Some relevant parameters affecting the glass transition of supported ultra-thin polymer films

We have measured, the thickness dependence of the glass transition temperature T-g(h), using ellipsometry at variable temperature, for poly (methyl-methacrylate) (PMMA) of various tacticity In confined geometry. We report that several factors significantly affect T-g(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 T-g(h)? Does T-g(h) depend on the modifications of conformational parameters of the chains (their entropy)? What is the correlation between local dynamics and T-g(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.