Thickness and composition dependence of the glass transition temperature in thin random copolymer films

Abstract

Glass transition temperatures ( T g) of thin poly(styrene-co-methyl methacrylate) and poly(2-vinyl pyridine-co-styrene) films coated on a native oxide surface of Si wafer (100) were measured by ellipsometry. The thickness dependence of T g can be properly fitted by previously suggested equation developed for homopolymers, based upon a continuous multi-layer model, although one component in thin random copolymer films demonstrates a slightly favorable interaction between a substrate and thin film, and another demonstrates a strongly favorable interaction. Surface and interface have a strong influence on T g of thin film coated on substrate: the surface has the effect of reducing T g, whereas the interface increases the T g according to the degree of interaction between a substrate and thin film. This degree of interaction can be quantified as an interaction parameter ( k), and is dependent on the composition of random copolymers. For the estimation of k values of thin random copolymer films, we proposed a parallel type additive function (1/ k ran= w 1/ k 1+ w 2/ k 2) where w is a weight fraction of component.