Some relevant parameters affecting the glass transition of supported ultra-thin polymer films
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We have measured, the thickness dependence of the glass transition temperature Tg(h), using ellipsometry at variable temperature, for poly(methyl-methacrylate) (PMMA) of various tacticity in confined geometry. We report that several factors significantly affect Tg(h): i) polymer microstructure (stereoregularity of PMMA) related to local dynamics; ii) interfacial interactions; iii) conformation of the polymer chains. These results raise many fundamental questions on the origin of the thickness-dependent glass transition. Why and how do the interactions with the substrate significantly affect Tg(h)? Does Tg(h) depend on the modifications of conformational parameters of the chains (their entropy)? What is the correlation between local dynamics and Tg(h) in thin films? The aim of this paper is to summarise these open questions, which should stimulate further investigations in the thin polymer film scientific community.
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