Evolution of Chain Conformation and Entanglements during Startup Shear

Using Brownian dynamics simulation, we determine chain dimensions in an entangled polymer melt undergoing startup shear at a rate lower than the reciprocal of the Rouse time yet higher than the reciprocal reptation time. Here the tube model expects negligible chain stretching. In contrast, our simulation shows the deformed coil to conform closely to affine deformation. We find that the total number of entanglements decreases with increasing shear. Remarkably, up to many Rouse time, the decline in the number of initial entanglements is slower than that under the quiescent condition. These results point to fundamental deficiencies in the molecular picture of the tube model for startup shear.