Molecular Mechanisms for Conformational and Rheological Responses of Entangled Polymer Melts to Startup Shear

In this work, we have carried out Brownian dynamics simulation to describe the detailed characteristics of conformational and rheological responses to startup shear. In addition to the evaluation of the contour length of the primitive chain, the state of chain entanglement and the first normal stress difference as a function of strain, three methods are applied to determine the time-dependent shear stress. Our results show significant stretching of the primitive chain up to many Rouse times, followed by retraction, as the primary origin of stress overshoot for deformation rates lower than the reciprocal of the Rouse time but higher than the reciprocal of the reptation time. The analysis of such results reveals heterogeneous local chain stretching, demonstrating the coupling between stretching and orientation that extends to times considerably longer than the Rouse time. Explicit comparison between the simulation and the theoretical description from the GLaMM theory shows marked differences. For example, the simulation indicates a slower decline in the number of original entanglements than that at equilibrium up to many Rouse times whereas the GLaMM theory predicts a faster decrease. Moreover, contrary to the simulation that depicts a nearly constant slope in the stress–strain relationship during startup shear, the GLaMM theory shows an immediate and precipitous strain softening.