Shear-induced structures in semidilute solution of ultrahigh molecular weight polyethylene at temperature close to equilibrium dissolution temperature

The shear-induced structure of a semidimute solution of ultrahigh molecular weight polyethylene (UHMWPE) with a paraffin wax as a solvent was investigated with shear small-angle light scattering (shear SALS) and shear microscopy. At a temperature of 124 degrees C, which is much higher than the nominal melting temperature (similar to 118 degrees C) and is approximately equivalent to or slightly lower than the equilibrium dissolution (crystal melting) temperature of the UHMWPE solution at quiescent state, the characteristic "butterfly" pattern was observed in shear SALS when the solution was subjected to shear flow at a shear rate larger than critical shear rate (gamma) over circle (c,butt). Lack of optical anisotropy in the sheared solution and disappearance of the butterfly pattern after cessation of the shear suggest that the pattern is attributed to shear-enhanced concentration fluctuations or phase separation. When further increased above a critical value c,streak, a strong, sharp streaklike scattering pattern appeared perpendicular to the flow in addition to the butterfly pattern. The streaklike scattering patterns were observed under crossed polarizers so that the pattern arises from optically anisotropic fibrils oriented along the flow axis. The shear rate dependence of the scattering patterns revealed that the shear-enhanced concentration fluctuations developed at lower shear rates triggered formation of optically anisotropic fibrils aligning along the flow direction upon a further increase of shear rate above c,streak.