Nonlinear elongational flows are commonly used in polymer processing but understanding their dynamics remains a challenging problem in polymer rheology. It is understood that strong nonlinear flows alter the entanglement and alignment of flowing melts, but competing microscopic theories and experimentally derived constitutive models make different assumptions about how chain entanglement & alignment couple to each other and to chain dynamics during flow. Testing the assumptions of different frameworks requires detailed knowledge of the microscopic dynamics of chains during flow. To this end, we’ve performed extensive molecular dynamics simulations of startup elongational flows of monodisperse linear polymers with Z=9 & 18 entanglements per chain. We deform melts at constant Rouse Weissenberg numbers = 0.1-52, out to a maximum Hencky strain of 4.0. We measure the extensional viscosity and resolve the linear viscoelastic envelope for low Wi, which we compare to the analytic predictions of tube & slip-link theories. At large Wi, we observe strain hardening and resolve the steady state viscosity. We measure the rate dependent deviation of the stress from the stress-optical rule for large Wi and investigate its microscopic origins.

Molecular dynamics simulation of nonlinear elongational flows in entangled polymer melts

254th National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.