Generalized Approach for Fabricating Monodisperse Anisotropic

Article pubs.acs.org/Macromolecules

Generalized Approach for Fabricating Monodisperse Anisotropic Microparticles via Single-Hole Swelling PGMA Seed Particles Lei Tian, Xiangjie Li, Panpan Zhao, Xin Chen, Zafar Ali, Nisar Ali, Baoliang Zhang, Hepeng Zhang, and Qiuyu Zhang* The Key Laboratory of Space Applied Physics and Chemistry, School of Science, Northwestern Polytechnical University, Xi’an 710072, China S Supporting Information *

ABSTRACT: A unique phenomenona single hole appeared gradually on the surface of the seed particles and grew with the proceeding of swellinghas been observed during swelling poly(glycidyl methacrylate) (PGMA) particles. PGMA particles suffer the function of the swelling agent and styrene monomer and form a single hole on their surface. SEM and TEM were utilized to observe and demonstrate the forming process. Inconceivably, monodisperse poly(glycidyl methacrylate)/polystyrene (PGMA/PS) anisotropic microparticles, including Janus, raspberry-shaped, acorn-shaped, and hollow with open mouth, have been fabricated by the seed polymerization of these single-hole microparticles as a generalized approach. The morphology evolution of PGMA/PS is investigated by regulating the amount of monomer and monomer/seed weight ratio. Moreover, the surface of PGMA/PS microparticles are rich in epoxy groups, which has bright prospects in application in biomacromolecule immobilization and superhydrophilic/superhydrophobic coating.

1. INTRODUCTION Over the past few decades, anisotropic microparticles with controllable morphologies and narrow size distribution, including raspberry-like,1−4 acorn-like,5 Janus,6,7 bowl-like,8−11 and hollow with open mouth,12−14 connected with the outside, have attracted substantial research interest since their promising applications in fields such as sensors,15,16 building blocks for self-assembly,17 drug delivery and controlled release,18,19 solid surfactants,20,21 carriers for immobilizing biological macromolecules,22,23 catalysts,24 surface-enhanced Raman scattering (SERS),25 and superhydrophobic materials.26,27 Among them, in particular, raspberry-shaped microparticles with hierarchical structures have been researched more deeply and still have drawn particular attention of the researchers due to their fascinating characteristics, such as unique morphologies, higher surface roughness, large specific surface area, and light scattering.27 Based on this point, many methods have been developed to synthesize anisotropic microparticles. Only in the case of raspberry-shaped microparticles, for example, two strategies here are provided: (a) The seed (or core) particles are preprepared; subsequently, the second (or corona) particles are formed onto the seed or core particles by a certain methods, such as emulsion polymerization,28 soap-free emulsion polymerization,29−31 Pickering emulsion polymerization,32−34 heterocoagulation methods,35,36 and assembly techniques.37,38 Among these methods, emulsion polymerization is the most common choice to control microstructures of particles.39,40 For example, Yang et al.41 prepared popcorn-like, molecule-like, and mushroom-like particles on the basis of the polystyrene (PS) © XXXX American Chemical Society

seed particles by altering the amount of swelling solvent and the type of cross-linker. (b) Raspberry-shaped particles are synthesized in situ via a one-step procedure in the presence of the seed (or core) particles and second (or corona) particles.30,31 For that system, one should note that it is rigorous for the requirements of thermodynamic control and precise regulation of microscopic phase separation. SiO2 particles with different functional group are normally utilized as seeds. Raspberry-shaped microparticles are prepared by a supramolecular linker or reaction between the groups to assemble functional nanoparticles onto SiO2 seeds.42,43 Among the above-mentioned methods, seeded emulsion polymerization (SEP) is undoubtedly the most basic means to synthesize anisotropic particles44−47 because of its briefness, large scale, and controllability. Popularly, monodisperse crosslinked polymer particles are required and dispersed in the sodium dodecyl sulfate (SDS) aqueous solution to be swollen by monomers. After heating to initiate polymerization, the swollen polymer particles generate phase separation and liquid protrusions of monomer on the seed particles because of the relaxation of stretched cross-linked polymer network. When liquid protrusions polymerize, the protrusions aggregate by reason for immiscibility, and dimers or Janus particles are formed. However, there still exists some imperfection restricting the development of SEP: (1) As for some other anisotropic particles, traditional SEP is powerless besides Received: June 17, 2015 Revised: September 25, 2015

A

DOI: 10.1021/acs.macromol.5b01319 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules specific conditions like γ-ray radiation.48,49 (2) The selection range of the seeds in SEP is narrow, although the method has been systematically studied. To our knowledge, PS,39,50 poly(methyl methacrylate),51,52 and SiO226,53 are the most widely used seeds. Recently, sulfonated polystyrene particles have also been a popular selection.29,54 Nevertheless, note that PS is difficult to be modified which hinders its extensive application; normally, that SiO2 could not be swollen by monomers determined it is almost impossible to realize raspberry-shaped particles in the strict sense. (3) Up to now, the detailed investigation of seeded emulsion polymerization system with PGMA as a seed particle remains unclear.30 However, the abundant epoxy groups of PGMA, which can build a bridge between biomacromolecule/medicine molecule and carrier, is of great significance in biology and biomedicine. (4) The swelling process in SEP still lacks a detailed tracking study. (5) Most of all, a generalized approach for synthesizing anisotropic particles with asymmetric geometry and surface property is under-reported. In this work, we improved the seed swelling process and explored a generalized approach to fabricate PGMA/PS anisotropic particles including raspberry-shaped, acorn-shaped, Janus, and hollow with open mouth. Excitingly, a fresh swelling seed particle with a single hole connected to the outside and inside was generated in the improved swelling process of PGMA seeds. It is significant that the single hole caused by increased osmotic pressure and interaction of monomer and swelling agent promotes further swelling of the monomer. Unlike traditional seed emulsion polymerization, PGMA seeds were not cross-linked. But the geometrical morphology of prepared microparticles could be regulated by adjusting the amount of monomers, monomer/seed weight ratio, and reaction conditions. Above all, our discovery will further improve the mechanism of SEP and will share a generalized approach for synthesizing anisotropic particles. In addition, the reference method we provided is large-scale, which will accelerate the industrialization process.

Table 1. Detailed Experimental Conditions for the Preparation of PGMA/PS Microparticles with Controllable Morphologies sample

seeds (g)

SDS (wt %)

Ss (g)a

Sp (g)b

AIBN (wt %)

time (h)

morphology

MAMs-1 MAMs-2 MAMs-3 MAMs-4 MAMs-5 MAMs-6 MAMs-7 MAMs-8 MAMs-9 MAMs-10 MAMs-11 MAMs-12 MAMs-13 MAMs-14

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25

0.0 1.0 0.0 0.5 1.0 1.0 0.5 0.0 1.0 1.5 2.0 2.5 1.5 1.0

0.5 0.0 1.0 1.0 0.5 1.0 2.0 3.0 2.0 1.5 1.0 0.5 2.0 5.0

1 1 1 1 1 1 1 1 1 1 1 1 1 1

24 24 24 24 24 24 24 24 24 24 24 24 24 24

Janus sphere single hole multihole multihole raspberry raspberry raspberry raspberry raspberry raspberry raspberry raspberry acorn

a

Ss states here the styrene monomer added in the swelling stage. bSp is the styrene monomer mixed in the polymerization stage (similarly hereinafter). The amount of swelling agent was equal to the PGMA seeds. was added into an Erlenmeyer flask. The emulsifier (SDS) concentration was fixed at 0.25 wt %. Subsequently, 0.5 g of swelling agent DBP and 1.0 g of S monomer were introduced. After swelling at 40 °C for 24 h, the above seeded emulsion was poured into a threenecked flask and added 1.0 g of styrene dissolved with 0.02 g of AIBN. Then, the reaction system was immersed into a water bath at 80 °C. After the polymerization reaction was continued for 8 h, the final products were washed with ethyl alcohol for one time and water for two times by centrifugation and dried by vacuum freeze-drying. The detailed experimental conditions in this article are listed in Table 1. MAMs-n, a general abbreviation for these monodisperse anisotropic microparticles, is employed. 2.4. Functionalization of PGMA/PS Microparticles. The amino-functionalized PGMA/PS microparticles were prepared first. The procedure was presented as follows. 1.0 g of PGMA/PS MAMs-6 microparticles was dispersed in 100 mL of ethyl alcohol. Subsequently, hexamethylenediamine (3.0 g) was melted at 60 °C and added to the above mixture. The functionalization process was performed at 70 °C for 9 h. The final product was achieved by centrifugation and washed with ethyl alcohol and water. 2.5. Preparation of PGMA/PS-Ag Microparticles.55 0.2 g of amino-functionalized PGMA/PS MAMs-6 microparticles was dispersed in 20 mL of water with 0.3 g of AgNO3. The mixture was stirred with a magnetic stirrer at 500 rpm at 50 °C for 4 h. Then, these microparticles were washed by centrifugation with water to remove excess AgNO3. When the above particles were redispersed into water with hydrazine hydrate to reduce Ag+ to Ag, PGMA/PS-Ag microparticles were achieved by centrifugation and washed with water. 2.6. Characterization. OM (Optical Microscope). The microstructure of particles could be simply observed by a metallographic microscope (DMM 300C). 1.0 mg/mL of the sample under ultrasonic dispersion was on the slide to observe their morphology on the objective table. The sample was dispersed in purified water. SEM (Scanning Electron Microscope). The morphology of particles was precisely observed 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. TEM (Transmission Electron Microscopy). TEM was employed to investigate the internal structure of PGMA/PS microparticles, which were carried out on an H-600 transmission electron microscope (Hitachi, Japan) at an accelerating voltage of 75 kV. Samples were embedded in epoxy resins, and the mixture was polymerized at 70 °C

2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (S, 99%, Shanghai Shan Pu Chemical Co. Ltd.) and glycidyl methacrylate (GMA, 95%, Sartomer Company) were distilled under reduced pressure and refrigerated for spar. Azobis(isobutyronitrile) (AIBN, Shanghai Mountain Pu Chemical Co., Ltd.) was purified by recrystallization in methanol solution. Polyvinylpyrrolidone (PVP, BASF Co.), sodium dodecyl sulfate (SDS, Sinopharm Chemical Reagent Co., Ltd.), dibutyl phthalate (DBP, Tianjin Bo Di Chemical Co., Ltd.), and anhydrous ethanol (Sinopharm Chemical Reagent Co., Ltd.) were used directly without further purification. Deionized water was ultrapure produced by an apparatus for pharmaceutical purified water (Aquapro Co. Ltd.). 2.2. Preparation of PGMA Seed Particles. Monodisperse PGMA seed particles were prepared by dispersion polymerization as follows: 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 roundbottomed flask. Subsequently, 0.3 g of AIBN dissolved in 20.0 g of GMA was added to the above system. After being purged with nitrogen for 30 min, the system was heated to 80 °C and kept for 24 h. The obtained product was washed with ethyl alcohol for one time and water for two times aided by centrifugation and dried by vacuum freeze-drying for 12 h. 2.3. Generalized Approach for Fabricating Shape-Controlled PGMA/PS Microparticles. PGMA/PS microparticles with different eccentric morphologies were synthesized by seeded emulsion polymerization. A typical procedure was as follows (Table 1, line 6). First, 50 mL of PGMA seeded emulsion with a solid content of 1 wt % B


Article

Macromolecules

Figure 1. Optical microscope image (A) and DLS size distribution (B) of PGMA seed particles. The dispersion polymerization of GMA was carried out at 80 °C for 24 h. Scale bar: 10 μm.

Figure 2. SEM images of swelling PGMA seed particles in the present of DBP and S at the swelling time of 0 (A), 1 (B), 7 (C), and 18 h (D). The weight ratios of DBP/PGMA and S/PGMA were 1:1 and 2:1, respectively. The swelling process was carried out at 40 °C. These swelling PGMA seed particles were washed with ethyl alcohol for one time and water for two times aided by centrifugation and dried by vacuum freeze-drying. Scale bar: 2 μm. The insets are the corresponding TEM images of swelling PGMA particles (scale bar: 0.4 μm). for 3 days. Then, after samples were microtomed into ∼100 nm thick ultrathin sections using a LKBV ultratome, they were collected on TEM grids and stained in RuO4 vapor for 30 min for observation. DLS (Dynamic Light Scattering). Average diameter and particle size distribution of PGMA/PS microparticles were determined at room temperature by an LS13320 laser particle size analyzer (Beckman Coulter). Samples were redispersed in water by ultrasonication with the concentration of 1 mg/mL.

FTIR (Fourier Transform Infrared Spectroscopy). FTIR spectra were acquired on a TENSOR27 FTIR spectrometer (Bruker). A sample of the powder on the tabulating machine was pressed into sheets. XPS (X-ray Photoelectron Spectroscopy). The XPS spectrum was measured 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 referenced to the C 1s line at 284.7 eV from adventitious carbon.56 C


Article

Macromolecules

Figure 3. SEM images of swelling PGMA seed particles at the swelling time of 24 h in the presence of DBP and S (A) and only DBP (B) or S (C). The weight ratio of DBP/PGMA was 1:1, and S/PGMA was 2:1. Scale bar: 2 μm.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Single-Hole Swelling PGMA Microparticles. Monodisperse shapecontrolled PGMA/PS anisotropy microparticles were synthesized by modified seeded emulsion polymerization, selecting PGMA as the seed particles. Non-cross-linked PGMA seed particles were obtained by dispersion polymerization as exhibited in Figures 1A and 2A(a). From the optical microscope and SEM images, one can see the surface of seed particles was smooth, and the inside contrast was uniform. Furthermore, the seed particles with an average diameter of about 1.6 μm (in Figure 1B) showed fine sphericity and monodispersity. Then, the swelling process of PGMA seeds which were dispersed in 0.25% SDS aqueous solution was carried out with the equal mass of DBP and S. Different from traditional swelling process, swelling PGMA particles with single hole connected inside to outside were formed. For clarity of the phenomenon, SEM and TEM were used to monitor the whole procedure. The corresponding swelling PGMA particles with different swelling time after being washed with ethyl alcohol and water are shown in Figure 2. Figure 2B shows the SEM image of the PGMA particles after swelling 1 h. From Figure 2B, we can find that part of the particle surface appeared as small holes or mutilations. Nonetheless, it was not obvious in the TEM image (Figure 2b). In Figure 2b, uniform contrast of the PGMA particle illustrates the holes or mutilations existed only on the outer surface. When the swelling time extended to 7 h, consequently and interestingly, obvious holes appeared on

the surface of most PGMA particles. And each particle possessed only one single hole which had a diameter of ∼0.8 μm as shown in Figure 2C. For the analysis of the distribution of the single-hole structure inside the PGMA particles, Figure 2c shows the TEM image at the moment. From Figure 2c, the obvious cavity was shown inside the PGMA particles with sharp contrast compared with the surroundings. With the extension of swelling time, almost all of the PGMA particle surface were appeared a single-hole structure as shown in Figure 2D. Figure 2d is the TEM image of PGMA particles after swelling 18 h. The cavity inside the seed particle further increased to a third of the particle. To continue the swelling process, the single-hole structure did not change significantly when the swelling time was 24 h (Figure 3A). It suggests that the swelling process has been completed. To determine the role of DBP and S monomer, the same procedure was carried out with only DBP or S. The corresponding SEM images are shown in Figure 3B,C. When DBP was used, the swelling PGMA particles in Figure 3B maintained the intact spherical morphology. The degree of sphericility of the obtained particles weakened. However, part of PGMA particles prepared by the swelling agent of S as shown in Figure 3C appeared as a small hole. Moreover, the size distribution of swelling PGMA was broadened. It illustrates that both DBP and S were indispensable for the formation of single-hole PGMA particles, especially S which played an essential role, instead of DBP to control the uniformity of PGMA particles. D


Article

Macromolecules Scheme 1. Schematic Synthesis of the Single-Hole Swelling PGMA Seed Particlesa

a (i) PGMA particle suffered the swelling of the swelling agent DBP and S driven by the osmotic pressure difference and formed swelling PGMA seed particles (ii); swollen DBP gradually phase separated from seed polymers with the increasing swelling amount (iii) and removed from seed particles to form the single-hole swelling PGMA seed particle (iv).

Figure 4. Optical image of the PGMA/PS Janus microparticles (MAMs-1) prepared with the ratio of monomer (Ss/Sp) of 0:0.5 (A). SEM images of open-mouth microparticles (B) obtained after washing PGMA/PS Janus microparticles by ethanol and water. Schematic fabrication of shapecontrolled PGMA/PS microparticles from Janus to open mouth (C). Scale bar: 5 μm.

swell in the seed particles. According to Ugelstad’s work58,59 and the book by Guanghui Ma,60 the swelling agent (oligomer or low molecular weight compound) should swell into the seed particles through the aqueous phase, yet its solubility is very low. Thus, polarity organic solvents are added into the aqueous phase to promote the solubility of swelling agent and the absorption rate by seed particles. Therefore, as a result of addition of S which is served as a polar solvent for DBP (the

Combining the above analysis, the formation of the single hole could be attributed to the simultaneous addition of monomer and swelling agent which caused the increase of osmotic pressure as shown in Scheme 1. And S and DBP will swell into seed particles further driven by the osmotic pressure difference.57 The amount of swollen DBP increases to enhance the degree of swelling.58,59 Moreover, when polar solvent was added to the aqueous phase, there is a promotion for DBP to E


Article

Macromolecules

Figure 5. SEM images of PGMA/PS microparticles with different morphologies fabricated with various ratios of styrene monomer Ss/Sp of 1:0 (A), 0:1.0 (B), 0.5:1.0 (C), 1.0:0.5 (D), 1.0:1.0 (E), 0.5:2 (F), 0:3.0 (G), 1.0:2.0 (H), 1.5:1.5 (I), 2.0;1.0 (J), 2.5:0.5 (K), and 1.5:2.0 (L). The weight ratio of DBP/PGMA was 1:1. The concentration of seeds was 1%. Polymerization time was 8 h. Scale bar: 2.5 μm.

of as-prepared microparticles could be obviously observed by comparing the contrast. The hole forming process can be consulted in Figure 4C. At the swelling stage, only DBP swelled into the seeds due to the absence of S. Therefore, the swelling rate of DBP was relatively slow. A single hole did not appear on the surface of seed particles as shown in Figure 3B(i). When S and initiator were added in the polymerization stage, the amount of swollen DBP was increased. As the polymerization was initiated, phase separation would occur with the stretch of the polymer chain, which resulted in the removal of DBP. Because of immiscibility of DBP in water, it would be aggregate together with unreacted S on the surface of seed particles to generate Janus particles (ii) in Figure 4A. Then, after PGMA/ PS Janus particles washed with ethanol and water, the DBP region disappeared, and PGMA/PS microparticles with open mouth (iii) were formed (as shown in Figure 4B). It attributes to the character of DBP which is more likely to be soluble in ethanol. The PGMA/PS microparticles with different morphologies after the emulsion polymerization are presented in Figure 5. When S monomer was only fed in the swelling stage, normal particles with smooth surface were obtained only. As shown in Figure 5A, the hole of swelling PGMA particles was filled in via the polymerization of monomer swelled into the particles. Conversely, when S was added in polymerization stage, almost all particles were endowed with an ∼0.8 μm hole on the surface in the Figure 5B connected with an internal cavity just like swelling PGMA. These could be used as microcontainers for cargo transportation and drug delivery. Increased use of S, multihole microparticles (Figure 5C,D) were fabricated via the enhancive osmotic pressure. When the total quantity of S was

dissolving capacity of S with DBP can be confirmed by mixing experiment of DBP and S in the lab), the swelling rate of DBP was accelerated, also the monomer swelling capacity. Thus, a large amount of DBP diffused into PGMA particles until the chemical potential reaches equilibrium (i → ii in Scheme 1). Because of the immiscibility of DBP in the aqueous phase, swollen DBP with a certain amount of S monomer gradually phase separated from seed polymers (see iii in Scheme 1).57 With the further swelling of the monomer, the DBP constantly removed from seed particles and left different sized single hole (Figure 2B−D) on the surface (iv) until the inner cavity possessed a third volume of seed particles (as shown in Figure 2d). The droplets of phase-separated DBP in the aqueous phase were caught by an optical microscope as shown in Figure S1D (see the Supporting Information). The single hole plays an important role in the next emulsion polymerization. It not only facilitates the pressure release but also enhances monomer to adsorb and swell into the seeds. In addition, the method mentioned here is not only simple and effective but also economical and practical. Here, it is obliged to describe the location of S extended from Scheme 1. Because of the relatively hydrophilic PGMA particles, a portion of S swelled into the seed and a part of S existed in the form of monomer bulges on the surface.61 3.2. Fabrication of Shape-Controlled PGMA/PS Anisotropy Microparticles. In order to prove the function of styrene monomer (Ss and Sp) in the swelling and polymerization stages, we have done a series of experiments with different monomer proportions (Ss/Sp). When Ss/Sp was 0:0.5, PGMA/PS Janus particles appeared as shown in the optical image (Figure 4A). In Figure 4A, the Janus morphology F


Article

Macromolecules

Figure 6. Morphology evolution of PGMA/PS microparticles prepared at different polymerization times: 0.5 (A), 1.5 (B), 3.5 (C), 5.5 (D), 6.5 (E), and 12 h (F). The weight ratios of DBP/PGMA and Ss/Sp were both 1:1 and S/PGMA was 2:1. Scale bar: 10 μm.

Figure 7. SEM image of PGMA/PS-Ag raspberry-shaped microparticles (A) and its SEM-EDS spectrum (B). SEM images of amino-modified raspberry-shaped microparticles (C) and acorn-shaped microparticles (D) obtained with the weight ratios of Sp/seeds and Ss/Sp 10:1 and 1:1, respectively. Polymerization time was 8 h. Scale bar: 5 μm.

G


Article

Macromolecules

Figure 8. PGMA/PS microparticles prepared with various concentrations of surfactant: 0.25% (A), 0.38% (B), 0.50% (C), and 0.70% (D). The weight ratio of DBP/PGMA was 1:1, S/PGMA was 2:1, and Ss/Sp was 1:1. Polymerization time was 8 h. Scale bar: 10 μm.

more than 1.5 g, raspberry-shaped PGMA/PS microparticles (Figure 5E−L) were synthesized in succession. From SEM images, the protrusions evenly dispersed on the surface of MAMs with the size ranging from 1.0 to 2 μm. Further, the protrusions continued to increase when the amount of S was increased, which resulted in the protrusions no longer uniformly distributed on the surface (Figure 5L). The generation of large protrusions can be attributed to the swelling of those protrusions by the increased monomer which can also serve as a monomer “warehouse” for the growth of the protrusions. Therefore, the research of the total monomer in the system above-mentioned illustrates the amount of S monomer has a significant effect on the morphology of PGMA/PS microparticles. Further, morphology evolution of PGMA/PS anisotropy microparticles was tracked. Figure 6 shows PGMA/PS microparticles prepared at different polymerization times. From Figure 6A,B, PGMA/PS microparticles remained intact sphericity. But some floccules were adhered to the surface in the Figure 6B which was ascribed to the polymerization of the monomer adsorbed around the microparticles. When initiators infiltrated into the microparticles, the swelling monomer inside started to polymerize. Following phase separation which occurred between the polymerized PS with PGMA seed particle, as shown in Figure 6C−E, raspberry-shaped PGMA/ PS microparticles were formed. And we can also see that the protrusions gradually become obvious until the polymerization

time was 6.5 h. With the extended reaction time, strangely, golflike microparticles were synthesized at 12 h. It can be due to the complete phase separation of PS and PGMA. Therefore, the polymerization time was fixed at 8 h to synthesize raspberryshaped PGMA/PS microparticles. The results of monomer conversion are shown in Figure S1. From Figure S2, the curve of conversion−time is roughly S-shaped, and there is no induction period. Before the polymerization temperature was reached, S dissolved AIBN was added into the reaction system. Thus, free radicals from the decomposition of AIBN have plenty of time to meet with monomer. Once the polymerization temperature was reached, polymerization would begin. Therefore, it is reasonable to observe no induction period. When the conversion is more than 20% (5.5 and 6.5 h), there is an obvious autoacceleration. After the system viscosity increased with the raised conversion, double-base termination of chain radicals is limited, which led to accelerated polymerization. Furthermore, combining Figures 6D and 6E, we concluded it is a critical period for the formation of anisotropic microparticles. To analyze the surface composition of PGMA/PS microparticles, the epoxy group of PGMA was opened loop by hexamethylenediamine to load silver nanoparticles, as shown in Figure 7A. PGMA/PS/Ag raspberry-shaped microparticles were clearly different from amino-modified PGMA/PS microparticles. The EDS spectrum (Figure 7B) further suggests that silver nanoparticles were distributed on PGMA/PS microH


Article

Macromolecules

Figure 9. SEM images of PGMA/PS microparticles at different swelling temperatures: 40 (A), 50 (B), and 60 °C (C). The concentration of surfactant was 0.25%. The weight ratio of DBP/PGMA was 1:1, S/PGMA was 2:1, and Ss/Sp was 1:1. Polymerization time was 8 h. Scale bar: 5 μm.

Figure 10. (A) Infrared spectra of raspberry-shaped PGMA/PS microparticles. (B) Internal structure of PGMA/PS microparticles synthesized with the weight ratio of Sp/Ss = 1:1. The red arrows point to PS protrusions of PGMA/PS microparticles.

particles. It preliminarily illustrated that part of the relatively hydrophilic epoxy groups in PGMA seed particles migrated onto the surface of the PGMA/PS microparticles. In order to illuminate the role of concentrations of monomer and seeds, we increased the amount of monomer and seeds in the same system. When the ratio of seeds and monomer added up to be 1:10, interestingly, PGMA/PS microparticles finally became acorn-shaped (Figure 7D). With the conversion of monomers, the viscosity of microparticles was increased. The diffusion of monomers was limited to a certain extent. The phase separation of the protrusions ununiformly occurred on the surface seeds. Many monomer gathered in the single hole formed the large hemisphere of acorn-shaped PGMA/PS microparticles. 3.3. Effect of Experiment Parameter on the ShapeControlled PGMA/PS Anisotropy Microparticles. Figure 8 shows the obtained raspberry-shaped PGMA/PS microparticles with different surfactant concentrations. It is obvious from Figure 8 that the protrusions on the surface of microparticles reduced gradually until they disappeared with the increased dosage of surfactant. Meanwhile, particle size also decreased consecutively. The surface tension of monomer and water phase γM,A was reduced due to the addition of surfactant. Therefore, the contact angle of particles and monomer θ would decrease which led the protrusions to wear off. And when the surfactant reached a certain amount, the protrusions almost disappeared.

According to the principle of thermodynamics, when the temperature of the swelling stage system was enhanced, the degree of swelling of the PGMA seed particles was increased. The amount of DBP and S relative to seed polymer was larger. Moreover, the swollen S could dissolve part of PGMA particles and generate a relatively large cavity inside compared to Figure S3A. After the addition of S and AIBN, heterogeneous nucleation of monomer occurred quickly on the surface of seeds and solidified with the polymerization. Less monomer was swelled into the inside of the seed particles. Therefore, Figure 9C shows the capsule structure PGMA/PS microparticles when the swelling temperature was 60 °C, in stark contrast to Figure 9A,B. From the SEM image, the capsule structure microparticles present regional folding morphology. In the process of drying of the microparticles, rigid PS would not shrink with PGMA and results in the “patches” on the surface. 3.4. Formation Mechanism of Shape-Controlled PGMA/PS Anisotropy Microparticles. To identify the chemical constituents of the obtained raspberry-shaped PGMA/PS microparticles, typical FTIR spectra were first determined as shown in Figure 10A. The absorption peaks at 2920 and 2854 cm−1 were referred to saturated carbon− hydrogen bond (CH3 and CH2) of PGMA and PS. The stretching vibration caused by a carbonyl group (CO) was at 1730 cm−1, while 1159 cm−1 was assigned to stretching vibration of O−C−O. Moreover, the absorption peak at 906 I


Article

Macromolecules

Figure 11. XPS spectra of PGMA/PS microparticles (A) and core-level spectra of C 1s (B). The PGMA/PS microparticles were synthesized with the feed weight ratio of PGMA/S = 1/4.

cm−1 belonged to the absorption peak of the epoxy group. The absorption bands at 1450, 1494, and 1604 cm−1 referred to the stretch vibration of the benzene ring skeleton. The peaks at 759 and 700 cm−1 were attributed to single substituted benzene. Therefore, we could confirm PGMA/PS microparticles were formed along with the polymerization of styrene in the presence of PGMA seeds. To further investigate the microstructures and formation mechanism of PGMA/PS microparticles with different morphologies, we drew support from ultrathin cross sections of PGMA/PS microparticles with the assistance of an ultramicrotome. In Figure 10B, because the particles were coated into epoxy resins, the contrast in the TEM images should be close to the surrounding epoxy resin. Therefore, the gray regions were PGMA seed main bodies, and the protrusions of PS were colored dark. In Figure 10B, the dark regions were not uniform as a result of cutting directions, and part of the PS extended to the inside of the microparticles from the surface. The surface composition and element analysis for PGMA/PS microparticles were characterized by XPS. The peaks at 532.9 and 284.7 eV are attributed to O 1s and C 1s, respectively (Figure 11A). The content of oxygen in raspberry-shaped PGMA/PS microparticles was 12.86 wt % in accordance with which we could figure out the molar ratio of monomer unit (GMA and S) on the surface was 0.4. It further indicates that epoxy groups in PGMA seed particles migrated onto the surface of the PGMA/PS anisotropic microparticles. The C 1s peak can also be split into four peaks as shown in Figure 11B. The peaks at 284.7 eV (C1) and 285.4 eV (C2) correspond to the C−C and C−O bonds, respectively. Binding energy peaks located at 286.9 eV (C3) and 288.9 eV (C4) correspond to the epoxy group and O−CO. That revealed appreciable quantity of PGMA was distributed on the surface of raspberry-shaped PGMA/PS microparticles. In this work, we should emphasize that the PGMA seed particles were non-cross-linked and linear polymers. Therefore, DBP can be regarded as a plasticization solvent, also S, and liquefied the PGMA particles.61,62 But the function of DBP is not obvious form Figure S3A. When S dissolved with AIBN was added to the swelling system, the liquified PGMA seeds were distinct in Figure S3B−D. The property of PGMA particles under the microscope were the same with the droplets of oil. From Figure S3B−D, we further observed that the single hole

was gradually enriched with swollen S compared to Figure S3A. With the temperature increasing, heterogeneous nucleation of S occurred on the surface of swelling PGMA seed particles as shown in Figure S3E. When the polymerization was conducted at 80 °C for 1 h, S/PS oligomers protrusions were uniformly distributed to form raspberry-shaped microparticles (Figure S3F). Because of the low conversion in Figure S2 at this moment, however, the liquid protrusions can be removed after washing with ethanol and water (Figure 6A,B). With further polymerization, the protrusions were solidified and phase separated from PGMA seeds to form raspberry-shaped PGMA/ PS microparticles (v in Scheme 2). It should be noted that the surfactant plays an important role in stabilizing the protrusions. A variant Young’s equation for the three-phase system of seeds−monomer−aqueous phase was given by Gilbert:63 γP,A = γP,M + γM,A cos θ

(1)

Here, γP,A, γP,M, and γM,A are the interfacial tensions of the particle and the aqueous phase, the particle and the monomer, and the monomer and the aqueous phase, respectively. θ is the contact angle between the monomer and the seed particle. Under the action of surfactant, surface tension of aqueous phase was significantly reduced with a consequent result of decreasing θ according to formula 1. When θ increased with a smaller amount of surfactant, the protrusions should extend and become distinct. This conclusion was consistent with the results in Figure 8. Therefore, the protrusions with different sizes can be controlled by the surfactant. Moreover, alternatively, the solidified protrusions were further swelled by S fed in the polymerization stage and grew or controlled by the addition of S in swelling stage (vi in Scheme 2). The raspberry-shaped PGMA/PS microparticles with various protrusions are shown in Figure 5. Further increasing the amount of monomer, the protrusion which filled the single hole would continue to absorb monomers, and further phase separated from PGMA seeds to form a large hemisphere (vii in Scheme 2). The acorn-shaped PGMA/PS microparticles were fabricated (Figure 7D).

4. CONCLUSIONS In conclusion, a generalized approach for monodispersed PGMA/PS anisotropy microparticles with controllable morphologies has been developed via modified seeded emulsion J


Macromolecules

■

Scheme 2. Schematic Synthesis of the Morphology Transformation Processa

Article

AUTHOR INFORMATION

Corresponding Author

*Tel +86-029-88431675; Fax +86-029-88431653; e-mail [email protected] (Q.Z.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful for the financial support provided by the National High-Tech Research and Development Program of China (863 Program) (No. 2012AA02A404), 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), and basic research fund of Northwestern Polytechnical University (3102014JCQ01094, 3102014ZD).

■

(1) Telford, A. M.; Hawkett, B. S.; Such, C.; Neto, C. Chem. Mater. 2013, 25, 3472−3479. (2) Nandiyanto, A. B.; Ogi, T.; Okuyama, K. ACS Appl. Mater. Interfaces 2014, 6, 4418−4427. (3) Li, X.; He, J. ACS Appl. Mater. Interfaces 2012, 4, 2204−2211. (4) Lu, C. L.; Urban, M. ACS Nano 2015, 9, 3119−3124. (5) Fujimoto, K.; Nakahama, K.; Shidara, M.; Kawaguchi, H. Langmuir 1999, 15, 4630−4635. (6) Zhao, H.; Liang, F. X.; Qu, X. Z.; Wang, Q.; Yang, Z. Z. Macromolecules 2015, 48, 700−706. (7) Sun, Y. J.; Liang, F. X.; Qu, X. Z.; Wang, Q.; Yang, Z. Z. Macromolecules 2015, 48, 2715−2722. (8) Liu, X. Y.; Kim, J. S.; Wu, J.; Eisenberg, A. Macromolecules 2005, 38, 6749−6751. (9) Zhang, J. A.; Yang, J. J.; Wu, Q. Y.; Wu, M. Y.; Liu, N. N.; Jin, Z. L.; Wang, Y. F. Macromolecules 2010, 43, 1188−1190. (10) Pan, J. M.; Wu, R. R.; Dai, X. H.; Yin, Y. J.; Pan, G. Q.; Meng, M. J.; Shi, W. D.; Yan, Y. S. Biomacromolecules 2015, 16, 1131−1145. (11) Hu, Y.; Chou, T. M.; Wang, H. J.; Du, H. J. Phys. Chem. C 2014, 118, 16011−16018. (12) Tan, L. H.; Xing, S. X.; Chen, T.; Chen, G.; Huang, X.; Zhang, H.; Chen, H. Y. ACS Nano 2009, 3, 3469−3474. (13) Mandal, S.; Sathish, M.; Saravanan, G.; Datta, K. K. R.; Ji, Q. M.; Hill, J. P.; Abe, H.; Honma, I.; Ariga, K. J. Am. Chem. Soc. 2010, 132, 14415−14417. (14) Biswas, A.; Nagaraja, A. T.; McShane, M. J. ACS Appl. Mater. Interfaces 2014, 6, 21193−21201. (15) Edel, J. B.; Kornyshev, A. A.; Urbakh, M. ACS Nano 2013, 7, 9526−9532. (16) Segev-Bar, M.; Haick, H. ACS Nano 2013, 7, 8366−8378. (17) Huang, C.; Shen, X. Chem. Commun. 2014, 50, 2646−2649. (18) Wu, Y.; Lin, X.; Wu, Z.; Mohwald, H.; He, Q. ACS Appl. Mater. Interfaces 2014, 6, 10476−10481. (19) Tapeinos, C.; Efthimiadou, E. K.; Boukos, N.; Charitidis, C. A.; Koklioti, M.; Kordas, G. J. Mater. Chem. B 2013, 1, 194−203. (20) Tu, F.; Lee, D. J. Am. Chem. Soc. 2014, 136, 9999−10006. (21) Kim, J.-W.; Lee, D.; Shum, H. C.; Weitz, D. A. Adv. Mater. 2008, 20, 3239−3243. (22) Ortac, I.; Simberg, D.; Yeh, Y. S.; Yang, J.; Messmer, B.; Trogler, W. C.; Tsien, R. Y.; Esener, S. Nano Lett. 2014, 14, 3023−3032. (23) Sun, J.; Ge, J.; Liu, W.; Lan, M.; Zhang, H.; Wang, P.; Wang, Y.; Niu, Z. Nanoscale 2014, 6, 255−262. (24) Gao, W.; Pei, A.; Dong, R.; Wang, J. J. Am. Chem. Soc. 2014, 136, 2276−2279. (25) Yang, S.; Hricko, P. J.; Huang, P. H.; Li, S.; Zhao, Y.; Xie, Y.; Guo, F.; Wang, L.; Huang, T. J. J. Mater. Chem. C 2014, 2, 542−547. (26) Xu, D.; Wang, M.; Ge, X.; Hon-Wah Lam, M.; Ge, X. J. Mater. Chem. 2012, 22, 5784−5791.

a

(i) The swelling PGMA seed particles with a single hole (ii) was formed by the action of swelling agent and monomer. Then, the Janus (iii) and raspberry-shaped PGMA/PS microparticles (v) were obtained with different ratios of monomer/seeds; after washing with ethanol, the one face of Janus particles was removed and transformed into large-hole microparticles (iv), and the size of protrusions of raspberry-shaped microparticles can be regulated (vi). To further increase the ratio of monomer/seeds, acorn-shaped microparticles (vii) were formed along with a large hemisphere separated from raspberry-shaped microparticles.

polymerization. Interestingly and originally, a single hole was appeared gradually on the surface of PGMA seed particles during swelling, which assisted monomer in permeating into the inside of the seeds. The formation of single hole was attributed to phase separation of swelling agent with seed polymer. In addition, by means of increasing monomer feed ratio, Janus and raspberry-shaped PGMA/PS microparticles with various sizes of protrusions were fabricated. And the protrusions can be regulated by the amount of surfactant and the monomer feed ratio, alternatively. Further increasing the ratio of monomer/seed, acorn-shaped microparticles with a large hemisphere were formed. Moreover, the formation mechanism of PGMA/PS anisotropy microparticles was expounded by borrowing ideas from heterogeneous nucleation of monomer on the surface of seed particles. Above all, because of high surface area of these shape-controlled PGMA/PS microparticle generated by the high degree of the protrusions, it will play a significant and promising role in biocatalysis and superhydrophilic/superhydrophobic coating.

■

REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01319. Figures S1−S3 (PDF) K


Article

Macromolecules (27) Zhang, X.; Yao, X.; Wang, X.; Feng, L.; Qu, J.; Liu, P. Soft Matter 2014, 10, 873−881. (28) Suzuki, D.; Kobayashi, C. Langmuir 2014, 30, 7085−7092. (29) Pan, M.; Yang, L.; Wang, J.; Tang, S.; Zhong, G.; Su, R.; Sen, M. K.; Endoh, M. K.; Koga, T.; Zhu, L. Macromolecules 2014, 47, 2632− 2644. (30) Fan, X.; Jia, X.; Zhang, H.; Zhang, B.; Li, C.; Zhang, Q. Langmuir 2013, 29, 11730−11741. (31) Sun, Y.; Yin, Y.; Chen, M.; Zhou, S.; Wu, L. Polym. Chem. 2013, 4, 3020−3027. (32) Chen, M.; Zhou, S. X.; You, B.; Wu, L. M. Macromolecules 2005, 38, 6411−6417. (33) Haaj, S. B.; Thielemans, W.; Magnin, A.; Boufi, S. ACS Appl. Mater. Interfaces 2014, 6, 8263−8273. (34) Guan, Y. Y.; Meng, X. H.; Qiu, D. Langmuir 2014, 30, 3681− 3686. (35) Chenal, M.; Rieger, J.; Philippe, A.; Bouteiller, L. Polymer 2014, 55, 3516−3524. (36) Au, K. M.; Armes, S. P. ACS Nano 2012, 6, 8261−8279. (37) Kraft, D. J.; Vlug, W. S.; van Kats, C. M.; van Blaaderen, A.; Imhof, A.; Kegel, W. K. J. Am. Chem. Soc. 2009, 131, 1182−1186. (38) Xu, X. W.; Zhang, X. M.; Liu, C.; Yang, Y. L.; Liu, J. W.; Cong, H. P.; Dong, C. H.; Ren, X. F.; Yu, S. H. J. Am. Chem. Soc. 2013, 135, 12928−12931. (39) Tang, C.; Zhang, C.; Sun, Y.; Liang, F.; Wang, Q.; Li, J.; Qu, X.; Yang, Z. Macromolecules 2013, 46, 188−193. (40) Yuan, J. F.; Wang, L. X.; Zhu, L.; Pan, M. W.; Wang, W. J.; Liu, Y.; Liu, G. Langmuir 2015, 31, 4087−4095. (41) Yang, M.; Wang, G.; Ma, H. Chem. Commun. 2011, 47, 911− 913. (42) Lan, Y.; Wu, Y.; Karas, A.; Scherman, O. A. Angew. Chem., Int. Ed. 2014, 53, 2166−2169. (43) Carcouët, C. C. M. C.; Esteves, A. C. C.; Hendrix, M. M. R. M.; van Benthem, R. A. T. M.; de With, G. Adv. Funct. Mater. 2014, 24, 5745−5752. (44) Parvole, J.; Chaduc, I.; Ako, K.; Spalla, O.; Thill, A.; Ravaine, S.; Duguet, E.; Lansalot, M.; Bourgeat-Lami, E. Macromolecules 2012, 45, 7009−7018. (45) Park, J. G.; Forster, J. D.; Dufresne, E. R. J. Am. Chem. Soc. 2010, 132, 5960−5961. (46) Chu, F.; Siebenburger, M.; Polzer, F.; Stolze, C.; Kaiser, J.; Hoffmann, M.; Heptner, N.; Dzubiella, J.; Drechsler, M.; Lu, Y.; Ballauff, M. Macromol. Rapid Commun. 2012, 33, 1042−1048. (47) Konishi, N.; Fujibayashi, T.; Tanaka, T.; Minami, H.; Okubo, M. Polym. J. 2010, 42, 66−71. (48) Huang, H.; Liu, H. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 5198−5205. (49) Wang, F.-W.; Liu, H.-R.; Zhang, J.-D.; Zhou, X.-T.; Zhang, X.-Y. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 4599−4611. (50) Hou, H.; Yu, D.; Tian, Q.; Hu, G. Langmuir 2014, 30, 1741− 1747. (51) Liu, Y. D.; Fang, F. F.; Choi, H. J. Langmuir 2010, 26, 12849− 12854. (52) Shi, S.; Wang, T.; Tang, Y. T.; Zhou, L. M.; Kuroda, S. I. Chin. Chem. Lett. 2011, 22, 1127−1129. (53) Désert, A.; Chaduc, I.; Fouilloux, S.; Taveau, J.-C.; Lambert, O.; Lansalot, M.; Bourgeat-Lami, E.; Thill, A.; Spalla, O.; Ravaine, S.; Duguet, E. Polym. Chem. 2012, 3, 1130−1132. (54) Weng, H.; Huang, X.; Wang, M.; Ji, X.; Ge, X. Langmuir 2013, 29, 15367−15374. (55) Fan, X. L.; Zhang, Q. Y.; Zhang, H. P.; Zhang, B. L.; Li, C. M.; Li, X. J.; Lei, X. F. Particuology 2013, 11, 768−775. (56) Fan, X. L.; Jia, X. K.; Liu, Y.; Zhang, B. L.; Li, C. M.; Liu, Y. L.; Zhang, H. P.; Zhang, Q. Y. Polym. Chem. 2015, 6, 703−713. (57) Kim, S. H.; Hollingsworth, A. D.; Sacanna, S.; Chang, S. J.; Lee, G.; Pine, D. J.; Yi, G. R. J. Am. Chem. Soc. 2012, 134, 16115−16118. (58) Ugelstad, J.; Mork, P. C. Adv. Colloid Interface Sci. 1980, 13, 101−140.

(59) Ugelstad, J.; Mork, P. C.; Schmid, R.; Ellingsen, T.; Berge, A. Polym. Int. 1993, 30, 157−168. (60) Ma, G., Su, Z.-G., Eds.; Polymer Microsphere Materials; Chemical Industry Publishing House: Beijing, 2005; Vol. 2, p 65. (61) Niu, Q.; Pan, M. W.; Yuan, J. F.; Liu, X.; Wang, X. M.; Yu, H. F. Macromol. Rapid Commun. 2013, 34, 1363−1367. (62) Sacanna, S.; Korpics, M.; Rodriguez, K.; Colon-Melendez, L.; Kim, S. H.; Pine, D. J.; Yi, G. R. Nat. Commun. 2013, 4, 1−6. (63) Mock, E. B.; De Bruyn, H.; Hawkett, B. S.; Gilbert, R. G.; Zukoski, C. F. Langmuir 2006, 22, 4037−4043.

L


Generalized Approach for Fabricating Monodisperse Anisotropic

Recommend Documents