Roles of Branch Content and Branch Length in Copolyethylene Crystallization: Molecular Dynamics Simulations

Abstract

Molecular dynamics simulations of two series of single copolymer chain models with precisely controlled methyl branching and with branches of different lengths are performed. For the models with precisely controlled methyl branching, the collapse process and the morphology of the lamellar structure are similar to those of the random copolymer. In particular, it is found that the branch content plays an important role in determining whether a single chain can form a lamellar structure or not. With the decrease of the branch contents, more perfect lamellar structure is formed, and the crystallinity of the copolymer increases. For the models with branches of different lengths, the simulations show that the branch point (CH unit) is always expelled from the crystalline phase as a defect, while the behavior of the branch chain in the crystallization depends on the chain length. When the chain has less than 10 carbon atoms, the branch is rejected to the folded surface of the lamellar structure as a defect, whereas when the branch length is longer, the branch is folded back and cocrystallizes with the main chain. In this case, the branch chain is packed in a manner as in polyethylene.