Simulations of Startup Planar Elongation of an Entangled Polymer Melt

Abstract

Using the primitive chain network model, we have simulated the startup of steady planar elongation of a moderately entangled polymer melt at several elongation rates. The time dependence of the first normal stress growth coefficient, η⁺_p1, was similar to that under startup uniaxial elongation, η⁺_u, and exhibited significant strain hardening when the Rouse Weissenberg number, Wi_R ≳ 1. Analysis of the chain conformation revealed that planar elongation resulted in the loss of fewer entanglements and in a lower orientation anisotropy compared to uniaxial elongation, and these two effects nearly compensated each other resulting in the qualitative similarity between η⁺_p1 and η⁺_u. The second normal stress growth coefficient, η⁺_p2, showed only strain softening and resembled startup shear including the appearance of a stress maximum. Independent of the strain rate, this stress maximum occurred at approximately half the strain at the stress maximum under startup shear. The time dependence of the segment orientation revealed that the molecular origin of the stress maximum can be attributed to the maximum in the corresponding orientation anisotropy, in direct analogy to the behavior under shear.