The Impact of Nonpolymerizable Swelling Agents On The Synthesis of

Jul 3, 2015 - When we change the swelling agent to a sol−gel precursor, that is, tetraethyl orthosilicate, we can make polystyrene-silica hybrid par...
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The Impact of Nonpolymerizable Swelling Agents On The Synthesis of Particles With Combined Geometric, Interfacial, and Compositional Anisotropy Sijia Wang and Ning Wu* Department of Chemical and Biological Engineering, Colorado School of Mines, Golden, Colorado 80401, United States ABSTRACT: Seeded emulsion polymerization is by far the most successful synthetic method for making anisotropic particles with precise control and high throughput. However, this synthesis involves multiple steps and the types of anisotropic properties that have been made on particles are limited. Here, we demonstrate, by using two different types of nonpolymerizable swelling agents, that we can simplify this method while still producing colloidal dimers with combined anisotropic properties in geometry, interface, and composition. When we swell cross-linked polystyrene seed particles with a simple solvent toluene, without additional polymerization steps we can make dimers with asymmetric distribution of surface charges and roughness on two lobes by fast extraction of toluene. We further show that this toluene-swelling-extraction method can promote the surface modification of the second lobe selectively especially for hydrophilic and stimuli-responsive polymers, which was a significant challenge in dimer synthesis. When we change the swelling agent to a sol−gel precursor, that is, tetraethyl orthosilicate, we can make polystyrene-silica hybrid particles with different morphologies. Our method provides a facile synthetic platform for making colloidal particles with different types of anisotropic properties, which are expected to find important applications for colloidal surfactant, self-assembly, and artificial motors.

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limited size range or monodispersity.21−23 Second, most work reported so far for creating interfacial anisotropy involves modifying the surface of the original lobe by adding functional monomers in CPS. For example, Kim et al. added glycidyl methacrylate and showed that the epoxy groups can be reacted with poly(ethylenimine) selectively on the original lobe.10 Tang et al. performed a more sophisticated study by first making polyacrylonitrile (PAN) hollow spheres and then using them as seeds to make PAN-polystyrene dimers.17 There is, however, no concrete evidence in literature showing the successful functionalization of the second lobe, which might be inherently related to the monomer swelling process. Although a functional monomer as the swelling agent would mostly accumulate in the second lobe, it can still swell the original lobe slightly. Once polymerized, both lobes will contain the same functional polymer, hence reducing the degree of interfacial anisotropy.7,10 Therefore, the ability to independently tune the surface functionality on two lobes is highly desired. Last but not least, as can be inferred from literature and the above examples, the surface modification based on the seeded emulsion polymerization typically involves multiple steps, which is both time-consuming and difficult to control.14,23 In this work, we explore the possibility of using nonpolymerizable swelling agents such as toluene and tetraethyl

INTRODUCTION Synthesis of anisotropic particles has been an active pursuit for the past decade.1 Particles with either nonspherical shapes or heterogeneous surface properties have shown unprecedented assembly structures driven by different types of directional interactions.2,3 However, facile and scalable synthetic approaches that can produce anisotropic particles with high quality are still limited.4−7 The seeded emulsion polymerization is one of the most promising techniques for making monodisperse colloidal dimers with a high throughput.7−13 This method is based on swelling cross-linked polymer seeds (CPS) with monomers such as styrene. Although the enthalpy of mixing monomer and polymer is negative, the increase in elastic energy of the cross-linked polymer prevents the seed particles from swelling isotropically. Instead, the monomer is squeezed out of the seed and forms a second lobe, which can be solidified through an additional step of polymerization. Because this is a bulk synthetic method, the throughput can be high. In addition, the geometric anisotropy in terms of both size ratio and bond length between two lobes can be controlled precisely.14,15 However, the above swelling-polymerization method has a number of limitations. First, it can only be applied to certain types of monomers, which need to be compatible with the cross-linked seeds (the most common one is styrene). Therefore, most dimers are typically made of polymers on both lobes.16−20 Organic−inorganic hybrid dimers with compositional anisotropy have only been fabricated with © XXXX American Chemical Society

Received: May 29, 2015 Revised: July 1, 2015

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DOI: 10.1021/acs.langmuir.5b01982 Langmuir XXXX, XXX, XXX−XXX

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in a water/ethanol mixture four times at 2000 rpm. To further grow a thin silica layer selectively on the silica lobe, we dispersed 1 mL of PSSiO2 particle solution (∼5 mg/mL) into 10 mL of ethanol/water mixture (95:5 v/v) containing 200 μL of NH4OH (30%) with ultrasonication. A mixture of 1 mL of TEOS and 1 mL of ethanol was then added dropwise to the above solution under magnetic stirring at 20 °C with a rate of 1 mL/hour.25 Synthesis of Metallic Nanoparticles. We followed the previously reported method to make CTAB26 stabilized gold nanoparticles. Characterization. The size and morphology of all synthesized particles were characterized by scanning electron microscopy (JEOL JSM-7000F). Both bright-field and fluorescent images were taken by a color camera (Retiga 2000R) connected to an inverted microscope (Olympus IX71). Zeta sizer (Brookhaven 90Plus PALS) was used to measure the zeta potential of particles.

orthosilicate (TEOS), and investigate their impacts on making dimers with combined geometric, interfacial, or compositional anisotropy. Both the simple solvent toluene and sol−gel precursor TEOS can swell CPS into dimers. Surprisingly, by quickly extracting the excess toluene, we can obtain dimers with novel properties such as the asymmetric distribution of surface charges and roughness on two lobes. Combining toluene and functional monomers, we also successfully functionalize the second lobe selectively in dimers. After swelling the CPS with TEOS, we further utilize the hydrolysis/condensation reaction to convert the TEOS lobes into silica, making polystyrene-silica hybrid particles with compositional anisotropy. Our method not only simplifies the procedure for making dimers but also can be potentially extended to other types of swelling agents that will expand the existing catalogue of anisotropic particles.

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RESULTS AND DISCUSSION Polystyrene Dimers by Using Toluene as the Swelling Agent. The conventional method for making dimers is based on swelling of the CPS with styrene.8−10 Because of its negative mixing enthalpy, styrene has a tendency to enter CPS particles and to swell them into larger spheres. However, because the polymer chains inside CPS are mostly cross-linked, the increase in elastic energy prevents CPS from expanding isotropically. As a compromise, the excess amount of styrene will be expelled and form a second lobe attaching to the original CPS. This second lobe needs to be solidified by an additional step of polymerization. More details about this conventional styreneswelling-polymerization method has been discussed previously.14 Styrene is not the only swelling agent to make dimers. For example, other monomers such as methyl methacrylate and nbutyl acrylate have been used to synthesize heterogeneous PS− PMMA and PS−PBA dimers.16,27 In principle, one does not need to use monomer either. As long as the swelling agent has enough miscibility with polystyrene, it should be able to swell the CPS and form a second lobe. We have tried a series of simple liquid with different Hildebrand solubility parameters28 including decane (δ = 13.4), dodecane (δ = 16.2), cyclohexane (δ = 16.8), α-methylstyrene (δ = 17.4), and toluene (δ = 18.2). We find that when the liquid has a solubility parameter close to styrene (δ = 19), it can swell CPS and form a second lobe. Linear hydrocarbons, such as decane and dodecane, cannot swell CPS at all due to their poor miscibility with styrene. Among the liquids mentioned above, toluene is most effective to swell a wide range of CPS with different cross-linking densities probably because of its similar chemical structure to styrene. However, previous work does not use toluene to make dimers because the toluene lobe cannot be polymerized. In the following, we show our surprising results that one can conveniently make dimers with novel anisotropic properties simply by extracting the excess toluene in a proper way. Figure 1a shows the schematics of the synthetic route we have followed. We first cross-link presynthesized polystyrene spheres (with sulfonate functionality on surfaces) based on seeded emulsion polymerization.14 The CPS are then swollen by toluene droplets which are pre-emulsified in water, forming a second lobe. Under the optical microscopy (inset of Figure 1b), we observe dumbbell shape particles on which the second lobe is much larger than the original one and appears to be filled with toluene. In the final step, we remove the excess amount of toluene by quickly centrifuging the dumbbells solution in ethanol. What surprises us is that the dumbbells do not change

EXPERIMENTAL SECTION

Materials. Styrene, divinylbenzene (DVB), sodium 4-vinylbenzenesulfonate, polyvinylpyrrolidone (PVP, Mw, ∼40 000 and ∼10 000), sodium dodecyl sulfate (SDS), 3-aminopropyltriethoxysilane (APS), trisodium citrate, cetrimonium bromide (CTAB), ascorbic acid, NIsopropylacrylamide (NIPAm), ethidium bromide-N N′ bis(acrylamide), Pluronic F127, Rhodamine 6G, methacrylic acid, tetraethyl orthosilicate, poly(vinyl alcohol) (PVA, Mw, ∼13 000− 23 000), n-octylamine, and methyl methacrylate were purchased from Sigma-Aldrich. Toluene and 3-(trimethoxysily) propyl acrylate (TMSPA) were purchased from Tokyo Chemical Industry Co., Ltd. (TCI). The thermal initiator V65 was bought from Wako Chemicals. Hydrogen tetrachlororaurate (III) trihydrate (HAuCl4·3H2O, 99.9+%) and potassium tetrachloroplatinate (II) (K2PtCl4, 99.9%) were purchased from Alfa Aesar. All chemicals were used as received except that styrene and divinylbenzene were washed with aluminum oxide to remove the inhibitor before usage. Dimer Formation by Swelling with Toluene. Spherical polystyrene (PS) seed particles were first prepared by dispersion polymerization in methanol and were cleaned four times via centrifugation (4000 rpm, 30 min, IEC HT Centrifuge).24 After that, a mixture of 4 mL of 5 wt % PVP aqueous solution, 0.5 mL of 2 wt % SDS, 1 mL of styrene, 0.02 g of V65, varying amounts of DVB, and TMSPA were emulsified using ultrasonication (Branson digital sonifier 450). The emulsion was then used to swell 1 mL of polystyrene seed particles (10 wt % in deionized water) for 24 h. The swollen seeds were then polymerized and cross-linked in the reactor at 70 °C for another 24 h. Polystyrene dimers were formed from the CPS via a second swelling stage. We used 0.7 mL of toluene to swell 1 mL of CPS (1 wt %) with 4 mL of 5 wt % PVP and 0.5 mL of 2 wt % SDS for 3 h at different temperatures. Once dimers were formed during swelling, we removed the excess toluene via two different methods. The first was to slowly evaporate toluene in an open vial overnight while the second method was through centrifugation in ethanol for 20 min at 2000 rpm. To stabilize the dimers, we also added 5 mL of Pluronic F127 (2 wt %) in the original PVP and SDS solution. Functionalization of the Second Lobe with PNIPAm or pH Responsive Polymer. We dissolved 0.1 g of NIPAm and V65 (0.01 g/mL) in 0.7 mL of toluene and emulsified them in 4 mL of 5 wt % PVP and 0.5 mL of 2 wt % SDS solution. The emulsion was then used to swell CPS for 3 h. Subsequent polymerization was then carried out at 70 °C overnight. The above procedures can also be repeated for functionalization of pH responsive polymers such as poly(methacrylic acid) if we replace NIPAm with 50−100 μL methacrylic acid. After polymerization, centrifugation was performed four times at 2000 rpm for 20 min. Synthesis of PS-SiO2 Hybrid Particles. We emulsified 300 μL of TEOS in 4 mL of 5 wt % PVA solution and mixed it with 1 mL of CPS (2 wt %) solution. Within 30 min, the TEOS lobe appeared on CPS. We then added 100 μL of n-octylamine to catalyze the sol−gel reaction for hydrolysis and condensation of TEOS into silica. After 1 h, the reaction was stopped and particles were cleaned via centrifugation B

DOI: 10.1021/acs.langmuir.5b01982 Langmuir XXXX, XXX, XXX−XXX

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Langmuir R12 − x 2 + h1 = R1 − V1 =

R 22 − x 2 + R1 + R 2 = L

R12 − x 2 ; h 2 = R 2 −

R 22 − x 2

4 3 π 2 4 π πR1 − h1 (3R1 − h1); V2 = πR 23 − h 22(3R 2 − h 2) 3 3 3 3 (1)

It is interesting to note that the dimer volume Vd is 10−20% less than the volume of CPS VC, which indicates that some polymers are lost during the centrifugation step. This can be understood because toluene is a good solvent of uncross-linked polystyrene. Potentially some polymer chains that can be dissolved in toluene are then removed by centrifugation. We also find that the speed of removing toluene can affect the particle morphologies significantly. In addition to fast centrifugation, we have also tried to evaporate toluene naturally in an open vial at room temperature overnight. This slow removal process can only produce tiny protrusions on CPS (Figure 1c-iii), while its volume is also equal to the volume of initial CPS. Moreover, the surfaces of both lobes are smooth. Another interesting observation is that the dimer morphology depends on the swelling temperature, too. For example, if we swell CPS with toluene at room temperature and directly wash (via centrifugation) them with ethanol, we obtain a mixture of spheres and dimers (Figure 1c-i). It is clear that the overlap between two lobes on those surviving dimers is very little, making them easily broken under weak sonication between sequential centrifugation steps. When the swelling temperature is increased to 70 °C, the bonding between two lobes is much stronger and no single sphere is observed (Figure 1c-ii). Hence the ideal conditions for making dimers with distinct lobes are at a higher swelling temperature and with a faster toluene removal speed. One notable morphology of our dimers is that one lobe is rough but the other is smooth. It is therefore important to find out which lobe originates from the CPS particle. Because silane molecules (TMSPA) are incorporated during the synthesis of the original CPS spheres (Figure 1d-i), we can detect their positions following our previously reported method.14 In brief, we first attach APS to the dimers based on silane conjugation chemistry. We then mix the modified dimers with negatively charged gold nanoparticles, which specifically adsorb on the APS modified surfaces due to the electrostatic interaction and amine-metal complexation. Clearly, gold nanoparticles mainly coat on the rough lobe (Figure 1d-ii), which indicates the location of TMSPA. Therefore, the rough lobe comes from the original CPS sphere. As for the dimers obtained with slow toluene evaporation, the larger lobe is also coated primarily by gold nanoparticles. This is also consistent with our observation that the newly formed lobe is always smaller than the original (CPS) lobe. On the basis of these findings, we propose that some of the uncross-linked polymer chains (typically located in the core of CPS) migrate to the newly formed and tolueneswollen lobe. When the toluene removal speed is slow, for example, through natural evaporation, most of the chains can migrate back to the seeded lobe and only a small protrusion is left after toluene is evaporated completely. In contrast, when the toluene removal speed is fast (e.g., via centrifugation) there is little time for the polymer chains in the second lobe to migrate back and a significant second lobe is preserved. On the CPS lobe, there is a mismatch in terms of the elastic moduli between the shell and core, where the shell modulus is higher than the core. Therefore, mechanical buckling develops during

Figure 1. Dimers formed by swelling CPS with toluene. (a) The schematics for our toluene swelling-extraction method. (b) A large field of view of the monodisperse rough-smooth dimers after toluene extraction. Scale bar: 2 μm. Inset: An optical image of the CPS after swelling with toluene but before the toluene extraction. Scale bar: 5 μm. (c) Both swelling temperature and toluene removal speed affect the dimer morphology. Scale bar: 1 μm. (i) 20 °C and fast centrifugation; (ii) 70 °C and fast centrifugation; (iii) 70 °C and slow evaporation. The arrows indicate the original lobe coming from the CPS. (d) SEM of (i) the CPS spheres; (ii) citrate-gold nanoparticles coated on the APS modified dimers obtained by the toluene-swelling-centrifugation method; (iii) citrate-gold nanoparticles coated on the APS modified dimers obtained by the toluene-swellingevaporation method. Scale bar: 1 μm.

back to the spherical shape of CPS as the toluene lobe is removed completely and we obtain highly monodisperse dimers (Figure 1b). Examination of 100 dimers reveals that two lobe sizes are 1.43 ± 0.05 and 1.19 ± 0.05 μm. Moreover, these new dimers look very differently from conventional dimers where both lobe surfaces are smooth. Instead, Figure 1b shows that one of the two lobes is consistently buckled, while the other surface is smooth, forming the rough-smooth dimers. We can calculate the volumes of two intersecting spheres in the dimer based on the following formula, where all geometric parameters are illustrated in Figure 1a C

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%). So far, CPS in Figure 2a-v shows the best dimer morphology after toluene removal. Therefore, for the rest of studies, we use CPS that has 5% DVB and 10% TMSPA incorporated. When pure toluene is used as the swelling agent, the smooth second lobe can be easily fused to each other under centrifugation, forming triangular particles shown in Figure 3a. To enhance the particle stability, a triblock copolymer,

the fast removal of toluene, leaving rough surfaces on the original lobe. The similar mechanism has also been found when creating highly folded microspheres.29 The Effect of CPS Composition. We also systematically study other parameters that can influence the morphologies of dimers for process optimization. The first critical parameter is the CPS composition. Figure 2a shows the swelling stage of a

Figure 3. Stabilization of dimers. (a) Fusion between toluene lobes making triangular colloids but reducing the monodispersity of dimers. (b) The addition of pluronic F127 during swelling enhances particle stability dramatically. Scale bars: 2 μm.

Pluronic F127, can be added as a costabilizer in the toluene-inwater emulsion. During swelling, the hydrophobic segment, poly(propylene glycol), of Pluronic F127 embeds into the toluene droplet, while the hydrophilic part, poly(ethylene glycol), lies at the interface as a steric stabilizer. This strategy was first developed for stabilizing polystyrene spheres31 and works well for our dimers, too (Figure 3b). Anisotropy in Surface Charge Distribution. The roughsmooth dimers made from our toluene-swelling-extraction method exhibit surprising anisotropy in surface charge distribution as well. The original CPS particles are negatively charged because of the addition of sodium 4-styrenesulfonate monomer during synthesis, which enhances the particle stability in water. When we mix our dimers with a positively charged dye, Rhodamine 6G,14 we find much stronger fluorescence intensity on the larger lobe (Figure 4a). This indicates that the negative charges remain on the original lobe and they do not migrate to the second lobe. In contrast, we find that dimers produced via the conventional method, that is, styrene-swellingpolymerization, have symmetric charge distribution on both lobes regardless of the size ratio (Figure 4b). Apparently, the charged polymer chains, that is, poly(sodium 4-styrenesulfonate), in CPS migrate toward the second lobe possibly during the polymerization stage. By avoiding the additional step of polymerization, we make dimers with a new type of interfacial anisotropy, the anisotropy in surface charges. This is an exciting result because particles with asymmetric charge distribution are considered important building blocks for directed-assembly under externally applied electric fields. Moreover, the asymmetric distribution in surface charges can be utilized for further surface modification based on the electrostatic interaction. For example, we can coat positively charged CTAB-gold nanoparticles26 (zeta potential ξ ∼ 64 mV) selectively on the original lobe of the dimers (inset of Figure 4a-ii). In comparison, the same nanoparticles are coated uniformly on conventional dimers (inset of Figure 4b). This demonstrates a significant advantage of our dimers because

Figure 2. Impact of CPS composition on the morphology of dimers. (a) Optical images of CPS swollen by toluene with different amounts of cross-linker (DVB) and TMSPA in CPS. Scale bar: 5 μm. (b) SEM images of the corresponding particles after slow evaporation of toluene. Scale bar: 1 μm.

series of CPS with different amount of functional silane (TMSPA) and cross-linker (DVB). Figure 2b shows the same particles after the removal of toluene through slow evaporation. Cross-linked polymer network is necessary for building elastic stresses during swelling,9 and the hydrophilic silane30 also promotes dewetting of toluene on CPS. Both help maintain the toluene lobe itself to be distinct from the CPS. The first row in Figure 2a shows toluene-swollen CPS that have the same amount of cross-linker but a different amount of TMSPA. The first column in Figure 2a shows toluene-swollen CPS that have no TMSPA but with 10 and 20% cross-linker. It is clear that the incorporation of silane is essential to form dimers. This is different from the conventional styrene-swelling method, where CPS without silane is still able to form “snowman” shaped particles.14 DVB plays a role too. Too much DVB will lead to a highly cross-linked polymer matrix, which makes the migration of the uncross-linked polymer chains to the second lobe difficult (Figure 2a-iv). If no DVB is incorporated, only a relatively small second lobe can be observed during swelling (Figure 2a-vi) and it disappears after toluene removal (Figure 2b-vi). Therefore, Figure 2 reveals that the optimal composition of CPS for making dimers with distinct lobes should have sufficient amount of DVB (≥5 vol %) and TMSPA (≥10 vol D

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Figure 4. Surface charge characterization on dimers. (a) The brightfield and corresponding fluorescent images of polystyrene dimers formed via the toluene-swelling-extraction method, coated with Rhodamine 6G. Inset: coating of positively charged CTAB-gold nanoparticles. (b) Fluorescent images of conventional dimers indicating uniform surface charge distribution. The second lobe is highlighted in blue circles. Inset: coating of CTAB-gold nanoparticles. Scale bar for all images: 5 μm.

previous strategies to make anisotropic metal−organic hybrid dimers typically relied on silane conjugation14 where multiple steps are involved. Functionalization of the Second Lobe. Using toluene as a swelling agent can also help create functionalities on the second lobe. For example, we can mix toluene with functional monomers during the swelling stage and perform subsequent polymerization in the presence of toluene with the goal to add functionality specifically on the second lobe. We purposely choose monomer NIPAm because as a hydrophilic monomer it is hard to be incorporated into polystyrene dimers and the resulting anisotropy in terms of functionality is ill-defined in previous efforts.32 To probe the location of poly(NIPAm) on dimer, we also add a trace amount of fluorescent monomer that shares a similar chemical structure with NIPAm, that is, ethidium bromide-N,N′-bis(acrylamide). As shown in Figure 5a, after polymerization, we observe a distinct contrast in fluorescence intensity between two lobes. The smaller lobe appears to be strongly fluorescent. For better clarity, we purposely show the area where a trimer is formed by the fusion of the second (toluene) lobes between two dimers, highlighted in Figure 5a-ii; the fused lobe shows strong fluorescence, indicating that poly(NIPAm) is primarily located on the surface of the second lobe. Different from dimers with toluene only, we find that the addition of NIPAm significantly improves particle stability, which also indicates the incorporation of the hydrophilic polymer on the second lobe surface. Moreover, the contrast in surface charges between two lobes is still preserved (Figure 5b-ii). Therefore, the dimers shown in Figure 5a,b possess both physical and chemical anisotropy in terms of surface charge and surface functionality. If we use the conventional method, that is, substituting the swelling agent toluene by styrene, after polymerization we observe fluorescence on both lobes with much weaker contrast in fluorescence intensity (Figure 5c). Therefore, our tolueneswelling method can promote the surface modification of the second lobe selectively, especially for hydrophilic and stimuli-

Figure 5. Selective functionalization of the second lobe. (a) Dimers with both NIPAm and a fluorescent monomer (ethidium bromideN,N′-bis(acrylamide)) incorporated via the toluene-swelling-polymerization-extraction method. (i) Optical and (ii) the corresponding fluorescent image indicating the anisotropy in surface functionality. Scale bar: 5 μm. (b) (i) SEM image of dimers; and (ii) Fluorescent image of the same dimers coated with Rhodamine 6G indicating the anisotropy in surface charge. Scale bar: 2 μm. (c) Dimers with both NIPAm and ethidium bromide-N,N′-bis(acrylamide) incorporated via the styrene-swelling-polymerization method. (i) Optical and (ii) the corresponding florescent images indicating much weaker anisotropy. Scale bar: 5 μm.

responsive polymers, which is a significant challenge in dimer synthesis. Encouraged by the above results, we have also tried to incorporate pH-responsive polymers such as poly(methacrylic acid) (PMAA) into dimers. We mix different amounts of methacrylic acid with toluene, which is then used to swell CPS particles. Figure 6a shows the SEM images of dimers after polymerizing MAA and removing excess toluene. It is clear that the MAA polymerization induces significant mechanical buckling and the second lobe morphology (i.e., collapsed shells) is dramatically different from conventional dimers when the MAA concentration is relatively high (≥10 vol %). This is reasonable because MAA polymerization might only take place at the interface between toluene and water due to the hydrophilic nature of PMAA. The morphology difference between the original and second lobes in Figure 6a-i indicates possible interfacial anisotropy where the highly buckled lobe consists of primarily PMAA and the original (spherical) lobe is primarily polystyrene. To test this, we add the dimers in a 50:50 E

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Figure 6. Synthesis and interfacial activity of pH responsive dimers. (a) Dimers with different amount of MAA incorporated during toluene swelling. (i) 12.5 vol %; (ii) 10 vol %; and (iii) 6.7 vol %. Scale bar: 2 μm. (b) (i) Interfacial activity of dimers incorporated with 12.5 vol % MAA at different pH in hexane-water mixture. The dye Oil Red O is added in the hexane phase. (ii) Optical image of oil-in-water emulsions stabilized by dimers at pH 12. Scale bar: 100 μm. (iii) Dimers without MAA incorporated in a hexane-water mixture.

vol % mixture of hexane (with dye Oil Red O) and water at different pH and vortex the solution for 5 min. After that, the vials are kept still on bench. Also, the emulsion stabilities are examined. As shown in Figure 6b-i, at pH 2 the carboxylic groups in PMAA are protonated. Therefore, both lobes of the dimers are hydrophobic and they prefer to stay in the oil phase. At pH 7, some of the carboxyl groups on dimer surfaces are partially deprotonated since the pKa of PMAA is ∼5.5.33 Therefore, more particles can be suspended in the water phase. The particles are, however, not amphiphilic enough to stabilize emulsions for long time. Therefore, oil and water are phaseseparated. At pH 12, essentially all carboxylic groups are deprotonated and the PMAA lobes are hydrophilic. The contrast in hydrophobicity between the polystyrene and PMAA lobes becomes significant enough so that oil-in-water emulsions (Figure 7b-ii) can be stabilized over a period of at least one month. In comparison, the dimers without PMAA incorporated in the second lobe do not show any pH responsiveness and cannot stabilize emulsions either. Synthesis of Polystyrene−Silica Hybrid Particles. Inspired by our work using toluene as the swelling agent, we have also tried another type of swelling agent, TEOS, with the aim to make hybrid particles composed of polystyrene and silica. We choose TEOS because of its compatibility with styrene and it is a commonly used precursor for silica synthesis via sol−gel reactions.34,35 As shown schematically in Figure 7a, we first prepare emulsions of TEOS in poly(vinyl alcohol) solution, which are then used to swell CPS. The TEOS lobe emerges shortly after swelling. Thirty minutes later, we add an organic base (octylamine) to initiate and catalyze the sol−gel reaction of TEOS, which eventually converts into solid silica via hydrolysis and condensation. We have tried several different types of base including ammonium hydroxide, triethylamine, and octylamine. We find that both the strength and amount of the base are important. A hydrophilic base such as ammonia hydroxide typically leads to dissolution of the TEOS lobe in

Figure 7. Synthesis and characterization of PS-SiO2 hybrid particles. (a) Schematic of the synthetic route. (b) SEM images of the hybrid particles with (i) acorn and (ii) snowman shapes. Scale bar: 2 μm. Insets are corresponding TEM images. (iii) EDX analysis of silica and polystyrene lobes. (c) The optical image of PS-SiO2 dimers shows clear contrast in refractive indices between two lobes. Scale bar: 10 μm. (d) SEM of the remaining silica lobes after annealing the dimers (b-i and ii) at 200 °C. Scale bar: 1 μm.

water. Therefore, a relatively weak organic base is necessary so that the TEOS hydrolysis/condensation can take place at the TEOS/water interface slowly. High amount of strong base (e.g., triethylamine) causes particle aggregation due to fast TEOS hydrolysis. Meanwhile, insufficient amount of base leads to porous and hollow silica shells, which are fragile and easily broken during the cleaning (e.g., centrifugation and sonication) stage. F

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Langmuir Under appropriate conditions (see Experimental Section for details), polystyrene-silica (PS-SiO2) hybrid particles with different shapes such as acorn and snowman can be synthesized (Figure 7b). The energy-dispersive X-ray spectroscopy (EDX) in Figure 7b-iii further confirms that the brighter compartment is rich in both silicon and oxygen signals (the gold signal comes from sputtering for SEM sample preparation and has been subtracted as background in the elemental analysis). In contrast, the darker part is primarily polystyrene, where the oxygen signal comes from the sulfonate groups added during the synthesis of polystyrene seeds. The TEM images (insets in Figure 7b) also clearly show the contrast between the silica and polystyrene lobes. In fact, the distinct polystyrene and silica compartments can also be observed in bright-field optical microscopy (Figure 7c). The silica lobe is much more transparent than polystyrene because its refractive index (1.45) is closer to water (1.38). We note that there is some carbon in the silica lobe (∼6.6 wt %). It is possible that some polystyrene chains have migrated from the CPS to the silica lobe. In addition, considering the amount of octylamine (C/Si ∼ 3.6 in mole) added, we also suspect that some are trapped in the amorphous silica pores during the sol−gel reaction. We further notice that it is crucial to minimize the swelling time of TEOS, which helps create an asymmetric distribution in silica. When the swelling time is increased to 3 h, the silicon signal on the polystyrene lobe can be as high as 24 atom %. We also anneal the hybrid particles at 200 °C to remove the polystyrene compartment. The SEM images in Figure 7d show that the remaining silica lobes are in the form of dimpled spheres, which are consistent with the TEM images revealed in Figure 7b. Therefore, our method makes not only hybrid PS-silica particles but also dimpled silica spheres which are important building blocks for the study of “lock and key” interactions.2 The morphology of PS-SiO2 particles can be tuned by controlling either the CPS composition or the relative amount of TEOS to CPS. Figure 8a (i-iv) shows the impact of increasing the amount of cross-linker (DVB) in CPS. Clearly, a higher cross-linking density yields a larger elastic modulus in CPS and creates more distinct lobes (from acorn to dumbbell shapes). The surface hydrophilicity of CPS is also important, because it determines the wettability of TEOS on CPS. For example, TEOS swelling in CPS that has been incorporated with a hydrophobic monomer methyl methacrylate (MMA) yields distinct lobes of silica and polystyrene (Figure 8a-v) because the hydrolyzed TEOS wets poorly on hydrophobic surfaces.30 In comparison, when we add a hydrophilic monomer methacrylic acid (MAA) in CPS, the silica shell almost encapsulates the polystyrene lobe (Figure 8a-vi). We have also varied the relative amount of TEOS to CPS. As shown in Figure 8b, higher TEOS/CPS mass ratio leads to a larger silica lobe. We note that the absolute amount of TEOS used for swelling should be kept relatively high (300 μL), because it keeps diffusing into water during hydrolysis. Therefore, by adjusting the CPS composition and TEOS/ CPS mass ratio, our TEOS-swelling-condensation method allows convenient control in both morphology and size ratio between the polystyrene and silica compartments.

Figure 8. Morphology of the PS-SiO2 hybrid particles.(a) The impact of CPS composition on morphology. (i)−(iv): CPS with an increasing DVB concentration (5, 10, 15, and 18 vol %); (v) CPS incorporated with 10 vol % DVB and 10 vol % MMA; and (vi) CPS incorporated with 10 vol % DVB and 10 vol % MAA. (b) The impact of TEOS/CPS mass ratio on morphology: (i) 28:1, (ii) 14:1, and (iii) 7:1. Scale bar is the same for all images: 1 μm.

remove toluene quickly, indicating the migration of uncrosslinked polymer chains toward the second lobe during swelling. The benefits of replacing the conventional swelling agent styrene with toluene are surprisingly rich. Our method not only simplifies the synthetic procedures compared with the conventional method but also produces dimers with new types of interfacial anisotropy. For example, we demonstrate that the surface charges are asymmetrically distributed between two lobes. Such an anisotropy allows us to further modify one of the two lobes selectively based on strong electrostatic interactions, making metal−organic hybrid dimers conveniently. Moreover, the dimers obtained via the toluene-swelling-extraction method also exhibit anisotropy in terms of surface roughness, which can be potentially utilized for making colloidal micelles based on depletion interactions. Furthermore, by mixing toluene with a small amount of hydrophilic monomers, we successfully functionalize one lobe with temperature- and pH-responsive hydrophilic polymers, which is difficult to achieve in previous attempts. The other swelling agent we have explored is tetraethyl orthosilicate. Via the well-established sol−gel reaction, we have successfully synthesized dimers with compositional anisotropy, that is, the polystyrene-silica hybrid particles. By controlling both cross-linking density and hydrophobicity of the polystyrene seeds, we can tune the dimer morphology from acorn to dumbbell shapes. Overall, by utilizing nonpolymerizable swelling agents, we can make particles with combined interfacial and compositional anisotropy through a robust bulk synthetic approach. These particles could find important applications as colloidal emulsifiers, self-assembly building blocks, or microscopic motors.

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CONCLUSIONS We employ two different types of nonpolymerizable swelling agents to fabricate colloidal dimers with a wide range of anisotropic properties. When we swell cross-linked seeds with toluene, dimers emerge. The morphology is preserved after we G

DOI: 10.1021/acs.langmuir.5b01982 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

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(21) Qiang, W.; Wang, Y.; He, P.; Xu, H.; Gu, H.; Shi, D. Synthesis of Asymmetric Inorganic/polymer Nanocomposite Particles via Localized Substrate Surface Modification and Miniemulsion Polymerization. Langmuir 2008, 24, 606−608. (22) Lu, W.; Chen, M.; Wu, L. One-Step Synthesis of OrganicInorganic Hybrid Asymmetric Dimer Particles via Miniemulsion Polymerization and Functionalization with Silver. J. Colloid Interface Sci. 2008, 328, 98−102. (23) Liu, B.; Liu, J.; Liang, F.; Wang, Q.; Zhang, C.; Qu, X.; Li, J.; Qiu, D.; Yang, Z. Robust Anisotropic Composite Particles with Tunable Janus Balance. Macromolecules 2012, 45, 5176−5184. (24) Zhang, F.; Cao, L.; Yang, W. Preparation of Monodisperse and Anion-Charged Polystyrene Microspheres Stabilized with Polymerizable Sodium Styrene Sulfonate by Dispersion Polymerization. Macromol. Chem. Phys. 2010, 211, 744−751. (25) Zhang, S. W.; Zhou, S. X.; Weng, Y. M.; Wu, L. M. Synthesis of Silanol-Functionalized Latex Nanoparticles through Miniemulsion Copolymerization of Styrene and ??- Methacryloxypropyltrimethoxysilane. Langmuir 2006, 22, 4674−4679. (26) Narayanan, R.; Lipert, R. J.; Porter, M. D. Cetyltrimethylammonium Bromide-Modified Spherical and Cube-like Gold Nanoparticles as Extrinsic Raman Labels in Surface-Enhanced Raman Spectroscopy Based Heterogeneous Immunoassays. Anal. Chem. 2008, 80, 2265−2271. (27) Skelhon, T. S.; Chen, Y.; Bon, S. A. F. Synthesis of “Hard−Soft” Janus Particles by Seeded Dispersion Polymerization. Langmuir 2014, 30, 13525−13532. (28) Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters, 2nd ed.; Taylor & Francis: Oxford, U.K., 1991. (29) Zhao, T.; Qiu, D. One-Pot Synthesis of Highly Folded Microparticles by Suspension Polymerization. Langmuir 2011, 27, 12771−12774. (30) Mock, E. B.; De Bruyn, H.; Hawkett, B. S.; Gilbert, R. G.; Zukoski, C. F. Synthesis of Anisotropic Nanoparticles by Seeded Emulsion Polymerization. Langmuir 2006, 22, 4037−4043. (31) Kim, A. J.; Manoharan, V. N.; Crocker, J. C. Swelling-Based Method for Preparing Stable, Functionalized Polymer Colloids. J. Am. Chem. Soc. 2005, 127, 1592−1593. (32) Kraft, D. J.; Hilhorst, J.; Heinen, M. A. P.; Hoogenraad, M. J.; Luigjes, B.; Kegel, W. K. Patchy Polymer Colloids with Tunable Anisotropy Dimensions. J. Phys. Chem. B 2011, 115, 7175−7181. (33) Zhang, J.; Peppas, N. A. Synthesis and Characterization of pHand Temperature-Sensitive Poly(methacrylic acid)/Poly(N-Isopropylacrylamide) Interpenetrating Polymeric Networks. Macromolecules 2000, 33, 102−107. (34) Stö ber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62−69. (35) Hench, L. L.; West, J. K. The Sol-Gel Process. Chem. Rev. 1990, 90, 33−72.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Donors of the American Chemical Society Petroleum Research Fund (PRF No. 53638-DN110) for the support of this research.

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DOI: 10.1021/acs.langmuir.5b01982 Langmuir XXXX, XXX, XXX−XXX