Polyethylene Cross-link Density Effects on Crystallization and Shape Memory Performance*

In pursuit of a model of microstructure-property relationships in cross-linked, semi-crystalline Shape Memory Polymers (SMPs), we investigate the effect of cross-link density on thermally-activated SMP performance characteristics. Specifically, we aim to establish a relationship between crystal size distribution and shape fixity, shape recovery rate, and shape recovery temperature. United-atom Molecular Dynamics (UAMD) simulations of the shape memory cycle for cross-linked polyethylene are carried out at varying cross-link densities between 150 and 800 g/mol. Crystal size distribution is investigated as a function of cross-link density using order parameter and a density-based clustering algorithm. Both shape fixity and shape recovery rate are found to increase with lower cross-link density down to 150 g/mol, due to the uniform crystal size distribution resulting from unimpeded crystal growth.