Thermally Sensitive Block Copolymer Particles Prepared via Aerosol Flow Reactor Method: Morphological Characterization and Behavior in Water

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• 作者：Antti Nyk盲nen ; Antti Rahikkala ; Sami-Pekka Hirvonen ; Vladimir Aseyev ; Heikki Tenhu ; Raffaele Mezzenga ; Janne Raula ; Esko Kauppinen ; Janne Ruokolainen
• 刊名：Macromolecules
• 出版年：2012
• 出版时间：October 23, 2012
• 年：2012
• 卷：45
• 期：20
• 页码：8401-8411
• 全文大小：621K
• 年卷期：v.45,no.20(October 23, 2012)
• ISSN：1520-5835

文摘

This work describes properties of thermosensitive submicrometer-sized particles having the same chemical composition but different morphologies. These particles have been prepared with an aerosol technique using dimethylformamide solutions of linear polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene, PS-b-PNIPAM-b-PS. The particles were characterized by cryo-electron microscopy, microcalorimetry, and light scattering. Block copolymers self-assembled within the particles forming onion-like, gyroid-like, and spherical morphologies having poly(N-isopropylacrylamide) matrix and physically cross-linking polystyrene domains. The particles were dispersed in aqueous media, and their behavior in water was studied both below and above the lower critical solution temperature of poly(N-isopropylacrylamide). We found out that the particles with spherical and gyroid-like morphologies swell considerably in water at 20 掳C, whereas at 40 掳C the particles resemble more of those studied without water treatment. Light scattering experiments showed that the particles gradually aggregate and precipitate with time at 40 掳C. Microcalorimetric studies revealed for all three studied morphologies that PNIPAM undergoes a two-step transition due to the different hydration levels of PNIPAM inside and outside the particles. Thicknesses of the PS and PNIPAM layers within the onion-like particles were analyzed using the TEM micrographs by fitting a model of electron density to the integrated electron intensity data. The surface layer of the particles was found out to be PNIPAM, which was supported by light scattering and microcalorimetry. It was also found out from the TEM micrograph analysis that the width of the outmost PS layer is considerably thinner than the one in the dry state prior to immersion in water, and a degradation scheme is proposed to explain these results.