Rheological Influence of Short-Chain Branching for Polymeric Materials under Shear with Variable Branch Density and Branching Architecture

Abstract

An important objective in polymer science is to manipulate the material properties of polymers by altering their molecular architecture. To this end, understanding of the fundamental role of chain branches along the polymer backbone is crucial. Although the dynamics of linear and long-branched polymers have been thoroughly investigated over the past decades, a comprehensive understanding of branching effects has not yet been obtained, particularly because of a serious 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 have comprehensively studied the general effect of short-chain branching on the rheological properties of polymeric materials using Brownian dynamics simulations for a series of SCB polymers with systematically varied branch densities and branching architectures along the chain backbone. Our results demonstrate that the short branches, via their fast random motions, give rise to a more compact and less deformed chain structure in response to the applied flow, eventually reducing the shear-thinning behavior in viscosity and the first normal stress coefficient. Most importantly, by altering the distribution of short branches along the backbone, the structural and rheological properties of the SCB system are dramatically changed. We discuss the physical origins and molecular mechanisms underlying these effects and present a detailed interpretation using a molecular-level analysis of individual chain dynamics. This information is valuable, showing us how to systematically tune the material properties by controlling the molecular architecture of branched polymers under various flow conditions.