Crystallization of the polyethylene block in polystyrene-b-polyethylene-b-polycaprolactone triblock copolymers, 1. Self-nucleation behavior

Abstract

The self-nucleation of branched polyethylene chains of different degrees of chain mobility was studied. The polyethylene block (PE block) within poly(styrene-b-ethylene-b-caprolactone) triblock copolymers (SEC) of varying compositions was studied. Differential scanning calorimetry was used to determine the self-nucleation domains as a function of the self-nucleation temperature (Ts). The self-nucleation behavior of PE chains within SEC block copolymers was found to be anomalous in comparison to the classical self-nucleation behavior exhibited by homopolymers. When the degree of chain constraint is high, as in the case where the SEC copolymer only contains 15% of PE, domain II (only self-nucleation domain) completely disappears and annealing can take place before self-nucleation occurs. This means that chain constraint complicates the self-nucleation process and this situation persists until, upon decreasing the self-nucleation temperature (Ts), annealing has generated crystals that are big enough to act as self-nuclei for the less restricted portions of the chain. If the PE content in the copolymer is very low (15%), two crystal populations can be distinguished. This may reflect the differences in diffusion of the PE chain segments close to the interfaces with the other two blocks and those segments that are close to the middle of the PE block. The influence of chain constraint on determining the difficulty of the chains to self-nucleate was further explored using a crosslinked low-density polyethylene (XLDPE). In this case, crosslinking junctions instead of covalent links with other blocks restrict chain mobility. Nevertheless, a similar difficulty in self-nucleation was found as in the case of the PE block within SEC triblock copolymers in contrast to neat LDPE, a polymer that exhibited the classical self-nucleation behavior with the usual three domains.