Article pubs.acs.org/Macromolecules
Reorientation of Microphase-Separated Structures in WaterSuspended Block Copolymer Nanoparticles through Microwave Annealing Takeshi Higuchi,†,∥ Masatsugu Shimomura,†,‡ and Hiroshi Yabu*,‡,§ †
WPI Research Center, Advanced Institute for Materials Research (AIMR), Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan ‡ Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ABSTRACT: We describe here the first report for developing microphaseseparated structures in poly(styrene-block-isoprene) (PS-b-PI) block copolymer nanoparticles by microwave annealing in a nonsolvent, water. A random structure in the nanoparticles successfully transformed to thermodynamically stable structures within several minutes though it takes several days by using the conventional thermal annealing process.
technique.22 In that method, block copolymer films are annealed using appropriate solvent vapors that are generated through microwave irradiation of the solvent. Well-ordered microphase-separated structures were formed within several minutes. These method can be applied to block copolymer thin films; however, development of phase separation in nanostructured block copolymer materials, which include imprinted surface patterns, fibers, and spherical particles, is one of the still challenging topics. We have reported on block copolymer nanoparticles with microphase-separated structures, such as cylindrical and lamellar morphologies, prepared by a simple solvent evaporation process. In the method, block copolymers are precipitated as fine particles through the evaporation of a good solvent from block copolymer solutions containing nonsolvents and good solvents.23 Some other groups also intended to create block copolymer and end-funtionalized polymer nanoparticles with phase-separated structures by using similar techniques.24−26 Thermal annealing and solvent annealing of spherical block copolymer particles led to phase transition of block copolymer particles from disordered to ordered structures (unidirectionally stacked lamellar and onionlike structures) as a result of thermal annealing in a nonsolvent, even though the annealing temperature was lower than Tg of the polymer segment.27 Moreover, addition of a small amount of good solvent into water-suspended block copolymer nanoparticles was also useful to develop microphase-separated
1. INTRODUCTION Self-assembled block copolymer structures consisting of different covalently bound polymer segments have been receiving much attention.1 Block copolymers form various phase-separated structures at the nanometer scale due to the low compatibility of their polymer segments. These structures have been utilized in a range of scientific fields,2 including etching masks for lithography.3−5 The microphase separation of block copolymers has been applied to nanostructures in a wide variety of materials, such as fibers and microspheres.6−11 To use block copolymer nanostructures for specific applications, control of the orientation and uniformity of the phase-separated structures is important. Topological effects12−19 and chemically patterned surfaces20 are effective methods for controlling the orientation and uniformity of the structures. Even if a specific substrate is used as a template, a block copolymer film forms a disordered structure without further treatment. To reach an equilibrium structure annealing is necessary, and several annealing methods have been reported. The simplest method to restructure a film is thermal annealing. Block copolymer films are thermally annealed at temperatures higher than their glass transition temperature (Tg). Typically, it takes several days to reach a thermodynamically stable structure. Cavicchi and Russell have developed a technique that shortens the anneal time by exposing a block copolymer film to the vapor of a good solvent.21 By using this solvent annealing method, the duration time can be shortened to 30−60 min. Taking into account the various uses for structured block copolymer films, it is very important for a technique to be developed that further accelerates and simplifies annealing. Recently, Zhang et al. have reported on a microwave-assisted solvent annealing © XXXX American Chemical Society
Received: March 26, 2013 Revised: May 2, 2013
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structures in them.28 However, the phase transition took place over several weeks or days because of the moderate annealing conditions. In this report, we demonstrate a method for microwave annealing of hydrophobic block copolymer nanoparticles suspended in water, which is nonsolvent of block copolymer. Microwave annealing leads to stable structures of block copolymer nanoparticlesthat retain their spherical shape after only several minutes of treatment. By controlling the annealing temperature, the trajectory of the morphological change in a system using a symmetrical block copolymer was observed. Eventually, the block copolymer transitioned to an onion-like structure from a unidirectionally stacked lamellar structure through microwave annealing. These results indicate that microwave annealing can effectively accelerate the phase separation of polymer moieties, not only at the particle interface but also in the central region of a nanoparticle.
redispersed in pure water by ultrasonication. A drop of the stained particles suspended in water was placed on the surface of a carbon membrane on a Cu mesh and dried at room temperature. The phaseseparated structures in the particles were then observed by TEM (accelerating voltage: 100 kV; H-7650, Hitachi Ltd., Japan) and STEM (S-5200 equipped with TEM detector; accelerating voltage: 40 kV; Hitachi Ltd.).
3. RESULTS AND DISCUSSION TEM images of preannealed PSt-PI-43 nanoparticles are shown in Figure 2a. The diameters of the nanoparticles were several
2. EXPERIMENTAL SECTION Three kinds of block copolymers with PSt and PI segments (PSt-PI43: Mn(PSt) = 17 800, Mn(PI) = 12 000, f PI = 0.43, Mw/Mn = 1.02; PStPI-44: Mn(PSt) = 16 100, Mn(PI) = 11 200, f PI = 0.44, Mw/Mn = 1.03; PSt-PI-28: Mn(PSt) = 40 800, Mn(PI) = 10 400, f PI = 0.28, Mw/Mn = 1.06), which were purchased from Polymer Source Inc., Ltd., Canada, were employed. The block copolymers were dissolved in THF with stabilizer (EP, Wako Pure Chemical Industries, Ltd., Japan) to a final concentration of 0.1 g/L. Twenty milliliters of pure water was added to 10 mL of the THF solution of each block copolymer while stirring at 10 °C in a water bath. After stirring, the block copolymers were precipitated as nanoparticles through the rapid evaporation of THF using a rotary evaporator.
Figure 2. TEM images of PSt-PI-43 nanoparticles (a) before and (b) after microwave annealing at 80 °C for 30 min (inset: magnified images).
hundred nanometers. Typically, their size distribution range was 10−20%.29 In the image, the bright and dark regions correspond to the unstained PSt moieties and the stained PI moieties, respectively. Disordered structures were observed in the nonannealed nanoparticles. Disordered structures of preannealed PSt-PI-43 nanoparticles storing at 10 °C have not changed for several weeks, which means that there is no effect of solvent annealing of residual THF, even if it may be remained in the dispersion. Figure 2b shows a TEM image of nanoparticles after microwave annealing at 80 °C for 30 min. An onion-like structure was observed in the nanoparticles. From the magnified images of a nanoparticle, the outermost region of the particle was a stained PI layer (inset image in Figure 2b). Since a PI segment has lower interfacial tension with water than that of a PSt segment, the PI segments covered the particle surface to reduce the interfacial tension between the particle and water, which was the dispersion medium. This result shows that microwave annealing effectively induces morphological transformations from a disordered phase to a thermodynamically stable onion-like phase.22 To control the internal structure of nanoparticles and investigate their transformation dynamics, the holding temperature and microwave annealing time were studied. First, symmetric PSt-PI-44 nanoparticles, having disordered structures, were prepared similarly to the PSt-PI-43 nanoparticles; then the nanoparticle dispersions were microwave-annealed at different temperatures (60−80 °C) for up to 5 min. The dispersions were then kept at those temperatures for various holding times (5−60 min). From the TEM images, the internal structures of the nanoparticles were determined (Figure 3a−d) and are summarized in the plot (Figure 3e). When the nanoparticle dispersion was annealed at 60 °C for 30 min, a disordered structure remained. A longer microwave annealing time (60 min) led to the formation of the onion-like structure. When the nanoparticle dispersion was annealed at 70 °C, three different ordered structures were observed depending on the holding time. After annealing for 5 min, a unidirectionally
Figure 1. Schematic illustration of method for preparing block copolymer microwave-annealed nanoparticles. The water is added to the block copolymer THF solution while stirring at 10 °C. After stirring, THF is rapidly evaporated by a rotary evaporator until THF has completely evaporated. The block copolymers precipitate as nanoparticles. Nanoparticle dispersions were transferred to microwave vessels (MV-7, GL Science Inc., Japan), and then the nanoparticles were annealed under microwave irradiation (NE-S300F; output power: 700 W; frequency: 2.45 GHz; National, Japan). After several minutes of irradiation, the dispersions were gradually cooled to room temperature. To control the microwave annealing conditions, a microwave system was employed (Topwave; output power: 0−1000 W; frequency: 2.45 GHz; Analytik Jena, Germany), which can control temperature and measure pressure in the vessels. The nanoparticle dispersions were heated to various temperatures (60, 70, 80, and 90 °C) within 5 min and then kept at those temperatures for various holding times (5−60 min). After microwave annealing, the nanoparticle dispersions were gradually cooled to room temperature. To observe phase-separated structures in block copolymer nanoparticles, the nanoparticles in dispersions were stained with OsO4, which selectively reacts with the double bonds in PI segments. The suspensions of particles (0.5 mL) were stained with 0.2 wt % OsO4 (0.5 mL) for 2 h at room temperature. Afterward, the stained particles were centrifuged (12 000 rpm, 5 °C, 15 min) and washed with distilled water to remove excess OsO4. After washing, the stained particles were B
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Figure 4. STEM images of (a, b) PSt-PI-43 and (c, d) PSt-PI-28 nanoparticles: (a, c) before annealing and (b, d) after microwave annealing for 2 min with constant output power (700 W). Figure 3. Morphological transformations in the PSt-PI-44 nanoparticles by microwave annealing. The nanoparticle dispersion was annealed at 60−80 °C for different holding times. (a−d) From the TEM images, the internal structures of the nanoparticles were determined. (e) In the plot, internal structures are indicated by different markers: disordered structures (green triangle), unidirectionally stacked lamellar structures (blue square), transformed lamellar structures (purple diamond), and onion-like structures (red circle).
stacked lamellar structure and a transformed lamellar structure were formed in the nanoparticles. After 25 min, the transformed lamellar and onion-like structures were observed in the nanoparticles. In the case of microwave annealing at 80 °C, transformed lamellar and onion-like structures were immediately formed in the nanoparticles after 5 min of annealing. After 25 min at 80 °C, all of the nanoparticles had an onion-like structure. Annealing at 90 °C resulted in the formation of the onion-like structure after only 5 min. These results show that microwave annealing can transform the internal structures of nanoparticles from disordered structures to onion-like structures via unidirectionally stacked lamellar structures. These results are consistent with our previous report that discusses the long-duration low-temperature thermal annealing of particles in water.22 Similarly to the long-duration thermal annealing, the morphological transformation is induced even though the annealing temperature is lower than Tg of the PSt segments. In the nanoparticles, Tg of the block copolymers is assumed to be considerably lower than that in the bulk state because nanoparticles have a larger surface-to-volume ratio. A microwave system with a constant output power (700 W) was used to shorten the time of thermal annealing for the PStPI-43 and PSt-PI-28 nanoparticles with random structures. Scanning TEM (STEM) images of the PSt-PI-43 and PSt-PI-28 nanoparticles before and after microwave annealing are shown in Figure 4. In the STEM images, the bright and dark regions correspond to the unstained PSt moieties and the stained PI moieties, respectively. As in the above results, preannealed nanoparticles of both polymers have a random structure (Figure 4a,b). After 1 min of microwave annealing, an onionlike structure was formed in the surface region of the PSt-PI-43 nanoparticles, but the phase-separated structure was unclear in the central region (Figure 5). The dispersion temperature was elevated to ca. 80 °C in this case. After 2 min of microwave annealing, an onion-like structure formed throughout the entirety of the nanoparticles (Figure 4b). These results indicate
Figure 5. STEM image of PSt-PI-43 nanoparticles after microwave annealing for 1 min with a constant output power (700 W). As indicated by the black arrow, the phase-separated structure in the center of the nanoparticle was unclear.
that phase separation propagates from the surface to the center of nanoparticles subjected to microwave annealing. Two minutes of microwave annealing at 700 W is needed to form the thermodynamically stable structure in the nanoparticles, that is, the onion-like structure. In the case of PSt-PI-28 nanoparticles, a hexagonally packed cylindrical structure was observed after 2 min microwave annealing (Figure 4c,d). This result shows that microwave annealing is effective for any block copolymer morphology. In long-duration low-temperature thermal annealing for block copolymer nanoparticles, at least several weeks are required for the structure to transform from a random structure to an onion-like structure.22 Thus, microwave annealing of a dispersion rapidly leads to a thermodynamically stable structure in block copolymers regardless of the form they take, be it film or nanoparticle. We suggest two reasons why the rearrangement of block copolymer molecules is effectively promoted by microwaves: one, the dispersion medium (i.e., water) is rapidly and uniformly heated; and two, the molecular motion of block copolymers is directly activated under microwave irradiation. From the results shown in Figure 3, the interior structure of nanoparticles changed from disordered structures to unidirectionally stacked lamellae and then transformed to onion-like structures in the case of annealing at low temperature (60−70 °C). On the other hand, the structure directly transformed to onion-like structure by high temperature annealing (over 80 C
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°C). As shown in Figure 5, high temperature annealing made nanoparticles phase separate from the surface since heat transferred from the heated water to nanoparticles via nanoparticle/water interfaces and phase separation proceeded from the surface to the center of nanoparticles; the thermodynamically stable onion-like structure was selectively formed. At lower temperature, the whole part of nanoparticles gradually annealed, and nanoparticles have kinetically trapped and formed the unidirectionally stacked lamellae structure. After further annealing, the stacked lamellae structure finally transformed to the onion-like structure same as shown in the previous report.27 In general, materials having a large dielectric constant can effectively absorb microwaves. In the case of hydrocarbon polymers such as PSt and PI, they weakly adsorb microwave radiation because of their lower dielectric constants (PSt: 2.6; PI: 2.7) as compared with water, which has a dielectric constant of ∼80. Thus, the microwave radiation is mostly absorbed by the water. However, even though only a small amount of radiation is absorbed by the polymers, its effect is not negligible because the block copolymers transitioned to ordered structures despite the temperatures used being below their glass transition temperatures.22
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4. CONCLUSION In this paper we presented the results for microwave annealing of block copolymer nanoparticles in water dispersions. This is the first report of microwave annealing to produce microphaseseparated structures of block copolymers in a dispersion. The random structures formed in the nanoparticles transformed to thermodynamically stable structures within several minutes through this method. We also posit that microwave annealing in a non- solvent would be useful for block copolymer films as well. This would be especially true for a thick film because it requires a long annealing time with current methods. Because the method reported here allows for a specimen to be annealed without disturbing its shape, even a patterned block copolymer film prepared by nanoimprinting can be annealed under microwave irradiation, which is not possible by solvent annealing.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address ∥
T.H.: Institute for Materials Chemistry (IMCE), Kyushu Univer-sity, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been partially supported by Grant-in-Aid for Priority Area “Metamaterials”, MEXT, Japan (No. 23109502). We thank Prof. H. Oikawa and Prof. A. Masuhara, IMRAM, Tohoku University, for helping in the experiment of microwave annealing.
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REFERENCES
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