9746 Chem. Commun., 2013, 49, 9746--9748 This journal is c The Royal Society of Chemistry 2013

Cite this: Chem. Commun.,2013,49, 9746

Synthesis of Janus particles via kinetic control of phaseseparation in emulsion droplets†

Bing Liu,*a Helmuth Mohwalda and Dayang Wang*b

A reaction-induced phase separation strategy in emulsion droplets

has been developed to fabricate non-spherical Janus particles. The

final morphology of the resulting Janus particles is controlled by a

reaction sequence. These Janus particles are amphiphilic and show

regio-selective adsorption orientation at oil/water interfaces.

Janus particles are expected to self-assemble in similar ways toamphiphilic molecules thanks to the spatial separation of the twochemically different domains within the particles and on theirsurfaces in particular.1,2 This not only advances our understandingof fundamentalprocesses in condensedmattersuch ascrystallization3

and protein folding,4 but also results in unprecedented particlebehaviors such as self-propelling and innovative collective materialproperties engineered by tailored spatial and temporal interparticleconfigurations.5 Many methods have been developed to synthesizeJanus particles such as regio-selective surface modification with andwithout the assistance of templates such as oil/water interfaces, phaseseparation of polymer blends or block copolymers, template-assistedself-assembly, and microfluidic methods.6 Among these methods, thephase separation of polymer blends in emulsion droplets is envi-sioned fairly facile and especially useful for massive production ofJanus particles with excellent colloidal stability. It usually involves anoil-in-water (o/w) emulsion system, in which oil droplets comprise twoincompatible polymers, for instance polystyrene (PS) and poly(methylmethacrylate) homogeneously blended with the aid of a co-solvent.After evaporation of the co-solvent, phase separation leaves behindtwo, spatially well-separated, incompatible polymer domains – Janusstructures – in the resulting particles. In terms of surface wettability,however, the surfaces of as-prepared Janus particles remain isotropicdue to the hydrophobic nature of the two constituent polymerdomains, which need further regio-selective surface modification tobecome Janus.7 To date, phase separation has been rarely used forsynthesis of polymer/inorganic Janus particles as it is difficult to

perform polymerization of monomers and sol–gel reaction of inor-ganic precursors in the same emulsion droplet. For instance, when aconventional precursor such as tetraethyl orthosilicate (TEOS) is used,the resulting inorganic (SiO2) particles are too hydrophilic and readilyescape from templating emulsion droplets. Herein we present directsynthesis of hydrophilic SiO2/hydrophobic PS Janus particles based onphase separation of the emulsion dropletsof the mixture ofstyrene (St)and octadecyltrimethoxylsilane (ODTS) in water. ODTS is utilized asthe SiO2 precursor because it is a poor solvent for PS while its long alkylchains make ODTS miscible with St. The resulting PS–SiO2 compositeparticles can be readily transformed between Janus and core–shellstructures by simply adjusting the sequence of ODTS hydrolysis toSiO2 and St polymerization to PS.

A mixture of ODTS and St, bearing 2-azoisobutylnitrile (AIBN,2.5 wt% with respect to St), was emulsified in water with cetyltri-methylammonium bromide (CTAB) to form o/w emulsions. Nospecial attention was paid to narrowing the size distribution ofas-prepared emulsion droplets, which was not only for experimentalsimplification but also for generation of fairly large emulsion droplets(>5 mm) for the benefit of observation using confocal laser scanningmicroscopy (CLSM). Note that there are a number of techniquesavailable in the literature such as membrane-assisted emulsificationto easily manipulate the sizes of emulsion droplets and to significantlynarrow their size distribution.8 The resulting emulsions were purgedwith argon to remove oxygen, and then heated in an oil bath to initiateSt polymerization at 80 1C. Several small phase-separated domainsappeared in the emulsion droplets after 14 min St polymerization andcoalesced to single large domains residing at the interfaces of theemulsion droplets after 2.5 h St polymerization, which remained littlechanged in terms of size and shape during further St polymerization(Fig. 1a). These newly formed domains remained stable even when theemulsion droplets occasionally collapsed (Fig. 1b). This implies thatthe new domains are made of PS derived via St polymerization bytaking into account the exceedingly slow hydrolysis of ODTS in neutralaqueous media. After single PS domains were formed and separatedfrom the ODTS phases, we added ammonia into the continuous waterphase to accelerate the hydrolysis of ODTS to SiO2. The scanningelectron microscopy (SEM) image shows that as-prepared PS-SiO2

particles consisted of two, well-separated domains, indicating the

a Department of Interfaces, Max-Planck Institute of Colloid and Interfaces, 14476,

Potsdam, Germany. E-mail: liubing@iccas.ac.cnb Ian Wark Research Institute, University of South Australia, Mawson Lakes,

SA 5095, Australia. E-mail: dayang.wang@unisa.edu.au

† Electronic supplementary information (ESI) available: Experimental details andFig. S1–S3. See DOI: 10.1039/c3cc45813j

Received 30th July 2013,Accepted 28th August 2013

DOI: 10.1039/c3cc45813j

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This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 9746--9748 9747

Janus structural characteristics (Fig. 2a). The SiO2 particles obtainedvia hydrolysis of the droplets of ODTS, emulsified in water with CTAB,usually show noticeably rough surfaces (Fig. S1, ESI†). Thus, the largedomains with rough surfaces should be made of SiO2 derived viaODTS hydrolysis and the small ones with smooth surfaces of PSderived via St polymerization, although they are difficultly distin-guished simply based on the SEM imaging contrast. To further clarifythe structure, we have selectively stained the PS domains in theresulting PS-SiO2 particles by adding Nile red – a hydrophobicdye – into the ODTS–St mixture droplets. CLSM imaging clearlydemonstrates that as-prepared PS-SiO2 particles consisted of small,fluorescent PS domains and large, non-fluorescent SiO2 domains(inset in Fig. 2a). Fig. 2 indicates that the PS domain sizes ofas-prepared PS-SiO2 Janus particles noticeably increase with theSt-to-ODTS volume ratio used. The smallest St-to-ODTS volume ratioused in our work is 0.25; the resulting PS domains display a ratherrough surface with small knobs atop probably due to incompletephase separation (Fig. 2d).

When the St polymerization is carried out before the ODTShydrolysis, as shown in Fig. 3 and Fig. S2 (ESI†), the Janus structures

are obtained and little changed when the polymerization timedecreases from 16 h (Fig. 3a) to 1 h (Fig. 3c). But further reductionof the St polymerization time causes the formation of a non-negligible amount of particles consisting of small PS cores and thickSiO2 shells (Fig. 3d). It is plausible that low polymerization conver-sion is due to precipitation of newly formed PS in ODTS, which isquite common during precipitation polymerization.9 When the Stpolymerization is conducted after >6 h ODTS hydrolysis, only PS-SiO2

core–shell particles are obtained. The formation of core–shell parti-cles is obviously due to preferential accumulation of hydrophilic SiO2

derived from organic ODTS hydrolysis (before St polymerization) atthe oil/water interfaces. When the ODTS hydrolysis time is o6 hbefore the St polymerization, mixtures of Janus and core–shellparticles are obtained (Fig. 3d and e). This can be rationalized bythe fact that the hydrolysis of ODTS, similar to other condensationpolymerization, takes longer time to achieve a high molecular weightthan the radical polymerization of St and a larger amount of ODTScan act as the co-solvent to improve the compatibility of SiO2

oligomers with St. Fig. 4 summarises the dependence of the phaseseparation structures of as-prepared PS–SiO2 composite particles onthe kinetics of St polymerization and ODTS hydrolysis and thesequences of these two reactions.

Since ODTS is a poor solvent for PS, AIBN-initiated Stpolymerization in the St/ODTS droplets can be considered as aprecipitation polymerization process, which is expected to producepolymers with a low molecular weight and a low conversion.9 Toincrease the conversion of PS and thus foster the phase separation ofPS and SiO2 domains, we added toluene as a nonreactive co-solventinto the St–ODTS mixture droplets. Compared with those obtained

Fig. 1 (a) A time series of CLSM images recorded after initiation of St polymerizationof the St/ODTS droplets stabilized by CTAB in water. During the polymerization, newlysynthesized PS forms small isolated domains, which gradually coalesce into a largephase separated from the ODTS phase in the mixture droplet. (b) A typical CLSMimage of a collapsed emulsion droplet, consisting of well separated PS and ODTSphases, indicating that the solid phase is PS and the fluid phase is ODTS.

Fig. 2 SEM images of the PS-SiO2 Janus particles obtained via consecutivepolymerization and hydrolysis of CTAB-stabilized St–ODTS mixture droplets inwater. The St to ODTS volume ratio in the emulsion droplets is 24 (a), 9 (b), and1 (c), and 0.25 (d). The St polymerization time is 24 hours prior to ODTS hydrolysis.The PS domains are indicated by white arrows. CLSM images of the resultingJanus particles are shown in the insets. The PS domains are stained with Nile red.

Fig. 3 CLSM images of PS/SiO2 particles derived from the St/ODTS dropletsemulsified by CTAB in water. The PS domains are stained with Nile red. The St toODTS volume ratio is 9. The particles shown in a–e are obtained via ODTShydrolysis after 16 h (a), 3 h (b), 1 h (c), 0.25 h (d) and 0 h (e) St polymerization.The particles shown in (f) and (g) are obtained via St polymerization after 24 h (f)and 6 h (g) ODTS hydrolysis. Scale bar: 8 mm.

Fig. 4 Summary of phase separation structures of PS-SiO2 particles versus Stpolymerization time: core–shell structures in region a, coexistence of core–shelland Janus structures in region b, and Janus structures in region c. The time toinitiate the St polymerization is set as t = 0. The ‘‘minus time’’ represents the timeof ODTS hydrolysis before the St polymerization.

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9748 Chem. Commun., 2013, 49, 9746--9748 This journal is c The Royal Society of Chemistry 2013

in the absence of toluene (Fig. 2), the PS domains of as-preparedJanus PS-SiO2 particles obtained in the presence of toluene werenoticeably larger (Fig. S3, ESI†). This encouraged us to fabricatePS-SiO2 Janus particles by using the toluene solution of PS instead ofSt in CTAB-stabilized emulsion droplets in water. After consecutivetoluene evaporation and ODTS hydrolysis, PS-SiO2 Janus particlescould be produced (Fig. 5).

After removal of the emulsion stabilizer (CTAB), the surfaces ofthe SiO2 domains of as-prepared PS-SiO2 Janus particles are expectedto be hydrophilic, while the surface of the PS domains is hydro-phobic especially taking into account that the St polymerization istriggered by an organic initiator, AIBN. Furthermore, the surfaces ofSiO2 domains are expected to be negatively charged as a resultof ODTS hydrolysis induced by ammonia in our work. This isconfirmed by the fact that poly(allylamine hydrochloride) (PAH)labeled by fluorescein isothiocyanate (FITC), denoted as FITC-PAH,is observed to selectively adsorb onto the surfaces of the SiO2

domains of as-prepared PS-SiO2 Janus particles (Fig. 6). After intro-ducing as-prepared PS-SiO2 Janus particles into an octanol–waterbiphasic system, we observe that the particles readily absorb onto theoctanol/water interface with the SiO2 domains dominantly exposedto the water phase (Fig. 7). Thus, we can conclude that the surfaces ofas-prepared PS-SiO2 Janus particles is clearly divided into two well-separated surface domains, one is hydrophobic (PS) and the other ishydrophilic (SiO2).

In conclusion, we have demonstrated a simple reaction-inducedphase separation approach to produce PS–SiO2 composite particles

from the emulsion droplets consisting of mixtures of ODTS and St.The structures of as-prepared particles can be tuned from Janus tocore–shell by manipulation of the sequences of St polymerizationand ODTS hydrolysis and the kinetics of these reactions. The presentapproach allows massive production of colloidal particles not onlywith Janus structural and chemical characteristics but also withJanus surface hydrophobicity. The clear separation of the hydropho-bic (PS) and hydrophilic (SiO2) domains on their surfaces enables theparticles to adsorb onto the oil/water interfaces with a definedconfiguration, similar to amphiphilic molecules. Taking advantageof new developments in organic sol–gel chemistry,10 our ensuingresearch focuses on applying the present strategy for large scaleproduction of Janus particles composed of different hydrophobicpolymers and metal oxides to enrich our toolbox of Janus particles forbetter mimicking self-assembly behavior of surfactants at interfaces.2

The Max Planck Society is acknowledged for financial sup-port. This work was partially supported by the AustralianResearch Council (DP 110104179 and DP 120102959).

Notes and references1 P. G. de Gennes, Rev. Mod. Phys., 1992, 64, 645.2 A. Kumar, B. J. Park, F. Tu and D. Lee, Soft Matter, 2013, 9, 6604.3 (a) Q. Chen, S. C. Bae and S. Granick, Nature, 2011, 469, 381;

(b) Y. Wang, Y. Wang, D. R. Breed, V. N. Manoharan, L. Feng,A. D. Hollingsworth, M. Weck and D. J. Pine, Nature, 2012, 491, 51;(c) S. C. Glotzer and M. J. Solomon, Nat. Mater., 2007, 6, 557.

4 (a) G. Villar, A. W. Wilber, A. J. Williamson, P. Thiara, J. P. K. Doye,A. A. Louis, M. N. Jochum, A. C. F. Lewis and E. D. Levy, Phys. Rev.Lett., 2009, 102, 118106; (b) T. Chen, Z. Zhang and S. C. Glotzer, Proc.Natl. Acad. Sci. U. S. A., 2007, 104, 717.

5 (a) S. Fournier-Bidoz, A. C. Arsenault, I. Manners and G. A. Ozin,Chem. Commun., 2005, 441; (b) S. Yin, C. Wang, Z. Yu, J. Wang, S. Liuand S. Chen, Adv. Mater., 2011, 23, 2915.

6 (a) A. Perro, S. Reculusa, S. Ravaine, E. Bourgeat-Lamic and E. Duguet,J. Mater. Chem., 2005, 15, 3745; (b) J. Hu, S. Zhou, Y. Sun, X. Fanga andL. Wu, Chem. Soc. Rev., 2012, 41, 4356; (c) A. Walther and A. H. E. Muller,Soft Matter, 2008, 4, 663; (d) F. Wurm and A. F. M. Kilbinger, Angew.Chem., Int. Ed., 2009, 48, 2; (e) B. Liu, W. Wei, X. Qu and Z. Yang, Angew.Chem., Int. Ed., 2008, 47, 3973; ( f ) L. Hong, S. Jiang and S. Granick,Langmuir, 2006, 22, 9495; (g) B. Liu, C. Zhang, J. Liu, X. Qu and Z. Yang,Chem. Commun., 2009, 3871; (h) J. Kim, L. He, Y. Song, Y. Yin andS. Kwon, Chem. Commun., 2012, 48, 6091.

7 (a) T. Tanaka, R. Nakatsuru, Y. Kagari, N. Saito and M. Okubo,Langmuir, 2008, 24, 12267; (b) T. Higuchi, A. Tajima, K. Motoyoshi,H. Yabu and M. Shimomura, Angew. Chem., Int. Ed., 2009, 48, 5125;(c) Y. Wang, B. Guo, X. Wan, J. Xu, X. Wang and Y. Zhang, Polymer,2009, 50, 3361; (d) T. Kietzke, D. Neher, M. Kumke, O. Ghazy,U. Ziener and K. Landfester, Small, 2007, 3, 1041.

8 (a) T. Yanagishita, R. Fujimura, K. Nishio and H. Masuda, Langmuir,2010, 26, 1516; (b) C. Charcosset and H. Fessi, Rev. Chem. Eng., 2005,21, 1; (c) Q. Xu and M. Nakajima, Appl. Phys. Lett., 2004, 85, 3726;(d) T. G. Mason and J. Bibette, Phys. Rev. Lett., 1996, 77, 3481.

9 G. Odian, Principle of Polymerizations, John Wiley & Sons, New Jersey,4th edn, 2004.

10 M. Niederberger and G. Garnweitner, Chem.–Eur. J., 2006, 12, 7282.

Fig. 5 (a) Optical and (b) SEM images of PS-SiO2 Janus particles obtained via theevaporation of co-solvent toluene and following ODTS hydrolysis. The volume ratio ofODTS to the toluene solution of PS is 1 : 9. Insets: SEM images of the PS particles (top-left)and SiO2 domains (top-right) obtained after detachment of the other phase. (c) SEMimages of the PS-SiO2 Janus particles obtained with a varied volume ratio of ODTS to thePS toluene solution, in turn from left to right: 1 : 45, 1 : 18, 1 : 6, 1 : 3 and 3 : 1.

Fig. 6 Transmission (a) and fluorescence CLSM images (b) of a PS/SiO2 Janusparticle with FITC-PAH selectively coated on the SiO2 domain. (c) Overlaid imageof the fluorescence and transmission CLSM images. The Janus particles arederived from CTAB-stabilized emulsion droplets of ODTS and PS toluene solution.The volume ratio of ODTS to PS toluene solution is 1 : 9. The scale bar is 5 mm.

Fig. 7 Transmission CLSM image of as-prepared PS–SiO2 Janus particles at theoctanol/water interface. The orientation of the SiO2 domains is indicated bywhite arrows. The scale bar is 30 mm.

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