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

TL;DR

This paper presents a thermodynamic mean field model to describe the distribution of relaxation times in confined semicrystalline polymers, highlighting the influence of crystalline domains on segmental relaxation behavior.

Contribution

It introduces a novel thermodynamic model linking confinement effects to the distribution of relaxation times in semicrystalline polymers, emphasizing the role of crystalline domains.

Findings

Distribution of local constraining conditions is quasi-Poissonian.

Average free energy barrier is comparable to its dispersion.

Larger cooperative regions are observed in oriented samples.

Abstract

The effect of confinement in the segmental relaxation of polymers is considered. On the basis of a thermodynamic model we discuss the emerging relevance of the fast degrees of freedom in stimulating the much slower segmental relaxation, as an effect of the constraints at the walls of the amorphous regions. In the case that confinement is due to the presence of crystalline domains, a quasi-poissonian distribution of local constraining conditions is derived as a result of thermodynamic equilibrium. This implies that the average free energy barrier $\Delta F$ for conformational rearrangement 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 to the analysis of the $\alpha$-relaxation as observed by dielectric broad band spectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallized from either an isotropic or an oriented glass. It is found that in the latter case the regions of cooperative rearrangement are significantly larger than in the former.