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. KremerUniversity of Leipzig, Department of Physics, D-04103 Leipzig, Germany kremer@physik.uni-leipzig.de
(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 (
-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
