Methyl branch effects on rheological behaviours of short-chain polypropylene under steady shear studied via nonequilibrium molecular dynamics simulations

Communication: Role of short chain branching in polymer structure and dynamics

A comprehensive understanding of chain-branching effects, essential for establishing general knowledge of the structure-property-phenomenon relationship in polymer science, has not yet been found, due to a critical lack of knowledge on the role of short-chain branches, the effects of which have mostly been neglected in favor of the standard entropic-based concepts of long polymers. Here, we show a significant effect of short-chain branching on the structural and dynamical properties of polymeric materials, and reveal the molecular origins behind the fundamental role of short branches, via atomistic nonequilibrium molecular dynamics and mesoscopic Brownian dynamics by systematically varying the strength of the mobility of short branches. We demonstrate that the fast random Brownian kinetics inherent to short branches plays a key role in governing the overall structure and dynamics of polymers, leading to a compact molecular structure and, under external fields, to a lesser degree of structural deformation of polymer, to a reduced shear-thinning behavior, and to a smaller elastic stress, compared with their linear analogues. Their fast dynamical nature being unaffected by practical flow fields owing to their very short characteristic time scale, short branches would substantially influence (i.e., facilitate) the overall relaxation behavior of polymeric materials under various flowing conditions.