Crystal-inclusion of Short Chain Branches in Ethylene-alpha-olefin Random Copolymers Studied by Molecular Dynamics Simulations
Abstract
We use atomistic molecular dynamics simulations to determine the equilibrium fraction of short-chain branches grafted to periodic polyethylene chains on the surface of a crystalline slab that are included within the crystalline region that grows onto the crystalline slab after a temperature quench. From this and the Boltzmann principle, we estimate the corresponding crystal-inclusion free energy as a function of temperature and of branch length ranging from methyl to hexyl groups. Snapshots show that, if included into the crystal, side groups larger than methyl need to be accommodated by defects including crystal distortion, chain folding, or chain ends, where the latter indicates a dependence on molecular weight of the crystal-inclusion energy. The different mechanisms of short-chain accommodation may be responsible for our observations of the effect of branch length and temperature on the crystal-inclusion energy. At 360 K, the crystal-inclusion energy increases from methyl to ethyl branches but remains almost unchanged at around 25 kJ/mol for larger side groups. For a methyl branch, the crystal-inclusion energy increases as quench temperature decreases from 360 to 300 K while the larger branches (ethyl and hexyl groups) show the opposite trend.