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

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<ul><li><p>Available online at</p><p>Colloids and Surfaces A: Physicochem. Eng. Aspects 313314 (2008) 8790</p><p>Preparation of lamella-structured blocand their irreversible lamella-disord</p><p>b,c,</p><p>hima-ku,</p><p>b In ty, 2-1ako,T), J32-0027 A</p><p>07</p><p>Abstract</p><p>In this re strucnanoparticle drofb-isoprene), in which each polymer segment has almost same length. Scanning transmission electron microscope (STEM) observation of theblock-copolymer nanoparticles and their cross-section image revealed that the consecutive lamellar structure was formed one-directionally in thenanoparticles. When the suspension of lamellar-structured nanoparticles was annealed at 50 C for 10 h, the lamellar phase changed to the disorderstructures. This transition was irreversible. This result shows the lamellar structure formed in nanoparticles is less stable than that in the planarfilm. 2007 Else</p><p>Keywords: B</p><p>1. Introdu</p><p>Block-clently bonkinds of mblock-copomolecular wblock-coporial sciencebe applicabventionallyinvestigatetion and phfor coresh</p><p> CorresponAdvanced MMiyagi 980-8</p><p>E-mail adshimo@tagen</p><p>0927-7757/$doi:10.1016/jvier B.V. All rights reserved.</p><p>lock-copolymer; Nanoparticle; Self-organization; Phase transition; Annealing</p><p>ction</p><p>opolymer is a polymer which consists of cova-ded more than two polymer segments. Variousicro-phase separation structures are formed in thelymer film depending on their compatibilities andeight ratio of each polymer segment. In recent years,</p><p>lymers received a great attention in the fields of mate-and nanotechnology because block-copolymers canle to one of the new building blocks [1,2]. Con-</p><p>, the micro-phase separation structures have beend in their films. There is few report of prepara-ase separation of block-copolymer particles exceptell type block-copolymer micelles prepared from</p><p>ding author at: Institute of Multidisciplinary Research foraterials, Tohoku University, 2-1-1 Katahira, Aobaku, Sendai,677, Japan. Tel.: +81 22 217 5329; fax: +81 22 217 5329.dresses: (T. Higuchi), (M. Shimomura).</p><p>amphiphilic macromers in an aqueous phase [3]. It is diffi-cult to prepare block-copolymer particles by using conventionalpreparation method including emulsion polymerization becausemost block-copolymers require highly controlled non-aqueouspolymerization conditions (e.g., anionic polymerization).</p><p>On the other hand, we found that fine particles of variouskinds of polymers (e.g., engineering plastics, biodegradablepolymers, etc.) can be prepared by adding a poor solvent (e.g.,water) into a polymer solution [4]. After evaporation of a goodsolvent (e.g., tetrahydrofuran), suspensions of nanoparticles in apoor solvent were obtained. By using this method which namedsolvent evaporation exchange method, a diameter of particles iscontrolled by changing the concentration of a polymer solu-tion and ratio of a good/poor solvent. We have successfullyprepared the nanoparticles with periodic lamellar structures ofpoly(styrene-b-isoprene) [5]. Usually, in order to prepare thehighly ordered micro-phase separation structures in the cast filmof block-copolymer, the film of the block-copolymer is annealedover glass transition temperature (Tg) for a long time and thengradually cooled down below Tg [6], or exposed with vapor of</p><p> see front matter 2007 Elsevier B.V. All rights reserved..colsurfa.2007.04.076Takeshi Higuchi a,c, Hiroshi YabuToyoki Kunitake c, Masatsugu S</p><p>a Graduate School of Science, Hokkaido University, N10W8 Kitstitute of Multidisciplinary Research for Advanced Materials, Tohoku Universi</p><p>c Frontier Research System, RIKEN, 2-1 Hirosawa, Wd Core Research for Evolutional Science and Technology (CRES</p><p>4-1-8 Honcho, Kawaguchi, Saitama 3Received 6 November 2006; accepted</p><p>Available online 31 May 20</p><p>port, we investigated the effect of annealing on the phase separations with the lamellar structures were prepared by evaporation of tetrahyk-copolymer particleser phase transition</p><p>Shinya Onoue c,omura b,c,d,</p><p>Sapporo, Hokkaido 060-0810, Japan-1 Katahira, Aobaku, Sendai, Miyagi 980-8677, Japan</p><p>Saitama 351-0198, Japanapan Science and Technology (JST),12, Japanpril 2007</p><p>tures formed in the block-copolymer nanoparticles. Theuran (THF) from the THF/water solution of poly(styrene-</p></li><li><p>88 T. Higuchi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313314 (2008) 8790</p><p>a good solvent [7]. In this report, we investigated the effect ofannealing on the micro-phase separation structures formed in theblock-copolymer nanoparticles, and we found the irreversiblephase transition, which is different from that in the films.</p><p>2. Experimental</p><p>The schematic illustration of the experimental procedure wasdescribed 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-b-PI was dissolved in tetrahydrofuran (THF) to prepare 0.1, 0.15and 0.2 mg/ml solutions. Two milliliters of pure water was addedto 1 ml of the block polymer solution with stirring the polymersolution. The supplying speed of water was 1 ml/min. THF wasevaporatedpassed afteWhen 2 daevaporatedwater as na</p><p>The avedynamic liics Co. Ltdby transmistained witwas selectiof OsO4 anparticles (1room tempcles were seand washedAfter washwater withstained parbrane placTo investigof the partannealing,30 min. Ththen, the ssame proceseparationscanning tr</p><p>Hitachi Ltd., Japan). The surface structures of the particles wereobserved by using a SEM (secondary electron mode) imagesand the inner structures of the particles were observed by darkfield images (scattering mode).</p><p>The stained particles (sample 1) were embedded in the epoxyresin (Epok-812, Wako Pure Chemical Industries Ltd., Japan).This epoxy resin was cured at 60 C for 12 h. The particleembedded 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 embed-ded epoxy resin was fixed on a Cu mesh covered by a carbonmembrane. The cross-section of the particles was observed byusing a transmission electron microscope (TEM, JEM-2100F,JEOL Ltd., Japan).</p><p>ults</p><p>par2. Tds ocopocopoes on. 3(bes (sic coes saEMwas</p><p>m teters,artic</p><p>c) shcopo, thepectcopores acast</p><p>le 2)</p><p>showifor</p><p>iningat room temperature after stop stirring. When 10 hr starting evaporation, the solution became opaque.ys after starting evaporation, THF was completely, and the block-copolymer molecules precipitated innoparticles.rage hydrodynamic particle size was measured byght scattering (DLS, FDLS-3000, Otsuka Electron-., Japan). To observe the inner structures of particlesssion electron microscope (TEM), the particles wereh osmium tetraoxide (OsO4). The isoprene moietyvely stained with OsO4 due to cross-linking reactiond the double bonds of isoprene. The suspension ofml) was stained by 0.2 wt% OsO4 (1 ml) for 2 h at</p><p>erature (sample 1). After staining, the stained parti-parated by centrifugation (12,000 rpm, 5 C, 15 min)twice with pure water to eliminate the excess OsO4.</p><p>ing, the stained particles were re-dispersed in pureapplying ultrasonic. The water suspension of the</p><p>ticles was dropped onto the surface of collodion mem-ed on a Cu mesh and dried at room temperature.ate the effect of annealing, the water suspensions</p><p>icles were annealed at 30 and 50 C for 10 h. Afterthe temperature was cooled to room temperature fore annealed particles were stained with OsO4, andamples for TEM observation were prepared by thedures described above (samples 2 and 3). The phasestructures of the particles were observed by using aansmission electron microscope (STEM, HD-2000,</p><p>3. Res</p><p>Thein Fig.hundreblock-block-wrinkl</p><p>Figparticlperiodparticltheir SimageosmiuPSt layof the pFig. 3(block-imageers, res</p><p>block-structuas the(sampkept.</p><p>Asnon-un</p><p>Fig. 1. Schematic illustration of the preparation annealing and staand discussion</p><p>ticle size distributions measured by DLS are shownhe average diameter was controlled from the severalf nm to 1m by changing the concentration of thelymer solution. Fig. 3(a) shows the SEM image of thelymer particles. The spherical particles with periodicthe surfaces were formed.</p><p>) shows the dark field image of the block-copolymerample 1) before annealing. The black and whitentrast of the lamella structure was observed in theme as the periodic surface structures observed inimage. The white and black part in the dark fieldattributed to the isoprene moieties, which stained byraoxide and electrons did not go through this part, andrespectively. In order to observe the inner structuresles, the cross-sections of the particles were prepared.</p><p>ows the cross-section TEM image (bright field) of thelymer particles (sample 1). Contrary to the dark fieldwhite and black part was attributed to PSt and PI lay-ively. The stripe patterns were clearly observed in thelymer particles. This result shows that the lamellarre formed one-directionally in the particles as wellfilms. When the suspension was annealed at 30 C, the same lamellar phase separation structure was</p><p>n in Fig. 3(d), the lamellar structures disappeared andm web-like structures were observed after annealing</p><p>of the block-copolymer nanoparticles.</p></li><li><p>T. Higuchi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313314 (2008) 8790 89</p><p>Fig. 2. The particle size distributions of the block-copolymer nanoparticlesprepared from 0.1, 0.15 and 0.2 g/l solutions.</p><p>(sample 3). This result shows that the lamellar phase structureformed in the block-copolymer particles turned to the disorderphase by annealing at 50 C. Hashimoto and co-worker [8] andFloudas et al. [9] reported that in the bulk state the lamellar phaseturns to disorder phase by annealing over the phase transitiontemperature described as</p><p>TODT = 213 + 1.00742Nn (1)</p><p>where TODT and Nn are the orderdisorder transition tem-perature and the number average degree of polymerization ofthe block-copolymers, respectively. From Eq. (1), TODT of theblock-copolymer in the bulk film is estimated to 290 C. Becausethe orderdisorder transition in the bulk film is reversible, rapidquenching from the phase transition temperature is required tofix the disorder structures at room temperature.</p><p>Fig. 3 shows that TODT in the particles is much lower thanthat in the film whose TODT is estimated to 290 C. Althoughthe block-copolymer tends to form the highly ordered phaseseparation structures in the films after annealing, the regularity inthe particle, however, becomes worse after annealing. Moreover,the phase transition in the particles is irreversible though that inthe film is reversible. These results show the lamellar structureformed in the particles is not stable but metastable state.</p><p>This instability of the lamellar structures formed in the parti-cles relates to the formation mechanism of the particles by usingour method. It is known that the phase separation structure ofblock-copohydrophilicstrate in aspontaneou[10]. Russefilled in th</p><p>Fig. 3. (a) STEM (SEM mode) image of the block-copolymer nanoparticles. (b) STEM (dark fieldnanoparticles before annealing (sample 1) and (d) after annealing (sample 3).lymers is affected by the interface. For example, apolymer segment tends to face the hydrophilic sub-</p><p>phase separation structures. By using this property,s patterning of block-copolymers can be realizedll et al. reported that when the block-copolymer ise anodized alumina pores, the concentric lamellar</p><p>) image, (c) the cross-section TEM image of the block-copolymer</p></li><li><p>90 T. Higuchi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313314 (2008) 8790</p><p>structures along the walls of pores were formed [11]. Consid-ering the phase separation structures in the block-copolymerparticles based on this concept, the onion-like lamellar structureis the most suitable to reduce the surface free energy. However,the lamellar structures were formed one-directionally in the par-ticles in this experiment. We observed the particle formationprocess in our method and found that the polymers were gradu-ally precipitated as particles near the air/solution interface [12].When formation of lamellar structure and precipitation of parti-cle are simultaneously occurred near the air/solution interface,anisotropic phase separation structures can be formed in theparticles. The anisotropic lamellar structure is thermodynami-cally less stable than the isotropic concentric sphere structures.Therefore, the irreversible phase transition with very low transi-tion temperature was occurred in the block-copolymer particles.</p><p>4. Conclusion</p><p>In this paper, the anisotropic lamellar structure in the block-copolymer particles were prepared by using solvent evaporation</p><p>exchange method. We found that the transition temperature fromlamellar to disorder was much lower than that in the films andthe transition was irreversible.</p><p>References</p><p>[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.</p><p>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, Macro-</p><p>molecules 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.</p><p>Chem. Phys. 104 (1996) 2083.[10] G.M. Wilmes, D.A. Durkee, N.P. Balsara, J.A. Liddle, Macromolecules 39</p><p>(2006) 2435.[11] H. Xiang, K. Shin, T. Kim, S.I. Moon, T.J. McCathy, T.P. Russell, Macro-</p><p>molecules 37 (2004) 5660.[12] T. Higuchi, H. Yabu, M. Shimomura, Colloid Surf. A 284285 (2006) 250.</p><p>Preparation of lamella-structured block-copolymer particles and their irreversible lamella-disorder phase transitionIntroductionExperimentalResults and discussionConclusionReferences</p></li></ul>


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