Article pubs.acs.org/Langmuir
Direct One-Pot Synthesis of Chemically Anisotropic Particles with Tunable Morphology, Dimensions, and Surface Roughness Yanan Liu, Wang Liu, Yuhong Ma, Lianying Liu,* and Wantai Yang* Beijing Engineering Research Center for the Synthesis and Applications of Waterborne Polymers, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *
ABSTRACT: Previously, synthesis of anisotropic particles by seeded polymerizations has involved multiple process steps. In conventional one-pot dispersion polymerization (Dis.P) with a cross-linker added, only spherical particles are produced due to rapid and high cross-linking. In this Article, a straightforward one-pot preparation of monodisperse anisotropic particles with tunable morphology, dimensions, surface roughness, and asymmetrically distributed functional groups is described. With a cross-linker of divinylbenzene (DVB, 8%), ethylene glycol dimethacrylate (EGDMA, 6%), or dimethacryloyloxybenzophenone (DMABP, 5%) added at 40 min, shortly after the end of nucleation stage in Dis.P of styrene (St) in methanol and water (6/4, vol), the swollen growing particles are inhomogeneously cross-linked at first. Then, at low gel contents of 59%, 49%, and 69%, corresponding to the cases using DVB, EGDMA, and DMABP, respectively, the growing particle phase separates and snowman- or dumbbell-like particles are generated. Thermodynamic and kinetic analyses reveal that moderate cross-linking and sufficient swelling of growing particles determine the formation and growth of anisotropic particles during polymerization. Morphology, surface roughness, sizes, and cross-linking degrees of each domain of final particles are tuned continuously by varying start addition time and contents of crosslinkers. The snowman-like particles fabricated with DVB have a gradient cross-linking and asymmetrical distribution of pendant vinyl groups from their body to head. The dumbbell-like particles prepared using DMABP have only one domain cross-linked; i.e., only one domain contains photosensitive benzophenone (BP) groups. With addition of glycidyl methacrylate (GMA) or propargyl methacrylate (PMA) together with DVB or EGDMA, epoxy or alkynyl groups are asymmetrically incorporated. With the aid of these functional groups, carboxyl, amino, or thiol groups and PEG (200) are attached by thiol−ene (yne) click and photocoupling reactions.
1. INTRODUCTION In the past decade, anisotropic particles with asymmetric geometry and specific surface reactive groups have attracted tremendous attention because of their fascinating properties including surface amphiphilicity, optical and magnetic properties, etc. Many strategies, typically including phase separation-, Pickering emulsion-, and microfluid-based techniques, etc., have been developed for their fabrication, as addressed in recent review articles.1−6 Of much more interest in recent times is the exploration of relatively easy and reproducible processes7−11 that can provide high yields of uniform anisotropic particles suitable for self-assembly and new applications. Among such processes, controlled internal phase separation in seeded polymerizations is considered to be a prevalent technique.12−29 Spherical or nonspherical particles are usually taken as starting materials (seeds). When cross-linked seed particles are swollen with monomers12−15,22−29 and heated, phase separation (expelling of monomers as a liquid protrusion from seeds) is induced by relieving elastic stress built up on network. Subsequent polymerization of the expelled monomers © 2014 American Chemical Society
enhances the phase separation and results in formation of anisotropic particles. However, this technique still has some limitations. For instance, multiple process steps, including synthesis of seeds, additional modification to endow surface with hydrophilicity to increase interfacial tensions,23,26 cross-linking and swelling of seeds, and subsequent polymerization, are required.12−14,23−25 The swelling and repeated separating processes are timeconsuming. These could limit the achievable yields. Moreover, the seed domain of resultant anisotropic particle usually retains its smooth or coarse surface,22 while the newly formed domains are all smooth-bulb-shaped.22−24 In such cases, there are difficulties in control of surface roughness, shape, and sizes in the course of forming anisotropic particles. Besides, if different functionalities are desired on different compartments, a series of reactions, such as introduction of specific functional groups Received: November 3, 2014 Revised: December 28, 2014 Published: December 30, 2014 925
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Langmuir Scheme 1. Formation of Anisotropic Particles with Asymmetrically Distributed Functional Groups
to seeds, conversion of the functional groups, and postfunctionalization of anisotropic particles, need to be carried out.29 The asymmetrically rough surface and distributed functional groups are important to self-assembly and new applications of anisotropic particles.30 So, the preparation of chemically anisotropic particles through a simple and efficient approach involving fewer steps is still strongly demanded. It is known that dispersion polymerization (Dis.P) is a very simple method for synthesis of monodisperse polymer microspheres with functionalities. Its process can be divided into a nucleation stage and a particle growth stage. The growing particles can be considered as dynamically changing seeds. They remain swollen by monomer and oligomers throughout the particle growth stage, until the very end of reaction.31−33 When the swollen growing particles are cross-linked via a delayed addition (two-stage Dis.P) technique developed by Winnik’s group,31−33 the formed polymer network can be inhomogeneous. This tends to localize contraction forces. However, to date, the swelling ability and inhomogeneity in cross-linking of growing particles have not been utilized for inducing phase separation and synthesizing anisotropic particle, except for preparing spherical core−shell (cross-linked) particles.34−36 According to a thermodynamic model developed by Sheu et al.,12,13 it is thought that whether or not the phase separation of growing particle can occur depends on a balance between dynamically changing contraction (cross-linking) and expansion (swelling) of growing particles during polymerization. Usually, high reactivities of cross-linkers such as divinylbenzene (DVB) or ethylene glycol dimethacrylate (EGDMA) will promote consumption of monomers, and bring about rapid, high cross-linking, and excessive contraction of growing particles. Moreover, mobility of high molecular weight polymers is poor, and their expelling from growing particles is difficult. Therefore, only spherical particles rather than
anisotropic particles are produced. If the cross-linking, swelling, and interior polymer mobility of growing particles can be properly manipulated during Dis.P, an alternative method for straightforward production of anisotropic particles is possible. Our recent work37 demonstrated the possibility, and core− shell, multiple-compartment anisotropic particles were prepared by a modified one-pot Dis.P of styrene (St), in which ethylene glycol (EG) and water was used as medium, water-soluble initiator of ammonium persulfate (APS) and hydrophilic monomer of vinyl acetate (VA) were added to improve surface hydrophilicity of particles, and St was fed with DVB to enhance the swelling ability of growing particles. In this Article, considering the well-swollen state of growing particles at a low monomer conversion, we explore more simple diverse ways to control the cross-linking, swelling, and interior polymer mobility of growing particles, in order to prepare monodisperse anisotropic particles containing asymmetrically distributed functional groups. As depicted in Scheme 1, to induce phase separation of growing particles, (1) a polar medium of methanol (MeOH) and water [6/4, vol, solubility parameter δmix = 37.3 (J/cm3)1/2, calculated by δmix = δMeOHφMeOH + δH2OφH2O] is used, (2) a commonly used DVB or EGDMA cross-linker, or a special low-reactivity cross-linker of dimethacryloyloxybenzophenone (DMABP), is applied shortly after the nucleation stage, and (3) start addition time and crosslinker (DVB, EGDMA, or DMABP) content are varied in Dis.P of St. The growing particles are thought to be contracted in a polar medium. Hydrophilic surface of growing particles is formed by copolymerization of St with sodium p-styrenesulfonate (NaSS). Moreover, in the diffusion of cross-linker, cross-linking immediately occurred in exterior of growing particles. These factors are unfavorable to the diffusion. Therefore, the crosslinker content is high in exterior and low in interior of growing particles, producing mitigated and spatially inhomogeneous 926
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99.9%), propargyl methacrylate (PMA, 98%), ethylene glycol dimethacrylate (EGDMA, 98%), and divinylbenzene (DVB, 55% mand p-DVB, 42% ethylvinylbenzenes, 3% diethylbenzenes) were purchased from Alfa Aesar and used without further purification. Dimethacryloyloxybenzophenone (DMABP) was synthesized and characterized as previously reported.34 Azobis(isobutyronitrile) (AIBN, 99.9%, Sinopharm Chemical Reagent Co., Ltd.) was purified by successive recrystallizations from methanol (MeOH). 3-Mercaptopropionic acid (HSCH2CH2COOH, 99%, Alfa Aesar), 2-aminoethanethiol (NH2CH2CH2SH, 96%, Beijing Bailingwei Chemical Technology Co., Ltd), 1,6-hexanedithiol (HS(CH2)6SH, 96%, Heowns Biochem Technologies. LLC. Tianjin), and polyethylene glycol 200 (PEG (200), 98%, Tianjin Guangfu Fine Chemical Research Institute) were used as received. MeOH and tetrahydrofuran (THF) were obtained from Beijing Chemical Reagents Co., Ltd. Deionized (DI) water was employed in all experiments. 2.2. Preparation of Anisotropic Particles. Briefly, St (2.5 mL), AIBN (0.0453 g), NaSS (0.0453 g), MeOH (15 mL), and deionized water (10 mL) were added to a 100 mL three-neck round-bottom flask equipped with a reflux condenser and a mechanical stirrer. The flask was then immersed in a water bath. The mixture was stirred at a speed of 200 rpm. The polymerization was started at 75 °C. After 40 min, (I) DVB (4-10%), (II) EGDMA (4-24%), (III) DMABP (1-6%) dissolved in St (0.5 mL), or (IV) DVB (6%) and GMA (6%), (V) EGDMA (6%) and PMA (6%) was added dropwise. The contents of reagents added were based on the weight of initial St. The polymerization was conducted for 400 min. Final particles, denoted as (I) poly(St-coDVB), (II) poly(St-co-EGDMA), (III) poly(St-co-DMABP), (IV) poly(St-co-DVB-co-GMA), and (V) poly(St-co-EGDMA-co-PMA), respectively, were separated by centrifugation (9000 rpm, 15 min), washed with MeOH for several times, and dried overnight at 25 °C in a vacuum oven. To convert the residual pendant vinyl groups (I) on growing particles, HSCH2CH2COOH (4%) was injected when polymerization had run for 340 min. NH2CH2CH2SH (4%) or HS(CH2)6SH (4%) was added at 340 min to transform epoxy (IV) or alkynyl (V) groups. The resultant particles were denoted as poly(St-co-DVB)-COOH, poly(St-co-DVB-co-GMA)-NH2, and poly(St-co-EGDMA-co-PMA)SH, respectively. The incorporated BP groups (III) on final particle (0.05 g) were coupled with PEG (200, 0.5 g) in MeOH (20 mL) under UV irradiation (375 W high-pressure mercury lamp, incident light intensity 12.5 W/m2, λ = 254 nm) for 30 min. During the exposure to UV light, nitrogen gas was bubbled. The resultant particles were purified with DI water. 2.3. Characterization. Dried particles (W0) were immersed in THF (renewed 3 times) at room temperature for 24 h under stirring. After that, the particles were centrifuged at 10 000 rpm for 15 min, washed 3 times with excess THF and 1 time with MeOH, dried at 40 °C overnight in a vacuum oven, and weighed (W1). Gel content of particles (the fraction of cross-linked polymers in particles), representing the cross-linking degree of particles, was determined by the following formula:
cross-linking. When monomer conversion is not very high, such cross-linked growing particles can still be sufficiently swollen by monomers continuously diffusing into them, and by oligomers formed inside them. Their phase separation is therefore allowed to release viscoelastic stress created on polymer network; that is, the monomers and oligomers or low molecular weight polymers can be expelled from the growing particles, and snowman- or dumbbell-like particles are generated (Scheme 1, I−III). Hydrophilic surface of the cross-linked and swollen growing particles and presence of excessive polar medium around them promote their elastic retraction, and due to a high contact angle between the surface and protrusion,23,26 a distinct protrusion rather than a shell is formed. This direct one-pot Dis.P for preparation of anisotropic particles essentially differentiates from the seeded polymerization in which presynthesized, linear polymer spherical particles are used, swollen with a monomer/cross-linker mixture, and then undergo polymerization. In the seeded polymerization, the seed particles contain high molecular weight polymers. Usually, they are uniformly and fully swollen at first to maintain particle monodispersity and reaction stability, and rapidly and homogeneously cross-linked afterward, producing monodisperse large spherical particles with interpenetrating network (IPN) structure.12,13,28 The rapid consumption of monomer and cross-linker and formation of uniform IPN hinder the phase separation of seed particles.13 Using the resultant cross-linked spherical particles as seeds and by swelling, heating, and polymerization, anisotropic particles are synthesized.12−14,28,29 In the present system, as polymerization proceeds, surface roughness and sizes of each domain grow, and the newly formed domain is cross-linked gradually. The cross-linking occurred first in growing particle and then in new domain. This helps to maintain the stability and monodispersity of final particles. With the addition of other monomers such as glycidyl methacrylate (GMA) or propargyl methacrylate (PMA), desired functional groups are incorporated onto/into final anisotropic particles (Scheme 1, IV−V). The functional groups such as pendant vinyl group, epoxy or alkynyl group, and photosensitive BP groups are distributed mainly or entirely on the growing particle domain, depending on cross-linking behaviors. They can serve as reactive sites for further surface modification. Compared with previous seeded polymerization approaches involving multiple steps, this one-pot method for synthesis of anisotropic polymer particles with asymmetrically incorporated functional groups is Dis.P-based, simple, straightforward, and thus advantageous to achievement of high yields. Cross-linking density, morphology, surface roughness and functionalities, and dimension of each domain of particles can be tuned by controlling key factors such as start addition time and contents of cross-linkers (which greatly affect the swelling and crosslinking of growing particles). Here, the influences of dynamically changing swelling ability and cross-linking degree of growing particles on formation of anisotropic particles are studied. Some examples of anisotropic particles with asymmetrically functionalized surfaces are shown.
gel content (%) =
W1 × 100 W0
Morphology of particles was observed using scanning electron microscope (SEM, Hitachi S-4700, at 20 kV accelerating voltage) and transmission electron microscope (TEM, JEOL JEM-2100, at 200 kV accelerating voltage). Elemental analysis was done under STEM mode using an energy dispersive X-ray (EDX) detector. Samples were prepared by placing a droplet of particle suspension in MeOH onto a clean glass slide, or by spreading diluted particle suspension in MeOH onto carbon-coated copper grids, and then evaporating solvent at ambient temperature. Samples for SEM observation were sputtercoated with platinum. Average particle size was evaluated statistically by counting at least 30 individual particles on SEM images using image processing software Nano Measure 1.2. FTIR analyses were performed on a Nexus 670 spectrometer. The data were collected with a resolution of 4 cm−1 from 4000 to 400 cm−1. XPS analyses were
2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (St, 98%, Tianjin Yongda Chemical Reagent Co., Ltd.) was distilled under reduced pressure before use. Glycidyl methacrylate (GMA, 96%) was supplied by Beijing Bailingwei Chemical Technology Co., Ltd. Sodium p-styrenesulfonate (NaSS, 927
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Figure 1. Morphology evolutions of particles produced during polymerization with DVB (8%, a1−a5), EGDMA (6%, b1−b5), or DMABP (5%, c1− c5) added at 40 min. The upper and lower right inserts are TEM images of particles before and after immersion in THF. Scale bars in SEM images are 500 nm. Scale bars in TEM images are 200 nm. conducted on a Thermo VG ESCALAB 250 system operating in constant analyzer energy mode (Al Kα radiation source, 9 × 10−9 mmHg of analysis chamber pressure).
boundaries were not so distinct. After being immersed in THF, the snowman-like particles obtained at 100 min lost their small heads, while the later produced particles retained most of their heads. It seemed that the top of head was removed by THF. The body size reduced a little, while the head size reduced more. These suggested a gradient, incomplete cross-linking which started from the growing particle and then occurred in the newly formed domain. In addition, particle gel content increased rapidly upon addition of DVB, and reached a maximum value at 120 min. After 120 min, the gel content showed some decrease and then became almost level (Figure 2d). When EGDMA (6%) was added, a small protrusion appeared at 85 min (Figure 1b1). Later, sizes of the head and body displayed no evident changes, and particle aspect ratio (L/D) reached around 1.14 (Figure 2b). The body turned to a distinct raspberry shape after 100 min (Figure 1b2−b5). After immersion in THF, the small head was not dissolved entirely, but its size showed a great reduction (Figure 2b), while the body showed only a slight reduction in size. This indicated that the growing particle domain had an almost complete and denser cross-linking than the new domain. Particle gel content
3. RESULTS AND DISCUSSION 3.1. Formation of Anisotropic Particles. In the polymerization of St in MeOH and water (6/4, vol), copolymer of NaSS and St acted as a stabilizer. When the polymerization had run for 40 min and reaction mixture became opal (conversion, 18%; particle size, 200 nm; Mn, 13.9−14.2 × 103; Supporting Information Figure S1 and Table S1), a cross-linker of DVB, EGDMA, or DMABP was added (Scheme 1, I−III). As the evolutions of particles during polymerization were monitored (Figure 1, Supporting Information Figure S2, and Figure 2), it was seen that when DVB (8%) was added, a small bulge protruded from growing particle at 100 min, forming snowmanlike particles with smooth head and body (Figure 1a1). From 100 to 120 min, both head and body (the original growing particle) grew, particle aspect ratio (L/D) reached about 1.2, and head grew faster than body (Figure 2a). The body turned to be raspberry-like after 120 min (Figure 1a2−a5). Nodules greater than 100 nm were anchored on surface, but their 928
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Figure 2. Evolutions of body size (D, D′), head size (d, d′), aspect ratio (L/D) (a−c), and gel content (d) of snowman-like particles prepared during polymerization with (a) DVB (8%), (b) EGDMA (6%), or (c) DMABP (5%) added at 40 min.
separation of growing particles (expelling monomers and oligomers or relatively low molecular weight polymers) occur. After the addition of DVB (8%), or EGDMA (6%), or DMABP (5%), cross-linking and consumption of monomers inside growing particles increased gradually (Figure 2d, Supporting Information Figure S1). This led to a contraction of growing particles, increase of ΔG̅ el, and simultaneous decrease of ΔG̅ m. The decrease of ΔG̅ m was against the improvement of ΔG̅ m,p and thus against the phase separation of growing particles. However, when particle cross-linking degree (indicated by gel content) and monomer conversion were low, residual monomers in continuous medium were allowed to continuously diffuse into the cross-linked growing particles, promoting their expansion, causing an increase of ΔG̅ m. Consequently, a positive ΔG̅ m,p could be achieved and phase separation of the growing particles could occur. For example, when DVB was added and polymerization had been performed for about 100 min, particle gel content and monomer conversion were only about 59% and 61%, respectively. The growing particles were still sufficiently swollen, and therefore, a small protrusion on them was formed (Figure 1a1). Similarly, protrusions appeared at 85 min (Figure 1b1), at a gel content of 49% and monomer conversion of 45% when EGDMA was added, or appeared at 120 min (Figure 1c1), at a gel content of 69% and monomer conversion of 46% when DMABP was added. As polymerization proceeded, the size of growing particle domain increased (Figure 2a,b), and its surface became coarse (Figure 1a2−a5,b2−b5). This signified an increase of crosslinking in/on growing particle and hence an increase of phase separation degree (size of head). Moreover, the new linear domain that protruded from growing particles was cross-linked due to a preferential diffusion of cross-linker into it (Figure 1a2,b2). Both the cross-linking and polymerization in the two
increased rapidly once EGDMA was added, and after 100 min, this increase tended to slow down (Figure 2d). When DMABP (5%) was added, snowman-like particles were formed at 120 min (Figure 1c1). Their heads grew very quickly and were comparable to bodies in size at 160 min (Figures 1c3 and 2c), forming dumbbell-shaped particles. After 160 min, the two domains showed only some increase in size, and particle aspect ratio (L/D) reached 1.4. After 240 min, the new domain surface appeared to be a little bit coarse (Figure 1c4,c5), while the original domain surface remained smooth. The new domain was dissolved in THF, revealing that it was un-cross-linked (Figure 1c1−c5). The original domain displayed an obvious reduction in size after immersion in THF (Figure 2c), indicating that it was not completely crosslinked. Particle gel content increased fast at first, reached a maximum at 140 min, and then decreased and became level (Figure 2d). 3.2. Analyses for Formation of Anisotropic Particles. To clarify the above evolutions of particles, the dynamic changes in cross-linking and swelling of growing particles during polymerization are discussed. Upon addition of a cross-linker at 40 min, the swollen growing particles started to be cross-linked. In thermodynamic aspect, the free energy of monomers and oligomers within the cross-linked growing particles (ΔG̅ m,p) correlated closely with the monomer-polymer mixing energy (ΔG̅ m) and polymer network elastic-retractile energy (ΔG̅ el), according to a thermodynamic model developed by Sheu et al.12,13 ΔG̅ el made a positive contribution to ΔG̅ m,p (restraining particle expansion), and was determined by cross-linking degree of growing particles. ΔG̅ m made a negative contribution to ΔG̅ m,p (promoting particle expansion), and was governed by swelling ability of growing particles. Only when ΔG̅ m,p > 0 could phase 929
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Figure 3. SEM and TEM images (the upper right inserted) of particles produced with DVB (8%, a1−a4), EGDMA (6%, b1−b4), and DMABP (5%, c1−c4) added at various reaction times. The lower right inserts are TEM images that indicate morphologies of particles after being immersed in THF. Scale bars in SEM images are 500 nm, and scale bars in TEM images are 200 nm.
DVB isomers had different reactivities, and after one double bond of DVB was reacted, the pendant vinyl group was less reactive than either monomer. These caused insufficient crosslinking of growing particles (Figure 2a). Upon phase separation, DVB in continuous medium could diffuse into the new domain, making it cross-linked (Figure 1a2). Since the residual DVB content was low, the new domain was partially cross-linked (Figure 2a). Especially, the top of the new domain was un-cross-linked (Figure 1a2−a5). This phenomenon suggested that phase separation of the cross-linked new domain occurred as that of growing particle, but the degree was quite low due to low cross-linking and difficult extrusion of polymer.12,13,23 As a result, merely cross-linked and un-crosslinked (top) portions were separated; that is, gradient crosslinking from the growing particle to new domain was formed. After 120 min, particle gel content decreased (Figure 2d), which was ascribed to the rapid consumption of DVB and enhanced homopolymerization of St. In addition, the rapid cross-linking could affect the swelling ability of growing particles and thus impede further entry of DVB. When DVB at interfacial layer and in continuous medium was copolymerized, and the resultant copolymers were grafted from or coagulated/precipitated onto the growing particles, nodules were formed on the surface (Figure 1a2−a5).
domains contributed to their growth, and to the increases of particle gel content and monomer conversion (Figure 2, Supporting Information Figure S1). After 120 min (DVB added) or 100 min (EGDMA added), further consumption of monomers gave rise to an insufficient expansion of particles. So, extrusion of monomers and polymers from the growing particle became difficult. There was no further increase in phase separation degree and no evident changes in sizes. When DMABP was added, the phase separation and growth of two domains of particles were quite different. This was related to different cross-linking behaviors. From the kinetic aspect, diffusion and reactivity of crosslinkers determined where and how fast the cross-linking occurred in growing particles and therefore controlled the evolutions of particle morphology, surface roughness, and sizes. In our experiments, growing particles were well-swollen at 40 min. This allowed the added cross-linker to diffuse into and produce cross-linking inside (Figure 1). However, this crosslinking was not complete and dense (Figure 1a1, b1, b2 c1-c5, the lower right inserted TEM images) since the diffusion of cross-linker was impeded by contraction, surface hydrophilicity, and immediate exterior cross-linking of growing particles. When DVB was added, it was consumed rapidly (Figure 2d) owing to its high reactivity ratio for copolymerization with St (rst/rp‑DVB = 0.15/1.00, rst/rm‑DVB = 0.6/0.88).32 The p- and m930
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Figure 4. Body size (D, D′), head size (d, d′), aspect ratio (L/D) (a−c), and gel content (d) of particles prepared by polymerization with (a) DVB (8%), (b) EGDMA (6%), or (c) DMABP (5%) added at various times.
new domain size was enlarged (Figures 1c2 and 2c). After 140 min, particle cross-linking was reduced (Figure 2d) due to increase of homopolymerization. This means a decrease of ΔG̅ el and a reduction of phase separation degree. However, the new domain grew continuously, and dumbbell-shape particles were formed at 160 min (Figures 1c3 and 2c). This was likely to be related to the further absorption and polymerization of St in the new domain. As for the growing particle domain, there were two factors influencing its size. One was the volume reduction caused by extrusion of monomers and oligomers. Another was the volume enlargement that resulted from absorption and polymerization of monomers. A balance of these two factors led to an almost constant size of growing particle domain at 120− 160 min. After 160 min, both of the two domains grew bigger. 3.3. Tuning of Particle Morphology, Dimension, and Surface Roughness. During polymerization, with the consumption of monomers, polymer molecular weights (Supporting Information Table S1) and viscosity in growing particles increased. The swelling ability of growing particles reduced gradually, and diffusion of monomers into the growing particles became somewhat difficult. Therefore, when and how much cross-linker was added could greatly influence the swelling and cross-linking of growing particles, and in turn influence the formation of anisotropic particle. As shown in Figure 3, Supporting Information Figure S3, and Figure 4, when DVB (8%), EGDMA (6%), or DMABP (5%) was added after 40 min, denser cross-linking appeared at the edge of growing particles (Figure 3a1−a4,b1−b3,c1−c2). Considering the difficulty in diffusion of cross-linker at high conversions, the content of cross-linker at the edge was higher than that in the center of growing particles.34 This uneven distribution of cross-linker resulted in the denser cross-linking at edge. The protrusion on growing particles became smaller
Compared with DVB, EGDMA has a more favorable reactivity ratio for reaction with St (rst/rEGDMA = 0.40/ 0.64).32 Reactivities of two methacrylate groups of EGDMA are essentially identical. So, reaction of EGDMA led to higher cross-linking degrees before 100 min (Figure 2d) and an almost complete cross-linking of growing particle (Figure 2b). This allowed an achievement of higher ΔG̅ el, and thereby enabled the growing particle phase to separate earlier (Figure 1b1). However, the high cross-linking severely hindered the expelling of monomers and oligomers from growing particles, giving rise to small head (Figures 1b1−b5 and 2b). The graft or coagulation/precipitation of EGDMA copolymer or very tiny cross-linked nuclei yielded distinct nodules smaller than dozens of nanometers on the surface (Figure 1b2−b5). The occasional presence of one or two relatively larger nodules (Figure 1b3− b5) suggested a tendency toward formation of multiple protrusions under high cross-linking. DMABP is a kind of benzophenone (BP)-containing crosslinker. It can be used as a binder for photochemical immobilization of polymer or biomolecules.35 From the study on copolymerization of 4-methacryloyloxybenzophenone (MABP) with St (rst = rMABP = 0.33),38 it is assumed that there is some tendency toward alternating copolymerization of DMABP with St. Therefore, at a low content relative to St, DMABP (5%) was consumed first. Concurrently, marked homopolymerization of St occurred. These resulted in low, insufficient cross-linking of growing particle (Figures 1c1−c5 and 2c,d), and hence low ΔG̅ el and late phase separation. Moreover, the newly formed domain was not cross-linked. Therefore, the two domains of particles maintained good swelling ability, permitting monomers to diffuse into them for growth. After 120 min, with the increase of cross-linking, phase separation degree of growing particles was improved, and the 931
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Figure 5. SEM and TEM images (the upper right inserted) of particles produced with various contents of DVB (4%, 6%, 10%), EGDMA (4%, 12%, 18%), or DMABP (1%, 2%, 4%) added at 40 min. The lower right inserts are TEM images of particles after immersion in THF. Scale bars in SEM images are 500 nm, and scale bars in TEM images are 200 nm.
Figure 6. Body size (D, D′), head size (d, d′), aspect ratio (L/D) (a−c), and gel content (d) of particles prepared by polymerization with various contents of DVB (a), EGDMA (b), or DMABP (c) added at 40 min.
decreased (Figure 4d) due to improved amount of homopolymer presented in growing particles. This decrease brought about a reduction of ΔG̅ el, and hence a diminution of phase separation degree. Additionally, at high monomer conversions, molecular weights of linear polymers and viscosity inside the growing particles were high, and thus the mobility and extrusion of the polymers was difficult. This was also
(Figures 3a1-c1,a2−c2 and 4a−c), and was inconspicuous when cross-linkers were added after 90 min (Figure 3a3−c3,a4−c4). Only nearly spherical particles with asymmetrical surface roughness and cross-linking density (partially, sparsely crosslinked, or partially uncross-linked) were presented, and particle aspect ratio (L/D) was reduced to 1. When the cross-linkers were added later, the cross-linking degree of particles was 932
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6c). Head size increased evidently at first, and then showed only slight increase with the increase of DMABP content. Body size changed a little when DMABP content was increased from 1% to 2%. This could be ascribed to a balance between the shrinkage caused by extrusion of monomers and oligomers or polymers and the growth of growing particles. When the extrusion was enhanced, the body size reduced while head size increased, and hence a dumbbell-like particle formed. The addition of more DMABP was favorable to the growth of two domains, and this caused a decrease of particle aspect ratio (L/ D). The newly formed domain became somewhat coarse when DMABP content was higher than 4% (Figures 5c3 and 1c5). Gel content of particles displayed a tendency to increase with the increase of DMABP content (Figure 6d). 3.4. Asymmetrical Incorporation of Functional Groups. Dis.P is an efficient and simple approach for incorporation of functional groups onto particles by addition of reactive agents. In our experiments, the snowman-like particles produced using DVB contained residual, pendant vinyl groups. These vinyl groups can be utilized for further reaction, such as a click reaction with HSCH2CH2COOH to fix carboxyl groups (−COOH) on particle surface (Scheme 1, I). After the reaction, the particles displayed characteristic absorption of carbonyl groups (OCO) at 1716 cm−1 on FTIR spectrum (Supporting Information Figure S5a, spectrum 3). The ratio of peak area at 1716 cm−1 (Aν−O−CO) to that at 760 cm−1 (Aν−CH‑, CH in PSt, as a reference), Aν−O−CO/ Aν−CH−, reached 0.31 (Table 1, sample 2), confirming the
unfavorable to the phase separation of growing particles. The protrusion was mainly composed of high molecular weight polymers. The high viscosity in protrusion hindered the subsequent diffusion of cross-linker, giving rise to lower cross-linking of the protrusion. The widened differences between sizes of the protrusion before and after being immersed in THF demonstrated the reduction in cross-linking of the protrusion (Figure 4a,b). The growing particle domain showed a slight growth when the cross-linker addition started earlier than 120 min, and a slight reduction over much delayed addition (Figure 4a−c). When DVB or EGDMA was added later, because of enhanced homopolymerization, the differences between the sizes of growing particle domain before and after immersion in THF enlarged (Figure 4a,b). When a cross-linker was added at 40 min and its content was varied, particle morphology, surface roughness, and dimensions were tuned. As shown in Figure 5, Supporting Information Figure S4, and Figure 1, when DVB content was increased from 4% to 8% (Figures 5a1,a2 and 1a5), final particles changed from spherical raspberry shape into snowman shape with smooth head and coarse body. This indicated that the phase separation degree of growing particles increased with the increase of DVB content. The extrusion of monomers and oligomers or polymers from growing particles resulted in a slightly reduced body size (Figure 6a). Particle gel content increased gradually (Figure 6d). Both the head and body were not entirely crosslinked. The roughness of body surface was improved under high DVB contents. When DVB content was increased to 10%, snowman-like particles were also fabricated, but conglutination between particles presented (Figure 5a3), suggesting deterioration of reaction stability. Due to the presence of conglutination, TEM images of particles were not included here. When EGDMA content was 4%, only rough, spherical particles were generated (Figure 5b1). Increasing EGDMA content to 6%, snowman-like particles with smooth head and raspberry-like body were produced (Figure 1b5). With an increase of EGDMA content to 12%, both the head and body of snowman-like particles grew bigger (Figures 5b2 and 6b), and nodules with a clear boundary were observed on the body. When EGDMA content was further increased, spherical particles which looked more like a raspberry with nodules greater than 100 nm were fabricated (Figure 5 b3). Under a high EGDMA content, there seemed to be a tendency of forming conglutination between particles. Gel content of particles displayed a gradual increase when EGDMA content was less than 12%, and then showed slight changes (Figure 6d). The increased cross-linking of growing particles facilitated their phase separation. But too much cross-linking impeded the extrusion of monomers and oligomer or polymers, and thus made a negative effect on the phase separation of growing particles. Differing from the situations using DVB and EGDMA, when DMABP content was 1%, smooth spherical particles with core− shell (densely cross-linked) structure appeared (Figure 5c1). When DMABP content was greater than 1%, marked phase separation and formation of snowman- or dumbbell-like particles were observed (Figures 5c2,c3 and 1c5). No crosslinked shell (dense exterior cross-linking) of growing particles was clearly shown, owing to a promoted diffusion of the crosslinker at high contents and hence enhanced interior crosslinking of growing particles. When DMABP content was increased to 4%, particle aspect ratio (L/D) reached 1.6 (Figure
Table 1. FTIR, XPS Analyses on Anisotropic Particles with Functional Groups XPS anisotropic particle samples 1. poly(St-co-DVB) 2. poly(St-co-DVB)COOH 3. poly(St-co-DVB-coGMA) 4. poly(St-co-DVB-coGMA)-NH2
FTIR
C/O
C/S
18.63 4.43
148.41 68.61
8.24
182.75
6.05
164.83
11.48
184.42
7.35
78.54
ν−O−CO 1740 cm−1
11.04
188.56
νAr−CO−Ar 1667 cm−1 AνAr−CO−Ar/Aν−CH‑ 0.06 ν−O−CO 1740 cm−1
8.20
209.76
Aν−O−CO/Aν‑CH‑ 0.31 ν−C−O−C− 1082−1200 cm−1 Aν−O−CO/Aν−CH− 0.43 ν−C−O−C− 1082−1200 cm−1 Aν−O−CO/Aν−CH− 0.45
5. poly(St-coEGDMA) 6. poly(St-coEGDMA)-COOH 7. poly(St-coEGDMA-co-PMA) 8. poly(St-coEGDMA-co-PMA)COOH 9. poly(St-coEGDMA-co-PMA)SH 10. poly(St-coDMABP)
11. poly(St-coDMABP)PEG(200)
AνCC−H/Aν−CH‑ 0.27 Aν−O−CO/Aν−CH‑ 1.62 AνCC−H/Aν−CH− 0.18 Aν−O−CO/Aν−CH‑ 1.67 AνCC−H/Aν−CH‑ 0.16
νAr−CO−Ar 1667 cm−1 AνAr−CO−Ar/Aν−CH‑ 0.02 933
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Langmuir incorporation of COOH groups by thiol−ene coupling. XPS analysis indicated the improvements in contents of O and S (decreases of C/O and C/S ratios, Table 1, sample 2), also proving the attachment of COOH groups. SEM observation revealed that the body surface of snowmanlike particles became much coarser (Figure 7a, Supporting
shown in Figure 8a,b, when Fe3O4 or Ag nanoparticles (NPs) were loaded on snowman-like particles with −COOH groups, their enrichment on the body was shown in bright-field images. EDX spectrum analyses on 6 randomly selected particles indicated that Fe or Ag content on the body (average C/Fe ≈ 84−86, or average C/Ag ≈ 130−150, area 2, 3) was higher than that on the head (average C/Fe ≈ 161, or average C/Ag ≈ 313, area 1). The snowman-like particles containing asymmetrically distributed −COOH groups can be easily dispersed in water (Supporting Information Figure S8a). In contrast, the snowman-like particles without −COOH groups cannot be dispersed in water (Supporting Information Figure S8b), but can be dispersed in a mixture of MeOH and water (Supporting Information Figure S8c). These results implied an improvement in surface wettability of snowman-like particles with the incorporation of −COOH groups. When a functional monomer, such as GMA (6%) was added together with DVB (6%), snowman-like particles with coarse body (raspberry-like) and smooth head were produced (Figure 7b and Supporting Information Figure S6b). The head (178 nm) was smaller than that of particles fabricated without using GMA (Figure 5a2, Supporting Information Table S2). The cross-linking of body barely changed (values of D-D′ were comparable), while the cross-linking of head was reduced (value of d-d′ was enlarged), implying a greater cross-linking gradient. Particle gel content showed a slight decrease. The FTIR spectrum presented the absorption of OCO at about 1732 cm−1 (Supporting Information Figure S5a, spectrum 4). Aν−O−CO/Aν−CH− reached 0.43 (Table 1, sample 3). New absorptions related to COC appeared at about 1082−1200 cm−1. Further, amino groups were introduced onto particles by a click reaction with thiol groups of NH2CH2CH2SH (Scheme 1, IV). Ninhydrin colorization test, in which the color of the sample changed to navy blue (Supporting Information Figure S9), proved the introduction of amino groups. To introduce functional groups onto/into snowman-like particles prepared using EGDMA (6%), PMA (6%) was added at 40 min (Scheme 1, V). Snowman-like particles were also produced (Figure 7c, Supporting Information Figure S6c). These particles had almost the same coarse body and smooth head as particles obtained when only EGDMA was used (Figure 1b5), but their body and head were much bigger (Supporting Information Table S2). The body showed no marked changes in cross-linking (just a slight reduction of DD′). The head displayed a clear (un-cross-linked) core− (crosslinked) shell structure (Figure 7c, the lower right inserted TEM
Figure 7. SEM and TEM images of particles synthesized with (a) DVB (8%), (b) DVB (6%) and GMA (6%), (c) EGDMA (6%) and PMA (6%), or (d) DMABP (6%) added at 40 min, and incorporated groups of (a) −COOH, (b) −NH2, or (c) −SH by thiol−ene coupling, or of (d) PEG (200) by photocoupling. The lower right inserted are TEM images of particles after immersion in THF. Scale bars in SEM images are 250 nm, and scale bars in TEM images are 200 nm.
Information Figure S6a) compared with that of particles without −COOH groups (Figure 1a5, Supporting Information Figure S2a). The head surface turned from smooth to coarse. After immersion in THF, the body became less coarse, and the head became smooth again (Supporting Information Figure S7a). These phenomena suggested some precipitation of copolymers containing −COOH groups, and asymmetrical distribution of −COOH groups on body and head. From the evolution of particles during polymerization (Figures 1 and 2), it was seen that, first, high cross-linking occurred in growing particle domain, and then low cross-linking occurred in new domain. Especially, no cross-linking was produced on the top of the new domain. This led to a gradient distribution of pendant vinyl groups and the later introduced −COOH groups. The asymmetrical distribution of functional groups was also manifested from STEM observation and elemental analysis. As
Figure 8. STEM bright-field images and EDX spectra of particles attached with Fe3O4 or Ag NPs: (a) poly(St-co-DVB)-COOH-Fe3O4 NPs, (b) poly(St-co-DVB)-COOH-Ag NPs, and (c) poly(St-co-EGDMA-co-PMA)-SH-Ag NPs. 934
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Langmuir
steps. By conventional two-stage Dis.P using DVB or EGDMA, only spherical cross-linked particles were produced. In the present work, snowman- or dumbbell-like particles were generated on the basis of one-pot, two-stage Dis.P. The method is simple, time-saving, and thus advantageous to production of anisotropic particles with high yields. The key to phase separation of growing particles during polymerization is the control of cross-linking behavior. Adding a cross-linker with low reactivity, such as DMABP, or varying the time when crosslinkers were added (onset of cross-linking) and the content of a cross-linker relative to main monomer, the cross-linking, swelling, and interior polymer mobility of growing particles were manipulated. The morphology, surface roughness, and dimensions of anisotropic particles were hence continuously tuned. Moreover, when a functional monomer was added together with cross-linker, anisotropic particles with functional groups such as epoxy or alkynyl were synthesized conveniently. The sequential cross-linking of growing particle and newly formed domain occurred during polymerization, resulting in gradient distributions of functional groups on/in particles. Particularly, when DMABP was used, only the original domain of resultant dumbbell-like particles was cross-linked, confining photosensitive BP groups to a specific domain. Further, the asymmetrically distributed functional groups were used as active sites for exemplified click or photocoupling reactions. Our one-pot technique provides a simple and efficient way for synthesis of anisotropic particles with diverse morphologies and various functional groups.
image), quite different from the structure formed using only EGDMA. This gave rise to a low value of d-d′. Particle gel content was reduced from 75% to 71%. In the presence of PMA, the cross-linking caused by EGDMA could be alleviated. This allowed more monomers to diffuse into growing particles, forming a big body. Moreover, the extrusion of linear oligomers or polymers became easy, consequently producing a big head. The subsequent cross-linking in the head was difficult owing to impeded diffusion of EGDMA into the head which contained more polymers, producing a cross-linked shell and an un-crosslinked core of the head, and resulting in a reduction of particle gel content. The above changes in cross-linking, dimensions, and structure of snowman-like particle were different from those that resulted from the addition of GMA and DVB. When DVB was added, the cross-linking of growing particles was lower (higher D-D′ value), compared with the case when the same amount of EGDMA was added (Figure 6a,b, Supporting Information Table S2). The addition of GMA could reduce the cross-linking of growing particles, diminishing their phase separation degree and forming a smaller head than that obtained using DVB only. In addition, the head was less crosslinked and presented no evident shell. Upon the incorporation of PMA, the characteristic absorption of ≡CH at 3290 cm−1 appeared in the FTIR spectrum of particles (Supporting Information Figure S5b, spectrum 4). Then, COOH or SH was introduced to the surface through (HSCH2CH2COOH, or HS(CH2)6SH) thiol−yne reaction. The absorption strength of ≡CH was reduced. AνC−H/ Aν−CH− was reduced from 0.27 to 0.18 or 0.16 (Table 1, samples 7−9). Correspondingly, Aν−O−CO/Aν−CH− was increased (Table 1, samples 7, 8). Furthermore, the contents of element O and S were increased (C/O and C/S ratios were decreased, Table 1, sample 8) after the coupling of COOH groups. STEM images showed that more Ag NPs were attached on the body (Figure 8c). EDX spectrum analyses showed that Ag content on the body (average C/Ag ≈ 53−66, areas 2 and 3) was higher than that on the head (average C/Ag ≈ 222, area 1). As for anisotropic particles generated using DMABP, the newly formed domain was un-cross-linked (Figures 1, 3, 5, and 7), and therefore, photosensitive BP groups presented only in the original growing particle domain. The absorptions at about 1667 cm−1(νAr−CO−Ar) and 1740 cm−1(ν−O−CO) on FTIR spectrum confirmed the incorporation of BP groups (Supporting Information Figure S5c, spectrum 2). With the aid of these BP groups, an extensively used antifouling polymer of PEG (200) was coupled (Scheme 1, III) by hydrogen abstraction and coupling reaction under UV-irradiation.39 The appearance of absorptions at about 3390 cm−1(ν−OH) (Supporting Information Figure S5c, spectrum 3), the reduction of AνAr−CO−Ar/Aν−CH−, and the increase of element O content (Table 1, samples 10, 11) testified to the coupling of PEG (200). Consequently, the anisotropic particles changed from hydrophobic into amphiphilic ones, which were used as a solid surfactant to stabilize oil drops in water (Supporting Information Figure S10).
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ASSOCIATED CONTENT
S Supporting Information *
Details about materials, preparation of spherical and anisotropic particles by dispersion polymerization, attachment of Fe3O4 or Ag nanoparticles (NPs), ninhydrin colorization test (digital photos of samples), and stabilization of toluene drops in water by anisotropic particles (digital photo and optical micrograph). Monomer conversion and particle size with polymerization, molecular weight, and its distribution of PSt in microspheres. FTIR spectra, SEM and TEM images of particles, digital photos of dispersions. This material is available free of charge via the Internet at http://pubs.acs.org
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors acknowledge funding from National Natural Science Foundation of China (Grants 20974008, 51273012) and State Key Laboratory of Chemical Resource Engineering (0-6463, CRE-2012-C-205).
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4. CONCLUSION A facile and straightforward one-pot preparation of anisotropic particles with tunable morphology, dimensions, surface roughness, and asymmetrically incorporated functional groups was demonstrated. In previous works, synthesis of anisotropic particles by seeded polymerizations involved multiple process
REFERENCES
(1) Du, J. Z.; O’Reilly, R. K. Anisotropic Particles with Patchy, Multicompartment and Janus Architectures: Preparation and Application. Chem. Soc. Rev. 2011, 40, 2402−2416. (2) Lee, K. J.; Yoon, J.; Lahann, J. Recent Advances with Anisotropic Particles. Curr. Opin. Colloid Interface Sci. 2011, 16, 195−202.
935
DOI: 10.1021/la504317m Langmuir 2015, 31, 925−936
Article
Langmuir (3) Hu, J.; Zhou, S. X.; Sun, Y.Y.; Fang, X. S.; Wu, L. M. Fabrication, Properties and Applications of Janus Particles. Chem. Soc. Rev. 2012, 41, 4356−4378. (4) Kaewsaneha, C.; Tangboriboonrat, P.; Polpanich, D.; Eissa, M.; Elaissari, A. Janus Colloidal Particles: Preparation, Properties, and Biomedical Applications. ACS Appl. Mater. Interfaces 2013, 5, 1857− 1869. (5) Walther, A.; Müller, A. H. E. Janus Particles: Synthesis, SelfAssembly, Physical Properties, and Applications. Chem. Rev. 2013, 113, 5194−5261. (6) Kim, J. H.; Jeon, T. Y.; Choi, T. M.; Shim, T. S.; Kim, S. H.; Yang, S. M. Droplet Microfluidics for Producing Functional Microparticles. Langmuir 2014, 30, 1473−1488. (7) Liu, B.; Wei, W.; Qu, X. Z.; Yang, Z. Z. Janus Colloids Formed by Biphasic Grafting at a Pickering Emulsion Interface. Angew. Chem., Int. Ed. 2008, 120, 4037−4039. (8) Nie, L.; Liu, S.; Shen, W.; Chen, D.; Jiang, M. One-Pot Synthesis of Amphiphilic Polymeric Janus Particles and Their Self-Assembly into Supermicelles with a Narrow Size Distribution. Angew. Chem., Int. Ed. 2007, 46, 6321−6324. (9) Lin, C. C.; Liao, C. W.; Chao, Y. C.; Kuo, C. Fabrication and Characterization of Asymmetric Janus and Ternary Particles. ACS Appl. Mater. Interfaces 2010, 2 (11), 3185−3191. (10) Zhang, C. L.; Liu, B.; Tang, C.; Liu, J. G.; Qu, X. Z.; Li, J. L.; Yang, Z. Z. Large Scale Synthesis of Janus Submicron Sized Colloids by Wet Etching Anisotropic Ones. Chem. Commun. 2010, 46, 4610− 4612. (11) Tang, C.; Zhang, C. L.; Sun, Y. J.; Liang, F. X.; Wang, Q.; Li, J. L.; Qu, X. Z.; Yang, Z. Z. Janus Anisotropic Hybrid Particles with Tunable Size from Patchy Composite Spheres. Macromolecules 2012, 46, 188−193. (12) Sheu, H. R.; El-Aasser, M. S.; Vanderhoff, J. W. Phase Separation in Polystyrene Latex Interpenetrating Polymer Networks. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 629−651. (13) Sheu, H. R.; El-Aasser, M. S.; Vanderhoff, J. W. Uniform Nonspherical Latex Particles as Model Interpenetrating Polymer Networks. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 653−667. (14) Kim, J. W.; Larsen, R. J.; Weitz, D. A. Uniform Nonspherical Colloidal Particles with Tunable Shapes. Adv. Mater. 2007, 19, 2005− 2009. (15) Kim, J. W.; Larsen, R. J.; Weitz, D. A. Synthesis of Nonspherical Colloidal Particles with Anisotropic Properties. J. Am. Chem. Soc. 2006, 128, 14374−14377. (16) Park, J. G.; Forster, J. D.; Dufresne, E. R. High-Yield Synthesis of Monodisperse Dumbbell-Shaped Polymer Nanoparticles. J. Am. Chem. Soc. 2010, 132, 5960−5961. (17) Liu, B.; Liu, J. G.; Liang, F. X.; Wang, Q.; Zhang, C. L.; Qu, X. Z.; Li, J. L.; Qiu, D.; Yang, Z. Z. Robust Anisotropic Composite Particles with Tunable Janus Balance. Macromolecules 2012, 45 (12), 5176−5184. (18) Yu, H. K.; Mao, Z.; Wang, D. Genesis of Anisotropic Colloidal Particles via Protrusion of Polystyrene from Polyelectrolyte Multilayer Encapsulation. J. Am. Chem. Soc. 2009, 131, 6366−6367. (19) Nagao, D.; Hashimoto, M.; Hayasaka, K.; Konno, M. Synthesis of Anisotropic Polymer Particles with Soap-Free Emulsion Polymerization in the Presence of a Reactive Silane Coupling Agent. Macromol. Rapid Commun. 2008, 29, 1484−1488 33. (20) Kim, S. H.; Hollingsworth, A. D.; Sacanna, S.; Chang, S. J.; Lee, G.; Pine, D. J.; Yi, G. R. Synthesis and Assembly of Colloidal Particles with Sticky Dimples. J. Am. Chem. Soc. 2012, 134, 16115−16118. (21) Tang, C.; Zhang, C. L.; Liu, J. G.; Qu, X. Z.; Li, J. L.; Yang, Z. Z. Large Scale Synthesis of Janus Submicrometer Sized Colloids by Seeded Emulsion Polymerization. Macromolecules 2010, 43, 5114− 5120. (22) Meng, X. H.; Guan, Y. Y.; Zhang, Z. D.; Qiu, D. Fabrication of a Composite Colloidal Particle with Unusual Janus Structure as a HighPerformance Solid Emulsifier. Langmuir 2012, 28, 12472−12478.
(23) 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. (24) Mock, E. B.; Zukoski, C. F. Emulsion Polymerization Routes to Chemically Anisotropic Particles. Langmuir 2010, 26, 13747−13750. (25) Kraft, D. J.; Hilhorst, J.; Heinen, M. A.; Hoogenraad, M. J.; Luigjes, B.; Kegel, W. K. Patchy Polymer Colloids with Tunable Anisotropy Dimensions. J. Phys. Chem. B 2010, 115, 7175−7181. (26) Kraft, D. J.; Vlug, W. S.; Kats, C. M.; Blaaderen, A.; Imhof, A.; Kegel, W. K. Self-Assembly of Colloids with Liquid Protrusions. J. Am. Chem. Soc. 2008, 131, 1182−1186. (27) Park, J. G.; Forster, J. D.; Dufresne, E. R. Synthesis of Colloidal Particles with the Symmetry of Water Molecules. Langmuir 2009, 25, 8903−8906. (28) Peng, B.; Blaaderen, A.; Imhof, A. Direct Observation of the Formation of Liquid Protrusions on Polymer Colloids and their Coalescence. ACS Appl. Mater. Interfaces 2013, 5, 4277−4284 34. (29) Ravensteijn, B. G. P.; Kamp, M.; Blaaderen, A.; Kegel, W. K. General Route toward Chemically Anisotropic Colloids. Chem. Mater. 2013, 25, 4348−4353. (30) Song, J. S.; Tronc, F.; Winnik, M. A. Two-Stage Dispersion Polymerization toward Monodisperse, Controlled Micrometer-Sized Copolymer Particles. J. Am. Chem. Soc. 2004, 126, 6562−6563. (31) Kraft, D. J.; Ni, R.; Smallenburg, F.; Hermes, M.; Yoon, K.; Weitz, D. A.; Blaaderenb, A.; Groenewolda, J.; Dijkstra, M.; Kegel, W. K. Surface Roughness Directed Self-Assembly of Patchy Particles into Colloidal Micelles. Proc. Natl. Acad. Sci. U.S.A. 2012, 109 (27), 10787− 10792. (32) Song, J. S.; Winnik, M. A. Cross-Linked, Monodisperse, MicronSized Polystyrene Particles by Two-Stage Dispersion Polymerization. Macromolecules 2005, 38, 8300−8307. (33) Song, J. S.; Tronc, F.; Winnik, M. A. Monodisperse, Controlled Micron-Size Dye-Labeled Polystyrene Particles by Two-Stage Dispersion Polymerization. Polymer 2006, 47, 817−825. (34) Peng, B.; Wee, E.; Imhof, A.; Blaaderen, A. Synthesis of Monodisperse, Highly Cross-Linked, Fluorescent PMMA Particles by Dispersion Polymerization. Langmuir 2012, 28, 6776−6785. (35) Wang, S. L.; Yue, K.; Liu, L. Y.; Yang, W. T. Photoreactive, Core-Shell Cross-Linked/Hollow Microspheres Prepared by Delayed Addition of Cross-linker in Dispersion Polymerization for Antifouling and Immobilization of Protein. J. Colloid Interface Sci. 2013, 389, 126− 133. (36) Wang, S. L.; Yang, X. F.; Liu, L. Y.; Yang, W. T. High Immobilization of Antibacterial Moieties onto Monodisperse Microspheres by Dispersion Polymerization Using Bicationic Viologen Surfmer. Chin. J. Polym. Sci. 2012, 30, 865−872. (37) Liu, Y. N.; Yang, Q.; Zhu, J. M.; Liu, L. Y.; Yang, W. T. Facile Synthesis of Core-Shell, Multiple Compartment Anisotropic Particles via Control of Cross-Linking and Continuous Phase Separations in One-Pot Dispersion Polymerization. Colloid Polym. Sci. 2014, DOI: 10.1007/s00396-014-3444-0. (38) Morales, C.; Castro, E.; Kaluyzhnaya, E.; Cuerrero-Santos, R. Synthesis of Graft Copolymers. Part I. Synthesis of Macroinitiators. J. Appl. Polym. Sci. 1995, 57, 997−1004. (39) Deng, J. P.; Wang, L. F.; Liu, L. Y.; Yang, W. T. Developments, New Applications of UV-induced Surface Graft Polymerizations. Prog. Polym. Sci. 2009, 34, 156−193.
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DOI: 10.1021/la504317m Langmuir 2015, 31, 925−936
