Eur. Phys. J. E 8, 145-154 (2002)
Molecular dynamics in thin films of isotactic poly(methyl methacrylate)L. Hartmann, W. Gorbatschow, J. Hauwede and F. Kremer
University of Leipzig, Department of Physics, D-04103 Leipzig, Germany email@example.com
(Received 12 August 2001 and Received in final form 16 November 2001)
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 ( -relaxation) and a (local) secondary -relaxation. While the latter does not depend on the dimensions of the sample, the dynamic glass transition becomes faster ( decades) with decreasing film thickness. This results in a shift of the glass transition temperature 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 -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, was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements yield a gradual increase of with decreasing film thickness. The findings concerning the different thickness dependences of are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the shifts incorporates recently developed models to describe the glass transition in thin polymer films.
64.70.Pf - Glass transitions.
68.60.-p - Physical properties of thin films, nonelectronic.
77.22.Gm - Dielectric loss and relaxation.
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2002