The effect of interbranch spacing on structural and rheological properties of hyperbranched polymer melts

Nonequilibrium molecular dynamics simulations were performed for a family of hyperbranched polymers of the same molecular weight but with different chain lengths between branches. Microscopic structural properties including mean squared radius of gyration, distribution of beads from the center of mass and from the core and the interpenetration function of these systems were characterized. A relationship between the zero shear rate mean squared radius of gyration and the Wiener index was established. The molecular and bond alignment tensors were analyzed to characterize the flow birefringence of these hyperbranched polymers. The melt rheology was also studied and the crossover from the Newtonian to non-Newtonian behavior was captured for all polymer fluids in the considered range of strain rates. Rheological properties including the shear viscosity and normal stress coefficients obtained from constant pressure simulations were found to be the same as those from constant volume simulations except at high strain rates due to shear dilatancy. A linear dependence of zero shear rate viscosities on the number of spacer units was found. The stress optical rule was shown to be valid at low strain rates with the stress optical coefficient of approximately 3.2 independent of the topologies of polymers.