pubs.acs.org/Langmuir

Fabrication of Polymer Microspheres Using Titania as a Photocatalyst and

Pickering Stabilizer

Xiaomei Song, Yongliang Zhao, Haitao Wang,* and Qiangguo Du*

Key Laboratory of Molecular Engineering of Polymers of Ministry of Education,Department of Macromolecular Science, Fudan University, Shanghai 200433, PR China

Received November 27, 2008. Revised Manuscript Received January 16, 2009

Facile photocatalytic emulsion polymerization was developed to fabricate polystyrene (PS) micro-spheres using a transparent anatase titania hydrosol both as a photocatalyst and stabilizer. Underthe appropriate conditions, PS microspheres with a well-defined particle size distribution can be easilyproduced from 100 to 830 nm. The effects of cross-linking agent ethylene glycol dimethacrylate (EGDMA)and coupling agent acrylic acid (AA) on the particle size and the size distribution of PS microsphereswere investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), andother characterization means. It is proven that EGDMA and AA play importance roles in the morphologyof microspheres. In addition, AA bonds a large number of titania nanoparticles on the surface of PSmicrospheres because its carboxyl group forms inorganic armored polymer microspheres. This interfacialinteraction between titania nanoparticles and PS chains causes the elevated glass-transition temperatureof microspheres.

Introduction

It is well known that polymer microspheres havespecial properties, such as surface effects, volumeeffects, magnetic effects, and perfect biocompatibility,which form the basis of many applications.1 A considerableamount of work had been dedicated to preparingdiameter-controlled microspheres with good monodispersity.Various polymerization methods are utilized in thisprocess, such as some traditional methods of emulsionpolymerization, dispersion polymerization, and suspensionpolymerization and some new methods of micro-emulsion polymerization, reaction-induced phase separation,and so on.1,2

About a century ago, a novel method called Pickeringemulsion was proposed. Pickering3 and Ramsden4

described paraffin-water emulsions with solid colloids,which generate self-assembly at the interface betweenthe two immiscible phases, inhibiting the coalescenceof the emulsion drops. Pieranski5 reported the theoreticalassembly behavior of the colloidal particles at theoil/water interface, which was determined by a decreasein the total free energy. The stability of the emulsions dependson the particle size, particle-particle interaction, andparticle-water and particle-oil interactions.5,6 Withthese thoughts in mind, many have reported various colloidal

systems with particles of different size and surfacechemistry.6-10

Recently, Pickering emulsions stabilized by solid particleshave increasingly attracted interest.11-16 The emulsion colloi-dosomes17 have been used as polymerization vessels forpreparing novel structures. Percy et al.18 synthesized vinylpolymer-silica nanocomposite particles using commercialalcoholic silica sols. The research group of Bon preparedpolymer-clay composites and polymer-titania hybrid hol-low microspheres via inorganic filler-stabilized Pickeringemulsion polymerization.19,20

Titania nanoparticles as one of the most studied semicon-ductors have attracted a great amount of attention forexhibiting various properties such as a high refractiveindex, UV light absorption, and photocatalysis. Titania hasbeen widely used in the photoinduced decompositionof organic pollutants in water and in air21 and forphotoelectrochemical solar cells.22,23Other researchon titania

*To whom correspondence should be addressed. E-mail: wanght@fudan.edu.cn; qgdu@fudan.edu.cn. Phone: +86 21 6564 2392. Fax:+86 21 6564 0293.

(1) Ma, G. H.; Su, Z. G. Polymer Microspheres Materials, 1st ed.;Chemical Industry Press: Beijing, 2005; Chapter 2.

(2) Kawaguchi, H. Prog. Polym. Sci. 2000, 25, 1171.(3) Pickering, S. U. J. Chem. Soc. Trans 1907, 91, 2001.(4) Ramsden, W. Proc. R. Soc. London, Ser. A 1903, 72, 156.(5) Pieranski, P. Phys. Rev. Lett. 1980, 45, 569.(6) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2374.(7) Binks, B. P.; Whitby, C. P. Langmuir 2004, 20, 1130.(8) Horozov, T. S.; Aveyard, R.; Clint, J. H.; Binks, B. P. Langmuir 2003,

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1014.(14) Fujii, S.; Armes, S. P.; Binks, B. P.;Murakami, R.Langmuir 2006, 22,

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(20) Chen, T.; Colver, P. J.; Bon, S. A. F. Adv. Mater. 2007, 19, 2286.(21) Fujishima, A.; Honda, K. Nature 1972, 238, 37.(22) Regan, B. O.; Gratzel, M. Nature 1991, 353, 737.(23) Gratzel, M. Nature 2001, 414, 338.

Published on Web 3/5/2009

© 2009 American Chemical Society

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has focused on the photocatalytic syntheses of organiccompounds24 and the initiation of polymerization.25-33 Tita-nia has been employed to initiate the photopolymerization ofacrylates,25,30 methacrylates,26-33 and other vinylics.30 Inthese systems, when titania particles are illuminated by UVlight, electrons are excited from the valence band into theconduction band, and the positive holes remain in the valenceband. Subsequently, these photogenerated charge carriersmigrate from the catalyst surface to reactants (such as mono-mers). The excited state of themonomer dissociates into a freeradical by intramolecular bond cleavage, followed by free-radical propagation.25,26,32

In this article, a novel, practical, and convenient approachwas used to prepare a stable, transparent anatase titaniahydrosol. We synthesized PS microspheres with differentdiameters by using a self-made titania hydrosol. In thisprocess, the titania nanoparticles were utilized to stabilizethe emulsiondroplets, and they initiated the polymerizationofstyrene. The effects of EGDMA and AA on the morphologyof the PS microspheres were investigated extensively.

Experimental Section

Materials. Titanium(IV) chloride (TiCl4, g 98.0%), ethanol(EtOH, g 99.7%), acrylic acid (AA, g 98.0%), and ethyleneglycol dimethacrylate (EGDMA, g 98.0%) were purchasedfrom Shanghai Chemical Reagent Co. (China). Styrene (St, g99.0%) was distilled under vacuum before use. Other reagentswere used as received. Deionized water was used throughout theexperiments.

Preparation of Titania Nanoparticles. The anatase titaniananoparticles synthesized by the controlled nonhydrolytic sol-gel method was obtained according to our previous report34

with some modifications. In this procedure, TiCl4 (25 g) wasmixed with a certain amount of anhydrous ethanol (75 g) withconstant stirring for 12 h at room temperature. The mixture wasdried at 80 �C for 20 h, and the dried particles were subsequentlyground with a ball mill (QM-3SP04, Nanjing University Instru-ment Plant) for 24 h to yield white nanosized titania particles.Finally, these nanoparticles were dispersed in water to form atransparent, stable sol with a concentration of 1.0 wt %.

Preparation of Polystyrene Microspheres. PS microsphereswere prepared by photocatalytic polymerization as follows: theanatase titania hydrosol (10 g) with different amounts of styrenewas added to a 25mL glass flask, whichwas bubbled withN2 for10 min and sealed. The mixture was then stirred magnetically atroom temperature for 5 h in the dark. The polymerization wascarried out by exposing the flask to the XQ 500W Xe lamp (thedistance from the glass flasks to the Xe lamp was 20 cm) withconstant stirring for 48 h at room temperature. If necessary,

EGDMA and AA were premixed with styrene and titaniahydrosol, respectively. The PS microspheres were centrifuged(15 000 rpm, 10 min) from the emulsions, washed with ethanoland water three times, and then redispersed in ethanol for thefollowing measurements. Details of these experiments are givenin Table 1. The content of titania hydrosol was 1.0 wt % in allrecipes.

Characterization. The morphologies of titania hydrosol andthe obtained PSmicrospheres were imagedwith aHitachiH-600TEM. They were dropped onto the carbon-coated copper gridsfor TEM observation. The dispersion of the titania particles inthe obtained spheres was observed using a JEOL JEM2011HR-TEM operating at 200 kV. The absorption spectrum of theanatase titania hydrosol was carried out on a Perkin-ElmerLambda 35UV/vis spectrometer. AMalvern ZetasizerNanoZSwith a universal dip cell in a glass cuvette was used for thedetermination of the zeta potential of the acrylic acid-functio-nalized titania hydrosol. Crystal structure identification wasperformed on anX’Pert PRO (PANalytical) diffractometer withCu KR radiation and 2θ ranging from 20 to 80�. FTIR spectrawere taken on a Nicolet Nexus-470 FTIR spectrometer. Thepolymer microspheres and the functionalized titania particleswith acrylic acid were centrifuged and washed with absoluteethanol and water, dried in a vacuum oven, and then pressedinto KBr pellets for FTIR measurements. The morphologies ofthe PS microspheres were further characterized by TESCAN5136MM SEM. The dispersions of the PS microspheres werediluted with absolute ethanol and dried on a cover glass,followed by sputter coating with gold. The particle size andparticle distribution were determined by the measurement ofabout 300 particles. Differential scanning calorimetry (DSC)tests were performed at a heating rate of 10 �C/min under anitrogen atmosphere using a Perkin-Elmer DSC-7 apparatus.The titania content and thermal stability of PS/titania nano-composites was investigated by thermogravimetric analysis(TGA) using a Perkin-Elmer Pyris 1 thermogravimetric analy-zer at a heating rate of 20 �C/min under an air atmosphere.

Results and Discussion

Characterization of Anatase Titania Hydrosol. The titaniawas prepared via a controlled nonhydrolytic sol-gel route.34

The obtained nanoparticles can be well dispersed in water toform a stable titania hydrosol. Figure 1 shows the absorptionspectrum of the transparent titania sol. The 1.0 wt % titaniahydrosol has strong absorption in the UV region, whereas itis almost transparent in the visible region.No precipitation isfound after about half a year, which suggests that the titaniahydrosol is very stable at room temperature.

The XRD patterns in Figure 2 are presented for theoriginal titania nanoparticles obtained via the controllednonhydrolytic sol-gel process, as well as for the particlesthat had been dispersed in water and subsequently dried

Table 1. Recipes in 100 g of 1.0 wt % Titania Hydrosol for the

Preparation of PS Microspheres by Photocatalytic Emulsion Poly-

merization

sample styrene/g EGDMA/g AA/g

1 2.0 0 02 4.0 0 03 8.0 0 04 10.0 0 05 4.0 0.06 06 4.0 0.12 07 10.0 0.15 08 4.0 0 0.059 4.0 0 0.5

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prior toXRDanalysis. It is clearly seen that both of them arepure anatase.35 The crystallinity is excellent, considering thelow synthesis temperature of 80 �C. It is also proven that thecrystalline state of the anatase titania nanoparticles is notaffected after dispersion in water. The crystal size (calculatedfrom the (101) reflection using the Scherrer equation) isabout 10 nm, which is in good agreement with the particlesize observed by TEM as shown in Figure 3. Good crystal-linity of titania nanoparticles in the hydrosol is crucial totheir application in photocatalysis.36,37

Characterization of Synthesized PS Microspheres. Che-mical Structures. The FTIR spectra of PS prepared viaphotocatalytic polymerization are shown in Figure 4. Thecharacteristic absorption peaks of PS at 1601, 1491, 1452,757, and 698 cm-1 are clearly seen,38 confirming that PSindeed has been synthesized by the photocatalysis of the self-

made anatase titania hydrosol. In addition, the strong peaksat 1724 cm-1 in Figure 4b and at 1716 cm-1 in Figure 4cascribed to the stretching of CdO39 indicate the successfulincorporation of EGDMA and AA into the final polymermicrospheres. The rise of the spectrum baseline below 1100cm-1, especially in Figure 4c, is due to the strong absorptionof Ti-O bonds in the titania lattice.40 The broad peak atabout 3400 cm-1 is caused by the stretching of-OH groupsin the titania particles,41 and the peak at 3730 cm-1 isassigned to the hydroxyl groups chemisorbed on the surfaceof the titania particles.42 The strong absorption peaks oftitania in Figure 4c imply that the final obtained particlescontain a large number of titania particles, which would begiven as further evidence below.

To investigate the effects of the amount of styrene and theaddition of a cross-linking agent and coupling agent on the

Figure 1. UV/vis spectrum of a 1.0 wt % anatase titania hydrosol.

Figure 2. Comparison of the XRD patterns of (a) original titaniananoparticles and (b) particles that had been dispersed in water andsubsequently dried.

Figure 3. TEM image of the anatase titania hydrosol.

Figure 4. FTIR spectra of (a) sample 2, (b) sample 5, and (c)sample 8.

(35) Hong, X. T.; Wang, Z. P.; Cai, W.M.; Lu, F.; Zhang, J.; Yang, Y. Z.;Ma, N.; Liu, Y. J. Chem. Mater. 2005, 17, 1548.

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A: Polym. Chem. 2001, 39, 1634.

(39) Pettibone, J. M.; Cwiertny, D. M.; Scherer, M.; Grassian, V. H.Langmuir 2008, 24, 6659.

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(41) Park,H.K.; Kim,D.K.; Kim, C.H. J. Am.Ceram. Soc. 1997, 80, 743.(42) Janus, M.; Inagaki, M.; Tryba, B.; Toyoda, M.; Morawski, A. W.

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morphology of the obtained polymer microspheres, a seriesof experiments were carried out.

Amount of Styrene. Figure 5 shows the effect of styrenecontent (samples 1-4) on the morphology of PS micro-spheres with a constant titania concentration of 1.0 wt %in water. As shown in Figure 5a, the microspheres have acomparatively uniform distribution, and the particle size isabout 520 nm.With the increase in styrene content in sample2, the PSmicrospheres exhibit a broad distribution in the sizerange of 200 nm to 1.4 μm shown in Figure 5b. Furtherincreases in monomer content in samples 3 and 4 lead to abroad particle distribution and the interparticle aggregation(Figure 5c,d). These results indicate that the limited numberof titania nanoparticles insufficiently protects the emulsionstability. As the monomer concentration increased, theemulsion system tends to obtain larger spherical particles,and the distributions become much broader.

In this article, the titania particles are nanosized, enrichedin-OH groups (absorption peak around 3400 cm-1, see theSupporting Information), and well dispersed in water as aresult of the structure of the double electronic shell. Theemulsion system is stabilized via the synergic effect ofelectrostatic repulsion and self-assembly at the oil/water

interface from the titania nanoparticles. We speculate onthe stability of this kind of emulsion by examining the finalobtained PS emulsions. As shown in Figure 5, with theincreased content of the monomer, the system stabilitydecreases and the formation of secondary particle nucleationbecomes easier, which leads to increased particle size with abroadening of the size distribution.

Effect of EGDMA.As shown in Figure 5, the obtained PSmicrospheres have a relatively wide size distribution. To getmicrospheres with a uniform size distribution, cross-linkingagent EGDMA was introduced into the emulsion system.Some researchers have employed EGDMA with high reac-tivity to prepare microsized cross-linked microspheres orseed latex particles.43,44 The effect of EGDMA on the sizeand morphology of PS microspheres is shown in Figure 6.Compared with the emulsion systems of sample 2, sample 5has an increasedmicrosphere size of 830 nm in diameter witha relatively narrow size distribution by the addition of 0.06 g

Figure 5. SEM images of PS microspheres for samples 1-4 (a-d).

(43) Zhang, H. T.; Huang, H.; Lv, R.; Chen, M. Colloids Surf., A 2005,253, 217.

(44) Durant, Y. G.; Sundberg, E. J.; Sundberg, D. C. Macromolecules1997, 30, 1028.

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of EGDMA (Supporting Information). As shown inFigure 5d, the emulsion system with a 10:1 by mass ratio ofSt/titania is likely to agglomerate. As a comparison, theobtained microspheres of sample 7 by adding 0.15 g ofEGDMA (Figure 6c) show uniform spherical size, besidesa small amount of emulsion conglutination that occurs.These results indicate that the cross-linking agent improvesthe morphology and particle size distribution of PS micro-spheres, as previous work reported.1 However, with furtherincreases in theEGDMAcontent (sample 6), the aggregationof microspheres and conglutination are found (Figure 6b).Therefore, the content of the cross-linking agent in the finalparticles plays an important role in themorphology of the PSparticles. The formed compact network induced by EGD-MA inhibits the diffusivity of monomer inside and leads tothe formation of a larger initial nucleus, which results in theformation of larger spherical particles with a narrow sizedistribution.45,46 When the amount of EGDMA furtherincreases to a certain extent, the content of the cross-linking

Figure 6. SEM images of the PS microspheres for samples 5-7 (a-c).

Figure 7. FTIR spectra of the functionalized titania particles withvarious amounts of acrylic acid: (a) 0, (b) 0.05, and (c) 0.5 g.

(45) Kim, J. W.; Lee, C. H.; Jun, J. B. Colloids Surf., A 2001, 194, 57.(46) Lee, K. C.; Lee, S. Y. Macromol. Res. 2007, 15, 244.

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agent in the resulting polymer microspheres also increases(Supporting Information). It is difficult for the monomer todiffuse into the primary PS particles for further growth. As aconsequence, the absorption of oligomers and/or smallparticles precipitated from the water phase becomes domi-nant for particle propagation, which leads to the aggregationand conglutination of microspheres, as well as inhomogene-ity in the size distribution.

Effect of Acrylic Acid (AA). In previous reports,47,48 somecarboxylic monomers have been used for the modification ofthe titania particle surface. Khaled et al.48 employedmethacrylic acid to functionalize titania in order to improvethe interfacial strength between the polymer and the nano-filler. In this study, acrylic acid was utilized to functionalizeself-made titania particles. In Figure 7, the FTIR spectra ofthe functionalized titania particles shows very strong peaksnear 1530, 1430, and 1400 cm-1, which are due to thebridging bidentate coordination between the titania particles

and acrylic acid.48 The peaks at 1700 cm-1 corresponds tothe CdO bonds in acrylic acid. The FTIR spectra confirmthat acrylic acid has certainly been introduced onto thesurfaces of the titania nanoparticles. The functionalizedtitania nanoparticles with a very small amount of AA wereeffectively used to stabilize the emulsion drops. As shown inFigure 8a (sample 8), the size of the obtained PS sphericalparticles is largely reduced to 100 nm. This is attributed tocoupling agent AA, which has both a carboxyl group forcoordination to the titania nanoparticles through the brid-ging bidentate mode49-52 and a vinyl group to make thetitania organophilic48 and is used for the subsequent copo-lymerization.

We studied the stability of the emulsion by measuring thezeta potential of the titania hydrosol functionalized byacrylic acid. There is a reduction in zeta potential from+71 mV for the pure titania hydrosol to +56 mV for the

Figure 8. TEM images of PS particles prepared by various amounts of AA: (a) sample 8, (b) sample 9, and (c) HR-TEM image of sample 8.

(47) Chen, H. J.; Wang, L. Y.; Chiu, W. Y.; Don, T. M. Ceram. Int. 2008,34, 467.

(48) Khaled, S. M.; Sui, R. H.; Charpentier, P. A.; Rizkalla, A. S.Langmuir 2007, 23, 3988.

(49) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227.(50) Gratzel, M. Inorg. Chem. 2005, 44, 6841.(51) Foster, A. S.; Nieminen, R. M. J. Chem. Phys. 2004, 121, 9039.(52) Zhang, H. Z.; Penn, R. L.; Hamers, R. J.; Banfield, J. F. J. Phys.

Chem. 1999, 103, 4656.

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functionalized titania hydrosol at pH 2.5 (Supporting In-formation). Binks and Rodrigues53 have investigated theemulsion system stabilized by positively charged silica par-ticles after the addition of a negatively charged anionicsurfactant. During the process, the particles become weaklyflocculated, and the zeta potential of the particles decreases,which have been confirmed to make the emulsions stable toboth creaming and coalescence.10 The functionalized titaniahydrosol provides the emulsion system with better stability,and the particle size sharply decreases. However, as theamount of AA rises from 0.05 to 0.5 g in our reaction system,besides the formation of large spherical particles, lots ofhairy materials due to PAA appear as observed by TEM inFigure 8b. This can be explained in that the rate of homo-polymerization increases as the dissociative AA content inwater increases.

Furthermore, we find that more titania nanoparticles areabsorbed on or bonded to the PS nanoparticle surface as seenfrom the rough surfaces of the particles in the TEM image(Figure 8a) after the obtained PS nanospheres are cleanedadequately with ethanol andwater. This is in agreement with

the strong absorbance of titania below 1100 cm-1 for sample8 as shown in Figure 4c. To further visualize the dispersion oftitania in the final PS particles, HR-TEM was employed toobserve sample 9 as shown in Figure 8c. The rough surfacesof the particles are clearly seen, and the titania nanoparticlesare uniformly coated onto the surface of PS particles. There-fore, this provides a novel route to preparing inorganicarmored polymer nanospheres and other specific structures.

Thermal Characterization of PS Microspheres. Comparedwith samples 1-4, the use of functional monomer AA canlead to more titania nanoparticles being bonded to thesurface of PS nanospheres. To examine the titania contentsin the obtained PS spheres, TGA analysis was conducted.Before TGA tests, the PS particles were purified by repeatedwashing/centrifugation and dried at 80 �C overnight undervacuum. As shown in Figure 9, the TGA thermograms of thePS particles exhibit a major decomposition event at 410 �Cthat is due to the decomposition of PSpolymer.After heatingto 550 �C, 17.6 wt % titania remained in sample 8, which isconsistent with the initial theoretical content (20 wt %),whereas sample 2without anyAAmonomer has only a smallamount of titania remaining (4.3 wt %). In addition, theDSC thermograms of PS composite spheres with and with-out AA are compared in Figure 9. The glass-transitiontemperature obviously increases from 88 to 97 �C by theuse of 0.05 g of AA in sample 8. This can be explained in thatcoupling agent AA significantly enhances the interfacialinteractions between the titania nanoparticles and polymers.

Conclusions

In summary, we have prepared a stable, transparent titaniahydrosol by a facile, controlled nonhydrolytic sol-gel route.The obtained titania nanoparticles show perfect anatasepatterns with a size of about 10 nm, which provides supportfor its use as a Pickering stabilizer and photocatalyst. Subse-quently, PS microspheres were synthesized via photocatalyticemulsion polymerization for the first time. It is found that thepolymermicrospheres tend to obtain larger spherical particlesand a broadening of the size distribution with the increase inmonomer concentration. The addition of EGDMA can sig-nificantly improve the polymer particle distribution. Func-tional monomer AA induces a strong interfacial interactionbetween the organic and inorganic states, which revealssmaller polymer particles of about 100 nm, high titaniacontent in the final composites, and an enhanced endothermicconversion temperature for PS. This process produces PSspheres with various particle sizes as well as inorganic ar-mored polymer nanospheres. In addition, the reaction can beinitiated by irradiation with a Xe lamp as well as solar light,which is capable of making full use of natural resources.

Acknowledgment. This work was funded by the NationalNatural Science Foundation of China (NSFC) (no.20704007).

Supporting Information Available: FTIR spectra of thepure titania particles and samples 5 and 6. Bar charts of thesize distribution of the PS particles (samples 1, 2, 5, and 8).Zeta potential of functionalized titania hydrosols. Thismaterial is available free of charge via the Internet athttp://pubs.acs.org.

Figure 9. TGA and DSC curves of (a) sample 2 and (b) sample 8.

(53) Binks, B. P.; Rodrigues, J. A. Langmuir 2007, 23, 7436.

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