Effect of short-chain branching on the tie chains and dynamics of bimodal polyethylene: Molecular dynamics simulation

Abstract

Molecular dynamics simulations were performed on bimodal polyethylene (BPE) in order to shed light on the molecular mechanism of short-chain branching on BPE nucleation process, crystallization process and especially the formation process of tie chains. The study contemplates two kinds of BPE model chains with the same branch content (BC), branch length (BL) but with different short-chain branching distribution (SCBD): short chain branches incorporated in either the long chain or the short chain. For the nucleation process, nucleation time of all models increased with the increase of BC. Short branches distributed on the long chain provoke a delay of the nucleation as compared to the case of short branches distributed on the short chain. Additionally the nucleation process is further delayed as the BL increases. Crystallization rate and crystallinity decrease as BC increases. The crystallinity of the BPE mode chains with branches placed on the long chain were lower than the case of branches placed on the short chain above a critical BC (5/1000C), and this critical BC was independent with BL. When branches distribute on the long chain, short chain is more likely to locate inside the long chain, while for the case of branches distribute on the short chain, the results are opposite. The concentration of tie chains increased with the increase of BC. For BPE model chains with branches placed on the long chains are more likely to form tie chains when compared with the case of branches placed on the short chain. The concentration of tie chains is insensitive to BL. A modified tie chains formation process mechanism is proposed to explain the SCBD effect on tie chains.