  In addition to the terminal flow (the region I) and the shear thinning (the
region II), we discover two new flow regions in capillary flow at the wall
stress higher than the plateau modulus of the polymer. The region III violates
the empirical Cox-Merz rule with a significantly weaker shear thinning than the
region II, and the region IV exhibits unexpected shear thickening. Moreover,
the crossover shear rates between the regions II and III and between the
regions III and IV scale with the number of entanglement per chain, Z=M_w/M_e,
as Z^(-2.0) and Z^(-1.2) respectively. We attribute the weakening in shear
thinning and the emergence of shear thickening to the deformation-induced
non-Gaussian stretching of polymers. These observations offer the first
experimental quantification of the deformation behaviors of polymer melts at
high-stress shear.
