle diameter were measuredby a particle size analyzer (LS230, Beckman Coulter, USA).To measure the zeta potential, the rinsed composite particleswere redispersed in water (pH of nearly 7.0) and measurementwas performed on a zeta-potential analyzer (Zetasizer4 1308,Malvern). The migration behavior of the polystyrene/silicacomposite particles in distilled water was monitored by measur-ing the change in the backscattering of incident monochromaticlight (λ = 880 nm) from the suspension employing an opticalanalyzer, Turbiscan LAB (Formulaction, France). Suspensionin flat-bottomed cylindrical glass tubes (height 70 mm, exter-

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Fig. 3. Particle size distributions of polystyrene/silica composite particles withdifferent addition times of silica at 12.5 wt% relative to styrene: (a) 0, (b) 10,(c) 20, (d) 30, and (e) 60 min.

nal diameter 27.5 mm) was placed in the instrument and thebackscattering or transmission of light from suspensions wasthen periodically measured along the height axis with every40-µm interval at room temperature. The results are presentedas the sedimentation profile, i.e., Δ backscattering versus timealong the sample height. The rinsed composite particles were

dried in a vacuum oven and thermogravimetric analysis wasperformed on TGA Q50 (TA Instrument, USA). Conversionwas measured gravimetrically.

3. Results and discussion

3.1. Addition time of silica

The particle formation mechanism was addressed in Sec-tion 1. The existence of large, oppositely charged species inthe polymerization system causes aggregation with the initiallyformed charged oligomers. These aggregates influence parti-cle nucleation greatly. The surface potential of the particlesis decreased by the oppositely charged ions and the particleaggregation is facilitated. Furthermore, particle growth occursmore rapidly with particle agglomeration and is promoted bythe presence of the oppositely charged ions. Thus, the additionof oppositely charged surfactants controls the aggregation be-havior to produce micrometer-sized polymer particles [27,28].For example, Konno and co-workers [29,30] synthesized fairlymonodisperse polystyrene microspheres by adding a cationicsurfactant, cetyltrimethylammonium bromide (CTAB), in thesoap-free emulsion polymerization initiated by an anionic ini-tiator, KPS. They added CTAB 0.5–3.0 min after the initiation.The particle size was marginally influenced by the CTAB ad-dition time and the particle size distribution became broader inthe presence of CTAB, since the agglomeration took place for alonger period of time. This principle was extended to the use ofan amphoteric surfactant, dimethyldodecylbetaine, which pos-sesses both cationic and anionic moieties [30,31].

The formation of silica-adsorbed polystyrene composite par-ticles based on the soap-free emulsion polymerization mech-anism is illustrated in Fig. 1. It is noted that the silica wasadded after the initiation. The net amount of silica was fixedat 12.5 wt% relative to styrene monomer to find the appropriatetime for the silica addition. The silica addition time was var-ied from 0 to 60 min after the initiation of the polymerization.Fig. 2 represents the morphology of the final silica/polystyrenehybrid particles with varying silica addition time. When the sil-ica sol is present at the beginning of the polymerization, 5.0-µmparticles with broad size distribution, 138% Cv, are prepared(Fig. 2a). However, the particles have insufficient colloidal sta-bility to give collapsed hollow hemisphere morphology. Whenthe silica is added after initiation and nucleation, i.e., forma-tion of aggregates, colloidally stable microspheres containingsilica at the surface are obtained, as shown in Figs. 2b–2e.The size distribution in Fig. 3 reveals that the ultimate particlesize is slightly affected by modulating the silica addition time,whereas the size distribution has a significant dependence. Theaverage particle size (µm)/Cv (%)’s are 0.64/75.3, 0.71/43.7,1.01/36.4, and 0.77/66.0 at the silica addition times of 10,20, 30, and 60 min, respectively. When the silica is presentat the beginning of polymerization, a bimodal size distribu-tion arising from the irreversible particle agglomeration occursat equal population intensity. Such an agglomerated particleis readily witnessed on the SEM microphotograph in Fig. 2a.When the silica is added after initiation and nucleation (after

116 J. Lee et al. / Journal of Colloid and Interface Science 310 (2007) 112–120

Fig. 4. SEM microphotographs of polystyrene/silica composite particles with different concentrations of silica injected after 30 min from the initiation: (a) 7.5,(b) 10, (c) 12.5, and (d) 15 wt% relative to styrene.

10 min), such large agglomerated particles of nearly 10 µmtend to disappear. At 30 and 60 min, unimodally distributedpolystyrene/silica composite particles are achieved. Althoughthe size variation is not significant, the average particle sizetends to increase as the silica addition time becomes later. Itis expected that the aggregation of growing oligoradicals is hin-dered if oppositely charged silica sol is present at the point ofinitiation and nucleation, resulting in unstable collapsed mor-phology. Once the polymerization system starts to nucleatewithout an exterior barrier species, the cationic silica parti-cles promote aggregation of anionic oligoradicals dissolvedin aqueous medium. Therefore, it is recommended that oppo-sitely charged silica sol be added after nucleation of soap-freeemulsion polymerization. The optimum silica addition time is30 min, judged from the average particle size and particle sizedistribution.

3.2. Varying concentrations of silica

Increasing amounts of silica from 7.5 to 15 wt% relative tostyrene were added at 30 min after initiation, as determined inthe previous section. The morphology and particle size distrib-ution of the polystyrene/silica composite particles are shown inFigs. 4 and 5, respectively. At these silica concentrations, col-loidally stable polystyrene/silica composite microspheres areobtained. It is noted that polystyrene particles prepared in theabsence of silica have diameter 0.55 µm and Cv 13% (Run 6 inTable 1). With varying silica concentration from 7.5 to 15 wt%,overall particle sizes are marginally affected. At a high silica

concentration, more silica particles are adsorbed onto the sur-faces of growing polystyrene particles, and in turn the surface-adsorbed silica attracts more polystyrene precursors by meansof electrostatic attraction. Finally, the size of ultimate primarycomposite particles grows to nearly 2.0 µm diameter at 15 wt%silica. Meanwhile the small secondary particles of 0.5 µm areproduced over a certain concentration of silica. After the sil-ica adhering on the surface of the primary polystyrene particlesreaches a certain level, the secondary particles can be gener-ated by the remaining silica. If all the silica particles added tothe polymerization system are adsorbed, the formation of sec-ondary particles can be avoided. However, the amount of silicaadsorbed on the polystyrene particles does not make a greatdifference even though the amount of added silica is increased(see TGA analysis), indicating that the birth of secondary parti-cles is attributed to the excess silica staying in the polymeriza-tion medium. Therefore, the particle size distribution at silica15 wt% is not narrow but bimodal, as seen in Table 1 andFigs. 4d and 5d. It is noted that particle size data (Nos. 7–9)in Table 1 are somewhat misleading, since the size representsa simple overall average without considering the primary andsecondary particles.

In addition, the final conversions in the presence of variousamounts of silica are listed in Table 1. For all polymerizationsin the presence of silica the final styrene conversion was lowercompared with the silica-free run. Obviously, the polymeriza-tion mechanism is influenced by the adsorbed silica spheres. Inaddition, the conversion tends to increase with the amount ofsilica since the silica acts as an additional stabilizer.

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Fig. 5. Particle size distributions of polystyrene/silica composite particles withdifferent concentrations of silica injected after 30 min from the initiation: (a) 0,(b) 7.5, (c) 10, (d) 12.5, and (e) 15 wt% relative to styrene.

3.3. Dispersion stability of polystyrene/silica compositeparticles

In this experiment, Turbiscan was usefully employed to in-vestigate the sedimentation behavior of the polystyrene/silicacomposite colloidal suspensions as a measure of the suspen-sions’ stability in water. The sedimentation behavior was mon-itored every 1 h and backscattering intensity profiles vs theheight in the sample at different times were obtained (Fig. 6).The interpretation of the backscattering profiles uses the changein the light backscattering caused by the sedimentation of the

particles occurring over a sample cell. When sedimentationtakes place in the suspension, the backscattering profiles varywith the height of the sample with time. On the other hand, ifthe dispersion is stable, no discernible change in the backscat-tering profile with time is observed over an entire sample cell.It is natural that sedimentation behavior of a colloidal system isaffected by the surface nature of a solute. The case of the sus-pension containing particles made of polystyrene and 7.5 wt%silica shows a representative example of poor dispersion stabil-ity causing sedimentation (Fig. 6a). Since the particles precipi-tate from the top layer, the concentration of particles graduallydecreases at the top, permitting more light to transmit throughthe suspension medium. This causes a stepwise decrease ofbackscattering intensity along the height of the cell. The step-wise interval between the profiles is proportional to the rateof sedimentation of suspended particles. At the same time, thebackscattering intensity at the bottom of the cell monotonicallyincreases due to the increase in the concentration of particleswith sedimentation. On the other hand, silica concentrationsabove 10 wt% greatly enhance the dispersion stability of thecomposite particles in water. The sedimentation takes place ata low rate, judged from the narrow profile intervals at the toplayer of the cell and the negligible increase in the backscatter-ing intensity at the bottom. The improved dispersion stabilityis related to the variation of the surface charge of the parti-cles.

The state of surface charge with the incorporated silicaamount is investigated by measuring the zeta potential ofpolystyrene/silica particles suspended in water (Fig. 7). Thezeta potentials of bare silica sol and bare polystyrene parti-cles prepared by KPS in the absence of silica are +8.2 and−41.8 mV, respectively. With increasing silica amount from7.5 to 15 wt%, the zeta potential linearly increases from −42.5to −26.2 mV. Compared with the zeta potential value of barepolystyrene particles (−41.8 mV), the value of bare silica ismuch weaker (+8.2 mV). Therefore, the composite particlesstill have negative zeta potential values. Another reason for thenegative zeta potential of the composite particles would be theincomplete coverage of polystyrene particles by silica, as seenin Figs. 8a–8b. Although the magnitude of the zeta potentialis reduced with the silica amount, improved dispersion sta-bilization is achieved from the balance between the negativepolystyrene particles and the positive silica sol particles.

3.4. Adsorption of silica on polystyrene particles

According to the analysis results from SEM micropho-tographs, particle size distribution, and dispersion stability, theoptimum production conditions for the polystyrene/silica com-posite particles are as follows: 12.5 wt% net amount of silicarelative to styrene and injection of silica at 30 min after ini-tiation. Fig. 8 represents the TEM microphotographs of thePS/silica composite particles synthesized at the optimum condi-tions. The adsorbed silica particles are clearly seen at the exte-rior of the particles. The thickness of the silica layer is approx-imately 30 nm. Considering the average size of silica sol par-ticles used, i.e., 25 nm, silica particles form a monolayer shell.

118 J. Lee et al. / Journal of Colloid and Interface Science 310 (2007) 112–120

Fig. 6. Backscattering intensity (Y axis) profiles along the sample height (X axis) at every 1 h. The polystyrene/silica composite particles are suspended in water atroom temperature. The composite particles are produced with different concentrations of silica injected after 30 min from the initiation: (a) 7.5, (b) 10, (c) 12.5, and(d) 15 wt% relative to styrene.

Fig. 7. Zeta potential of polystyrene/silica composite particles with differentconcentrations of silica in water: (2) bare polystyrene particles, (") poly-styrene/silica composite particles, and (Q) bare silica particles.

Although there is a 5-nm discrepancy between the two values,the difference would be caused by the average value, which isnot much different. The TGA analysis in Fig. 9 reveals that themain weight loss of the particles is monitored in the range of380–450 ◦C by the thermal decomposition of polystyrene. Af-ter the complete combustion of polystyrene above 500 ◦C, theresidual weight is considered to be the amount of incorporatedsilica. The silica amounts are 12.8, 14.2, 13.9, and 15.1 wt%for the composite particles prepared with 7.5, 10.0, 12.5, and15.0 wt%, respectively. As the added silica is increased twofoldfrom 7.5 to 15 wt%, the amount of adsorbed silica is only in-creased from 12.8 to 15.1 wt%, meaning that the adsorption iscontrolled by the degree of surface charge of bare polystyreneparticles. At 15 wt% added silica, a significant amount of silicais still present in the polymerization medium, and the remainingsilica accelerates the generation of small secondary particles, asproven in Fig. 4d. In addition, the TGA curves shift to the rightin the decomposition region when the silica content exceeds10 wt%, because of the strong interaction between polystyreneand silica. Therefore, the composite particles are reinforced interms of thermal stability by the adsorption of silica when thesilica content is high [10].

J. Lee et al. / Journal of Colloid and Interface Science 310 (2007) 112–120 119

Fig. 8. TEM micrographs of polystyrene/silica composite particles with 12.5 wt% silica relative to styrene injected after 30 min from the initiation: (a and a′)exterior, (b and b′) cross section.

Fig. 9. TGA curves of polystyrene/silica composites with different concentra-tions of silica injected after 30 min from the initiation.

4. Conclusion

Core-polystyrene/shell-silica nanocomposite particles aresynthesized by simple soap-free emulsion polymerization witha positively charged silica sol (+8.2 mV zeta potential). Thepolymerization is initiated with inexpensive and conventionalanionic KPS. It is found that the presence of silica at the begin-ning of the polymerization results in the formation of collapsedhollow hemisphere morphology of the composite particles. Thesilica sol should be added after the initiation and nucleation ofthe soap-free emulsion polymerization in order to obtain col-loidally stable composite particles. The excess amount of silicapromotes the formation of a second particle generation, givinga broader particle size distribution. The addition of silica affectsthe particle size distribution rather than the size of the compos-ite particles. Above 10 wt% silica relative to styrene, dispersionstability of the composite particles in water is significantly im-proved, showing a reduced sedimentation rate as observed by anovel optical analyzer, Turbiscan. The silica particles of aver-age diameter 25 nm form almost a monolayer shell of thicknessapproximately 30 nm on the polystyrene particles. An almostsimilar amount of silica, 12.8–15.1 wt% relative to polystyrene,

120 J. Lee et al. / Journal of Colloid and Interface Science 310 (2007) 112–120

is adsorbed, although the initially added silica is varied twofold.The incorporation of silica particles also gives rise to the en-hancement of thermal stability when the silica concentrationexceeds 10 wt%, due to the strong interaction between silicaand polystyrene molecules.

Acknowledgment

This work was financially supported by a Korea ResearchFoundation Grant funded by the Korean government(MOEHRD) (D00028).

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