Fabrication of Liquid Protrusions on Non-Cross-Linked Colloidal

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Fabrication of Liquid Protrusions on Non-Cross-Linked Colloidal Particles for Shape-Controlled Patchy Microparticles Lei Tian, Xue Li, Panpan Zhao, Zafar Ali, and Qiuyu Zhang* Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an 710072, China The Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, Northwestern Polytechnical University, Xi’an 710072, China S Supporting Information *

ABSTRACT: The use of single particles as building blocks for the design and construction of advanced materials is generally recognized as a promising approach. This paper reports a novel double-speed swelling (DSS) technique to fabricate solid−liquid asymmetric monomer-swollen particles (MSPs) with adjustable and removable liquid protrusions on the surface of non-cross-linked poly(glycidyl methacrylate) (PGMA) colloidal particles, although this procedure is thermodynamically unfavorable. Further, PGMA/polystyrene (PS) patchy microparticles with controllable morphologies are fabricated by a versatile and large-scale seeded emulsion polymerization (SEP) on the basis of these MSPs. The size and number of protrusions can be precisely regulated by the swelling ratio and the amount of polymerizable monomer. These patchy microparticles exhibit excellent light reflection and could be applied as new thermal barrier coatings. Additionally, the self-assembled “particle gel” membranes with superoleophilic properties have been employed to absorb model oil and exhibited efficient absorption performance.

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swollen particles, MSPs),34−36 and polymerize to form anisotropic particles with a variety of structures.37,38 The thermodynamic model of seed polymerization was established and improved.39 Next, the swelling polymerization of noncross-linked seed particles was explored in greater detail. Recently, the Yi group40 reported a temperature-controlled swelling process to produce dimpled particles with adjustable size and shape. Microwave-assisted alcohol-thermal treatment has also been proposed as a method to fabricate concave polystyrene@carbon (PS@C) particles, with the concavity attributed to the mechanical deformation of the PS@C particles.30 Moreover, non-cross-linking particles with hydrophilic domains have been selected as seeds, and anisotropic particles have also been synthesized by soap-free SEP.41 Thermodynamically, the monomer-swelling process relies on the seed cross-linking density, with a high cross-linking density contributing to the generation of liquid protrusions on the surface of seed particles.34 However, the high cross-linking density makes it difficult to reshape and recycle such functional particles and directly create functional membranes. Currently, building MSPs based on non-cross-linked seeds is thermodynamically challenging. In addition, SEP does not appear to be applicable to solid−liquid anisotropic particles and other complex particles because of its mechanism. The inability to perform seed swelling polymerization with non-cross-linked

INTRODUCTION Nature provides inspiration for scientists and artists. Learning from nature to meet practical needs is a permanent theme. Pollen and virus particles are among the best examples of sources of biological insipration.1−3 Scientists have designed and fabricated a wide variety of anisotropic particles that mimic the irregular morphologies of pollen and viruses through a range of methods, including physical processes (electrospray,4−6 microfluidics,7,8 lithography,9 solvent evaporation induced phase separation,10,11 and self-assembly;12−15) and chemical approaches, such as seeded polymerization (e.g., seeded emulsion polymerization [SEP]16−20 and seed dispersion polymerization21,22), emulsion polymerization (soapfree emulsion polymerization,23,24 and Pickering emulsion polymerization25,26), and dispersion polymerization.27−29 Seed polymerization is an attractive technique with advantages including flexibility, controllability, and rigorous experimental conditions.30 Researchers have performed systematic research on seed polymerization. Two-step seed swelling,31 dynamic swelling32 and droplet swelling polymerization33 were presented, establishing the theoretical foundations of seed polymerization. In this period, large monodisperse particles were produced for use as particle standards, catalyst carrier, and other applications. Subsequently, cross-linked polymer seeds were used to construct anisotropic particles. The elastic force generated by the swelling of the cross-linked polymer network can be released through the phase separation of the monomer from the seed polymer. As a result, a number of liquid protrusions emerge on the surface of seed particles (monomer© XXXX American Chemical Society

Received: September 21, 2016 Revised: November 14, 2016

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Figure 1. (a) Schematic Scheme of the morphology evolution of MSPs from Janus-like to single-hole particles during the DSS process. (b) Optical microscope (OM) images of MSPs extracted from swelling system over a range of reaction times. The scale bar represents 5 μm. The DBP/St ratio was 2.5/1.5. The swelling was performed at 40 °C for 24 h. (DBP, Tianjin Bo Di Chemical Co., Ltd., China), and anhydrous ethanol (Sinopharm Chemical Reagent Co., Ltd., China) were used directly without further purification. Isocyanate fluorescein (FITC) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used as received. Deionized water was produced by an apparatus for pharmaceutical purified water (Aquapro Co. Ltd., China). Preparation of PGMA Seed Particles. Monodisperse PGMA seed particles were prepared by dispersion polymerization: 4.0 g of PVP was dissolved in the mixture of 110 mL of anhydrous ethanol and 10 mL of water in a 250 mL three-neck round-bottomed flask. Subsequently, AIBN (0.3 g) dissolved in 20.0 g of GMA was added to the above system. After a 30 min nitrogen purge, the system was heated to 80 °C and maintained for 24 h. The resulting product was washed once by centrifugation with ethanol and twice with water and then freeze-dried under vacuum for 12 h. Fabrication of Janus Monomer-Swollen PGMA Particles via a DSS Procedure. A typical procedure was as follows. First, 0.5 g of PGMA was dispersed in 50 mL of aqueous solution containing 2.5 wt % SDS with magnetic stirring. Subsequently, 2.5 g of the swelling agent DBP and 1.0 g of St monomer were added. After swelling at 40 °C for 24 h, the seed particles were extracted to observe their morphology evolution by OM. Polymerization of Monomer-Swollen PGMA Particles To Obtain Patchy Microparticles. After 1.0 g of St and 0.02 g of AIBN were added to the swollen seed emulsion, the reaction system was immersed in a water bath at 80 °C. The polymerization was allowed to continue for 8 h, and then, the final product was washed once with ethanol and twice with water by centrifugation and then freeze-dried under vacuum. Preparation of Fluorescent Patchy Microparticles. Aminemodified PGMA/PS patchy microparticles were described previously. To obtain fluorescent-labeled particles, 0.05 g of modified PGMA/PS microparticles was dispersed in 20 mL of ethanol. After adding 10 mg of FITC dissolved in 20 mL of ethanol, the mixture was held at 40 °C for 8 h. The product was washed 3 times by centrifugation with ethanol and then dispersed in water for fluorescence detection. Preparation of Acid-Treated Patchy Microparticles. In a typical synthesis, 0.5 g of PGMA/PS patchy microparticles was dispersed in 60 mL of concentrated sulfuric acid. The mixture was allowed to stir in the magnetic stirrer at 50 °C for 5 h. Then, the product was washed with water and redispersed in water/ethanol for subsequent uses. Characterization. OM. The morphology and microstructure of the prepared microparticles were directly observed using a metallographic microscope (DMM 300C). To observe the particle morphology, 1.0 mg/mL of microparticles was ultrasonically dispersed in purified water and analyzed.

particles is the main constraint currently, which limits SEP to a great degree. Poly(glycidyl methacrylate) (PGMA) particles have shown unexpected results in previous studies.42−44 Here, we selected non-cross-linked PGMA as seed particles and developed a strategy, called double-speed swelling (DSS), to construct polymerizable MSPs with various asymmetric structures, such as Janus structures, despite this process being not thermodynamic favorable. Morphology evolution of MSPs from Januslike to single-hole particles was observed, and the sizes of liquid protrusions were controllable by the dibutyl phthalate/styrene (DBP/St) ratio. Furthermore, the polymerization of solid− liquid MSPs was initiated to construct shape-controlled PGMA/PS patchy microparticles. The size and number of solidified protrusions on the surface of the PGMA/PS microparticles could be precisely regulated by the DBP/S ratio and the amount of monomer added in the polymerization stage. The strong fluorescence intensity of amino-functional patchy microparticles modified with fluorescein dye demonstrates that the surface of patchy microparticles mainly consists of PGMA, consistent with the microscopic observations of ultrathin sections. These prepared patchy microparticles could present advantages for various applications, including biomacromolecule immobilization and catalyst carriers. In addition, PGMA/PS patchy microparticles exhibit significant light reflection because of the multiple protrusions, which could be a new approach to fabricate thermal barrier coating. Both untreated and acid-treated microparticles readily form selfassembled membranes with superoleophilic properties. Therefore, these materials can be used to efficiently adsorb model oil. More importantly, the unique preparation procedure described here is simple and controllable. Therefore, this patchy material can be fabricated on a large scale to absorb spilled oil, as a thermal barrier coating, and for other applications.

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EXPERIMENTAL SECTION

Materials. Glycidyl methacrylate (GMA, 95%, Sartomer Company, USA) and St (99%, Shanghai Shan Pu Chemical Co. Ltd., China) were distilled under reduced pressure and refrigerated for spar. Azobis(isobutyronitrile) (AIBN, Shanghai Mountain Pu Chemical Co., Ltd., China) was purified by recrystallization in methanol solution. Sodium dodecyl sulfate (SDS, Sinopharm Chemical Reagent Co., Ltd., China), polyvinylpyrrolidone (PVP, BASF Co., Germany), dibutyl phthalate B

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Figure 2. OM images of Janus-like MSPs with different-sized liquid protrusions. The swelling conditions were as follows: (a, a′) DBP/St weight ratio of 2.5/1.0; (b, b′) DBP/St ratio of 2.5/2.5; and (c, c′) DBP/St ratio of 2.5/5.0. Scale bars represent 10 μm in parts a, b, and c and 5 μm in parts a′, b′, and c′. SEM. The microparticle morphology was captured by SEM (AMERY-1000B). The samples were coated on the conductive adhesive and sputtered with platinum by a JFC-1600 auto fine coater at a current of 20 mA for 3 min before examination. DLS. The average diameter and particle size distribution of PGMA/ PS patchy microparticles were determined at room temperature using a LS13320 Laser Particle Size Analyzer (Beckman Coulter). The samples were dispersed in water by ultrasonication with a concentration of 1.0 mg/mL. FTIR. FTIR spectra were acquired on a TENSOR27 FTIR spectrometer (Bruker) using samples that had been pressed into sheets. XPS. The XPS spectra were collected at a 90° takeoff angle using an AXIS Ultra DLD spectrometer (Kratos Analytical Co. Ltd. UK) equipped with a 300 W monochromatic Al Kα X-ray source. The binding energy was determined in reference to the C 1s line at 282.0 eV from adventitious carbon. TEM. TEM was used to investigate the internal structure of PGMA/ PS patchy microparticles, which were imaged on an H-600 TEM

(Hitachi, Japan) at an accelerating voltage of 75 kV. The sample was embedded in epoxy resin, which was polymerized at 70 °C for 3 days. Then, after the sample was microtomed into ∼100-nm thick ultrathin sections using a LKBV ultratome, the sections were placed on TEM grids and stained in RuO4 vapor for 30 min for observation. Reflection Spectra. Reflection spectra were measured using a UV3600 spectrometer at wavelengths of 200−2000 nm, and BaSO4 was used as the spectral reference.

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RESULTS AND DISCUSSION Fabrication of Solid−Liquid Asymmetric MonomerSwollen PGMA Seed Particles. It is generally thermodynamically unfavorable to swell non-cross-linked seed particles into solid−liquid asymmetric monomer swollen particles. However, we recently developed a novel swelling technology called the DSS procedure, which is an efficient way to fabricate Janus-like MSPs based on non-cross-linked PGMA particles. In DSS, unlike the traditional swelling process, both the monomer C

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good solvent for swelling agent DBP. Therefore, the simultaneous addition of the monomer accelerates the absorption rate of both monomer and DBP, and promotes monomer swelling. Figures 1 and S1 illustrate the evolution of the seed particle morphology during DSS: First, monodisperse non-cross-linked PGMA particles are synthesized by dispersion polymerization. The morphology of these particles is shown in Figure S1a, with an average size of 3 μm (Figure S1b). Then, PGMA particles were swollen simultaneously by DBP and St in sodium dodecyl sulfate (SDS) emulsion to form MSPs (Figure 1). Figure 1b shows the transformation of spherical PGMA seed particles into Janus-like MSPs with a large liquid protrusion on the surface. After a sufficient amount of MSPs were extracted from the swelling system, the liquid protrusion began to separate from the seed particles, and this separation continued with time. When the seed emulsion was separated from the swelling environment, the liquid protrusions on the seed particles were unstable, leading to the phase separation of the seed body from the liquid protrusions. Thus, far, we have developed a DSS technology to fabricate solid−liquid MSPs with liquid protrusions growing from non-cross-linked seed particles. During the swelling process, the liquid protrusions are composed of DBP and the swollen monomer. Therefore, the size of the liquid protrusions can be controlled by varying the amount of DBP and St, as demonstrated in Figure 2, in which different amounts of St was added, and the DBP content was constant. When the DBP/St ratio was 2.5/1.0, the swollen PGMA particles (MSPs) exhibited a uniform Janus-like shape (Figure 2a,a′). As the St content increased, the liquid protrusions attached to the PGMA seeds grew continuously (Figure 2b,b′). Further increasing the St content decreased the size of the liquid protrusions but increased the size of MSPs (Figure 2c,c′). These results indicate that increasing the added amount of St further delayed the swelling equilibrium and increased the swelling ratio but accelerated the phase separation of the liquid protrusion and the seed polymer. It was difficult to dislodge the protrusions from the seeds. In addition to St, PGMA seed particles were similarly swelled by GMA. Asymmetric MSPs were also fabricated, including polymerliquid dimer particles (Figure S2). Although Janus particles have emerged as a highly active area of research in recent years, the fabrication of solid−liquid swollen Janus particles has remained challenging.45 DSS provides a simple, convenient, high-efficiency, and novel strategy to construct solid−liquid Janus and other anisotropic particles. Furthermore, if a photoinitiator is introduced into the liquid portion with the swollen monomer, the solid Janus particles are retained under ultraviolet (UV) irradiation. Synthesis of PGMA/PS Patchy Microparticles with Controlled Protrusions. Thus, far, we have solved the first problem: developing the technology to prepare asymmetric MSPs using non-cross-linked seed particles. On the basis of this result, flower-like PGMA/PS patchy microparticles (Figure 3) were obtained when an initiator, azobis(isobutyronitrile) (AIBN), was dissolved in St and added to the swollen seed emulsion. The prepared PGMA/PS microparticles were flowershaped clusters consisting of several units with diameters of 1− 3 μm and an average total size of approximately 5.0 μm. This result indicates that particles with more complicated structures can be fabricated by optimizing the experimental conditions. Using this strategy, the size and number of the protrusions on the surface of PGMA/PS patchy microparticles can be

Figure 3. Scanning electron microscopy (SEM) image of flower-like PGMA/PS patchy microparticles prepared with a DBP/St weight ratio of 1.5/1.5. The components were artificially dyed with different colors. The inset presents the dynamic light scattering (DLS) size distribution of flower-like PGMA/PS microparticles. The polymerization was initiated by adding 0.02 g of AIBN dissolved in 1.0 g of St.

Figure 4. PGMA/PS patchy microparticles of different sizes and with different numbers of protrusions prepared with various DBP/St weight ratios: (a) 5.0/2.5, (b) 5.0/5.0, (c) 2.5/1.0, (d) 2.5/2.5, (e) 2.5/5.0, (f) 1.0/1.0, and (g)1.0/2.5. Polymerization was initiated by adding 1% AIBN dissolved in 1.0 g of St.

and swelling agent are added simultaneously to the seed emulsion. The addition of the swelling agent activates the seed particles and promotes the swelling of the monomer; however, the swelling agent inefficiently diffuses into the seeds though the aqueous phase. It has been demonstrated that the addition of a polar organic solvent increases the solubility of the swelling agent, thereby enhancing the adsorption rate of seed particles.31,42 In this study, the swelling monomer (St) is a D

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Figure 5. Morphology evolution of PGMA/PS patchy microparticles prepared with different St contents added in the polymerization stage: (a) 1.5 g, (b) 2.0 g, (c) 2.5 g, (d) 3.0 g, and (e) 4.0 g. (f) Schematic depicting the reversible transformation process of large protrusions. The DBP/St ratio was fixed at 2.5/2.5, and the amount of AIBN was 1% relative to the monomer.

large and smooth spherical particles appeared to adsorb on the patchy microparticles (Figure 5, parts c and d). The liquid protrusions played a significant role in the generation of PGMA/PS patchy microparticles (Figure 5f). When 4.0 g of Sp was added, the protrusions on the patchy microparticles increased further, forming microparticle clusters with large building blocks (Figure 5e). When PGMA seed particles were replaced with PS and poly(methyl methacrylate) (PMMA), single-hole, solid−liquid Janus swollen particles were not observed, and patchy microparticles were not obtained (Figure S5, Supporting Information). The possible reason was that First, PGMA possesses the lowest glass transition temperature (Tg) (Figure S6, Supporting Information), the polymer chains are relatively easy to stretch in support of swelling and polymerization process. Then, PGMA seed particles are easier to form phase separation structure (Janus swollen particles) because PGMA is more immiscible with St compared with PS.11 The abovementioned results have shown that PGMA has the inherent unique properties. To determine the composition and internal structure of PGMA/PS patchy microparticles, Fourier transform infrared (FTIR) spectroscopy was used, as shown in Figure 6a. The absorption peak at 1160 cm−1 is ascribed to the stretching vibration of O−C−O, and that at 1724 cm−1 is attributable to the stretching vibration of CO. The peak at 906 cm−1 belongs to the absorption peak of the epoxy group. Furthermore, the absorption peaks at 1454, 1492, and 1600 cm−1 are attributed to the stretching vibration of the benzene ring skeleton, whereas those at 756 and 700 cm−1 are assigned to single substituted benzene. Thus, we were able to confirm that the prepared patchy microparticles were composed of PGMA and PS. X-ray photoelectron spectra (XPS) were also collected to determine the composition of the protrusions on the surface of the PGMA/PS patchy microparticles (Figure 6b).

regulated by changing the DBP/St ratio. When the amount of St is fixed, as shown in the third columns in Figure 4, the protrusions became smaller but increasingly abundant as the amount of DBP increases (1C, Figure 4, parts c and f; 2C, Figure 4, parts g, d, and a; 3C, Figure 4, parts e and b). In addition, as the amount of St increased (from left to right), the protrusions clearly increased in size but gradually decreased in number. However, no uniform trend was identified across the rows (fixed DBP). These results demonstrate that the addition of St increased the swelling rate when DBP was sufficient. Figure 4 indicates that the optimum amount of DBP is 2.5 g. When 5.0 g of St was used, a large smooth hemisphere appeared on one side of each microparticles, forming particles with a shape reminiscent of baby cuttlefish (Figure 4b). The polymeric monomer (Sp) added in emulsion polymerization stage also influenced the morphology and structure of PGMA/PS microparticles. The DBP/St ratio was fixed as 2.5/ 2.5, and the resulting PGMA/PS patchy microparticles are shown in Figure 5, which presents the unique morphology evolution that occurs during this process in detail. PGMA/PS microparticles maintained a single-hole structure at relatively low dosages of Sp (Figure 5, parts a and b), even after the completion of the polymerization. However, the surface of those microparticles were irregular (Figure 5a), and small particles that were distributed uniformly around the particles appeared (Figure 5b). In this case, the liquid protrusion on the MSPs served as monomer warehouses, supporting the polymerization in the presence of insufficient Sp, and the monomer expended to form single holes. Moreover, the polymerization of additional Sp and St in the liquid protrusions was initiated by heterogeneous nucleation on the surface of the PGMA seed particles. When the amount of Sp was increased further, the liquid protrusion also acted as monomer warehouse, undergoing in situ self-polymerization sufficient Sp was present to nucleate on the surface of the seed particles. Thus, E

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Figure 6. (a) IR spectrum of PGMA/PS patchy microparticles. (b) XPS spectra of PGMA seed particles and PGMA/PS patchy microparticles. (c) Photoluminescence spectra of FITC (☆) and FITC-modified flower-like PGMA/PS patchy microparticles (○). Insets present pictures of the FITCmodified PGMA/PS microparticles dispersed in water under sunlight (left) and UV light (right). (d) Core-level spectra of the O 1s of PGMA/PS patchy microparticles. (e) TEM image and (f) high resolution TEM image of an ultrathin section of PGMA/PS patchy microparticles. The microparticles were artificially dyed with different colors to delineate the interior and exterior structures.

like PGMA/PS patchy microparticles (Figure 3) were observed by transmission electron microscopy (TEM), as shown in Figure 6e. Because the microparticles were coated in epoxy resin, the dark domain corresponded to the PS, whereas the gray domain was the PGMA. Parts e and f of Figure 6 show that the PS was mainly distributed in the protrusions, with some PS nanoparticles in the interior of the microparticles. The XPS results (Figure 6b) indicated that the protrusions were composed of PS and PGMA, with the PGMA principally distributing on the surface. The Formation Mechanism of PGMA/PS Patchy Microparticles. The problem lingering in our mind is the formation mechanism of these PGMA/PS patchy microparticles. For this purpose, the polymerization process was tracked as shown in Figure S8 and S9, and has been discussed

The peaks at 529.0 and 282.0 eV are attributed to O 1s and C 1s, respectively. The oxygen contents in PGMA seed particles and PGMA/PS patchy microparticles were 27.03% and 25.05%, respectively. The decreased oxygen on the surface of the patchy microparticles resulted from the polymerization of swollen St. However, the high oxygen content illustrated the presence of abundant epoxy groups on the surface. Therefore, fluorescein isothiocyanate (FITC) reacted readily with the amine-functionalized PGMA/PS patchy microparticles. To confirm this reaction, the photoluminescence properties were determined by fluorescence spectroscopy (Figure 6c). The excitation peak of the fluorescent patchy microparticles was at 522 nm, and a red shift of 7 nm compared to the FITC molecule was observed.46 To further study the distribution of the two components (PGMA and PS), ultrathin sections of the flowerF

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Macromolecules Scheme 1. Formation Mechanism of PGMA/PS Patchy Microparticles with Low (a) and High (b) Dosages of Sp

Figure 7. Reflectance spectra of PGMA seed particles and PGMA/PS patchy microparticles. (a) Schematic diagram of PGMA/PS patchy microparticles irradiated by incident light. (b) Reflectance spectra of PGMA/PS patchy microparticles prepared with various DBP/St weight ratios and (c) different Sp contents.

G

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Figure 8. (a) Schematic illustration of the self-assembly of a membrane made of PGMA/PS patchy microparticles. (b) SEM image of a membrane consisting of self-organized acid-treated patchy microparticles. (c) Water contact angles of the as-synthesized and acid-treated patchy microparticles self-assembled at different temperature. PGMA/PS patchy microparticles were assembled at 120 °C, and acid-treated patchy microparticles were assembled at 60 and 120 °C; the inset presents a water droplet on the untreated particle membrane after the substrate was inverted. (d) Absorption of liquid paraffin that had been dyed yellow by the addition of dimethyl yellow to the water.

protrusions, as shown in Figure 7a. The corresponding reflectance spectra are presented in Figure 7b and c. The reflectance of the PGMA/PS patchy microparticles was relatively high at wavelengths ranging from 210 to 190 nm and 1.65 to 1.76 μm. Acorn-shaped PGMA/PS microparticles (Figure 5, parts c and d) exhibited stronger reflection than PGMA seed particles over the wavelength range of interest (Figure 7, parts b and c). Moreover, the flower-shaped microparticles also exhibited enhanced reflection (Figure 7b). These results indicated that PGMA/PS patchy microparticles could have application in, for example, light reflection and thermal barrier coating. To evaluate the reflective capacity of PGMA/PS patchy microparticles, the patchy coefficient β was defined and measured (Supporting Information). At wavelengths of 266 and 1680 nm, the PGMA/PS patchy microparticles exhibited high reflectance compared with PGMA seed particles. The reflectance increased with increasing β, as shown in Figure S10a and b (Supporting Information). However, at 266 nm, a rapid increase was also observed below β values of 4. These results indicate that the structure of particles with protrusions strongly influences the reflectance, with acorn-like shapes exhibiting an especially strong effect (Figure 5, parts c and d). For β values up to 4, the increase in reflectance was not distinct, and the patchy microparticles prepared with a DBP/St of 2.5/2.5 (Sp: 2.5) exhibited the highest reflectance. At 1680 nm, the rate of

in Supporting Information. We propose the formation mechanism of PGMA/PS patchy microparticles as shown in Scheme 1. When the dosage of Sp was low, the liquid protrusions on monomer-swollen PGMA particles served as monomer warehouses, supporting the polymerization in the presence of insufficient Sp. The liquid protrusions were disappeared as the polymerization performing (Scheme 1a), and single-hole patchy microparticles with small protrusions were formed (Figure 5a,b). When we continue to increase Sp, most liquid protrusions were solidified and survived by the polymerization in the presence of sufficient Sp to form irregular patchy microparticles (Scheme 1b). Then, these irregular patchy microparticles were further swollen by additional monomer, and polymerized into PGMA/PS patchy microparticles with small protrusions. Further, these protrusions regrew, and PGMA/PS patchy microparticles with large protrusions were synthesized (Figure 5). Application of PGMA/PS Patchy Microparticles. When applying particulate materials as thermal barrier coatings and photonic crystals, the particle morphology and size play important roles in determining the light reflection.47,48 Therefore, the ability to regulate the protrusions on the PGMA/PS patchy microparticles prepared here is an important advantage. The particles in Figure 5c and d are especially promising for these applications because of the two size distributions of intact PGMA/PS microparticles and their H

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increase in the reflectance did not exceed that at 266 nm, but acorn-shaped patchy microparticles exhibited the optimal reflectance. The PGMA/PS patchy microparticles also exhibit excellent performance for other potential applications. The patchy microparticles can easily self-assemble to form membranes at an appropriate temperature due to its nature of thermoplasticity (Figure 8). These assembled particle membranes show excellent hydrophobic performance, as confirmed by a water contact angle of 140° (Figure 8c). This high hydrophobicity is ascribed to the multiple protrusions structure of membrane. The mechanism of it has been explained by our group49 and other researchers.50 Moreover, the water droplet remained anchored on the membrane, even after the substrate was flipped 180° (inset in Figure 8c), indicating that the membrane shows strong adhesion to water. So what is the trick? It is protrusions in large scale. Every single protrusion provides a microscopic attracting force. Large amount of protrusions all working like a team, helps water drops to stick on the membrane. A more amazing property of these PGMA/PS patchy microparticles is that they will transformed easily into “particle gel” and readily formed membranes when they were treated by concentrated sulfuric acid, as shown in Figure 8b. The selfassembly process was expounded in Scheme S1 due to hydrogen bonding of hydroxyl groups which were generated by the hydrolysis of the epoxy ring of PGMA/PS patchy microparticles and the cross-linking network between the microparticles catalyzed by acid. The water contact angles were significantly reduced at different assembly temperatures. In contrast, an oil drop applied to the surface spread out instantaneously (Figure S11, Supporting Information). Therefore, this oleophilic acid-treated patchy material is expected to be an efficient absorbent for oil spills and oily wastewater treatment.51 To verify its absorption properties, liquid paraffin dyed with dimethyl yellow was selected as a model oil. The yellow oil was immediately absorbed and disappeared upon contact with the acid-treated patchy material (Figure 8d). Therefore, these PGMA/PS patchy microparticles are promising oil absorbents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02059. Experimental details and additional figures, Figures S1− S19 (PDF) Movie S1, oil contact angle of acid-treated patchy material (MPG) Movie S2, absorption of oil by acid-treated patchy material (MPG) Movie S3, acid-treated PGMA/PS patchy microparticles with gel structure (MPG)

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AUTHOR INFORMATION

Corresponding Author

*(Q.Z.) Telephone: +86-029-88431675. Fax: +86-02988431653. E-mail: [email protected]. ORCID

Lei Tian: 0000-0002-4039-4820 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the support from the Key Program of the National Natural Science Foundation of China (No. 51433008), the General Program of the National Natural Science Foundation of China (No. 51173146), the Excellent Doctorate Foundation of Northwestern Polytechnical University, and the Innovation Foundation for Doctorate Dissertation of Northwestern Polytechnical University (CX201623).

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CONCLUSION In summary, we developed a DSS strategy for the fabrication of asymmetric MSPs. Using this DSS technology, the liquid protrusions of prepared Janus-like MSPs can be fine-tuned by adjusting the DBP/St ratio and the adding amount. Furthermore, the morphology evolution of MSPs from Janus to single-hole particles was observed as the swelling time was increased. After MSPs polymerization, flower-like PGMA/PS patchy microparticles emerged with protrusions that could be precisely controlled by adjusting the DBP/St ratio and the amount of the polymeric monomer. The numerous protrusions on the smooth spherical particle also provided excellent light reflection. This discovery could contribute to the development of novel thermal barrier coatings. This DSS technique not only provides a new method for the preparation of patchy particles but also promotes the theoretical and practical development of seed polymerization. The resulting patchy microparticles with well-controlled protrusions have excellent prospects for various applications, including as building blocks for self-assembly, in superhydrophobic coatings, and for the absorption of spilled oil. I

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DOI: 10.1021/acs.macromol.6b02059 Macromolecules XXXX, XXX, XXX−XXX