Anomalous enhanced mobility in a semicrystalline random poly(butylene isophthalate/butylene adipate) copolyester.

An anomalous enhancement of the segmental dynamics upon crystallization is observed by analyzing the dielectric relaxation of a random copolymer formed by stiff aromatic and flexible aliphatic co-monomeric units. The corresponding aliphatic homopolymer is characterized by a low glass transition temperature. The results show that the characteristic self-confinement inherent to semicrystalline polymer systems, which significantly slows down the segmental dynamics as compared to that of pure amorphous ones, is not only avoided but pushed in the opposite direction. Although the effect formally resembles that observed in liquids confined within non-interacting environments, X-ray measurements have shown that the origin in the present case must be sought in an enrichment of the amorphous phase with the most flexible co-monomer upon crystallization.