Time Evolution of Structures under Shear-Induced Phase Separation and Crystallization in Semidilute Solution of Ultrahigh Molecular Weight Polyethylene

We investigated shear-induced structures for a semidilute solution of ultrahigh molecular weight polyethylene (UHMWPE) with a paraffin wax as a solvent by means of shear small-angle light scattering, shear microscopy, birefringence, and rheology. The solution has a reduced concentration (c/c* where c and c* is concentration and the overlap concentration, respectively) of 11 and hence well-developed entanglements. The structures were investigated at temperature of 124 °C, which is approximately equal to the equilibrium melting temperature of crystals in the solution at quiescent state. We found both shear-induced liquid−liquid phase separation and shear-induced crystallization, when the solution was subjected to shear flow at shear rate larger than the critical shear rate γ̇c,streak. Time change in the shear-induced structures after onset of the shear at γ̇ > γ̇c,streak or after cessation of the same shear illuminated that the shear-induced phase separation triggers the shear-induced crystallization into fibrous structures: crystallization occurs through regions rich in polymer concentration that are built up by the shear.