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Available online at www.sciencedirect.com

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

Preparation of lamella-structured block-copolymer particlesand 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,∗

a Graduate School of Science, Hokkaido University, N10W8 Kita-ku, Sapporo, Hokkaido 060-0810, Japanb 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, Japand 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 2007Available online 31 May 2007

bstract

In this report, we investigated the effect of annealing on the phase separation structures formed in the block-copolymer nanoparticles. Theanoparticles with the lamellar structures were prepared by evaporation of tetrahydrofuran (THF) from the THF/water solution of poly(styrene--isoprene), in which each polymer segment has almost same length. Scanning transmission electron microscope (STEM) observation of the

lock-copolymer nanoparticles and their cross-section image revealed that the consecutive lamellar structure was formed one-directionally in theanoparticles. When the suspension of lamellar-structured nanoparticles was annealed at 50 ◦C for 10 h, the lamellar phase changed to the disordertructures. This transition was irreversible. This result shows the lamellar structure formed in nanoparticles is less stable than that in the planarlm. 2007 Elsevier B.V. All rights reserved.

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eywords: Block-copolymer; Nanoparticle; Self-organization; Phase transition

. Introduction

Block-copolymer is a polymer which consists of cova-ently bonded more than two polymer segments. Variousinds of micro-phase separation structures are formed in thelock-copolymer film depending on their compatibilities andolecular weight ratio of each polymer segment. In recent years,

lock-copolymers received a great attention in the fields of mate-ial science and nanotechnology because block-copolymers cane applicable to one of the new building blocks [1,2]. Con-entionally, the micro-phase separation structures have been

nvestigated in their films. There is few report of prepara-ion and phase separation of block-copolymer particles exceptor core–shell type block-copolymer micelles prepared from

∗ Corresponding author at: Institute of Multidisciplinary Research fordvanced Materials, Tohoku University, 2-1-1 Katahira, Aobaku, Sendai,iyagi 980-8677, Japan. Tel.: +81 22 217 5329; fax: +81 22 217 5329.

E-mail addresses: higuchi@poly.es.hokudai.ac.jp (T. Higuchi),himo@tagen.tohoku.ac.jp (M. Shimomura).

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927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2007.04.076

ealing

mphiphilic macromers in an aqueous phase [3]. It is diffi-ult to prepare block-copolymer particles by using conventionalreparation method including emulsion polymerization becauseost block-copolymers require highly controlled non-aqueous

olymerization conditions (e.g., anionic polymerization).On the other hand, we found that fine particles of various

inds of polymers (e.g., engineering plastics, biodegradableolymers, etc.) can be prepared by adding a poor solvent (e.g.,ater) into a polymer solution [4]. After evaporation of a good

olvent (e.g., tetrahydrofuran), suspensions of nanoparticles in aoor solvent were obtained. By using this method which namedolvent evaporation exchange method, a diameter of particles isontrolled by changing the concentration of a polymer solu-ion and ratio of a good/poor solvent. We have successfullyrepared the nanoparticles with periodic lamellar structures ofoly(styrene-b-isoprene) [5]. Usually, in order to prepare the

ighly ordered micro-phase separation structures in the cast filmf block-copolymer, the film of the block-copolymer is annealedver glass transition temperature (Tg) for a long time and thenradually cooled down below Tg [6], or exposed with vapor of

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8 T. Higuchi et al. / Colloids and Surfaces A: P

good solvent [7]. In this report, we investigated the effect ofnnealing on the micro-phase separation structures formed in thelock-copolymer nanoparticles, and we found the irreversiblehase transition, which is different from that in the films.

. Experimental

The schematic illustration of the experimental procedure wasescribed in Fig. 1. Poly(styrene-b-isoprene) (PSt-b-PI, MnPSt =7, 800, MnPI = 12, 000, Mw/Mn = 1.02, volume fraction of PI:

PI = 0.51) was purchased by Polymer Source Inc., USA. PSt-b-I was dissolved in tetrahydrofuran (THF) to prepare 0.1, 0.15nd 0.2 mg/ml solutions. Two milliliters of pure water was addedo 1 ml of the block polymer solution with stirring the polymerolution. The supplying speed of water was 1 ml/min. THF wasvaporated at room temperature after stop stirring. When 10 hassed after starting evaporation, the solution became opaque.hen 2 days after starting evaporation, THF was completely

vaporated, and the block-copolymer molecules precipitated inater as nanoparticles.The average hydrodynamic particle size was measured by

ynamic light scattering (DLS, FDLS-3000, Otsuka Electron-cs Co. Ltd., Japan). To observe the inner structures of particlesy transmission electron microscope (TEM), the particles weretained with osmium tetraoxide (OsO4). The isoprene moietyas selectively stained with OsO4 due to cross-linking reactionf OsO4 and the double bonds of isoprene. The suspension ofarticles (1 ml) was stained by 0.2 wt% OsO4 (1 ml) for 2 h atoom temperature (sample 1). After staining, the stained parti-les were separated by centrifugation (12,000 rpm, 5 ◦C, 15 min)nd washed twice with pure water to eliminate the excess OsO4.fter washing, the stained particles were re-dispersed in pureater with applying ultrasonic. The water suspension of the

tained particles was dropped onto the surface of collodion mem-rane placed on a Cu mesh and dried at room temperature.o investigate the effect of annealing, the water suspensionsf the particles were annealed at 30 and 50 ◦C for 10 h. Afternnealing, the temperature was cooled to room temperature for0 min. The annealed particles were stained with OsO4, and

hen, the samples for TEM observation were prepared by theame procedures described above (samples 2 and 3). The phaseeparation structures of the particles were observed by using acanning transmission electron microscope (STEM, HD-2000,

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Fig. 1. Schematic illustration of the preparation annealing

chem. Eng. Aspects 313–314 (2008) 87–90

itachi Ltd., Japan). The surface structures of the particles werebserved by using a SEM (secondary electron mode) imagesnd the inner structures of the particles were observed by darkeld images (scattering mode).

The stained particles (sample 1) were embedded in the epoxyesin (Epok-812, Wako Pure Chemical Industries Ltd., Japan).his epoxy resin was cured at 60 ◦C for 12 h. The particlembedded cured resin was slice to prepare thin films (thickness:a. 100 nm) by using an ultra-microtome (Leica Ultracut UCT,eica Microsystems) and then, the thin film of particle embed-ed epoxy resin was fixed on a Cu mesh covered by a carbonembrane. The cross-section of the particles was observed by

sing a transmission electron microscope (TEM, JEM-2100F,EOL Ltd., Japan).

. Results and discussion

The particle size distributions measured by DLS are shownn Fig. 2. The average diameter was controlled from the severalundreds of nm to 1 �m by changing the concentration of thelock-copolymer solution. Fig. 3(a) shows the SEM image of thelock-copolymer particles. The spherical particles with periodicrinkles on the surfaces were formed.Fig. 3(b) shows the dark field image of the block-copolymer

articles (sample 1) before annealing. The black and whiteeriodic contrast of the lamella structure was observed in thearticles same as the periodic surface structures observed inheir SEM image. The white and black part in the dark fieldmage was attributed to the isoprene moieties, which stained bysmium tetraoxide and electrons did not go through this part, andSt layers, respectively. In order to observe the inner structuresf the particles, the cross-sections of the particles were prepared.ig. 3(c) shows the cross-section TEM image (bright field) of thelock-copolymer particles (sample 1). Contrary to the dark fieldmage, the white and black part was attributed to PSt and PI lay-rs, respectively. The stripe patterns were clearly observed in thelock-copolymer particles. This result shows that the lamellartructures are formed one-directionally in the particles as wells the cast films. When the suspension was annealed at 30 ◦C

sample 2), the same lamellar phase separation structure wasept.

As shown in Fig. 3(d), the lamellar structures disappeared andon-uniform web-like structures were observed after annealing

and staining of the block-copolymer nanoparticles.

T. Higuchi et al. / Colloids and Surfaces A: Physi

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

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Fig. 3. (a) STEM (SEM mode) image of the block-copolymer nanoparticles. (b) STEnanoparticles before annealing (sample 1) and (d) after annealing (sample 3).

cochem. Eng. Aspects 313–314 (2008) 87–90 89

sample 3). This result shows that the lamellar phase structureormed in the block-copolymer particles turned to the disorderhase by annealing at 50 ◦C. Hashimoto and co-worker [8] andloudas et al. [9] reported that in the bulk state the lamellar phase

urns to disorder phase by annealing over the phase transitionemperature described as

ODT = 213 + 1.00742Nn (1)

here TODT and Nn are the order–disorder transition tem-erature and the number average degree of polymerization ofhe block-copolymers, respectively. From Eq. (1), TODT of thelock-copolymer in the bulk film is estimated to 290 ◦C. Becausehe order–disorder transition in the bulk film is reversible, rapiduenching from the phase transition temperature is required tox the disorder structures at room temperature.

Fig. 3 shows that TODT in the particles is much lower thanhat in the film whose TODT is estimated to 290 ◦C. Althoughhe block-copolymer tends to form the highly ordered phaseeparation structures in the films after annealing, the regularity inhe particle, however, becomes worse after annealing. Moreover,he phase transition in the particles is irreversible though that inhe film is reversible. These results show the lamellar structureormed in the particles is not stable but metastable state.

This instability of the lamellar structures formed in the parti-les relates to the formation mechanism of the particles by usingur method. It is known that the phase separation structure oflock-copolymers is affected by the interface. For example, aydrophilic polymer segment tends to face the hydrophilic sub-

trate in a phase separation structures. By using this property,pontaneous patterning of block-copolymers can be realized10]. Russell et al. reported that when the block-copolymer islled in the anodized alumina pores, the concentric lamellar

M (dark field) image, (c) the cross-section TEM image of the block-copolymer

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tructures along the walls of pores were formed [11]. Consid-ring the phase separation structures in the block-copolymerarticles based on this concept, the onion-like lamellar structures the most suitable to reduce the surface free energy. However,he lamellar structures were formed one-directionally in the par-icles in this experiment. We observed the particle formationrocess in our method and found that the polymers were gradu-lly precipitated as particles near the air/solution interface [12].

hen formation of lamellar structure and precipitation of parti-le are simultaneously occurred near the air/solution interface,nisotropic phase separation structures can be formed in thearticles. The anisotropic lamellar structure is thermodynami-ally less stable than the isotropic concentric sphere structures.herefore, the irreversible phase transition with very low transi-

ion temperature was occurred in the block-copolymer particles.

. Conclusion

In this paper, the anisotropic lamellar structure in the block-opolymer particles were prepared by using solvent evaporation

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chem. Eng. Aspects 313–314 (2008) 87–90

xchange method. We found that the transition temperature fromamellar to disorder was much lower than that in the films andhe transition was irreversible.

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