Synthesis of polystyrene/silica composite particles …

1 Journal of Colloid and Interface Science 310 (2007) 112 of polystyrene / silica composite particles by soap-free emulsionpolymerization using positively charged colloidal silicaJeongwoo Leea, Chang Kook Hongb, Soonja Choea,SangEunShima, aDepartment of Chemical Engineering, Inha University, 253 Yonghyundong, Namgu, Incheon 402-751, South KoreabDigital Printing Division, Samsung Electronics Co., Ltd., 416 Maetandong, Yeongtongu, Suwon, Gyeonggido 443-742, South KoreaReceived 15 July 2006; accepted 6 November 2006 Available online 15 February 2007 AbstractCore- 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 .

2 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 : polystyrene ; silica ; Nanocomposite; Soap-free emulsion polymerization; Core shell1. IntroductionRecently, 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.

3 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 Shim).loidal silica at the beginning of heterogeneous polymerizations[11 16]such as emulsion, dispersion, and suspension 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.

4 To improve the encapsulation effi-ciency, , 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), wt% relative to polymer, is adsorbed. The use of an initiatoroppositely charged to silica enhances the adsorption efficiencyup to 40 wt%[13].

5 0021-9797/$ see front matter 2006 Elsevier Inc. All rights Lee et al. / Journal of Colloid and Interface Science 310 (2007) 112 120113 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 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.

6 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 , 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+ 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 MaterialsStyrene (99%, Kanto Chemical, Japan) was purified usingan inhibitor removal column (Aldrich, USA) and stored at 5 C prior to use.

7 Potassium persulfate (KPS, Aldrich, USA)was used as an initiator without further purification. Positivelycharged colloidal silica (NANOS-AS30) having a zeta potentialof+ 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 1. Mechanism of particle formation and growth of polystyrene / silica com-posite PolymerizationThe soap-free emulsion polymerization of styrene with sil-ica sol was conducted in doubly distilled water with KPS as anTa b l e 1 Average diameter and coefficient of variation of size of polystyrene / silica composite particles in each conditionRunConcentrationof silica (wt%)Addition timeof silica (min)Averagediameter ( m)Coefficient ofvariation (%)Conversion(%) , , , , , bimodal114J.

8 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 wt% relative to styrene: (a) 0,(b) 10, (c) 20,(d) 30, and (e) 60 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 g, 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 inTa -ble 1).

9 The net amount of silica was varied from ( 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 inTable 1. After comple-tion of the polymerization, the colloidal solution was rinsed offwith distilled water and methanol in order to remove CharacterizationA 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 ) and measurementwas performed on a zeta-potential analyzer (Zetasizer4 1308,Malvern).

10 The migration behavior of the polystyrene /silicacomposite particles in distilled water was monitored by measur-ing the change in the backscattering of incident monochromaticlight ( =880 nm) from the suspension employing an opticalanalyzer, Turbiscan LAB (Formulaction, France). Suspensionin flat-bottomed cylindrical glass tubes (height 70 mm, exter-J. Lee et al. / Journal of Colloid and Interface Science 310 (2007) 112 120115 Fig. 3. Particle size distributions of polystyrene / silica composite particles withdifferent addition times of silica at wt% relative to styrene: (a) 0, (b) 10,(c) 20, (d) 30, and (e) 60 diameter mm) was placed in the instrument and thebackscattering or transmission of light from suspensions wasthen periodically measured along the height axis with every40- m interval at room temperature.