Article pubs.acs.org/Langmuir
Raspberry-Shaped Composite Microgel Synthesis by Seeded Emulsion Polymerization with Hydrogel Particles Daisuke Suzuki* and Chiaki Kobayashi Graduate School of Textile Science & Technology, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan S Supporting Information *
ABSTRACT: A series of raspberry-shaped composite microgels were synthesized by the seeded emulsion polymerization of styrene with hydrogel particles. Thermoresponsive microgels of poly(N-isopropylacrylamide) cross-linked with N,N′methylenebis(acrylamide) acted as cores for the polymerization. During the surfactant-free polymerization, the core microgels shrank at 70 °C to provide thermoresponsive composite microgels, and the polystyrene particles attached to core microgels became bigger with increasing styrene concentration. Conversely, composite microgels synthesized with sodium dodecyl sulfate (SDS) ([SDS] > 6.5 mM) did not exhibit thermoresponsive deswelling behavior because polystyrene particles covered the core microgels. In particular, polystyrene particles formed composites on the microgel surface as well as inside the microgels when the SDS concentration exceeded a critical value for core microgel swelling at 70 °C. A mechanism is proposed based on these results for the seeded emulsion polymerization of water-immiscible monomers with microgels.
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first synthesized by conventional emulsion polymerization and then coated with a hydrogel shell by seeded precipitation polymerization.2,4,5 More recently, Lyon et al. produced pNIPAm core/poly(NIPAm-co-acrylic acid) shell microgels and an inverse core/shell structure, prompting much interest in core/shell microgels, in which core and shell comprise hydrogels.4,31 Hard core/hydrogel shell and core/shell microgels have been synthesized by seeded precipitation (or dispersion) polymerization: the solvent dissolves the monomer for shell synthesis but precipitates the polymer during the polymerization. Conversely, few studies have dealt with the seeded emulsion polymerization in the presence of soft microgels, in which water sparingly dissolves the monomer during the polymerization.40−42 Recently, multilayered composite microgels were obtained by the surfactant-free seeded emulsion polymerization of glycidyl methacrylate (GMA), a relatively water-miscible oil monomer (solubility 120 mM), with the deswollen pNIPAm microgel.42 However, the detailed mechanism of the seeded emulsion polymerization involving soft microgels remains unclear. To the best of our knowledge, no systematic studies have been performed to fill this gap. In this work, raspberry-shaped composite microgels were synthesized by the seeded emulsion polymerization of styrene with pNIPAm microgels. Styrene was chosen as a monomer because of its extensive use in emulsion polymerization and its
INTRODUCTION Hydrogel particles, or microgels, are cross-linked waterswellable polymeric particles that exhibit interesting stimuliresponsive physical/chemical properties. These properties change in response to external stimuli, such as temperature, pH, ionic strength, light, presence of biomolecules, and chemical reaction,1−9 making microgels suitable for various applications. In particular, microgels are expected to become central to the development of drug/gene delivery carriers,10−13 chemical/biological separation systems,14,15 photonic crystals,16−19 microreactors,20−22 and sensors.23,24 Poly(N-isopropylacrylamide) (pNIPAm), a representative thermoresponsive polymer exhibiting a low critical solution temperature (LCST) at around 31 °C, has extensively been exploited in stimuliresponsive microgels.25,26 Typically, pNIPAm-based microgels, which were first prepared by Pelton and Chibante,27 exhibit a volume phase transition temperature (VPTT) around the LCST of pNIPAm chains.4 Alternatively, Kawaguchi et al. have synthesized monodisperse and pH-responsive microgels using acrylamide, methacrylic acid, and a cross-linker by precipitation polymerization in several alcohols.28 The design and control of chemical composition and microgel morphology play an important role for realizing specific applications. Highly complex microgels displaying spatially separated regions composed of different polymers have been obtained by copolymerization,29,30 multistage polymerization,31−35 or postpolymerization modifications.36−39 There are a number of reports on the preparation of hard core/hydrogel shell particles. In these particles, the hard core, consisting of solid materials such as polystyrene particles, was © XXXX American Chemical Society
Received: May 8, 2014 Revised: May 31, 2014
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Figure 1. (a) Hydrodynamic diameters of N microgels as a function of temperature. (b) SEM images of N microgels dried at 25 °C on solid substrates. (c) TEM images of N microgels dried at 25 °C on solid substrates. Scale bars are 1 μm. (d) Photograph of an N microgel colloidal crystal in a rectangular capillary tube formed at 25 °C. flask. Next, KPS (0.027 g) dissolved in water (5 mL) was injected to the flask, and after 5 min, a mixture of styrene (2.299 μL, 200 mM) and water (50 mL) was poured into the flask to start the polymerization. Here, the water was bubbled for 30 min with nitrogen gas before mixing with styrene. The polymerization was allowed to proceed for 24 h, and the resulting microgel dispersion was cooled to room temperature. Composite microgels were purified with water two times using a RCF of 20000g at 20 °C and by dialysis for more than 4 days. The composite microgels are denoted as NSX(Y) in the remainder of the article. In this code, N and S represent NIPAm and styrene, respectively. The numbers X and Y represent the mole percentage of styrene in the feed and the SDS concentration, respectively. Characterization. Hydrodynamic diameters of individual particles were determined by dynamic light scattering (DLS) using a ZetasizerNanoS (Malvern Instruments Ltd., UK).42 The DLS data correspond to an average of 15 independent measurements of the intensity autocorrelation acquired over 30 s. These measurements were performed at a microgel concentration of ∼0.01 wt %, unless otherwise noted. The total salt concentration was adjusted to 1 mM using NaCl. The samples were allowed to equilibrate thermally at desired temperature for 10 min before measurements. Hydrodynamic diameters were calculated from the measured diffusion coefficients using the Stokes−Einstein equation (Malvern, Zetasizer software v. 6.12). The microgels in the solid state were examined by field emission scanning electron microscope (FE-SEM, Hitachi Ltd., S-5000). Diluted microgel dispersions were dried on a polystyrene substrate, thoroughly rinsed with water, and sputtered with Pt/Pd before observation. Note that certain samples for FE-SEM observation were not purified by centrifugation because the presence of secondary particles needed confirmation.42 Microgels were dried on a carbon-coated copper grid (Okenshoji Co., Ltd.) and observed by TEM using a JEOL2010 instrument operating at 200 kV. Furthermore, the lyophilized composite microgels were stained for 30 min by vaporizing 0.5% RuO4 in water and dispersed in epoxy matrices. Matrices were cured at 40 °C for 24 h and 60 °C for 24 h and were microtomed into ultrathin cross sections for the observation of TEM. To check the monodispersity, the pNIPAm core microgels were transferred into Vitrotube borosilicate rectangular capillaries (0.1 × 2.0 mm2) by capillary action and treated by thermal annealing to form colloidal crystals.44 Electrophoretic mobilities (EPMs) were measured using a ZetasizerNanoZS (Malvern, Zetasizer software v. 4.20).42 Samples at a concentration of ca. 0.0003 wt % were allowed to equilibrate thermally at the desired temperature for 10 min before measurements. The total salt concentration was adjusted to 1 mM using NaCl.
poor solubility in water (2.9 mM at 25 °C),43 which is much lower than for GMA used previously.42 pNIPAm-based core microgels were synthesized by aqueous free-radical precipitation polymerization, and the seeded emulsion polymerization of styrene was conducted on these cores with or without the anionic surfactant sodium dodecyl sulfate (SDS). The resulting composite microgels were characterized by dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), laser Doppler velocimetry, and X-ray photoelectron spectroscopy (XPS). In particular, the effects of styrene and SDS concentrations during the seeded polymerization on composite microgel morphologies were investigated.
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EXPERIMENTAL DETAILS
Materials. N-Isopropylacrylamide (NIPAm, purity 98%), N,N′methylenebis(acrylamide) (BIS, 97%), styrene (99%), potassium peroxodisulfate (KPS, 95%), ammonium peroxodisulfate (APS, 98%), and sodium dodecyl sulfate (SDS, 95%) were purchased from Wako Pure Chemical Industries, Ltd., and used as received. N,N,N′,N′Tetramethylethylenediamine (TEMED, 99%) was purchased from Sigma-Aldrich and used as received. A 0.5% ruthenium tetroxide (RuO4) stabilized aqueous solution was purchased from Electron Microscopy Sciences and used as received. For all experiments, water was distilled and ion-exchanged (EYELA, SA-2100E1) before use. Synthesis of pNIPAm Microgels. The chemically cross-linked pNIPAm-based microgels were synthesized by aqueous free-radical precipitation polymerization.2,42 A mixture of NIPAm (12.900 g, 95 mol %), BIS (0.925 g, 5 mol %), and water (800 mL) was poured into a 1000 mL three-neck round-bottom flask equipped with a mechanical stirrer, a condenser, and a nitrogen gas inlet. The monomer solution was bubbled for 30 min with nitrogen gas at 70 °C to purge the oxygen. Under a stream of nitrogen and with constant stirring at 250 rpm, the initiator KPS (0.432 g) dissolved in water (10 mL) was injected to the flask to start the polymerization. The reaction was allowed to proceed for 4 h, and the microgel dispersion was cooled to room temperature. The microgels were purified by centrifugation/ redispersion with water two times using a relative centrifugal force (RCF) of 50000g at 20 °C and dialyzed for a week. These microgels are denoted as N in the remainder of the article. Seeded Emulsion Polymerization of Styrene with pNIPAm Microgels. The seeded emulsion polymerization of styrene was conducted under various conditions using N microgels as cores. The pNIPAm dispersion (solid content: 0.17 g) was poured into a 200 mL three-neck round-bottom flask equipped with a mechanical stirrer and a condenser and heated to 70 °C with constant stirring at 250 rpm. In some cases, SDS (final concentration: 0−50 mM) was added to the B
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Figure 2. Electron micrographs of (a, b) NS50(0), (c, d) NS100(0), and (e, f) NS200(0) dried on the substrates at 25 °C. (a, c, e) FE-SEM images. Scale bars are 1 μm. (b, d, f) TEM images. Scale bars represent 300 nm. XPS spectra of lyophilized samples were acquired using a KRATOS AXIS Ultra DLD instrument equipped with a Mg Kα X-ray source (Shimazu Co.).
styrene particles for NS50(0) were undetectable because of the low amount of composited polystyrene. Polystyrene particle diameters equaled 77 ± 14 and 137 ± 22 nm for NS100(0) and NS200(0), respectively, showing that polystyrene particles became bigger with increasing styrene concentration. Finally, the resulting composite microgels generated sediments because of aggregation at a styrene concentration of 250 mM during the seeded emulsion polymerization (data not shown). Interestingly, no secondary polystyrene particles were observed after the seeded emulsion polymerization regardless of styrene concentrations (50−200 mM, Figure 2), suggesting that all the styrene was combined with the N core microgels. A similar tendency was noted in a previous study using GMA as a monomer.42 SEM images also indicate that NS50(0) and NS100(0) adopt non-close-packed ordered structures, consistent with previous investigations of dilute pNIPAm microgel dispersions dried at room temperature on solid substrates.2,27 In addition, this agrees with previous results showing that these microgels adsorbed at the air/water interface when the small dispersion droplets dried.44 Therefore, NS50(0) and NS100(0) were expected to present pNIPAm layers on their surfaces. Conversely, pNIPAm layers were difficult to distinguish on the NS200(0) composite microgels, which did not show non-closepacked ordered structures. These composite microgels were synthesized at 70 °C, resulting in deswollen core pNIPAm microgels in water. Thus, in control experiments, the seeded emulsion polymerization was performed using APS and TEMED as redox initiators at 25 °C, where the core pNIPAm microgels were swollen in water. However, in these cases, the polymerization was unsuccessful and led to microgel aggregation (Supporting Information, Table S1). Figure 3 shows the hydrodynamic diameters of NS50(0), NS100(0), and NS200(0) composite microgels as a function of temperature. All three composite microgels exhibited deswelling with increasing temperature. The swelling ratio (Vh20 °C/ Vh70 °C) amounted to 7.5, 4.4, 3.6, and 1.5 for N, NS50(0), NS100(0), and NS200(0), respectively, indicating a decrease
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RESULTS AND DISCUSSION Synthesis and Characterization of pNIPAm Core Microgels. A systematic investigation of the seeded emulsion polymerization with microgels requires monodisperse microgels. Therefore, core microgels were synthesized by aqueous free-radical precipitation polymerization using thermosensitive pNIPAm. In this conventional method for preparing thermosensitive microgels, the monomer is soluble under polymerization conditions, but the polymer undergoes a phase separation into globules.2,27,44 Figure 1a shows the hydrodynamic diameters of N microgels as a function of temperature. N microgel diameters changed from 505 nm at 25 °C to 266 nm at 70 °C, consistent with the thermoresponsiveness of pNIPAm. Figures 1b and 1c show SEM and TEM images of N microgels dried at 25 °C on the substrate, respectively.44 These images demonstrated that the dried N microgels consisted of monodisperse particles exhibiting a mean diameter of 357 ± 13 nm (n = 30, from SEM image). This diameter is much smaller than hydrodynamic diameters (i.e., 505 nm at 25 °C), determined in dispersions because of the dried state of the SEM samples. Note that these monodisperse N microgels assembled into colloidal crystals in water (Figure 1d). Effect of Styrene Concentration on the SurfactantFree Seeded Emulsion Polymerization. Next, the surfactant-free seeded emulsion polymerization of styrene was conducted at 70 °C using N microgels as cores. First, the effect of styrene monomer concentration on the resulting NS composite microgels during the polymerization was determined. Figure 2 shows the representative electron micrographs of NS composite microgels. These images clearly demonstrate that except for NS50(0), all NS composite microgels formed raspberry-shaped particles composed of microgels surrounded by small polystyrene particles. Presumably, the small polyC
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NS200(0) microgels did not show non-close-packed ordered structures when the microgels were dried at 25 °C. The appearance of the resulting dried structure (Figure 4d) was similar to that observed when NS100(0) microgels were directly dried at 25 °C (Figure 2c). In addition, no hysteresis was observed in the deswelling/swelling curves for NS200(0) (Supporting Information, Figure S1). These results clearly demonstrate that the composite structures remained intact upon heating/cooling treatment. Seeded Emulsion Polymerization of Styrene with pNIPAm Microgels. As mentioned above, the seeded emulsion polymerization of styrene was successful when N microgels were deswollen at 70 °C. To investigate the influence of the degree of core microgel swelling in this system, the anionic surfactant SDS was used for the seeded emulsion polymerization at 70 °C. In general, SDS binds to pNIPAm microgels, dramatically enhancing microgel swelling.45−48 Figure 5 shows the hydrodynamic diameters of the pNIPAm
Figure 3. Hydrodynamic diameters of N (black), NS50(0) (yellow), NS100(0) (blue), and NS200(0) microgels (red) as a function of temperature.
when the amount of added styrene increased. This suggests that the polystyrene particles did not completely cover the pNIPAm cores in these composite microgels but restricted swelling. In addition, the deswollen composite microgel dispersions were dried at 70 °C and examined by FE-SEM to further assess their thermosensitivity. NS50(0) (Figure 4a) and NS100(0)
Figure 5. Hydrodynamic diameters of pNIPAm microgels as functions of SDS concentration at 25 and 70 °C. Microgel concentrations equaled 0.17 wt % and were the same as during the seeded polymerization. We confirmed that hydrodynamic diameters are able to be determined by DLS at 0.17 wt % N microgel (Supporting Information, Figure S2).
microgels as a function of SDS concentration. The hydrodynamic diameter increased with increasing SDS concentration at 25 °C, where the pNIPAm microgels are highly swollen, in agreement with previous reports.45−48 In addition, the pNIPAm microgels swelled dramatically with SDS even at 70 °C, and in particular, their hydrodynamic diameters did not vary between 25 and 70 °C at 100 mM SDS. Next, SDS was added to the seeded emulsion polymerization system, and its influence on the polymerization was evaluated at 70 °C. Anionic surfactant and pNIPAm microgels were mixed for 10 min before initiating the polymerization to allow SDS to promote microgel swelling. Figure 6 shows the electron micrographs of the resulting composite microgels. Table 1 lists the composite microgel diameters measured from these FE-SEM images, along with those of their corresponding small polystyrene particles. While composite microgels became larger with increasing SDS concentration, the polystyrene particles became smaller. The dried NS200(0.5) structure (Figure 6a,b) was similar to that of NS200(0) at 25 °C (Figure 2e,f). In NS200(6.5), the small polystyrene particles were more uniformly distributed around the core microgels than in NS200(0). This result is a bit complicated to explain because the hydrodynamic diameters of core microgels did not change in the 0−8 mM SDS concentration range at 70 °C (Figure 5).
Figure 4. FE-SEM images of (a) NS50(0), (b) NS100(0), and (c) NS200(0) dried on a polystyrene substrate at 70 °C. (d) NS100(0) microgels dried at 25 °C after five successive heating/cooling cycles.
(Figure 4b) did not display non-close-packed ordered structures but aggregated composite microgels, indicating that the dispersions dried according to different mechanisms at 70 and 25 °C. No remarkable changes were observed in the NS200(0) dried structures between 70 °C (Figure 4c) and 25 °C (Figure 2e,f), suggesting that a large part of the deformable hydrogel core was composited with the polystyrene particles. When a dilute dispersion of NS100(0) was heated to 70 °C and subsequently cooled to 25 °C, the dried structures retrieved their swollen state, confirming their reversible thermoresponsiveness. This heating/cooling cycle was repeated five times before the dispersion was dried onto the solid substrate. Here, NS100(0) microgels were chosen for the experiment because D
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Figure 6. Electron micrographs of (a, b) NS200(0.5), (c, d) NS200(6.5), (e, f) NS200(16), and (g, h) NS200(50) dried on solid substrates at 25 °C. (a, c, e, g) FE-SEM images. Scale bars are 1 μm. (b, d, f, h) TEM images. Scale bars represent 300 nm.
Table 1. Diameters of Composite Microgels and Corresponding Small Polystyrene Particles Measured from FE-SEM Images composite microgels NS200(0) NS200(0.5) NS200(6.5) NS200(16) NS200(50)
diam/nm
CV/%
± ± ± ± ±
6.8 7.2 3.9 2.6 3.1
362 366 493 521 501
25 26 19 13 16
small polystyrene particles diam/nm
CV/%
± ± ± ± ±
16.3 19.7 19.7 24.7 21.2
137 133 110 39 34
22 26 22 10 7
Note that SDS and pNIPAm microgels were mixed in the flask at room temperature before heating at 70 °C. Therefore, at 70 °C, the deswollen microgels were bound to SDS even though the surfactant did not affect the hydrodynamic diameters. The role of the bound SDS may be to uniformly combine small polystyrene particles with the pNIPAm core. Conversely, in the case of NS200(16) and NS200(50), which were synthesized with 16 and 50 mM SDS, respectively, the core pNIPAm microgels were expected to swell in water during the seeded emulsion polymerization (Figure 5). The decrease in polystyrene particle diameter with increasing SDS concentration may be linked with small SDS aggregates in the microgels.47,48 However, these results did not display a marked relationship between the degree of pNIPAm core swelling and composite microgel structures. Figure 7 shows the hydrodynamic diameters of composite microgels synthesized with SDS. These composite microgels did not exhibit deswelling with increasing temperature, except NS200(0.5), suggesting that the pNIPAm cores were covered with continuous polystyrene particle layers. Similar to NS200(0), the temperature-induced deswelling was observed in NS200(0.5) because the polydisperse, relatively large polystyrene particles (∼133 nm, CV 19.7%), provided a discontinuous cover over the pNIPAm cores. In other words, the SDS concentration (0.5 mM) was very low to affect the morphology and deswelling behavior of the composite microgels. It is worthwhile mentioning that the glass transition temperature (Tg) of polystyrene (ca. 100 °C)49 is higher than the temperature range of the DLS measurement, resulting in sufficiently hard polystyrene layers to suppress core deswelling.
Figure 7. Hydrodynamic diameters of NS200(0.5) (red), NS200(6.5) (yellow), NS200(16) (green), and NS200(50) (blue) as a function of temperature.
Surface Structures of the Composite Microgels. Trilayered composite microgels comprising hydrogel core, solid shell, and hydrogel outer shell were previously synthesized by the surfactant-free seeded emulsion polymerization using GMA as a monomer.42 Therefore, EPMs of the styrene-based composite microgels studied were measured to elucidate the surface structures of the hydrogel core/solid shell composites. Table 2 summarizes the EPMs of the composite microgels, their parent N cores, and polystyrene particles. These polystyrene particles were synthesized by the surfactant-free emulsion polymerization, as described in the Supporting Table 2. Electrophoretic Mobilities of Core and Composite Microgels and Polystyrene Particles in 1 mM NaCl EPM/10−8 m2 V−1 s−1 sample N NS200(0) NS200(0.5) NS200(6.5) NS200(16) NS200(50) polystyrene particles E
25 °C −0.54 −1.06 −1.08 −3.64 −3.05 −3.03 −3.41
± ± ± ± ± ± ±
0.002 0.02 0.04 0.06 0.04 0.07 0.05
40 °C −3.44 −4.13 −3.32 −4.79 −3.35 −3.61 −3.02
± ± ± ± ± ± ±
0.11 0.03 0.03 0.26 0.05 0.11 0.16
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Figure 8. TEM images of ultrathin cross sections: (a) NS200(0), (b) NS200(0.5), (c) NS200(6.5), (d) NS200(16), and (e) NS200(50). Scale bars are 300 nm.
Information. Hydrogel layer-coated particles usually exhibit low EPMs,50 which may serve as indicators of the presence of a hydrogel layer on the particle surface. N, NS200(0), and NS200(0.5) surfaces displayed low EPM values ( 6.5 mM) became clear. NS200(6.5) comprised small polystyrene particles (ca. 110 nm) surrounding the pNIPAm core (Figure 8c). When the
SDS concentration increased during the seeded emulsion polymerization, such as in NS200(16) and NS200(50), the small polystyrene particles were formed on the outer and inner microgel surfaces (Figure 8d,e). Because not all composite microgels displayed small polystyrene particles over their entire structures, these polystyrene particles did not appear to exist at the center of the composite microgels for NS200(16) and NS200(50). In other words, we cannot choose the exact position of the composite microgels when samples are microtomed into ultrathin cross sections. This may be due to the presence of heterogeneous chemical cross-links in pNIPAm microgels synthesized by precipitation polymerization, leading to higher cross-linking density in the microgel center than on its surface.51 Based on all this data, a mechanism is speculated for the seeded emulsion polymerization of styrene with pNIPAm microgels. The pNIPAm microgels dispersions were heated to 70 °C, and the initiator KPS was injected into these dispersions before styrene monomer addition. Therefore, the styrene polymerization was expected to occur outside the pNIPAm microgels in the early stage. Kraft et al. have reported the nucleation of secondary particles during the seeded polymerization involving pNIPAm microgels and styrene with the liposoluble initiator AIBN, in which the core microgels were swollen by styrene.52 In contrast, the initiator used for the seeded emulsion polymerization was the water-soluble KPS. Therefore, polystyrene formation is expected to follow a surfactant-free emulsion polymerization mechanism. Surprisingly, however, no secondary particles were observed in the surfactant-free system although the SEM samples were not purified by centrifugation (Figure 2), indicating that polystyrene nucleated outside of the pNIPAm microgels may be adsorbed on these deswollen pNIPAm microgels during the seeded emulsion polymerization. When the styrene concentration increased, such as NS100(0) vs NS200(0) in Figure 2d,f, the number of small polystyrene particles decreased, and the diameters of the polystyrene particles became bigger. This implies that small polystyrene particles attached to deswollen F
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concentration exceeded a critical level to swell the core microgels. These uniquely shaped composite particles may find application as building blocks in optic materials, and the new raspberry-shaped particles, for which outer and inner surfaces are decorated with small particles (NS200(16)), may be used as an adsorbent for water-immiscible solvents.
cores served as seeds for further polystyrene particle growth. Morphologies of composite microgels synthesized with SDS differed from those synthesized without SDS. Gilányi et al. have demonstrated that above a critical surfactant concentration SDS binds to the outer shell of the pNIPAm microgels by forming small aggregates, resulting in microgel swelling.48 Furthermore, a higher aggregation number is achived when the equillibrium surfactant concentration exceeds a second critical value,48 in agreement with the results shown here. The SDS-bound pNIPAm microgels did not deswell, even above VPTT in pure water (Figure 5). Although the hydrodynamic diameters of the core microgels at 70 °C did not change in the 0−8 mM SDS concentration range, the morphology of NS200(6.5) changed significantly (Figures 6c,d and 8c) compared with NS200(0). In NS200(6.5), the pNIPAm core microgels did not swell, but SDS played an important role in polystyrene nucleation and growth around the deswollen pNIPAm microgels, resulting in the formation of raspberry-shaped composite microgels which are surrounded uniformly by small polystyrene particles (Figure 6c,d). Conversely, in NS200(16) and NS200(50), DLS measurements suggest that the pNIPAm core microgels were swollen in water during the seeded emulsion polymerization (Figure 5), promoting the development of small polystyrene particles in the microgels. However, small polystyrene particles were not present at the center of NS200(16) and NS200(50) microgels, which contradicts previous results by Gilányi et al.48 This discrepancy may originate from the diffusion of styrene monomer during the seeded emulsion polymerization, enabling polymerization outside the microgels. Interestingly, secondary polystyrene particles formed when styrene, SDS, and water were mixed before the seeded emulsion polymerization (Figure S4, Supporting Information), indicating that the polymerization procedure influence its outcome. Furthermore, systematic studies on the seeded emulsion polymerization using “hard” seed particles by Okubo et al. have produced unique polymer particles, such as golf-ball-like composite particles53,54 and multihollow polymer particles,55 suggesting that the seeded emulsion polymerization may generate more composite microgels in the presence of microgels. Although further investigations are necessary to completely understand this polymerization mechanism, this study has clearly demonstrated the synthesis of hybrid microgels comprising pNIPAm and polystyrene. Microgel morphologies and thermoresponsiveness were tuned by simply changing styrene monomer concentration or adding anionic surfactant SDS during the seeded emulsion polymerization.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details; Table S1 and Figures S1−S4. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected] (D.S.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS D.S. acknowledges Grant-in-Aids for (1) Challenging Exploratory Research (26620177) and (2) Scientific Research on Innovative Areas (26102517) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors thank Dr. Keigo Kinoshita from the Japan Agency for MarineEarth Science and Technology (JAMSTEC) for helpful discussions on composite microgel preparation. The authors also thank Masaki Murai for his help during SEM observation.
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REFERENCES
(1) Saunders, B. R.; Vincent, B. Microgel Particles as Model Colloids: Theory, Properties and Applications. Adv. Colloid Interface Sci. 1999, 80, 1−25. (2) Pelton, R. Temperature-Sensitive Aqueous Microgels. Adv. Colloid Interface Sci. 2000, 85, 1−33. (3) Kawaguchi, H. Functional Polymer Microspheres. Prog. Polym. Sci. 2000, 25, 1171−1210. (4) Nayak, S.; Lyon, L. A. Soft Nanotechnology with Soft Nanoparticles. Angew. Chem., Int. Ed. 2005, 44, 7686−7708. (5) Hellweg, T. Responsive Core−Shell Microgels: Synthesis, Characterization, and Possible Applications. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1073−1083. (6) Sershen, S. R.; Westcott, S. L.; Halas, N. J.; West, J. L. Temperature-Sensitive Polymer−Nanoshell Composites for Photothermally Modulated Drug Delivery. J. Biomed. Mater. Res. 2000, 51, 293−298. (7) Kim, J.; Nayak, S.; Lyon, L. A. Bioresponsive Hydrogel Microlenses. J. Am. Chem. Soc. 2005, 127, 9588−9592. (8) Suzuki, D.; Sakai, T.; Yoshida, R. Self-flocculating/Self-dispersing Oscillation of Microgels. Angew. Chem., Int. Ed. 2008, 47, 917−920. (9) Suzuki, D.; Taniguchi, H.; Yoshida, R. Autonomously Oscillating Viscosity in Microgel Dispersions. J. Am. Chem. Soc. 2009, 131, 12058−12059. (10) Nayak, S.; Lee, H.; Chmielewski, J.; Lyon, L. A. Folate-mediated Cell Targeting and Cytotoxicity Using Thermoresponsive Microgels. J. Am. Chem. Soc. 2004, 126, 10258−10259. (11) Bae, Y.; Jang, W.D.; Nishiyama, N.; Fukushima, S.; Kataoka, K. Multifunctional Polymeric Micells with Folate-mediated Cancer Cell Targeting and pH-Triggered Drug Releasing Properties for Active Intracellular Drug Delivery. Mol. Biosyst. 2005, 1, 242−250. (12) Tamura, A.; Oishi, M.; Nagasaki, Y. Enhanced Cytoplasmic Delivery of siRNA Using a Stabilized Polyion Complex Based on PEGylated Nanogels with a Cross-Linked Polyamine Structure. Biomacromolecules 2009, 10, 1818−1827.
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CONCLUSIONS The seeded emulsion polymerization of styrene was conducted with pNIPAm microgels. In the absence of surfactant, pNIPAm microgels were composited with polystyrene particles, and polystyrene particle diameters and amounts increased with styrene concentration. In addition, the polystyrene layers did not completely cover the pNIPAm core microgels, resulting in temperature-induced deswelling of the composite microgels. With SDS, the polystyrene particles covered the pNIPAm core microgels more homogeneously, and the composite microgels did not show thermoresponsive deswelling. The diameter of the polystyrene particles composited with core pNIPAm microgels decreased with increasing SDS concentration during the seeded emulsion polymerization. In particular, polystyrene particles formed within the pNIPAm core microgels when the SDS G
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(13) Guan, Y.; Zhang, Y. PNIPAM Microgels for Biomedical Applications: from Dispersed Particles to 3D Assemblies. Soft Matter 2011, 7, 6375−6384. (14) Morris, G. E.; Vincent, B.; Snowden, M. J. Adsorption of Lead Ions onto N-Isopropylacrylamide and Acrylic Acid Copolymer Microgels. J. Colloid Interface Sci. 1997, 190, 198−205. (15) Kawaguchi, H.; Kisara, K.; Takahashi, T.; Achiha, K.; Yasui, M.; Fujimoto, K. Versatility of Thermosensitive Particles. Macromol. Symp. 2000, 151, 591−598. (16) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Thermally Switchable Periodicities and Diffraction from Mesoscopically Ordered materials. Science 1996, 274, 959−960. (17) Hellweg, T.; Dewhurst, C. D.; Brúckner, E.; Kratz, K.; Eimer, W. Colloidal Crystals Made of Poly(N-isopropylacrylamide) Microgel Particles. Colloid Polym. Sci. 2000, 278, 972−978. (18) Lyon, L. A.; Debord, J. D.; Debord, S. B.; Jones, C. D.; McGrath, J. G.; Serpe, M. J. Microgel Colloidal Crystals. J. Phys. Chem. B 2004, 108, 19099−19108. (19) Okubo, T.; Suzuki, D.; Yamagata, T.; Katsuno, A.; Sakurai, M.; Kimura, H.; Tsuchida, A. Colloidal Crystallization of Thermo-Sensitive Gel Spheres of Poly (N-isopropyl acrylamide). Colloid Polym. Sci. 2011, 289, 291−299. (20) Zhang, J.; Xu, S.; Kumacheva, E. Polymer Microgels: Reactors for Semiconductor, Metal, and Magnetic Nanoparticles. J. Am. Chem. Soc. 2004, 126, 7908−7914. (21) Suzuki, D.; Kawaguchi, H. Modification of Gold Nanoparticle Composite Nanostructures Using Thermosensitive Core-Sell Particles as a Template. Langmuir 2005, 21, 8175−8179. (22) Lu, Y.; Mei, Y.; Drechsler, M.; Ballauff, M. Thermosensitive Core−Shell Particles as Carriers for Ag Nanoparticles: Modulating the Catalytic Activity by a Phase Transition in Networks. Angew. Chem., Int. Ed. 2006, 45, 813−816. (23) Kim, J.; Serpe, M. J.; Lyon, L. A. Photoswitchable Microlens. Arrays. Angew. Chem., Int. Ed. 2005, 44, 1333−1336. (24) Sorrell, C. D.; Carter, M. C. D.; Serpe, M. J. Color Tunable Poly(N-isopropylacrylamide)-co-Acrylic Acid Microgel−Au Hybrid Assemblies. Adv. Funct. Mater. 2011, 21, 425−433. (25) Heskins, M.; Guillet, J. E. Solution Properties of Poly(Nisopropylacrylamide). J. Macromol. Sci., Chem. 1968, A2, 1441−1455. (26) Schild, H. G. Poly(N-isopropylacrylamide): Experiment, Theory and Application. Prog. Polym. Sci. 1992, 17, 163−249. (27) Pelton, R. H.; Chibante, P. Preparation of Aqueous Lattices with N-Isopropylacrylamide. Colloids Surf. 1986, 20, 247−256. (28) Kawaguchi, H.; Yamada, Y.; Kataoka, S.; Morita, Y.; Ohtsuka, Y. Hydrogel Microspheres. 2. Precipitation Copolymerization of Acrylamide with Comonomers to Prepare Monodisperse Hydrogel Microspheres. Polym. J. 1991, 23, 955−962. (29) Hoare, T.; Pelton, R. Characterizing Charge and Crosslinker Distributions in Polyelectrolyte Microgels. Curr. Opin. Colloid Interface Sci. 2008, 13, 413−428. (30) Keerl, M.; Pedersen, J. S.; Richtering, W. Temperature Sensitive Copolymer Microgels with Nanophase Separated Structure. J. Am. Chem. Soc. 2009, 131, 3093−3097. (31) Jones, C. D.; Lyon, L. A. Synthesis and Characterization of Multiresponsive Core−Shell Microgels. Macromolecules 2000, 33, 8301−8306. (32) Tsuji, S.; Kawaguchi, H. Temperature-Sensitive Hairy Particles Prepared by Living Radical Graft Polymerization. Langmuir 2004, 20, 2449−2455. (33) Berndt, I.; Pedersen, J. S.; Richtering, W. Structure of Multiresponsive “Intelligent” Core−Shell Microgels. J. Am. Chem. Soc. 2005, 127, 9372−9373. (34) Lu, Y.; Ballauff, M. Thermosensitive Core−Shell Microgels: From Colloidal Model Systems to Nanoreactors. Prog. Polym. Sci. 2011, 36, 767−792. (35) Zeiser, M.; Freudensprung, I.; Hellweg, T. Linearly Thermoresponsive Core-Shell Microgels: Towards a New Class of Nanoactuators. Polymer 2012, 53, 6096−6101.
(36) Suzuki, D.; Tsuji, S.; Kawaguchi, H. Janus Microgels Prepared by Surfactant-Free Pickering Emulsion-Based Modification and Their Self-Assembly. J. Am. Chem. Soc. 2007, 129, 8088−8089. (37) Umeda, Y.; Kobayashi, T.; Hirai, T.; Suzuki, D. Effects of pH and Temperature on Assembly of Multiresponsive Janus Microgels. Colloid Polym. Sci. 2011, 289, 729−737. (38) Zha, L. S.; Zhang, Y.; Yang, W. L.; Fu, S. K. Monodisperse Temperature-Sensitive Microcontainers. Adv. Mater. 2002, 14, 1090− 1092. (39) Nayak, S.; Gan, D. J.; Serpe, M. J.; Lyon, L. A. Hollow Thermoresponsive Microgels. Small 2005, 1, 416−421. (40) Urakami, Y.; Kasuya, Y.; Fujimoto, K.; Miyamoto, M.; Kawaguchi, H. Phagocytosis of Microspheres with Modified Surfaces. Colloids Surf., B 1994, 3, 183−190. (41) Zhang, L.; Daniels, E. S.; Dimonie, V. L.; Klein, A. Synthesis and Characterization of PNIPAM/PS Core/Shell Particles. J. Appl. Polym. Sci. 2010, 118, 2502−2511. (42) Suzuki, D.; Yamagata, T.; Murai, M. Multilayered Composite Microgels Synthesized by Surfactant-Free Seeded Polymerization. Langmuir 2013, 29, 10579−10585. (43) SciFinder: https://scifinder.cas.org/scifinder. (44) Horigome, K.; Suzuki, D. Drying Mechanism of Poly(Nisopropylacrylamide) Microgel Dispersions. Langmuir 2012, 28, 12962−12970. (45) Tam, K. C.; Ragaram, S.; Pelton, R. H. Interaction of Surfactants with Poly(N-isopropylacrylamide) Microgel Latexes. Langmuir 1994, 10, 418−422. (46) Wu, C.; Zhou, S. Effects of Surfactants on the Phase Transition of Poly(N-isopropylacrylamide) in Water. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 1597−1604. (47) Mears, S. J.; Deng, Y.; Cosgrove, T.; Pelton, R. Structure of Sodium Dodecyl Sulfate Bound to a Poly(NIPAM) Microgel Particle. Langmuir 1997, 13, 1901−1906. (48) Gilányi, T.; Varga, I.; Meszaros, R.; Filipcsei, G.; Zrinyi, M. Interaction of Monodisperse Poly(N-isopropylacrylamide) Microgel Particles with Sodium Dodecyl Sulfate in Aqueous Solution. Langmuir 2001, 17, 4764−4769. (49) Rieger, J. The Glass Transition Temperature of Polystyrene Results of a Round Robin Test. J. Therm. Anal. 1996, 46, 965−972. (50) Makino, K.; Yamamoto, S.; Fujimoto, K.; Kawaguchi, H.; Ohshima, H. Surface-Structure of Latex-Particles Covered with Temperature-Sensitive Hydrogel Layers. J. Colloid Interface Sci. 1994, 166, 251−258. (51) Guillermo, A.; Addad, J. P. C.; Bazile, J. P.; Duracher, D.; Elaissari, A.; Pichot, C. NMR Investigations into Heterogeneous Structures of Thermosensitive Microgel Particles. J. Polym. Sci., Part B 2000, 38, 889−898. (52) Kraft, D. J.; Hilhorst, J.; Heinen, M. A.; Hoogenraad, M. J.; Luigjes, B.; Kegel, W. K. Patchy Polymer Colloids with Tunable Anisotropy Dimensions. J. Phys. Chem. B 2011, 115, 7175−7181. (53) Okubo, M.; Murakami, Y.; Fujiwara, T. Formation Mechanism of Anomalous “Golf Ball-like” Composite Polymer Particles by Seeded Emulsion Polymerization. Colloid Polym. Sci. 1996, 274, 520−524. (54) Okubo, M.; Fujiwara, T.; Yamaguchi, A. Morphology of Anomalous Polystyrene Polybutyl Acrylate Composite Particles Produced by Seeded Emulsion Polymerization. Colloid Polym. Sci. 1998, 276, 186−189. (55) Kobayashi, H.; Miyanaga, E.; Okubo, M. Preparation of Multihollow Polymer Particles by Seeded Emulsion Polymerization Using Seed Particles with Incorporated Nonionic Emulsifier. Langmuir 2007, 23, 8703−8708.
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Raspberry-Shaped Composite Microgel Synthesis by Seeded Emulsion Polymerization with Hydrogel Particles Daisuke Suzuki* and Chiaki Kobayashi Graduate School of Textile Science & Technology, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan S Supporting Information *
ABSTRACT: A series of raspberry-shaped composite microgels were synthesized by the seeded emulsion polymerization of styrene with hydrogel particles. Thermoresponsive microgels of poly(N-isopropylacrylamide) cross-linked with N,N′methylenebis(acrylamide) acted as cores for the polymerization. During the surfactant-free polymerization, the core microgels shrank at 70 °C to provide thermoresponsive composite microgels, and the polystyrene particles attached to core microgels became bigger with increasing styrene concentration. Conversely, composite microgels synthesized with sodium dodecyl sulfate (SDS) ([SDS] > 6.5 mM) did not exhibit thermoresponsive deswelling behavior because polystyrene particles covered the core microgels. In particular, polystyrene particles formed composites on the microgel surface as well as inside the microgels when the SDS concentration exceeded a critical value for core microgel swelling at 70 °C. A mechanism is proposed based on these results for the seeded emulsion polymerization of water-immiscible monomers with microgels.
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first synthesized by conventional emulsion polymerization and then coated with a hydrogel shell by seeded precipitation polymerization.2,4,5 More recently, Lyon et al. produced pNIPAm core/poly(NIPAm-co-acrylic acid) shell microgels and an inverse core/shell structure, prompting much interest in core/shell microgels, in which core and shell comprise hydrogels.4,31 Hard core/hydrogel shell and core/shell microgels have been synthesized by seeded precipitation (or dispersion) polymerization: the solvent dissolves the monomer for shell synthesis but precipitates the polymer during the polymerization. Conversely, few studies have dealt with the seeded emulsion polymerization in the presence of soft microgels, in which water sparingly dissolves the monomer during the polymerization.40−42 Recently, multilayered composite microgels were obtained by the surfactant-free seeded emulsion polymerization of glycidyl methacrylate (GMA), a relatively water-miscible oil monomer (solubility 120 mM), with the deswollen pNIPAm microgel.42 However, the detailed mechanism of the seeded emulsion polymerization involving soft microgels remains unclear. To the best of our knowledge, no systematic studies have been performed to fill this gap. In this work, raspberry-shaped composite microgels were synthesized by the seeded emulsion polymerization of styrene with pNIPAm microgels. Styrene was chosen as a monomer because of its extensive use in emulsion polymerization and its
INTRODUCTION Hydrogel particles, or microgels, are cross-linked waterswellable polymeric particles that exhibit interesting stimuliresponsive physical/chemical properties. These properties change in response to external stimuli, such as temperature, pH, ionic strength, light, presence of biomolecules, and chemical reaction,1−9 making microgels suitable for various applications. In particular, microgels are expected to become central to the development of drug/gene delivery carriers,10−13 chemical/biological separation systems,14,15 photonic crystals,16−19 microreactors,20−22 and sensors.23,24 Poly(N-isopropylacrylamide) (pNIPAm), a representative thermoresponsive polymer exhibiting a low critical solution temperature (LCST) at around 31 °C, has extensively been exploited in stimuliresponsive microgels.25,26 Typically, pNIPAm-based microgels, which were first prepared by Pelton and Chibante,27 exhibit a volume phase transition temperature (VPTT) around the LCST of pNIPAm chains.4 Alternatively, Kawaguchi et al. have synthesized monodisperse and pH-responsive microgels using acrylamide, methacrylic acid, and a cross-linker by precipitation polymerization in several alcohols.28 The design and control of chemical composition and microgel morphology play an important role for realizing specific applications. Highly complex microgels displaying spatially separated regions composed of different polymers have been obtained by copolymerization,29,30 multistage polymerization,31−35 or postpolymerization modifications.36−39 There are a number of reports on the preparation of hard core/hydrogel shell particles. In these particles, the hard core, consisting of solid materials such as polystyrene particles, was © XXXX American Chemical Society
Received: May 8, 2014 Revised: May 31, 2014
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Figure 1. (a) Hydrodynamic diameters of N microgels as a function of temperature. (b) SEM images of N microgels dried at 25 °C on solid substrates. (c) TEM images of N microgels dried at 25 °C on solid substrates. Scale bars are 1 μm. (d) Photograph of an N microgel colloidal crystal in a rectangular capillary tube formed at 25 °C. flask. Next, KPS (0.027 g) dissolved in water (5 mL) was injected to the flask, and after 5 min, a mixture of styrene (2.299 μL, 200 mM) and water (50 mL) was poured into the flask to start the polymerization. Here, the water was bubbled for 30 min with nitrogen gas before mixing with styrene. The polymerization was allowed to proceed for 24 h, and the resulting microgel dispersion was cooled to room temperature. Composite microgels were purified with water two times using a RCF of 20000g at 20 °C and by dialysis for more than 4 days. The composite microgels are denoted as NSX(Y) in the remainder of the article. In this code, N and S represent NIPAm and styrene, respectively. The numbers X and Y represent the mole percentage of styrene in the feed and the SDS concentration, respectively. Characterization. Hydrodynamic diameters of individual particles were determined by dynamic light scattering (DLS) using a ZetasizerNanoS (Malvern Instruments Ltd., UK).42 The DLS data correspond to an average of 15 independent measurements of the intensity autocorrelation acquired over 30 s. These measurements were performed at a microgel concentration of ∼0.01 wt %, unless otherwise noted. The total salt concentration was adjusted to 1 mM using NaCl. The samples were allowed to equilibrate thermally at desired temperature for 10 min before measurements. Hydrodynamic diameters were calculated from the measured diffusion coefficients using the Stokes−Einstein equation (Malvern, Zetasizer software v. 6.12). The microgels in the solid state were examined by field emission scanning electron microscope (FE-SEM, Hitachi Ltd., S-5000). Diluted microgel dispersions were dried on a polystyrene substrate, thoroughly rinsed with water, and sputtered with Pt/Pd before observation. Note that certain samples for FE-SEM observation were not purified by centrifugation because the presence of secondary particles needed confirmation.42 Microgels were dried on a carbon-coated copper grid (Okenshoji Co., Ltd.) and observed by TEM using a JEOL2010 instrument operating at 200 kV. Furthermore, the lyophilized composite microgels were stained for 30 min by vaporizing 0.5% RuO4 in water and dispersed in epoxy matrices. Matrices were cured at 40 °C for 24 h and 60 °C for 24 h and were microtomed into ultrathin cross sections for the observation of TEM. To check the monodispersity, the pNIPAm core microgels were transferred into Vitrotube borosilicate rectangular capillaries (0.1 × 2.0 mm2) by capillary action and treated by thermal annealing to form colloidal crystals.44 Electrophoretic mobilities (EPMs) were measured using a ZetasizerNanoZS (Malvern, Zetasizer software v. 4.20).42 Samples at a concentration of ca. 0.0003 wt % were allowed to equilibrate thermally at the desired temperature for 10 min before measurements. The total salt concentration was adjusted to 1 mM using NaCl.
poor solubility in water (2.9 mM at 25 °C),43 which is much lower than for GMA used previously.42 pNIPAm-based core microgels were synthesized by aqueous free-radical precipitation polymerization, and the seeded emulsion polymerization of styrene was conducted on these cores with or without the anionic surfactant sodium dodecyl sulfate (SDS). The resulting composite microgels were characterized by dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), laser Doppler velocimetry, and X-ray photoelectron spectroscopy (XPS). In particular, the effects of styrene and SDS concentrations during the seeded polymerization on composite microgel morphologies were investigated.
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EXPERIMENTAL DETAILS
Materials. N-Isopropylacrylamide (NIPAm, purity 98%), N,N′methylenebis(acrylamide) (BIS, 97%), styrene (99%), potassium peroxodisulfate (KPS, 95%), ammonium peroxodisulfate (APS, 98%), and sodium dodecyl sulfate (SDS, 95%) were purchased from Wako Pure Chemical Industries, Ltd., and used as received. N,N,N′,N′Tetramethylethylenediamine (TEMED, 99%) was purchased from Sigma-Aldrich and used as received. A 0.5% ruthenium tetroxide (RuO4) stabilized aqueous solution was purchased from Electron Microscopy Sciences and used as received. For all experiments, water was distilled and ion-exchanged (EYELA, SA-2100E1) before use. Synthesis of pNIPAm Microgels. The chemically cross-linked pNIPAm-based microgels were synthesized by aqueous free-radical precipitation polymerization.2,42 A mixture of NIPAm (12.900 g, 95 mol %), BIS (0.925 g, 5 mol %), and water (800 mL) was poured into a 1000 mL three-neck round-bottom flask equipped with a mechanical stirrer, a condenser, and a nitrogen gas inlet. The monomer solution was bubbled for 30 min with nitrogen gas at 70 °C to purge the oxygen. Under a stream of nitrogen and with constant stirring at 250 rpm, the initiator KPS (0.432 g) dissolved in water (10 mL) was injected to the flask to start the polymerization. The reaction was allowed to proceed for 4 h, and the microgel dispersion was cooled to room temperature. The microgels were purified by centrifugation/ redispersion with water two times using a relative centrifugal force (RCF) of 50000g at 20 °C and dialyzed for a week. These microgels are denoted as N in the remainder of the article. Seeded Emulsion Polymerization of Styrene with pNIPAm Microgels. The seeded emulsion polymerization of styrene was conducted under various conditions using N microgels as cores. The pNIPAm dispersion (solid content: 0.17 g) was poured into a 200 mL three-neck round-bottom flask equipped with a mechanical stirrer and a condenser and heated to 70 °C with constant stirring at 250 rpm. In some cases, SDS (final concentration: 0−50 mM) was added to the B
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Figure 2. Electron micrographs of (a, b) NS50(0), (c, d) NS100(0), and (e, f) NS200(0) dried on the substrates at 25 °C. (a, c, e) FE-SEM images. Scale bars are 1 μm. (b, d, f) TEM images. Scale bars represent 300 nm. XPS spectra of lyophilized samples were acquired using a KRATOS AXIS Ultra DLD instrument equipped with a Mg Kα X-ray source (Shimazu Co.).
styrene particles for NS50(0) were undetectable because of the low amount of composited polystyrene. Polystyrene particle diameters equaled 77 ± 14 and 137 ± 22 nm for NS100(0) and NS200(0), respectively, showing that polystyrene particles became bigger with increasing styrene concentration. Finally, the resulting composite microgels generated sediments because of aggregation at a styrene concentration of 250 mM during the seeded emulsion polymerization (data not shown). Interestingly, no secondary polystyrene particles were observed after the seeded emulsion polymerization regardless of styrene concentrations (50−200 mM, Figure 2), suggesting that all the styrene was combined with the N core microgels. A similar tendency was noted in a previous study using GMA as a monomer.42 SEM images also indicate that NS50(0) and NS100(0) adopt non-close-packed ordered structures, consistent with previous investigations of dilute pNIPAm microgel dispersions dried at room temperature on solid substrates.2,27 In addition, this agrees with previous results showing that these microgels adsorbed at the air/water interface when the small dispersion droplets dried.44 Therefore, NS50(0) and NS100(0) were expected to present pNIPAm layers on their surfaces. Conversely, pNIPAm layers were difficult to distinguish on the NS200(0) composite microgels, which did not show non-closepacked ordered structures. These composite microgels were synthesized at 70 °C, resulting in deswollen core pNIPAm microgels in water. Thus, in control experiments, the seeded emulsion polymerization was performed using APS and TEMED as redox initiators at 25 °C, where the core pNIPAm microgels were swollen in water. However, in these cases, the polymerization was unsuccessful and led to microgel aggregation (Supporting Information, Table S1). Figure 3 shows the hydrodynamic diameters of NS50(0), NS100(0), and NS200(0) composite microgels as a function of temperature. All three composite microgels exhibited deswelling with increasing temperature. The swelling ratio (Vh20 °C/ Vh70 °C) amounted to 7.5, 4.4, 3.6, and 1.5 for N, NS50(0), NS100(0), and NS200(0), respectively, indicating a decrease
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RESULTS AND DISCUSSION Synthesis and Characterization of pNIPAm Core Microgels. A systematic investigation of the seeded emulsion polymerization with microgels requires monodisperse microgels. Therefore, core microgels were synthesized by aqueous free-radical precipitation polymerization using thermosensitive pNIPAm. In this conventional method for preparing thermosensitive microgels, the monomer is soluble under polymerization conditions, but the polymer undergoes a phase separation into globules.2,27,44 Figure 1a shows the hydrodynamic diameters of N microgels as a function of temperature. N microgel diameters changed from 505 nm at 25 °C to 266 nm at 70 °C, consistent with the thermoresponsiveness of pNIPAm. Figures 1b and 1c show SEM and TEM images of N microgels dried at 25 °C on the substrate, respectively.44 These images demonstrated that the dried N microgels consisted of monodisperse particles exhibiting a mean diameter of 357 ± 13 nm (n = 30, from SEM image). This diameter is much smaller than hydrodynamic diameters (i.e., 505 nm at 25 °C), determined in dispersions because of the dried state of the SEM samples. Note that these monodisperse N microgels assembled into colloidal crystals in water (Figure 1d). Effect of Styrene Concentration on the SurfactantFree Seeded Emulsion Polymerization. Next, the surfactant-free seeded emulsion polymerization of styrene was conducted at 70 °C using N microgels as cores. First, the effect of styrene monomer concentration on the resulting NS composite microgels during the polymerization was determined. Figure 2 shows the representative electron micrographs of NS composite microgels. These images clearly demonstrate that except for NS50(0), all NS composite microgels formed raspberry-shaped particles composed of microgels surrounded by small polystyrene particles. Presumably, the small polyC
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NS200(0) microgels did not show non-close-packed ordered structures when the microgels were dried at 25 °C. The appearance of the resulting dried structure (Figure 4d) was similar to that observed when NS100(0) microgels were directly dried at 25 °C (Figure 2c). In addition, no hysteresis was observed in the deswelling/swelling curves for NS200(0) (Supporting Information, Figure S1). These results clearly demonstrate that the composite structures remained intact upon heating/cooling treatment. Seeded Emulsion Polymerization of Styrene with pNIPAm Microgels. As mentioned above, the seeded emulsion polymerization of styrene was successful when N microgels were deswollen at 70 °C. To investigate the influence of the degree of core microgel swelling in this system, the anionic surfactant SDS was used for the seeded emulsion polymerization at 70 °C. In general, SDS binds to pNIPAm microgels, dramatically enhancing microgel swelling.45−48 Figure 5 shows the hydrodynamic diameters of the pNIPAm
Figure 3. Hydrodynamic diameters of N (black), NS50(0) (yellow), NS100(0) (blue), and NS200(0) microgels (red) as a function of temperature.
when the amount of added styrene increased. This suggests that the polystyrene particles did not completely cover the pNIPAm cores in these composite microgels but restricted swelling. In addition, the deswollen composite microgel dispersions were dried at 70 °C and examined by FE-SEM to further assess their thermosensitivity. NS50(0) (Figure 4a) and NS100(0)
Figure 5. Hydrodynamic diameters of pNIPAm microgels as functions of SDS concentration at 25 and 70 °C. Microgel concentrations equaled 0.17 wt % and were the same as during the seeded polymerization. We confirmed that hydrodynamic diameters are able to be determined by DLS at 0.17 wt % N microgel (Supporting Information, Figure S2).
microgels as a function of SDS concentration. The hydrodynamic diameter increased with increasing SDS concentration at 25 °C, where the pNIPAm microgels are highly swollen, in agreement with previous reports.45−48 In addition, the pNIPAm microgels swelled dramatically with SDS even at 70 °C, and in particular, their hydrodynamic diameters did not vary between 25 and 70 °C at 100 mM SDS. Next, SDS was added to the seeded emulsion polymerization system, and its influence on the polymerization was evaluated at 70 °C. Anionic surfactant and pNIPAm microgels were mixed for 10 min before initiating the polymerization to allow SDS to promote microgel swelling. Figure 6 shows the electron micrographs of the resulting composite microgels. Table 1 lists the composite microgel diameters measured from these FE-SEM images, along with those of their corresponding small polystyrene particles. While composite microgels became larger with increasing SDS concentration, the polystyrene particles became smaller. The dried NS200(0.5) structure (Figure 6a,b) was similar to that of NS200(0) at 25 °C (Figure 2e,f). In NS200(6.5), the small polystyrene particles were more uniformly distributed around the core microgels than in NS200(0). This result is a bit complicated to explain because the hydrodynamic diameters of core microgels did not change in the 0−8 mM SDS concentration range at 70 °C (Figure 5).
Figure 4. FE-SEM images of (a) NS50(0), (b) NS100(0), and (c) NS200(0) dried on a polystyrene substrate at 70 °C. (d) NS100(0) microgels dried at 25 °C after five successive heating/cooling cycles.
(Figure 4b) did not display non-close-packed ordered structures but aggregated composite microgels, indicating that the dispersions dried according to different mechanisms at 70 and 25 °C. No remarkable changes were observed in the NS200(0) dried structures between 70 °C (Figure 4c) and 25 °C (Figure 2e,f), suggesting that a large part of the deformable hydrogel core was composited with the polystyrene particles. When a dilute dispersion of NS100(0) was heated to 70 °C and subsequently cooled to 25 °C, the dried structures retrieved their swollen state, confirming their reversible thermoresponsiveness. This heating/cooling cycle was repeated five times before the dispersion was dried onto the solid substrate. Here, NS100(0) microgels were chosen for the experiment because D
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Figure 6. Electron micrographs of (a, b) NS200(0.5), (c, d) NS200(6.5), (e, f) NS200(16), and (g, h) NS200(50) dried on solid substrates at 25 °C. (a, c, e, g) FE-SEM images. Scale bars are 1 μm. (b, d, f, h) TEM images. Scale bars represent 300 nm.
Table 1. Diameters of Composite Microgels and Corresponding Small Polystyrene Particles Measured from FE-SEM Images composite microgels NS200(0) NS200(0.5) NS200(6.5) NS200(16) NS200(50)
diam/nm
CV/%
± ± ± ± ±
6.8 7.2 3.9 2.6 3.1
362 366 493 521 501
25 26 19 13 16
small polystyrene particles diam/nm
CV/%
± ± ± ± ±
16.3 19.7 19.7 24.7 21.2
137 133 110 39 34
22 26 22 10 7
Note that SDS and pNIPAm microgels were mixed in the flask at room temperature before heating at 70 °C. Therefore, at 70 °C, the deswollen microgels were bound to SDS even though the surfactant did not affect the hydrodynamic diameters. The role of the bound SDS may be to uniformly combine small polystyrene particles with the pNIPAm core. Conversely, in the case of NS200(16) and NS200(50), which were synthesized with 16 and 50 mM SDS, respectively, the core pNIPAm microgels were expected to swell in water during the seeded emulsion polymerization (Figure 5). The decrease in polystyrene particle diameter with increasing SDS concentration may be linked with small SDS aggregates in the microgels.47,48 However, these results did not display a marked relationship between the degree of pNIPAm core swelling and composite microgel structures. Figure 7 shows the hydrodynamic diameters of composite microgels synthesized with SDS. These composite microgels did not exhibit deswelling with increasing temperature, except NS200(0.5), suggesting that the pNIPAm cores were covered with continuous polystyrene particle layers. Similar to NS200(0), the temperature-induced deswelling was observed in NS200(0.5) because the polydisperse, relatively large polystyrene particles (∼133 nm, CV 19.7%), provided a discontinuous cover over the pNIPAm cores. In other words, the SDS concentration (0.5 mM) was very low to affect the morphology and deswelling behavior of the composite microgels. It is worthwhile mentioning that the glass transition temperature (Tg) of polystyrene (ca. 100 °C)49 is higher than the temperature range of the DLS measurement, resulting in sufficiently hard polystyrene layers to suppress core deswelling.
Figure 7. Hydrodynamic diameters of NS200(0.5) (red), NS200(6.5) (yellow), NS200(16) (green), and NS200(50) (blue) as a function of temperature.
Surface Structures of the Composite Microgels. Trilayered composite microgels comprising hydrogel core, solid shell, and hydrogel outer shell were previously synthesized by the surfactant-free seeded emulsion polymerization using GMA as a monomer.42 Therefore, EPMs of the styrene-based composite microgels studied were measured to elucidate the surface structures of the hydrogel core/solid shell composites. Table 2 summarizes the EPMs of the composite microgels, their parent N cores, and polystyrene particles. These polystyrene particles were synthesized by the surfactant-free emulsion polymerization, as described in the Supporting Table 2. Electrophoretic Mobilities of Core and Composite Microgels and Polystyrene Particles in 1 mM NaCl EPM/10−8 m2 V−1 s−1 sample N NS200(0) NS200(0.5) NS200(6.5) NS200(16) NS200(50) polystyrene particles E
25 °C −0.54 −1.06 −1.08 −3.64 −3.05 −3.03 −3.41
± ± ± ± ± ± ±
0.002 0.02 0.04 0.06 0.04 0.07 0.05
40 °C −3.44 −4.13 −3.32 −4.79 −3.35 −3.61 −3.02
± ± ± ± ± ± ±
0.11 0.03 0.03 0.26 0.05 0.11 0.16
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Figure 8. TEM images of ultrathin cross sections: (a) NS200(0), (b) NS200(0.5), (c) NS200(6.5), (d) NS200(16), and (e) NS200(50). Scale bars are 300 nm.
Information. Hydrogel layer-coated particles usually exhibit low EPMs,50 which may serve as indicators of the presence of a hydrogel layer on the particle surface. N, NS200(0), and NS200(0.5) surfaces displayed low EPM values ( 6.5 mM) became clear. NS200(6.5) comprised small polystyrene particles (ca. 110 nm) surrounding the pNIPAm core (Figure 8c). When the
SDS concentration increased during the seeded emulsion polymerization, such as in NS200(16) and NS200(50), the small polystyrene particles were formed on the outer and inner microgel surfaces (Figure 8d,e). Because not all composite microgels displayed small polystyrene particles over their entire structures, these polystyrene particles did not appear to exist at the center of the composite microgels for NS200(16) and NS200(50). In other words, we cannot choose the exact position of the composite microgels when samples are microtomed into ultrathin cross sections. This may be due to the presence of heterogeneous chemical cross-links in pNIPAm microgels synthesized by precipitation polymerization, leading to higher cross-linking density in the microgel center than on its surface.51 Based on all this data, a mechanism is speculated for the seeded emulsion polymerization of styrene with pNIPAm microgels. The pNIPAm microgels dispersions were heated to 70 °C, and the initiator KPS was injected into these dispersions before styrene monomer addition. Therefore, the styrene polymerization was expected to occur outside the pNIPAm microgels in the early stage. Kraft et al. have reported the nucleation of secondary particles during the seeded polymerization involving pNIPAm microgels and styrene with the liposoluble initiator AIBN, in which the core microgels were swollen by styrene.52 In contrast, the initiator used for the seeded emulsion polymerization was the water-soluble KPS. Therefore, polystyrene formation is expected to follow a surfactant-free emulsion polymerization mechanism. Surprisingly, however, no secondary particles were observed in the surfactant-free system although the SEM samples were not purified by centrifugation (Figure 2), indicating that polystyrene nucleated outside of the pNIPAm microgels may be adsorbed on these deswollen pNIPAm microgels during the seeded emulsion polymerization. When the styrene concentration increased, such as NS100(0) vs NS200(0) in Figure 2d,f, the number of small polystyrene particles decreased, and the diameters of the polystyrene particles became bigger. This implies that small polystyrene particles attached to deswollen F
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concentration exceeded a critical level to swell the core microgels. These uniquely shaped composite particles may find application as building blocks in optic materials, and the new raspberry-shaped particles, for which outer and inner surfaces are decorated with small particles (NS200(16)), may be used as an adsorbent for water-immiscible solvents.
cores served as seeds for further polystyrene particle growth. Morphologies of composite microgels synthesized with SDS differed from those synthesized without SDS. Gilányi et al. have demonstrated that above a critical surfactant concentration SDS binds to the outer shell of the pNIPAm microgels by forming small aggregates, resulting in microgel swelling.48 Furthermore, a higher aggregation number is achived when the equillibrium surfactant concentration exceeds a second critical value,48 in agreement with the results shown here. The SDS-bound pNIPAm microgels did not deswell, even above VPTT in pure water (Figure 5). Although the hydrodynamic diameters of the core microgels at 70 °C did not change in the 0−8 mM SDS concentration range, the morphology of NS200(6.5) changed significantly (Figures 6c,d and 8c) compared with NS200(0). In NS200(6.5), the pNIPAm core microgels did not swell, but SDS played an important role in polystyrene nucleation and growth around the deswollen pNIPAm microgels, resulting in the formation of raspberry-shaped composite microgels which are surrounded uniformly by small polystyrene particles (Figure 6c,d). Conversely, in NS200(16) and NS200(50), DLS measurements suggest that the pNIPAm core microgels were swollen in water during the seeded emulsion polymerization (Figure 5), promoting the development of small polystyrene particles in the microgels. However, small polystyrene particles were not present at the center of NS200(16) and NS200(50) microgels, which contradicts previous results by Gilányi et al.48 This discrepancy may originate from the diffusion of styrene monomer during the seeded emulsion polymerization, enabling polymerization outside the microgels. Interestingly, secondary polystyrene particles formed when styrene, SDS, and water were mixed before the seeded emulsion polymerization (Figure S4, Supporting Information), indicating that the polymerization procedure influence its outcome. Furthermore, systematic studies on the seeded emulsion polymerization using “hard” seed particles by Okubo et al. have produced unique polymer particles, such as golf-ball-like composite particles53,54 and multihollow polymer particles,55 suggesting that the seeded emulsion polymerization may generate more composite microgels in the presence of microgels. Although further investigations are necessary to completely understand this polymerization mechanism, this study has clearly demonstrated the synthesis of hybrid microgels comprising pNIPAm and polystyrene. Microgel morphologies and thermoresponsiveness were tuned by simply changing styrene monomer concentration or adding anionic surfactant SDS during the seeded emulsion polymerization.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details; Table S1 and Figures S1−S4. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected] (D.S.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS D.S. acknowledges Grant-in-Aids for (1) Challenging Exploratory Research (26620177) and (2) Scientific Research on Innovative Areas (26102517) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors thank Dr. Keigo Kinoshita from the Japan Agency for MarineEarth Science and Technology (JAMSTEC) for helpful discussions on composite microgel preparation. The authors also thank Masaki Murai for his help during SEM observation.
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REFERENCES
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CONCLUSIONS The seeded emulsion polymerization of styrene was conducted with pNIPAm microgels. In the absence of surfactant, pNIPAm microgels were composited with polystyrene particles, and polystyrene particle diameters and amounts increased with styrene concentration. In addition, the polystyrene layers did not completely cover the pNIPAm core microgels, resulting in temperature-induced deswelling of the composite microgels. With SDS, the polystyrene particles covered the pNIPAm core microgels more homogeneously, and the composite microgels did not show thermoresponsive deswelling. The diameter of the polystyrene particles composited with core pNIPAm microgels decreased with increasing SDS concentration during the seeded emulsion polymerization. In particular, polystyrene particles formed within the pNIPAm core microgels when the SDS G
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