Primitive Chain Network Simulations for Pom-Pom Polymers in Uniaxial Elongational Flows

It is widely accepted that the nonlinear viscoelasticity of polymers with long chain branching can be described by the pom-pom theory [J. Rheol. 1998, 42, 81] that accounts for branchpoint withdrawal (BPW) as a nonlinear relaxation mechanism of the backbone. In spite of the remarkable success attained by refined theories derived from the original pom-pom model, there remain a few questions on the consistency with the theoretical development for linear polymers. For instance, convective constraint release (CCR) is neglected in the pompom theories. In this study, primitive chain network simulations were performed to investigate the details of molecular motion under uniaxial elongation. The simulation automatically includes thermal and convective constraint release via the multichain dynamics. The code that was assembled with finitely extensible nonlinear elasticity (FENE), BPW and the stretch-orientation/induced friction reduction (SORF) shows a reasonable agreement with literature data of linear viscoelasticity and of uniaxial elongational viscosity for monodisperse linear and pom-pom branched polystyrene (PS). Analysis of the simulations reveals that BPW is the dominant mechanism for the backbone relaxation under flow. CCR contributes to accelerate the reptative motion of the branchpoint along the backbone, but this contribution is rather small. SORF reduces the stretch of the arm and of the backbone, but it does not contribute to BPW. These results may help rationalizing the pom-pom theories in view of the dominance of BPW among the relaxation mechanisms.