Article pubs.acs.org/Langmuir
Preparation of Hemispherical Polymer Particles by Cleavage of a Janus Poly(methyl Methacrylate) /Polystyrene Composite Particle† Nobuko Yamashita,‡ Natsumi Konishi,‡ Takuya Tanaka,‡ and Masayoshi Okubo*,‡,§ ‡
Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan Smart Spheres Workshop Co., Ltd., 2-1-214-122, Koyo-Naka, Higashi-Nada, Kobe 658-0032, Japan
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ABSTRACT: Micrometer-sized, hemispherical particles were successfully prepared as a result of the cleavage of Janus PMMA/PS composite particles by dispersion into acetone/ water (9/1−10/0 v/v) media or a THF/water (8/2 v/v) medium. The spherical composite particles having a Janus structure were prepared by the slow evaporation of toluene from homogeneous PMMA/PS/toluene droplets dispersed in an aqueous medium in advance. It was clarified that the difference in affinity between PMMA and PS phases with respect to the media caused the cleavage of the composite particles. This method is expected to be a novel approach to the preparation of nonspherical polymer particles.
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INTRODUCTION Control of the morphology of composite polymer particles has become an increasingly important subject in achieving desirable physical properties for coatings, impact resistance plastics and other diverse applications. The particle morphology is determined by a competition between kinetic and thermodynamic factors.1,2 On the basis of these two factors, composite polymer particles with various morphologies have so far been prepared using multistep seeded polymerization (e.g., seeded emulsion polymerization3−17 and seeded dispersion polymerization18−21). Recently, we have prepared poly(methyl methacrylate) (PMMA)/polystyrene (PS) composite particles by the slow evaporation of toluene as a common good solvent from toluene droplets dissolving PMMA and PS dispersed in aqueous solutions of various surfactants (solvent evaporation method).22−24 The thermodynamically stable morphology of the PMMA/PS particles depending on the minimum total interfacial free energy could be controlled by tuning each interfacial tension in the dispersed system, resulting in spherical and “snowman-like” particles possessing the same interfacial areas between both polymer phases and the aqueous medium (so-called Janus particles). Moreover, we have already been able to prepare pH-responsive amphiphilic PMMA/poly(styrene-2(2-bromoisobutyryloxy)ethyl methacrylate) (P(S-BIEM))-gpoly(2-dimethylanimo ethyl methacrylate) (PDM)) particles utilizing the surface-initiated atom-transfer radical polymerization (ATRP) of 2-dimethylamino ethyl methacrylate (DM) with Janus PMMA/P(S-BIEM) composite particles having bromine (initiator) groups on one side of the surface prepared by the solvent evaporation method.25 In that study, we found that the Janus PMMA/P(S-BIEM) composite particles were cleaved at the interface between PMMA and P(S-BIEM) phases into two parts (hemispheres) in seconds via contact with © 2012 American Chemical Society
acetone and resulted in nonspherical particles. To our knowledge, there has been no previous report of such a phenomenon. The same cleavage also occurred in the case of Janus PMMA/PS composite particles. In this study, the formation mechanism of such hemisphere particles based on the cleavages of Janus PMMA/P(S-BIEM) and PMMA/PS composite particles is clarified.
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EXPERIMENTAL METHODS
Materials. Methyl methacrylate (MMA) and styrene were distilled under reduced pressure in a nitrogen atmosphere and stored in a refrigerator before use. Reagent-grade 2,2′-azobis(isobutyronitrile) (AIBN) was purified by recrystallization in methanol. Toluene, acetone, tetrahydrofuran (THF), and sodium dodecyl sulfate (SDS, Wako Pure Chemical Industries, Ltd., Osaka, Japan) were all reagent grade and used without purification. Water was purified using an Elix UV 3 system (Nihon Millipore K. K., Tokyo, Japan). Preparation of Janus PMMA/P(S-BIEM) Composite Particles. PMMA and P(S-BIEM) were synthesized by the solution (co)polymerization of MMA and S/BIEM (95/5 mol/mol), respectively, in toluene at 70 °C for 24 h using AIBN as the initiator. The weightaverage molecular weight (Mw) and polydispersity index (PDI) measured by gel permeation chromatography (GPC) were 5.4 × 104 and 1.8 for PMMA and 4.4 × 104 and 2.1 for P(S-BIEM), respectively. The molar ratio of P(S-BIEM) measured with 1H NMR was 99.6:0.4. A homogeneous toluene solution (3.3 g) of PMMA and P(S-BIEM) (PMMA/P(S-BIEM)/toluene = 1/1/24 w/w/w) was stored in a Teflon storage tank and permeated the Shirasu porous glass (SPG, SPG Technology Co. Ltd., Miyazaki, Japan) membrane having an average pore size of 3.9 μm into a 57.8 mM SDS aqueous solution under nitrogen pressure (ca. 0.03 MPa). Subsequently, toluene was slowly evaporated from the dispersion under gentle stirring with a magnetic stirrer at room temperature for 48 h in an uncovered beaker Received: June 15, 2012 Revised: July 28, 2012 Published: August 6, 2012 12886
dx.doi.org/10.1021/la302442t | Langmuir 2012, 28, 12886−12892
Langmuir
Article
(the interfacial area between air and the dispersion was 22 cm2). Obtained particles dispersed in the SDS aqueous solution were repeatedly washed by serum replacement with water. Preparation of Janus PMMA/PS Composite Particles. PMMA and PS were synthesized by the solution polymerization of MMA and S, respectively, in toluene at 70 °C for 24 h by employing AIBN as the initiator. Mw and PDI measured by GPC were 1.1 × 105 and 1.9 for PMMA and 1.1 × 105 and 1.8 for PS, respectively. A homogeneous toluene solution (0.8 g) of PMMA and PS (PMMA/PS/toluene = 1/ 1/24 w/w/w) was mixed with an aqueous solution (18.3 g) of SDS (63.6 mM), and the mixture was stirred vigorously using a homogenizer (Nihonseiki Kaisha Ltd., ABM-2, Tokyo, Japan) at 4000 rpm for 3 min in a 50 mL glass vial. Toluene was slowly evaporated from the dispersion under gentle stirring with a magnetic stirrer at room temperature for 24 h in an uncovered glass vial. (The interfacial area between air and the dispersion was 8 cm2.) Obtained particles dispersed in the SDS aqueous solution were repeatedly washed by serum replacement with water. PMMA or PS particles were prepared by the same method as the above Janus PMMA/PS composite particles. Observation of Particles. The dispersions of obtained particles were observed with an optical microscope (Nikon Corp., Eclipse 80i, Tokyo, Japan). The particles after drying were observed with a scanning electron microscope (SEM, S-2500, Hitachi Science Systems Ltd., Tokyo, Japan) at an acceleration voltage of 10 kV. Observation of Cleavage of Janus PMMA/P(S-BIEM) or PMMA/PS Composite Particles. A homogeneous solution of acetone/water or THF/water was dropped onto dried particles on a microscope slide for observation with an optical microscope (Nikon Corp., MICROPHOT-FXA, Tokyo, Japan) with a high-speed camera (Photron Ltd., FASTCAM-512PCI, Tokyo, Japan).
acetone, and (ii) 5 s later, a large amount of water was added to the mixture. Before the replacement, Janus PMMA/P(S-BIEM) (1/1 w/w) composite particles had a spherical shape. After that, hemispherical particles were observed. Because acetone is a good solvent for PMMA and a bad solvent for PS (and for P(SBIEM)), first it seemed that the formation of the hemispheres was based on the selective dissolution of the PMMA phase from the Janus particles, and the hemisphere particles consisted of P(S-BIEM). In actuality, when the dried PMMA/P(S-BIEM) composite particles were dispersed in acetone/water for 1 h, the PMMA phase completely dissolved and the P(S-BIEM) phase remained as ellipsoidal and spherical particles. The formation of nonspherical particles by selective dissolution from spherical composite particles with some solvent that dissolves one component but not the other in the composite particles is already known. If the hemispherical particles shown in Figure 1 are also obtained by selective dissolution, then there is no new topic on the phenomenon. However, the medium after the addition of a large amount of water in the second replacement carried out in Figure 1 was transparent. When water was added (second replacement) after the dried PMMA/ P(S-BIEM) composite particles had been dispersed in acetone/ water (first replacement) for 1 h, the medium was turbid. These suggest that most of the PMMA phase did not dissolve in the acetone/water medium (first replacement) within 5 s. That is, a Janus composite particle may cleave to two hemisphere particles. To our knowledge, there has been no report of such a cleavage phenomenon of composite particle, although we reported the cleavage phenomenon of nonspherical poly(nbutyl acrylate) (PBA)/PS composite particles having a raspberry-like shape in the aging period of seeded emulsion polymerization of styrene with spherical PBA seed particles at 90 °C with shaking.25 To confirm such a speculation, continuous optical micrographs of particles just after the dried Janus PMMA/P(SBIEM) (1/1 w/w) composite particles were dispersed in acetone/water (9/1 v/v) solution were taken with a high-speed camera as shown in Figure 2. The acetone/water (9/1 v/v) solution was adopted to make the operation focusing on the particles by slowing the cleavage easy, which will be described later. The micrographs obviously indicate that one side of the Janus structure got smaller gradually after dispersing in the acetone/water solution, and the particles cracked at the interface between PMMA and P(S-BIEM) phases, resulting in the cleavage of the composite particles. As shown in Figure 3, Janus PMMA/PS composite particles were also cleaved in the same way after dispersing the dried particles into the acetone/water (9/1 v/v) solution. This indicates that the BIEM component does not influence the cleavage directly. Therefore, we set about clarifying the cleavage mechanism of the Janus PMMA/PS composite particles. Figure 4 shows the relationship between the particle diameter and the required time (Tc) until the Janus PMMA/ PS composite particles are cleaved into two hemispheres by dispersion in the acetone/water (9/1 v/v) solution. The Tc value increased linearly with an increase in the particle diameter. Figure 5 shows the cleavage behavior of the Janus PMMA/ PS composite particles dispersed in acetone/water solutions as a function of the content of acetone in solution. The cleavage occurred above 88% acetone. Tc decreased linearly with an increase in the acetone content above 90%. In 100% acetone, the cleavage occurred instantly, separated PMMA hemispheres
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RESULTS AND DISCUSSION Figure 1 shows optical micrographs and SEM photographs of Janus PMMA/P(S-BIEM) (1/1 w/w) composite particles before and after the following two-step medium replacement: (i) the aqueous dispersion was added to a large amount of
Figure 1. Optical micrographs (a, b) and scanning electron microscope (SEM) photographs (a′, b′) of Janus PMMA/P(SBIEM) (S/BIEM =99.6/0.4 (mol/mol)) (1/1 w/w) composite particles (a, a′), which were prepared by the evaporation of toluene from polymer/toluene (1/12 w/w) droplets dispersed in an SDS aqueous solution (57.8 mM) and those (b, b′) that were first dispersed in acetone/water (high acetone content) solution by the addition of a large amount of acetone to the aqueous dispersion; 5 s later, a large quantity of water was added to the mixture. 12887
dx.doi.org/10.1021/la302442t | Langmuir 2012, 28, 12886−12892
Langmuir
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Figure 2. Continuous optical micrographs (via a high-speed camera) of the cleavage process of Janus PMMA/P(S-BIEM) (1/1 w/w) composite particles by dispersing the dried particles in an acetone/water (9/1 v/v) solution.
Figure 3. Continuous optical micrographs (via a high-speed camera) of the cleavage process of Janus PMMA/PS (1/1 w/w) composite particles by dispersing the dried particles into an acetone/water (9/1 v/v) solution.
Figure 5. Relationship between the acetone content (vol %) of the acetone/water solution used for the cleavage of Janus PMMA/PS (1/1 w/w) composite particles (7.0 ± 1.0 μm in diameter) and Tc.
Figure 4. Relationship between the diameter of the Janus PMMA/PS composite particles and the time required (Tc) to be cleaved into two hemispheres by dispersing the dried particles into an acetone/water (9/1 v/v) solution.
PMMA and PS hemispheres at the same time by cleaving of the Janus composite particle, it is important to stop the dissolution of the PMMA hemisphere and the deformation of the PS
quickly disappeared by dissolution, and the other PS hemispheres gradually became spherical. Therefore, to prepare 12888
dx.doi.org/10.1021/la302442t | Langmuir 2012, 28, 12886−12892
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indicates that the PMMA particles dissolved from the surface at >88% acetone. However, the volume of the PS particles did not change for 30 s up to 88% acetone content and drastically increased above 90%. On the basis of the above results, the reason that the Janus PMMA/PS composite particles were rapidly cleaved in the acetone/water (9/1) solution can be explained as follows. First, the higher swelling of the PMMA phase compared to that of the PS phase with acetone causes stress at the interface between the two phases. The partial dissolution of the PMMA surface reduces the interfacial area between the two phases, which decreases the interaction between them. The swelling of the PMMA and PS phases decreases the entanglement between PMMA and PS molecules at the interface, which also decreases the interaction between them. When the stress caused at the interface overcomes the interaction there, composite particles rapidly cleave to the PMMA and PS hemispheres. To confirm the idea and extend the cleavage conditions, some experiments were carried out. First, we examined whether THF would enable the cleavage of the Janus PMMA/PS composite particles. THF is also a good solvent for both PMMA and PS and has a higher affinity for PS than for PMMA, opposite to that of acetone. Figure 7 shows continuous optical micrographs of the Janus PMMA/PS composite particles just after being placed in contact with THF. The PS phase disappeared within 0.1 s, resulting in PMMA hemispheres. This indicates that THF has too strong an affinity for PS to cleave the Janus PMMA/PS composite particles. Figure 8 shows continuous optical micrographs of the Janus PMMA/PS composite particles just after being placed in contact with a THF/water (8/2 v/v) solution. In the composite particle that was framed with a circle in each photograph, the PS phase quickly swelled and deformed from hemispherical to spherical shape and Janus particles finally cleaved to one PMMA hemisphere and one PS sphere. The morphological change before the cleavage seems to be caused by decreasing the total interfacial free energy. Figure 9 shows changes in the swelling ratios (V/V0) of PMMA and PS particles with dispersion time in THF/water (8/2 v/v) solution. The swelling ratio of PMMA particles
Figure 6. Swelling ratios (V/V0) of PMMA (○) and PS (●) particles (initial diameter = 7.0 ± 1.0 μm) dispersed in acetone/water solutions for 30 s.
Scheme 1. Cleavage of Janus PMMA/PS Composite Particles by Dispersion in Acetone/Water (Acetone ≥90 vol %) Solutions in a Short Time (
