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The effect of confinement in the segmental relaxation of polymers isconsidered. On the basis of a thermodynamic model we discuss the emergingrelevance of the fast degrees of freedom in stimulating the much slowersegmental relaxation, as an effect of the constraints at the walls of theamorphous regions. In the case that confinement is due to the presence ofcrystalline domains, a quasi-poissonian distribution of local constrainingconditions is derived as a result of thermodynamic equilibrium. This impliesthat the average free energy barrier $\Delta F$ for conformationalrearrangement is of the same order of the dispersion of the barrier heights,$\delta (\Delta F)$, around $\Delta F$. As an example, we apply the results tothe analysis of the $\alpha$-relaxation as observed by dielectric broad bandspectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallizedfrom either an isotropic or an oriented glass. It is found that in the lattercase the regions of cooperative rearrangement are significantly larger than inthe former.

Segmental relaxation in semicrystalline polymers: a mean field model for the distribution of relaxation times in confined regimes