EPJ

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EPJ E - Soft Matter and Biological Physics
Soft Matter and Biological Physics


Eur. Phys. J. E 8, 145-154 (2002)
DOI: 10.1140/epje/i2001-10073-y

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

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(Received 12 August 2001 and Received in final form 16 November 2001)

Abstract
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 ( $\le 2$ decades) with decreasing film thickness. This results in a shift of the glass transition temperature $T_{\rm g}$ 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, $T_{\rm g}$ was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements yield a gradual increase of $T_{\rm g}$ with decreasing film thickness. The findings concerning the different thickness dependences of $T_{\rm g}$ are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the $T_{\rm g}$ shifts incorporates recently developed models to describe the glass transition in thin polymer films.

PACS
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

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