Molecular dynamics studies of the effects of branching characteristics on the crystalline structure of polyethylene

Molecular dynamics simulations were carried out on single chain models of l inear low-density polyethylene in vacuum to study the effects of branch len gth, branch content, and branch distribution on the polymer's crystalline s tructure at 300 K. The trans/gauche (t/g) ratios of the backbones of the mo deled molecules were calculated and utilized to characterize their degree o f crystallinity. The results show that the t/g ratio decreases with increas ing branch content regardless of branch length and branch distribution, ind icating that branch content is the key molecular parameter that controls th e degree of crystallinity. Although t/g ratios of the models with the same branch content vary, they are of secondary importance. However, our data su ggests that branch distribution (regular or random) has a significant effec t on the degree of crystallinity for models containing 10 hexyl branches/1, 000 backbone carbons. The fractions of branches that resided in the equilib rium crystalline structures of the models were also calculated. On average, 9.8% and 2.5% of the branches were found in the crystallites of the molecu les with ethyl and hexyl branches while C-13 NMR experiments showed that th e respective probabilities of branch inclusion for ethyl and hexyl branches are 10% and 6% [Hosoda , Polymer 1990, 31, 1999-2005]. However, the degree of branch inclusion seems to be insensitive to the branch content and bran ch distribution. (C) 2001 American Institute of Physics.