Molecular dynamics in thin films of isotactic poly(methyl methacrylate)

The molecular dynamics in thin films (18 nm-137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the dynamic glass transition (alpha-relaxation) and a (local) secondary beta-relaxation. While the latter does not depend on the dimensions of the sample, the dynamic glass transition becomes faster (less than or equal to 2 decades) with decreasing film thickness. This results in a shift of the glass transition temperature T-g to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and a decrease of the dielectric strength is observed for the alpha-relaxation. This enables to deduce a model based on immobilized boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, T-g was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements yield a gradual increase of T-g with decreasing film thickness. The findings concerning the different thickness dependences of T-g are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the T-g shifts incorporates recently developed models to describe the glass transition in thin polymer films.