Rheology of short-chain branched polyethylene melts under shear: Results from NEMD simulations and comparison with linear and H-shaped analogues

Jun Mo Kim (University of Tennessee, USA) and Chunggi Baig (University of Patras, Greece)

Considering the growing experimental and theoretical interest in the rheology of branched polymers, we have undertaken a systematic study of the role of chain branching on the structural and dynamical behavior of model polyethylene melts using nonequilibrium molecular dynamics (NEMD) simulations for shear flow. Three different model polyethylene liquids (all characterized by the same total chain length, equal to 178 carbon atoms) are considered: short branched chains consisting of small (up to 6 carbon atoms) branches frequently spaced along the main chain backbone, H-shaped chains, and linear chains. In order to avoid system-size effects, rather large simulation boxes are used, especially at high shear rates (containing up to 35,000 total atoms), since the chains are likely to be significantly oriented and deformed with respect to the flow direction. Also, in all cases, the simulations are carried out for sufficiently long intervals (especially, at low shear rates) in order to obtain reliable statistics. Results are analyzed for a number of structural and dynamical properties as a function of chain architecture, including the center-of-mass self-diffusion coefficient, the shear viscosity, the 1st and 2nd normal stress coefficients, and the conformation tensor.