Chain-End Modification: A Starting Point for Controlling Polymer Crystal Nucleation

This computational study illustrates how modifying the end groups of polymer chains offers a viable strategy to control polymer crystal nucleation in entangled melts. More specifically, altering chain ends to enhance chain-end aggregation enables crystal nucleation to occur at higher temperatures than with unmodified counterparts. Surprisingly, while chain-end aggregation does enhance crystal nucleation, chain-end clusters do not act as direct nucleating agents. As such, the crystal nucleation enhancement arising from chain-end modification differs from historical models, for example, via epitaxial matching. This study is based on coarse-grain molecular simulations of polyethylene crystallization in which chain-end modifications have been achieved by altering the Lennard-Jones parameters of the terminal coarse-grain beads of the polyethylene chains, thereby varying the size of chain ends and emulating chain termination with polycylic aliphatic moieties (e.g., adamantyl groups). The present approach thus leverages steric mismatch between chain-end and main-chain segments as a strategy to alter polyethylene crystal nucleation rather than relying on the utilization of directional (e.g., hydrogen bonding) or ionic interactions. The present computer simulations demonstrate an up to 20 K increase in the crystal nucleation temperature of entangled melts composed of n-C720H1442 polyethylene-like chains even though chain ends account for a small fraction (<1 mol %) of the sample. Seemingly, low chemical loadings are needed to control nucleation via end-group modification.