Preparation of lamella-structured block-copolymer particles and their irreversible lamella-disorder phase transition

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Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 87–90

Preparation of lamella-structured block-copolymer particles and their irreversible lamella-disorder phase transition Takeshi Higuchi a,c , Hiroshi Yabu b,c , Shinya Onoue c , Toyoki Kunitake c , Masatsugu Shimomura b,c,d,∗ b

a Graduate School of Science, Hokkaido University, N10W8 Kita-ku, Sapporo, Hokkaido 060-0810, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aobaku, Sendai, Miyagi 980-8677, Japan c Frontier Research System, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan d Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

Received 6 November 2006; accepted 27 April 2007 Available online 31 May 2007

Abstract In this report, we investigated the effect of annealing on the phase separation structures formed in the block-copolymer nanoparticles. The nanoparticles with the lamellar structures were prepared by evaporation of tetrahydrofuran (THF) from the THF/water solution of poly(styreneb-isoprene), in which each polymer segment has almost same length. Scanning transmission electron microscope (STEM) observation of the block-copolymer nanoparticles and their cross-section image revealed that the consecutive lamellar structure was formed one-directionally in the nanoparticles. When the suspension of lamellar-structured nanoparticles was annealed at 50 ◦ C for 10 h, the lamellar phase changed to the disorder structures. This transition was irreversible. This result shows the lamellar structure formed in nanoparticles is less stable than that in the planar film. © 2007 Elsevier B.V. All rights reserved. Keywords: Block-copolymer; Nanoparticle; Self-organization; Phase transition; Annealing

1. Introduction Block-copolymer is a polymer which consists of covalently bonded more than two polymer segments. Various kinds of micro-phase separation structures are formed in the block-copolymer film depending on their compatibilities and molecular weight ratio of each polymer segment. In recent years, block-copolymers received a great attention in the fields of material science and nanotechnology because block-copolymers can be applicable to one of the new building blocks [1,2]. Conventionally, the micro-phase separation structures have been investigated in their films. There is few report of preparation and phase separation of block-copolymer particles except for core–shell type block-copolymer micelles prepared from ∗

Corresponding author at: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aobaku, Sendai, Miyagi 980-8677, Japan. Tel.: +81 22 217 5329; fax: +81 22 217 5329. E-mail addresses: [email protected] (T. Higuchi), [email protected] (M. Shimomura). 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.076

amphiphilic macromers in an aqueous phase [3]. It is difficult to prepare block-copolymer particles by using conventional preparation method including emulsion polymerization because most block-copolymers require highly controlled non-aqueous polymerization conditions (e.g., anionic polymerization). On the other hand, we found that fine particles of various kinds of polymers (e.g., engineering plastics, biodegradable polymers, etc.) can be prepared by adding a poor solvent (e.g., water) into a polymer solution [4]. After evaporation of a good solvent (e.g., tetrahydrofuran), suspensions of nanoparticles in a poor solvent were obtained. By using this method which named solvent evaporation exchange method, a diameter of particles is controlled by changing the concentration of a polymer solution and ratio of a good/poor solvent. We have successfully prepared the nanoparticles with periodic lamellar structures of poly(styrene-b-isoprene) [5]. Usually, in order to prepare the highly ordered micro-phase separation structures in the cast film of block-copolymer, the film of the block-copolymer is annealed over glass transition temperature (Tg ) for a long time and then gradually cooled down below Tg [6], or exposed with vapor of

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a good solvent [7]. In this report, we investigated the effect of annealing on the micro-phase separation structures formed in the block-copolymer nanoparticles, and we found the irreversible phase transition, which is different from that in the films. 2. Experimental The schematic illustration of the experimental procedure was described in Fig. 1. Poly(styrene-b-isoprene) (PSt-b-PI, MnPSt = 17, 800, MnPI = 12, 000, Mw /Mn = 1.02, volume fraction of PI: fPI = 0.51) was purchased by Polymer Source Inc., USA. PSt-bPI was dissolved in tetrahydrofuran (THF) to prepare 0.1, 0.15 and 0.2 mg/ml solutions. Two milliliters of pure water was added to 1 ml of the block polymer solution with stirring the polymer solution. The supplying speed of water was 1 ml/min. THF was evaporated at room temperature after stop stirring. When 10 h passed after starting evaporation, the solution became opaque. When 2 days after starting evaporation, THF was completely evaporated, and the block-copolymer molecules precipitated in water as nanoparticles. The average hydrodynamic particle size was measured by dynamic light scattering (DLS, FDLS-3000, Otsuka Electronics Co. Ltd., Japan). To observe the inner structures of particles by transmission electron microscope (TEM), the particles were stained with osmium tetraoxide (OsO4 ). The isoprene moiety was selectively stained with OsO4 due to cross-linking reaction of OsO4 and the double bonds of isoprene. The suspension of particles (1 ml) was stained by 0.2 wt% OsO4 (1 ml) for 2 h at room temperature (sample 1). After staining, the stained particles were separated by centrifugation (12,000 rpm, 5 ◦ C, 15 min) and washed twice with pure water to eliminate the excess OsO4 . After washing, the stained particles were re-dispersed in pure water with applying ultrasonic. The water suspension of the stained particles was dropped onto the surface of collodion membrane placed on a Cu mesh and dried at room temperature. To investigate the effect of annealing, the water suspensions of the particles were annealed at 30 and 50 ◦ C for 10 h. After annealing, the temperature was cooled to room temperature for 30 min. The annealed particles were stained with OsO4 , and then, the samples for TEM observation were prepared by the same procedures described above (samples 2 and 3). The phase separation structures of the particles were observed by using a scanning transmission electron microscope (STEM, HD-2000,

Hitachi Ltd., Japan). The surface structures of the particles were observed by using a SEM (secondary electron mode) images and the inner structures of the particles were observed by dark field images (scattering mode). The stained particles (sample 1) were embedded in the epoxy resin (Epok-812, Wako Pure Chemical Industries Ltd., Japan). This epoxy resin was cured at 60 ◦ C for 12 h. The particle embedded cured resin was slice to prepare thin films (thickness: ca. 100 nm) by using an ultra-microtome (Leica Ultracut UCT, Leica Microsystems) and then, the thin film of particle embedded epoxy resin was fixed on a Cu mesh covered by a carbon membrane. The cross-section of the particles was observed by using a transmission electron microscope (TEM, JEM-2100F, JEOL Ltd., Japan). 3. Results and discussion The particle size distributions measured by DLS are shown in Fig. 2. The average diameter was controlled from the several hundreds of nm to 1 ␮m by changing the concentration of the block-copolymer solution. Fig. 3(a) shows the SEM image of the block-copolymer particles. The spherical particles with periodic wrinkles on the surfaces were formed. Fig. 3(b) shows the dark field image of the block-copolymer particles (sample 1) before annealing. The black and white periodic contrast of the lamella structure was observed in the particles same as the periodic surface structures observed in their SEM image. The white and black part in the dark field image was attributed to the isoprene moieties, which stained by osmium tetraoxide and electrons did not go through this part, and PSt layers, respectively. In order to observe the inner structures of the particles, the cross-sections of the particles were prepared. Fig. 3(c) shows the cross-section TEM image (bright field) of the block-copolymer particles (sample 1). Contrary to the dark field image, the white and black part was attributed to PSt and PI layers, respectively. The stripe patterns were clearly observed in the block-copolymer particles. This result shows that the lamellar structures are formed one-directionally in the particles as well as the cast films. When the suspension was annealed at 30 ◦ C (sample 2), the same lamellar phase separation structure was kept. As shown in Fig. 3(d), the lamellar structures disappeared and non-uniform web-like structures were observed after annealing

Fig. 1. Schematic illustration of the preparation annealing and staining of the block-copolymer nanoparticles.

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(sample 3). This result shows that the lamellar phase structure formed in the block-copolymer particles turned to the disorder phase by annealing at 50 ◦ C. Hashimoto and co-worker [8] and Floudas et al. [9] reported that in the bulk state the lamellar phase turns to disorder phase by annealing over the phase transition temperature described as TODT = 213 + 1.00742Nn

Fig. 2. The particle size distributions of the block-copolymer nanoparticles prepared from 0.1, 0.15 and 0.2 g/l solutions.

(1)

where TODT and Nn are the order–disorder transition temperature and the number average degree of polymerization of the block-copolymers, respectively. From Eq. (1), TODT of the block-copolymer in the bulk film is estimated to 290 ◦ C. Because the order–disorder transition in the bulk film is reversible, rapid quenching from the phase transition temperature is required to fix the disorder structures at room temperature. Fig. 3 shows that TODT in the particles is much lower than that in the film whose TODT is estimated to 290 ◦ C. Although the block-copolymer tends to form the highly ordered phase separation structures in the films after annealing, the regularity in the particle, however, becomes worse after annealing. Moreover, the phase transition in the particles is irreversible though that in the film is reversible. These results show the lamellar structure formed in the particles is not stable but metastable state. This instability of the lamellar structures formed in the particles relates to the formation mechanism of the particles by using our method. It is known that the phase separation structure of block-copolymers is affected by the interface. For example, a hydrophilic polymer segment tends to face the hydrophilic substrate in a phase separation structures. By using this property, spontaneous patterning of block-copolymers can be realized [10]. Russell et al. reported that when the block-copolymer is filled in the anodized alumina pores, the concentric lamellar

Fig. 3. (a) STEM (SEM mode) image of the block-copolymer nanoparticles. (b) STEM (dark field) image, (c) the cross-section TEM image of the block-copolymer nanoparticles before annealing (sample 1) and (d) after annealing (sample 3).

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structures along the walls of pores were formed [11]. Considering the phase separation structures in the block-copolymer particles based on this concept, the onion-like lamellar structure is the most suitable to reduce the surface free energy. However, the lamellar structures were formed one-directionally in the particles in this experiment. We observed the particle formation process in our method and found that the polymers were gradually precipitated as particles near the air/solution interface [12]. When formation of lamellar structure and precipitation of particle are simultaneously occurred near the air/solution interface, anisotropic phase separation structures can be formed in the particles. The anisotropic lamellar structure is thermodynamically less stable than the isotropic concentric sphere structures. Therefore, the irreversible phase transition with very low transition temperature was occurred in the block-copolymer particles. 4. Conclusion In this paper, the anisotropic lamellar structure in the blockcopolymer particles were prepared by using solvent evaporation

exchange method. We found that the transition temperature from lamellar to disorder was much lower than that in the films and the transition was irreversible. References [1] W.A. Lopes, H.M. Jaeger, Nature 414 (2001) 735. [2] K. Naito, H. Hieda, M. Sakurai, Y. Kamata, K. Asakawa, IEEE Trans. Mag. 38 (2002) 1949. [3] G. Battaglia, A.J. Ryan, J. Am. Chem. Soc. 127 (2005) 8757. [4] H. Yabu, T. Higuchi, K. Ijiro, M. Shimomura, Chaos 15 (2005) 047505. [5] H. Yabu, T. Higuchi, M. Shimomura, Adv. Mater. 17 (2005) 2062. [6] U. Jeong, D.Y. Ryu, J.K. Kim, D.H. Kim, X. Wu, T.P. Russell, Macromolecules 36 (2003) 10126. [7] S.-H. Kim, M.J. Misner, T.P. Russell, Adv. Mater. 16 (2004) 2119. [8] H. Tanaka, T. Hashimoto, Macromolecules 24 (1991) 5713. [9] G. Floudas, D. Vlassopoulos, M. Pisikalos, N. Hadjichristidis, M. Stam, J. Chem. Phys. 104 (1996) 2083. [10] G.M. Wilmes, D.A. Durkee, N.P. Balsara, J.A. Liddle, Macromolecules 39 (2006) 2435. [11] H. Xiang, K. Shin, T. Kim, S.I. Moon, T.J. McCathy, T.P. Russell, Macromolecules 37 (2004) 5660. [12] T. Higuchi, H. Yabu, M. Shimomura, Colloid Surf. A 284–285 (2006) 250.