Effects of Branching on Rheology of Polyethylene Combs: A Molecular Dynamics Simulation study

Linear, branched, and star polymers exhibit distinctive rheological behavior, critical to their processing, depending on their topology. Early studies by Graessley et al. have shown that the effects of branches differ below and above the entanglement length. Lohse and co-workers demonstrated that branches on the length scale of the entanglement length of the backbone results in increased viscosity. Using coarse grained (CG) molecular dynamics simulations we study the effects of degree of branching, including their length and density on the rheology of entangled polyethylene (PE) melts, with branch lengths above and below the entanglement length while branching density is varied. Our CG models are able to capture the flow properties observed by experimental studies and provide fundamental new correlations between branching length-densities and rheology for entangled branched PE. As expected branched PE chains diffuse slower than their linear analogs. For polymer melts with same M_W, diffusion is predominantly governed by the branch length and only slightly affected by the branching density. Beyond the new model for CG of branched polymers, this study has provided new insight into long-standing challenges in polymer rheology.