Primitive Chain Network Simulation of Elongational Flows of Entangled Linear Chains: Stretch/Orientation-induced Reduction of Monomeric Friction

Well-entangled monodisperse linear polystyrene melts exhibit monotonic thinning of the steady state elongational viscosity with increasing the strain rate (epsilon) over dot even beyond the Rouse relaxation frequency, tau(-1)(R). This behavior is quite different from the thinning followed by hardening at (epsilon) over dot > tau(-1)(R) observed for entangled semidilute solutions. We attempt to elucidate the molecular origin of this difference by focusing on the concept of stretch/orientation-dependent monomeric friction zeta recently proposed by Ianniruberto and co-workers. Specifically, literature data of the stress relaxation after cessation of transient elongational flow, reported for both PS melts and solutions, are analyzed to evaluate the stretch/orientation-dependent decrease of zeta. In our working hypothesis, zeta is expressed as a function of the factor F-so = (lambda) over tilde (2)(S) over bar, where (lambda) over tilde is the normalized stretch ratio of entangled subchains defined with respect to the fully stretched state, and S is an average orientational anisotropy of the components (polymer plus solvent if any) in the system. The factor F,0 was estimated from the stress decay data after flow cessation. The resulting functional form of zeta(F-so) was then used in the primitive chain network (PCN) simulation including finite extensible nonlinear elasticity (FENE) to examine the elongational behavior of melts and solutions. For melts the simulation indicates that zeta. decreases significantly under fast elongation because the entangled subchains are short and approach the fully stretched (and fully oriented) limit rather easily. Hence, the steady elongational viscosity eta(E) follows this decrease of C to exhibit the monotonic thinning even at (epsilon) over dot > tau(-1)(R). In contrast, for solutions, the simulated eta(E) exhibits thickening at (epsilon) over dot > tau(-1)(R) because the average anisotropy (S) over bar is governed by the solvent and remains small, thus overwhelming the increase of the subchain stretch A. The simulated results proved to be in satisfactory agreement with the experiments.