Recovery of stress overshoot under orthogonal shear after interrupted shear flow

Both entangled and unentangled polymer melts exhibit stress overshoots when subject to shear flow. The size of the overshoot depends on the applied shear rate and is related to relaxation mechanisms such as reptation, chain stretch and convective constraint release. Previous experimental work shows that melts subjected to interrupted shear flows exhibit a smaller overshoot when sheared again after allowing for some relaxation. The time scale for recovery of the maxima is about the timescale for relaxation of the stress for unentangled melts but is significantly longer for entangled melts. Here, we show results of molecular dynamics simulations of interrupted shear of polymer melts where the shear flow after the relaxation stage happens in an orthogonal direction to the original flow. We observe that the size of the stress overshoot is larger than predictions by the Rolie-Poly model, and larger than observed during a second shear in the same direction as the original for the same relaxation time. Similar differences are also observed in transient behavior of the normal stresses, chain end-to-end distance and number of entanglements per chain.