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

摘要

Molecular dynamics simulations were carried out on single chain models of linear low-density polyethylene in vacuum to study the effects of branch length, branch content, and branch distribution on the polymer's crystalline structure at 300 K. The trans/gauche (tlg) ratios of the backbones of the modeled molecules were calculated and utilized to characterize their degree of crystallinity. The results show that the tlg ratio decreases with increasing branch content regardless of branch length and branch distribution, indicating that branch content is the key molecular parameter that controls the degree of crystallinity. Although tlg ratios of the models with the same branch content vary, they are of secondary importance. However, our data suggests that branch distribution (regular or random) has a significant effect on the degree of crystallinity for models containing 10hexyl branches/1,OOO backbone carbons. The fractions of branches that resided in the equilibrium 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 molecules with ethyl and hexyl branches while C~(13) NMR experiments showed that the respective probabilities of branch inclusion for ethyl and hexyl branches are 10% and 6% [Hosoda et al., Polymer 1990, 31, 1999-2005] .However, the degree of branch inclusion seems to be insensitive to the branch content and branch distribution.