Preparation of Stimuli-Responsive “Mushroom-Like” Janus Polymer

Article pubs.acs.org/Langmuir

Preparation of Stimuli-Responsive “Mushroom-Like” Janus Polymer Particles as Particulate Surfactant by Site-Selective Surface-Initiated AGET ATRP in Aqueous Dispersed Systems Tomoe Yamagami,† Yukiya Kitayama,† 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

‡

S Supporting Information *

ABSTRACT: Micrometer-sized, monodisperse, “mushroom-like” J an us poly(m et hyl met hacrylate)/po ly (st yrene-2-(2bromoisobutyryloxy)ethyl methacrylate)-graf t-poly(2-(dimethyl amino)ethyl methacrylate) (PMMA/P(S-BIEM)-g-PDM) particles were successfully synthesized by site-selective surface-initiated activator generated by electron transfer for atom transfer radical polymerization in aqueous dispersed systems with spherical PMMA/P(S-BIEM) composite particles having controlled morphologies prepared using the solvent evaporation method. The anisotropic nonspherical shape of the obtained particles was controlled by changing the percentage of the surface area occupied by localized initiation sites (bromine group) at the surface of the PMMA/P(S-BIEM) composite particles with different P(S-BIEM) contents. Grafted PDM layer formed at the surface (contacting with water) of the P(S-BIEM) phase reversibly exhibited the volume phase transition in response to temperature and pH, which gave different nonspherical shapes (“open” or “closed” mushroom-cap). On the basis of such dual stimuli-responsive properties, the nonspherical particles effectively operated as particulate surfactant for Pickering emulsion, resulting in a stable 1-octanol-inwater emulsion at optimum temperature and pH value, and the Pickering emulsion could be easily unstabilized quickly by controlling them.

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electrohydrodynamic jetting process,9 and seeded monomer swelling and polymerization technique.6,26 Recently, we have proposed a versatile approach for the preparation of spherical and “snowman-like” hydrophobic Janus particles27,28 and “mushroom-like” amphiphilic Janus particles29 in aqueous dispersed systems. The synthetic procedure of the mushroom-like Janus particles was composed of the following two steps: (i) evaporation of toluene as a common good solvent from homogeneous poly(methyl methacrylate) (PMMA)/poly(styrene-2-(2-bromoisobutyryloxy)ethyl methacrylate) (P(S-BIEM))/toluene (1/1/24, w/w/w) droplets dispersed in an aqueous medium leads to the generation of spherical Janus PMMA/P(S-BIEM) composite particles possessing bromine groups, which works as an initiator of atom transfer radical polymerization (ATRP)30−33 on one side of the surface because of the internal phase separation therein. (ii) Surface-initiated activator generated by electron transfer for ATRP (AGET ATRP) of 2-(dimethylamino)ethyl methacrylate (DM) using the spherical Janus particles in an aqueous medium

INTRODUCTION

Morphology control of polymer particles is crucially important for the creation of functional colloidal materials. Extensive research for morphology control has thus been carried out based on thermodynamic and kinetic aspects.1−3 As a result, polymer particles with various morphologies have been designed and fabricated. In particular, Janus particles, defined as colloidal particles consisting of two surfaces of different chemistries and/or polarities, have attracted great attention because of their anisotropic wettabilities and optical, electric, and magnetic properties.4 Taking advantage of these anomalous properties, Janus particles have actually been used in diverse applications such as particulate surfactants,5−7 imaging nanoprobes,8,9 and self-motile colloidal materials,10 thereby demonstrating the practical usefulness of Janus particles. On the basis of this situation, extensive research has so far focused on the precise synthesis of Janus particles, for instance, toposelective surface modifications using the particle monolayer,5,8,10−15 Pickering emulsion,16−18 and partially masked particles19,20 as templates, various seeded polymerization methods,1−3 selective cross-linking and subsequent dissolution of microphaseseparated triblock copolymer,6,21 microfluidic technique,22−25 © 2014 American Chemical Society

Received: April 7, 2014 Revised: June 16, 2014 Published: June 16, 2014 7823

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Figure 1. Optical micrographs of PMMA/P(S-BIEM) composite particles with various P(S-BIEM) contents prepared by toluene evaporation from polymers/toluene (1/12, w/w) droplets, in which the polymer composition was different, dispersed in 1.0 wt % Emulgen 930 aqueous solution using a homogenizer. P(S-BIEM) contents (wt %): (a) 5; (b) 10; (c) 20; (d) 50; (e) 70; (f) 80.

critical micelle concentration was used at the solvent evaporation method. Before the polymerization because SDS is unsuitable for ATRP due to the toxicity to the copper complex,35 the Janus PMMA/P(S-BIEM) particles dispersed in SDS aqueous solution had to be repeatedly washed by serum replacement with nonionic surfactant aqueous solution to remove SDS under a stable colloidal state. However, this washing process is troublesome and not easy to apply in large scale. Therefore, at this time, initially we attempted a direct synthesis of PMMA/P(S-BIEM) particles with different percentages of surface area occupied by the localized bromine atoms using various nonionic surfactants instead of the SDS. In our previous work, we have clarified that the surfactant affects particle morphology.36 We investigated the particle morphology of PMMA/P(S-BIEM) particles prepared by several kinds of nonionic surfactants. Among several kinds of nonionic surfactants, the PMMA/P(S-BIEM) particles prepared by Emulgen 930 had morphology similar to those prepared by SDS. On the basis of the results, Emulgen 930 was selected as a substitute for SDS in this study to prepare such spherical composite particles. To investigate how P(S-BIEM) contents in the total polymer affect the particles’ morphology, we synthesized the PMMA/ P(S-BIEM) composite particles with various P(S-BIEM) contents prepared by toluene evaporation from polymer/ toluene (1/12, w/w) droplets dispersed in 1.0 wt % Emulgen 930 aqueous solutions as shown in Figure 1, where the droplets were synthesized by a homogenizer, resulting in the wide particle size distribution. We have clarified that the particle morphology was not dependent on the particle size in our previous reports.37,38 Actually, the particle morphology was quite similar in all confirmable particles prepared in the same condition, and the particle size did not affect particle morphology. In all systems, spherical PMMA/P(S-BIEM) composite particles were obtained, although the interfaces

causes the formation of mushroom-like Janus PMMA/P(SBIEM)-graf t-poly(DM) (-g-PDM) particles, which had temperature and pH responsive properties.34 Moreover, we have successfully applied the amphiphilic Janus particles as particulate surfactants for the formation of Pickering emulsion.34 For preparation of the stable Pickering emulsion, one important parameter of the particulate surfactant is hydrophilic/lipophilic balance (HLB) because the value affects the total interfacial free energy of particle/water, particle/oil, and oil/water interfaces when the particulate surfactants are adsorbed at the oil/water interface. We have also demonstrated the stimuli-responsive Pickering emulsion by the HLB control of the Janus particles utilizing the temperature and pH dependent property change of PDM layer as an hydrophilic phase in the Janus particles. One of the important advantages of our proposed synthetic method for amphiphilic Janus particles is easy controllability of the HLB balance by changing the composition of PMMA/P(SBIEM) composite particles as seeds, which should vary the percentage of surface area occupied by localized initiation sites (bromine atoms). In this article, we try to prepare amphiphilic Janus particles, where the shapes are controlled by varying the PMMA and P(S-BIEM) compositions in seed particles, resulting in the different morphologies from previously obtained particles29 Furthermore, stimuli-responsive properties of the obtained nonspherical particles and their applicability to particulate surfactants for the preparation of Pickering emulsion will be demonstrated.

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RESULTS AND DISCUSSION Preparation of Spherical PMMA/P(S-BIEM) Composite Particles by the Solvent Evaporation Method Using Nonionic Surfactant. In a previous study,29 in order to obtain spherical PMMA/P(S-BIEM) composite particles with a Janus structure, which work as macroinitiator particles for AGET ATRP, a sodium dodecyl sulfate (SDS) aqueous solution above 7824

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between PMMA and P(S-BIEM) phases were not flat except for the case of 5 wt % P(S-BIEM) content. The percentage of surface (contacting with water) occupied by the P(S-BIEM) phase was obtained as average values of many particles observed on optical micrographs. As shown in Figure 2, the percentage increased with increasing P(S-BIEM)

dependent on the total interfacial tensions including P(SBIEM)/water, PMMA/water, and P(S-BIEM)/PMMA interfaces; therefore, it seems that the increase of interfacial area of P(S-BIEM)/PMMA disturbed the linear increase of P(SBIEM)/water interfacial area. The results imply that the P(S-BIEM) phase may be more hydrophilic relative to the PMMA phase because of the BIEM unit. In this way, the percentage of particle surface occupied by the P(S-BIEM) phase, at which Br groups such as the ATRP initiation site should exist, could be controlled by changing P(SBIEM) content in the composite particles. XPS measurement is useful for the qualitative detection of surface Br groups as well as their quantitative detection. In our previous work, we have confirmed that the obtained composite particles had Br groups at the surface by XPS measurements. The optical micrographs and SEM photographs of PMMA/ P(S-BIEM) composite particles with 5 and 80 wt % P(S-BIEM) contents prepared by toluene evaporation from polymers/ toluene droplets prepared by SPG membrane emulsification, which was employed for the preparation of micrometer-sized, monodispersed droplets,39,40 and TEM photographs of ultrathin cross-sections of the RuO4-stained particles, where P(SBIEM) was preferentially stained by RuO4, are shown in Figure 3. In both systems, relatively monodispersed spherical particles were obtained without coagulation, and the percentages of the particle surfaces occupied by P(S-BIEM) were quite different. These results clearly show that the percentage of surface area occupied by the localized P(S-BIEM) phase having Br groups as ATRP initiation sites could be controlled by changing P(SBIEM) content in the composite particles. Synthesis of Nonspherical PMMA/P(S-BIEM)-g-PDM Particles by Surface-Initiated AGET ATRP. The obtained PMMA/P(S-BIEM) composite particles were subsequently employed as macroinitiators for surface-initiated AGET ATRP of DM. Figure 4 shows conversion−time plots for surface-

Figure 2. Relationship between P(S-BIEM) content and percentage of surface (contacting with water) occupied by P(S-BIEM) phase in PMMA/P(S-BIEM) composite particles prepared by toluene evaporation from polymers/toluene (1/12, w/w) droplets dispersed in 1.0 wt % Emulgen 930 aqueous solutions.

content. The particle surface was predominantly occupied by the P(S-BIEM) phase, for example, approximately 50% of the surface was occupied by the P(S-BIEM) phase even at 20 wt % P(S-BIEM) content; however, the relationship was not linear. In low P(S-BIEM) content, the P(S-BIEM)/PMMA interface was flat; however, the interface had curvature with increasing the P(S-BIEM) content to expose the P(S-BIEM) interface, indicating that the P(S-BIEM) phase was more hydrophilic than PMMA phase. Here, the particle morphology was

Figure 3. Optical micrographs (a,a′) and scanning electron microscope (SEM) photographs (b,b′) of PMMA/P(S-BIEM) composite particles with 5 (a,b,c) and 80 (a′,b′,c′) wt % P(S-BIEM) contents prepared by toluene evaporation from polymers/toluene (1/12, w/w) droplets dispersed in 1.0 wt % Emulgen 930 aqueous solutions by SPG membrane emulsification, and transmission electron microscope (TEM) photographs (c,c′) of ultrathin cross-sections of RuO4-stained composite particles. 7825

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Figure 5 shows optical micrographs of particles prepared for 2, 7, and 40 min. In both particles, PDM layers were clearly generated from a part of the particle surfaces, and the thickness of the PDM layer increased with increasing conversion. The grafted PDM chains should be stable from the hydrolysis because the hydrolysis of PDM is much slower compared to that of DM. Actually, in our previous study, we have clarified that the zeta potential of the PMMA/P(S-BIEM)-g-PDM particles was positive and approximately zero, respectively, in the low and high pH, indicating that the hydrolysis of PDM grafted PMMA/P(S-BIEM) particles were negligible in this study and that most of all components of grafted polymers were DM.29 Optical micrographs and SEM photographs of the particles prepared by the surface-initiated AGET ATRP of DM using the two kinds of PMMA/P(S-BIEM) composite particles with 5 and 80 wt % P(S-BIEM) contents for 1 h at 60 °C and TEM photographs of ultrathin cross-sections of the RuO4-stained particles are shown in Figure 6. In both systems, the nonspherical particles, which had different shapes from the previous particles prepared using Janus PMMA/P(S-BIEM) composite particles with 50 wt % P(S-BIEM) content,29 were obtained and PDM layers were only generated from the surface of the P(S-BIEM) phase, which was easily confirmed by TEM observation due to the selective staining of the S units by RuO4. The PMMA/P(S-BIEM) (20/80, w/w)-g-PDM particles had larger surface area covered by a PDM layer and a thinner thickness of the PDM layer than that of PMMA/P(S-BIEM) (5/95, w/w)-g-PDM particles. The results indicate that the particle morphology could be controlled by changing P(SBIEM) content in the composite particles as well as the monomer conversion, which controlled the PDM chain length. Demonstration of the Dual Stimuli-Responsive Property of Nonspherical PMMA/P(S-BIEM)-g-PDM Particles. PDM is a weak polybase (pKa ≈ 6.8) and exhibits lower critical solution temperature (LSCT) of approximately 34 °C in

Figure 4. Conversion−time plots for surface-initiated AGET ATRP of DM at 60 °C using PMMA/P(S-BIEM) composite particles with 5 (○) and 80 (●) wt % P(S-BIEM) contents.

initiated AGET ATRP of DM at 60 °C using two kinds of PMMA/P(S-BIEM) composite particles with 5 (a,b,c) and 80 (a′,b′,c′) wt % P(S-BIEM) contents. In both systems, polymerization proceeded quickly without any coagulation, but the conversions plateaued at approximately 30% and 70%, respectively, in 5 and 80 wt % P(S-BIEM) contents It seems that the phenomena were caused by the autocatalytic hydrolysis of DM to methacrylic acid (MAA) and 2-dimethylaminoethanol (DMAE) in water, where the hydrolysis of DM has been quantified by gas chromatography in our previous work.41 In the previous work,29 it was clarified that approximately 50% DM was hydrolyzed to MAA and DMAE for 30 min in water in the absence of initiator at 60 °C. MAA cannot be polymerized with ATRP because of irreversible reaction with copper complex to metal carboxylates.42 The difference in the plateaued conversions seems to be caused by the difference in the amounts of initiation site (Br atom) at the particle surfaces.

Figure 5. Optical micrographs of nonspherical PMMA/P(S-BIEM)-g-PDM particles prepared by surface-initiated AGET ATRP of DM using PMMA/P(S-BIEM) composite particles with 5 (a,b,c) and 80 (a′,b′,c′) wt % P(S-BIEM) content at 60 °C for 2 min (a,a′), 7 min (b,b′), and 40 min (c,c′). 7826

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Figure 6. Optical micrographs (a,a′) and SEM photographs (b,b′) of PMMA/P(S-BIEM)-g-PDM particles prepared by surface-initiated AGET ATRP of DM for 1 h at 60 °C using PMMA/P(S-BIEM) composite particles with 5 (a,b,c) and 80 (a′,b′,c′) wt % P(S-BIEM) content, and TEM photographs (c,c′) of ultrathin cross-sections of RuO4-stained composite particles.

Figure 7. Optical micrographs of PMMA/P(S-BIEM)-g-PDM particles with 5 (a,b,c,d) and 80 (a′,b′,c′,d′) wt % P(S-BIEM) contents in PMMA/ P(S-BIEM), dispersed at 25 °C in the aqueous media of pH values of 6.0 (a,a′), 2.0 (b,b′), 11 (c,c′), and 2.0 (d,d′), which were consecutively adjusted.

pH 6.0 taken alternatively at 25 and 60 °C repeatedly. In both particles, the PDM layers slightly shrank from 25 to 60 °C (Figure S1a-b and a′-b′, Supporting Information) and entirely recovered from 60 to 25 °C (Figure S1b-c and b′-c′, Supporting Information). In both particles, the volume phase transition by repeatedly changing the temperature was reversibly observed at every change in the temperature (Figure S2, Supporting Information). The shrinkage at 60 °C is due to the dehydration

water.43 On the basis of this knowledge, the change of the PDM layer in PMMA/P(S-BIEM)-g-PDM particles was examined at various pH values and temperatures. Figure S1 (Supporting Information) shows consecutive optical micrographs of the two kinds of PMMA/P(S-BIEM)g-PDM particles with 5 and 80 wt % P(S-BIEM) contents, which were, respectively, abbreviated as 5- and 8-wt %-P(SBIEM) particles hereafter, dispersed in the aqueous media at 7827

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ratio (3.0) of the molecular weight shown above. It is interesting that such ultrahigh molecular weight PDMs may be obtained in only 40 min. The PDM phase was cross-linked after polymerization (the reason is not clear),29 which disturbed the GPC analysis, however, because the polymerization rate in the aqueous phase was so high, and DM perhaps works as a reducing agent for copper species in ATRP. ATRP may proceed with significantly high polymerization rate.47 Application of the Nonspherical Particles as Particulate Surfactant to the Preparation of Pickering Emulsion. We demonstrated an applicability of the dual stimuli-responsive nonspherical 5-wt %-P(S-BIEM) particles as particulate surfactant for oil-in-water (o/w) emulsion. It is well known that solid particles having affinity toward both oil and water phases can adsorb at an oil/water interface and act like a surfactant, resulting in the formation of the so-called Pickering emulsion.48,49 When 1-octanol (0.4 g) and an aqueous dispersion (3.0 g) (pH 5.8) of the nonspherical particles (0.67 wt % solid content), which were carefully washed by water to remove unwanted Emulgen 930 for excluding unexpected interfacial behavior, were vigorously mixed for 1 min at 25 °C, and a stable oil-in-water emulsion was formed, where the 1-octanol/ water dispersion was selected for comparison with our previous work.50 Figure 8 shows optical micrographs of the obtained

of PDM above LCST. The modulus of volume change was larger in the 5-wt %-P(S-BIEM) particles than in the 80-wt %-(S-BIEM) particles, which seems to be caused by different lengths of the PDM chains in both particles. Figure 7 shows optical micrographs of the two kinds of PMMA/P(S-BIEM)-g-PDM particles with 5 (a,b,c) and 80 (a′,b′,c′) wt % P(S-BIEM) contents in PMMA/P(S-BIEM), dispersed at 25 °C in the aqueous media at various pH values, which were consecutively adjusted. In case of 80-wt %-P(SBIEM) particles (Figure 7a′−d′), change in the volume of the PDM layers by pH was not clearly observed visually. The phenomena would be caused by the rigidity of the PDM phase due to the cross-linking of the PDM chains on the particles, which have been clarified in our previous work.44 However, in the case of 5-wt %-P(S-BIEM) particles (Figure 7a−d), the PDM chains shrank at high pH (Figure 7c) and extended at low pH (Figure 7d), and the particle morphology clearly changed as if the mushroom-cap was opened completely at the low pH (Figure 7b). The mushroom-cap was clearly opened even after drying for SEM observation (Figure S3, Supporting Information). This is attributed to the protonation of tertiary amino groups of PDM. In the acidic region, the charge repulsion between protonated PDM chains grafted on the particles should cause the volume expansion, which leads to the “opened mushroom-cap” particle morphology. (Such a variation was not observed in the experiment of temperature change.) Even if the pH value was again returned from 2.0 to 11, at which the ionized tertiary amino groups are deprotonated, the opened mushroom-cap was not “closed” any more. Tsujii, Fukuda, and coworkers revealed that the densely packed grafted polymer was obtained by surface-initiated AGET ATRP.45 It seems that PDM chains were densely grafted at the surface of P(S-BIEM) phase; however, more detailed investigations are needed. The phenomena will be investigated in detail in the near future. The volume change was reversibly observed, although the mushroom-cap was irreversibly opened. The reason for the large volume change of the PDM layer of the 5-wt %-P(SBIEM) particles when changing the pH or temperature is that the PDM anchored area on the 5-wt %-P(S-BIEM) particle was smaller and that the thickness of the PDM layer was larger than those of the 80-wt %-P(S-BIEM) particles. The ratio of PDM chain length in the 5-wt %-P(S-BIEM) particles relative to the 80-wt %-P(S-BIEM) particles was calculated from these surface areas of the P(S-BIEM) phase (Figure 2) and plateaued conversions (Figure 4), assuming that the number of Br groups at unit surface area of the P(S-BIEM) phase was the same. On the basis of this calculation, the PDM chain length was approximately 3.0 times longer in the 5-wt %-P(S-BIEM) particles than in the 80-wt %-P(S-BIEM) particles. The thicknesses of the PDM chain layers, which were measured from optical micrographs, were 3.9 and 1.5 μm in the 5- and 80-wt %-P(S-BIEM) particles, respectively. Moreover, the thicknesses of the PDM chain layers at pH 2.0 and 25 °C were 5.6 and 1.7 μm in the 5 and 80-wt %-P(S-BIEM) particles, respectively. Calculated molecular weights of PDM layers at pH 2.0 and 25 °C were more than 3.5 × 106 and 1.0 × 106 in the 5and 80-wt %-P(S-BIEM) particles, respectively, which were calculated from the thickness of the PDM layer with assumption of a fully-stretched model of PDM chains.46 The ratio of thicknesses of the measured PDM layers in the 5 and 80-wt %-P(S-BIEM) particles (by optical micrographs) was 3.4, which was almost consistent with the corresponding calculated

Figure 8. Optical micrographs (low (a) and high (b) magnifications) of 1-octanol-in-water emulsion droplets stabilized by PMMA/P(SBIEM)-g-PDM particles with 5 wt % P(S-BIEM) contents in PMMA/ P(S-BIEM) in aqueous media at pH 5.8 at 25 °C.

emulsion. 1-octanol droplets were stabilized by the nonspherical particles, which were adsorbed at the 1-octanol/water interface. However, contrary to our prediction, i.e., that the PDM layer faces the aqueous phase after the adsorption at the oil/water interface, the particles were adsorbed at the interface with the mashroom-cap (PDM layer) facing the oil phase (shown in Figure 8b). This may be due to the higher affinity of PDM to 1-octanol than water. The interfacial activity of the 5-wt %-P(S-BIEM) particles for the Pickering emulsion was examined at three different pH values of 5.5, 5.8, and 6.0 (Figure S4, Supporting Information). At pH 5.5, the 5-wt %-P(S-BIEM) particles were slightly adsorbed at the surface of oil droplets, and the PDM phase existed in the aqueous phase. However, at pH 5.8 and 6.0, the PDM phase became incorporated into the oil phase, and the particles were strongly adsorbed at the oil droplets. These indicate that the adsorption of the particles at the interface of the oil droplets depends on the degree of protonation of dimethylamino groups in PDM. That is, the opened mushroom-cap was so hydrophilic at pH 5.5 that the particles could not adsorb at the interface and not stabilize the Pickering emulsion. However, at pH values of 5.8 and 6.0 the PDM phase 7828

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Figure 9. Consecutive optical micrographs of 1-octanol-in-water emulsion stabilized by PMMA/P(S-BIEM)-g-PDM particles with 5 wt % P(SBIEM) contents in PMMA/P(S-BIEM) at pH 5.8 at 25 °C (a), 33 °C (b), and 37 °C (c). Micrographs (a−c) were taken at the same visual field and at a rising rate of 5 °C/min. The scheme of whole behaviors of Pickering emulsion formed by stimuli-responsible “mushroom-like” Janus polymer particles (d).

particles transferred to the oil phase from the interface, and the droplets coalesced immediately. These results indicate that the amphiphilic property of the PMMA/P(S-BIEM)-g-PDM particles operated effectively to stabilize the 1-octanol droplets, even the PDM layer facing the 1-octanol phase, and that the stability of the Pickering emulsion should be controllable reversibly by changing the temperature due to the increase in the hydrophobicity of the PDM layer. Moreover, increasing pH also led to destabilization of the emulsion as shown in Figure S7 (Supporting Information). From these results, we concluded that the stability of the oil/ water emulsion prepared using the stimuli-responsive Janus particles could be controlled by altering temperature and pH (Scheme 1). That is, the PMMA/P(S-BIEM)-g-PDM particles were useful materials as dual-stimuli responsible for amphiphilic particulate surfactants.

has a good affinity to 1-octanol (by decreasing the hydrophilicity), resulting in the particles working well as particulate surfactants. Further, the stability of the Pickering emulsion was examined in the pH range of 6.2−6.5 (Figure S5, Supporting Information). At pH 6.2, the stability of the Pickering emulsion was slightly decreased compared to that at pH 6.0. As the pH value was increased, the stability was further decreased, and the droplets were collapsed at pH 6.5. These results also indicate that an appropriate hydrophilicity of the opened mushroom-cap (PDM phase) of the 5-wt %-P(S-BIEM) particles was necessary to stabilize the 1-octanol droplets (Figure S6, Supporting Information). The percentages of protonation of dimethylamino groups were calculated to be 95, 86, and 67% at pH values of 5.5, 6.0, and 6.5, respectively, using a pKa of 6.8, where we assumed that the protonation of PDM chains evenly occurred at pH 6.8 and neglected the pKa change affected by the close protonated PDM chains. They look to be too high. Because it is reported that pH values around a particle surface having high ionic charges is different from that in the aqueous medium,51 actual % protonation values seem to be much lower than those values. Figure 9 shows consecutive optical micrographs of the Pickering emulsion with rising temperature from room temperature at 5 °C/min. After rising above LCST, where the PDM phase became hydrophobic, the nonspherical

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CONCLUSIONS Micrometer-sized, monodisperse, nonspherical PMMA/P(SBIEM)-g-PDM particles were successfully synthesized utilizing the solvent evaporation method and surface-initiated AGET ATRP. The shape anisotropy of the obtained nonspherical particles could be controlled by changing the percentage of surface area occupied by the P(S-BIEM) phase having Br atoms as initiation sites of the polymerization, which was easily 7829

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Scheme 1. Preparation of Mushroom-Like Janus Polymer Particles by Site-Selective Surface-Initiated AGET ATRP in Aqueous Dispersed Systems (a) and Reaction Scheme of AGETATRP (b)

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research (B) (Grant 25288054) from the Japan Society for the Promotion of Science (JSPS).

changed by P(S-BIEM) content in the composite particles. Closed mushroom-cap-like PDM layer of the particles obtained in the 5-wt % (PS-BIEM) system showed a response to temperature and pH reversibly, resulting in opened mushroomcap-like particles. The dual stimuli-responsive nonspherical particles effectively operated as a particulate surfactant, resulting in a stable 1-octanol-in-water emulsion. The stability of the emulsion could be controlled by altering the temperature and pH, reflecting the nature of the nonspherical particles.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedures, characterization, materials, instrumentation details, and supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Okubo, M. Control of particle morphology in emulsion polymerization. Makromol. Chem., Macromol. Symp. 1990, 35-6, 307−325. (2) Dimonie, V. L.; Daniels, E. S.; Shaffer, O. L.; El-Aasser, M. S. In Emulsion Polymerization and Emulsion Polymers; Lovell, P. A., ElAasser, M. S., Eds.; John Wiley & Sons: New York, 1997; Chapter 9, pp 293−326. (3) Sundberg, D. C.; Durant, Y. G. Latex particle morphology, fundamental aspects: a review. Polym. React. Eng. 2003, 11, 379−432. (4) Walther, A.; Muller, A. H. E. Janus particles: synthesis, selfassembly, physical properties, and applications. Chem. Rev. 2013, 113, 5194−5261. (5) Casagrande, C.; Fabre, P.; Raphael, E.; Veyssie, M. Janus beads realization and behavior at water oil interfaces. Europhys. Lett. 1989, 9, 251−255. (6) Kim, J. W.; Lee, D.; Shum, H. C.; Weitz, D. A. Colloid surfactants for emulsion stabilization. Adv. Mater. 2008, 20, 3239−+. (7) Walther, A.; Hoffmann, M.; Muller, A. H. E. Emulsion polymerization using Janus particles as stabilizers. Angew. Chem., Int. Ed. 2008, 47, 711−714.

AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +81-78-858-2204. E-mail: [email protected]. Notes

The authors declare no competing financial interest. Part CCCLXII of the series Studies on Suspension and Emulsion. 7830

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(8) Choi, J.; Zhao, Y.; Zhang, D.; Chien, S.; Lo, Y.-H. Patterned fluorescent particles as nanoprobes for the investigation of molecular interactions. Nano Lett. 2003, 3, 995−1000. (9) Yoshida, M.; Roh, K. H.; Lahann, J. Short-term biocompatibility of biphasic nanocolloids with potential use as anisotropic imaging probes. Biomaterials 2007, 28, 2446−2456. (10) Howse, J. R. et al. Self-motile colloidal particles: From directed propulsion to random walk. Phys. Rev. Lett. 2007, 99. (11) Takei, H.; Shimizu, N. Gradient sensitive microscopic probes prepared by gold evaporation and chemisorption on latex spheres. Langmuir 1997, 13, 1865−1868. (12) Fujimoto, K.; Nakahama, K.; Shidara, M.; Kawaguchi, H. Preparation of unsymmetrical microspheres at the interfaces. Langmuir 1999, 15, 4630−4635. (13) Behrend, C. J.; Anker, J. N.; McNaughton, B. H.; Brasuel, M.; Philbert, M. A. Metal-capped Brownian and magnetically modulated optical nanoprobes (MOONs): Micromechanics in chemical and biological microenvironments. J. Phys. Chem. B 2004, 108, 10408− 10414. (14) Paunov, V. N.; Cayre, O. J. Supraparticles and “Janus” particles fabricated by replication of particle monolayers at liquid surfaces using a gel trapping technique. Adv. Mater. 2004, 16, 788−+. (15) Gangwal, S.; Cayre, O. J.; Velev, O. D. Dielectrophoretic assembly of metallodielectric Janus particles in AC electric fields. Langmuir 2008, 24, 13312−13320. (16) Takahara, Y. K.; et al. Asymmetrically modified silica particles: a simple particulate surfactant for stabilization of oil droplets in water. J. Am. Chem. Soc. 2005, 127, 6271−6275. (17) Hong, L.; Jiang, S.; Granick, S. Simple method to produce Janus colloidal particles in large quantity. Langmuir 2006, 22, 9495−9499. (18) Suzuki, D.; Tsuji, S.; Kawaguchi, H. Janus microgels prepared by surfactant-free pickering emulsion-based modification and their selfassembly. J. Am. Chem. Soc. 2007, 129, 8088−+. (19) Perro, A.; et al. Towards large amounts of Janus nanoparticles through a protection-deprotection route. Chem. Commun. 2005, 5542−5543. (20) Isojima, T.; Lattuada, M.; Vander Sande, J. B.; Hatton, T. A. Reversible clustering of pH- and temperature-responsive Janus magnetic nanoparticles. ACS Nano 2008, 2, 1799−1806. (21) Erhardt, R.; et al. Amphiphilic Janus micelles with polystyrene and poly(methacrylic acid) hemispheres. J. Am. Chem. Soc. 2003, 125, 3260−3267. (22) Nie, Z.; Li, W.; Seo, M.; Xu, S.; Kumacheva, E. Janus and ternary particles generated by microfluidic synthesis: design, synthesis, and self-assembly. J. Am. Chem. Soc. 2006, 128, 9408−9412. (23) Nisisako, T.; Torii, T.; Takahashi, T.; Takizawa, Y. Synthesis of monodisperse bicolored janus particles with electrical anisotropy using a microfluidic co-flow system. Adv. Mater. 2006, 18, 1152−1156. (24) Kim, S. H.; Jeon, S. J.; Jeong, W. C.; Park, H. S.; Yang, S. M. Optofluidic synthesis of electroresponsive photonic Janus balls with isotropic structural colors. Adv. Mater. 2008, 20, 4129−+. (25) Yuet, K. P.; Hwang, D. K.; Haghgooie, R.; Doyle, P. S. Multifunctional superparamagnetic Janus particles. Langmuir 2010, 26, 4281−4287. (26) van Ravensteijn, B. G. P.; Kamp, M.; van Blaaderen, A.; Kegel, W. K. General route toward chemically anisotropic colloids. Chem. Mater. 2013, 25, 4348−4353. (27) Saito, N.; Kagari, Y.; Okubo, M. Effect of colloidal stabilizer on the shape of polystyrene/poly(methyl methacrylate) composite particles prepared in aqueous medium by the solvent evaporation method. Langmuir 2006, 22, 9397−9402. (28) Tanaka, T.; Nakatsuru, R.; Kagari, Y.; Saito, N.; Okubo, M. Effect of molecular weight on the morphology of polystyrene/ poly(methyl methacrylate) composite particles prepared by the solvent evaporation method. Langmuir 2008, 24, 12267−12271. (29) Tanaka, T.; Okayama, M.; Kitayama, Y.; Kagawa, Y.; Okubo, M. Preparation of “mushroom-like” Janus particles by site-selective surface-initiated atom transfer radical polymerization in aqueous dispersed systems. Langmuir 2010, 26, 7843−7847.

(30) Okubo, M.; Minami, H.; Zhou, J. Preparation of block copolymer by atom transfer radical seeded emulsion polymerization. Colloid Polym. Sci. 2004, 282, 747−752. (31) Matyjaszewski, K.; Tsarevsky, N. V. Nanostructured functional materials prepared by atom transfer radical polymerization. Nat. Chem. 2009, 1, 276−288. (32) Ouchi, M.; Yoda, H.; Terashima, T.; Sawamoto, M. Aqueous metal-catalyzed living radical polymerization: highly active waterassisted catalysis. Polym. J. 2012, 44, 51−58. (33) Patrucco, E.; et al. Surface-initiated ATRP modification of tissue culture substrates: poly(glycerol monomethacrylate) as an antifouling surface. Biomacromolecules 2009, 10, 3130−3140. (34) Tanaka, T.; Okayama, M.; Minami, H.; Okubo, M. Dual stimuliresponsive “mushroom-like” janus polymer particles as particulate surfactants. Langmuir 2010, 26, 11732−11736. (35) Gaynor, S. G.; Qiu, J.; Matyjaszewski, K. Controlled/“living” radical polymerization applied to water-borne systems. Macromolecules 1998, 31, 5951−5954. (36) Saito, N.; Kagari, Y.; Okubo, M. Effect of colloidal stabilizer on the shape of polystyrene/poly(methyl methacrylate) composite particles prepared in aqueous medium by the solvent evaporation method. Langmuir 2006, 22, 9397−9402. (37) Yamashita, N.; Konishi, N.; Tanaka, T.; Okubo, M. Preparation of hemispherical polymer particles by cleavage of a Janus poly(methyl methacrylate)/polystyrene composite particle. Langmuir 2012, 28, 12886−12892. (38) Tanaka, T.; Okayama, M.; Okubo, M. Effect of polymer end group on the morphology of polystyrene/poly(methyl methacrylate) composite particles prepared by the solvent evaporation method. Macromol. Symp. 2010, 288, 55−66. (39) Ma, G. H.; Omi, S.; Dimonie, V. L.; Sudol, E. D.; El-Aasser, M. S. Study of the preparation and mechanism of formation of hollow monodisperse polystyrene microspheres by SPG (Shirasu Porous Glass) emulsification technique. J. Appl. Polym. Sci. 2002, 85, 1530− 1543. (40) Chaiyasat, P.; Ogino, Y.; Suzuki, T.; Minami, H.; Okubo, M. Preparation of divinylbenzene copolymer particles with encapsulated hexadecane for heat storage application. Colloid Polym. Sci. 2008, 286, 217−223. (41) Matsumoto, T.; Okubo, M.; Onoe, S. Studies on suspension and emulsion.19. Peculiar behavior of copolymerization of methylmethacrylate and dimethylaminoethyl methacrylate in an aqueousmedia with pottassium persulfate as initiator. Kobunshi Ronbunshu 1975, 32, 162−167. (42) Patten, T.E.; Matyjaszewski, K. Atom transfer radical polymerization and the synthesis of polymeric materials. Adv. Mater. 1998, 10, 901−+. (43) Okubo, M.; Ahmad, H. Synthesis of Temperature-Sensitive Submicron Size Composite Polymer Particles. Colloid Polym. Sci. 1995, 273, 817−821. (44) Tanaka, T.; Okayama, M.; Kitayama, Y.; Kagawa, Y.; Okubo, M. Preparation of “mushroom-like” Janus particles by site-selective surface-initiated atom transfer radical polymerization in aqueous dispersed systems. Langmuir 2010, 26, 7843−7847. (45) Tsujii, Y.; Ohno, K.; Yamamoto, S.; Goto, A.; Fukuda, T. In Surface-Initiated Polymerization I; Jordan, R., Ed.; Springer: New York, 2006; Vol. 197, pp 1−45. (46) Morinaga, T.; Ohkura, M.; Ohno, K.; Tsujii, Y.; Fukuda, T. Monodisperse silica particles grafted with concentrated oxetanecarrying polymer brushes: Their synthesis by surface-initiated atom transfer radical polymerization and use for fabrication of hollow spheres. Macromolecules 2007, 40, 1159−1164. (47) Matyjaszewski, K. Atom transfer radical polymerization (ATRP): current status and future perspectives. Macromolecules 2012, 45, 4015−4039. (48) Pickering, S. U. Emulsions. J. Chem. Soc. 1907, 91, 2001−2021. (49) Binks, B. P. Particles as surfactants - similarities and differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21−41. 7831

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Article

(50) Tanaka, T.; Okayama, M.; Minami, H.; Okubo, M. Dual stimuliresponsive “mushroom-like” Janus polymer particles as particulate surfactants. Langmuir 2010, 26, 11732−11736. (51) Sentenac, H.; Grignon, C. Effect of H+ excretion on the surface of corn root-cells elevated by using weak acid influx as a pH probe. Plant Physiol. 1987, 84, 1367−1372.

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