Tension-induced nematic phase separation in homopolymer melts

The nematic coupling parameter α, which quantifies the interactions between backbone tangents and nematic fields, governs the nematic phase behaviors and polymer alignment in bulk and at interfaces. Together with external tension, the nematic interactions can also drive phase separation of long chains from short ones in bidisperse homopolymer melts. Combining molecular dynamics (MD) simulations and analytical theory, we extract α for polyethylene (PE) oligomers under applied tension, and construct a mean-field free energy to predict the phase boundary for bidisperse melts in which the longer chains are stretched by uniaxial tension. Phase separation occurs when sufficient tension is applied, consistent with a previous prediction by Olmsted and Milner. By directly observing phase separation in MD simulations, we validate the phase diagram. Using non-equilibrium MD (NEMD) simulations, we also show that extensional and shear flow may lead to nematic phase separation in molten PE oligomers, because the flow can impose a stronger tension on the longer chains than the short ones. We expect the tension-induced nematic phase separation may affect chain configurations for polydisperse polymer melts under flow, and in turn affect flow-induced crystallization.