Reduction of the glass temperature of thin freely standing polymer films caused by the decrease of the coupling parameter in the coupling model

Dramatic reduction of the glass temperature, T-g, in freely standing polymer thin films has been found when the thickness of the films become approximately smaller than the average end-to-end distance of the unperturbed polymer chains. The observation of a large molecular weight dependence of the T-g reductions provides evidence for the importance of chain confinement effect on the glass temperature of thin polymer films. We propose that for freely standing polymer thin films of high molecular weights, there are: (a) induced orientations of the polymer chains when their average end-to-end distance becomes comparable to the film thickness and (b) decreases of the cooperative length scale for chain segments near the surfaces. As a consequence, the intermolecular constraints of local segmental motions for molecular units are reduced, particularly those located in the vicinity of the surfaces. The reduction of T-g is attributed to an attendant decrease of the coupling parameter in the coupling model. This explanation is consistent with a recent photon correlation spectroscopy (PCS) and quartz crystal microbalance (QCM) measurement of the dynamics of local segmental motions in polymer thin films. Calculation of the T-g of the thin film studied using the coupling model gives an order of magnitude estimate of the reduction of T-g, which is in agreement with experiment. (C) 1998 Elsevier Science B.V, All rights reserved.