Eur. Phys. J. E 8, 225-235 (2002)
DOI: 10.1140/epje/i2001-10062-2
Mobility in thin polymer films ranging from local segmental motion, Rouse modes to whole chain motion: A coupling model consideration
K.L. NgaiNaval Research Laboratory, Washington, DC 20375-5320, USA ngai@estd.nrl.navy.mil
(Received 1 August 2001 and Received in final form 1 December 2001)
Abstract
Large increases of mobility of local
segmental relaxation observed in polymer films as the film thickness is
decreased, as evidenced by decreases of the glass temperature, are not found
for relaxation mechanisms that have longer length scales including the Rouse
relaxation modes and the diffusion of entire polymer chains. We show that
the coupling model predictions, when extended to consider polymer thin
films, are consistent with a large increase of the mobility of the local
segmental motions and the lack of such a change for the Rouse modes and the
diffusion of entire polymer chains. There are two effects that can reduce
the coupling parameter of the local segmental relaxation in thin films. One
is the chain orientation that is induced parallel to the surface when the film
thickness
h becomes smaller than the end-to-end distance of the chains and
the other is a finite-size effect when
h is no longer large compared to the
cooperative length scale. Extremely thin (
nm) films obtained
by intercalating a polymer into layered silicates have thickness
significantly less than the cooperative length scale near the bulk polymer
glass transition temperature. As a result, the coupling parameter of the
local segmental relaxation in such thin films is reduced almost to zero.
With this plausible assumption, we show the coupling model can explain
quantitatively the large decrease of the local segmental relaxation time
found experimentally.
64.70.Pf - Glass transitions.
68.60.Bs - Mechanical and acoustical properties.
36.20.-r - Macromolecules and polymer molecules.
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2002
