Interrupted shear flow of unentangled polystyrene melts

Low molecular weight polystyrene melts were subjected to shearing flow whic h was periodically halted. Weak maxima in both the viscosity and normal str ess were observed upon startup of the flow, notwithstanding the absence of entanglements. The strain associated with the viscosity maxima was independ ent of shear rate, and consistent with the overshoot strains for entangled polymers. The stress overshoots disappeared or were of weaker intensity Whe n the flow was resumed after a brief interruption. For sufficiently long re st periods between Row, the original behavior was reproduced. The time scal e for recovery of the maxima was about the same as that for relaxation of t he stress from steady state flow. This differs from the behavior of entangl ed polymers. (C) 2001 The Society of Rheology.