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Freestanding thin polymer films with high molecular weights exhibit ananomalous decrease in the glass-transition temperature with film thickness.Specifically, in such materials, the measured glass-transition temperatureevolves in an affine way with the film thickness, with a slope that weaklydepends on the molecular weight. De Gennes proposed a sliding mechanism as thehypothetical dominant relaxation process in these systems, where stress kinkscould propagate in a reptation-like fashion through so called bridges, i.e.from one free interface to the other along the backbones of polymermacromolecules. Here, by considering the exact statistics of finite-sizedrandom walks within a confined box, we investigate in details the bridgehypothesis. We show that the sliding mechanism cannot reproduce the basicfeatures appearing in the experiments, and we exhibit the fundamental reasonsbehind such a fact.

On the bridge hypothesis in the glass transition of freestanding polymer films