Stress Relaxation of Comb Polymer Melts

Abstract

Branched polymers stress relaxation is at the center to their function as viscosity modifiers, though the fundamentals that underlie the correlation between the polymer topology and their impact on viscosity remains an open question. Here, the stress relaxation of short, branched polyethylene comb polymer melts is studied by molecular dynamics simulations. A coarsegrained model where four methylene groups constitute one bead is used, and the results are transposed to the atomistic level. For arms of length comparable to entanglement length n e of the linear polymer, we show that while increasing the number of branches with the same arm length decreases the plateau modulus, the terminal diffusive time does not change significantly. Increasing the arm length decreases the plateau modulus and increases the terminal time. As arms shorter than n e relax by the entanglement time, both the chain mobility and stress relaxation can be described by reptation of the backbone with an increased tube diameter and an increased friction coefficient; or in other words, the branches act as a solvent.