Direct All-Atom Molecular Dynamics Simulations of the Effects of Short Chain Branching on Polyethylene Oligomer Crystal Nucleation

Using all-atom molecular dynamics (MD) simulations, we demonstrate that short alkyl branches can hinder the nucleation of polyethylene (PE) oligomers. Although one methyl and ethyl branch in a 50-carbon oligomer may only slow the nucleation kinetics mildly, bulkier side chains, such as propyl, butyl, and hexyl branches, disturb the arrangement of neighboring backbone atoms, preventing these atoms from joining a growing crystal, and therefore significantly suppress the nucleation of PE crystals, with no clear evidence of nucleation being observed over a 20 ns simulation run when hexyl side chains are present. The degrees of branching and the distributions of short alkyl groups on PE backbones can also affect the crystallization kinetics, with well-spaced branches having a greater impact on crystallization than branches that are grouped within a shorter distance along the backbone that is similar to or shorter than the length of the branch. We show that the linear portions of PE crystallize first and the branched monomers may be regarded as "defects" that impede crystallization by slowing chain conformational and diffusive relaxations and limiting the lengthening of crystalline stems.