The emergence of cooperative dynamics in polymers as an effect of conformational restrictions: The case of crystallization and an example on heterogeneous confinement

Cooperativity in the segmental dynamics of polymers, as enhanced by crystal confinement, shows up even above the dynamic glass transition. Some related physical aspects are outlined and, on the basis of a statistical mechanical model, these effects are quantitatively analysed. In this model, which extends aspects of the Adam-Gibbs theory for glass formers, the frequency width of the conformational relaxation is intrinsically connected to a finite free energy barrier hindering readjustment. A possible thermodynamic criterion for crystallization arrest then follows from the analysis of dielectric relaxation data available in the literature. Low-temperature cold-crystallized poly(ethylene terephthalate) is then considered in particular; it is found from mechanical and dielectric relaxation analyses that a cooperativity increase accompanies both recrystallization and cooling towards the glass transition temperature. The application of the model to the case of poly(dimethyl siloxane) confined in nanoporous glasses is also illustrated as an example.