X-ray scattering investigation of highly concentrated poly(benzyl glutamate) solutions under shear flow

In situ x-ray scattering techniques are used to measure molecular orientation and investigate the coexistence of hexagonal and nematic phases in highly concentrated PBG/cresol solutions under shear flow. Beyond a threshold concentration, the diffuse nematic lobe in x-ray scattering patterns is accompanied by sharp reflections indicating lateral packing of molecules in a hexagonal arrangement. The threshold concentration for formation of this hexagonal phase depends on molecular weight, but occurs near 35 wt % PBG and corresponds directly to the onset of region I shear thinning. Applied shear acts to reduce the amount of hexagonal phase and ultimately, at high enough shear rates, return the solution to a fully nematic state. Relaxation experiments at high shear rates indicate that the final state of the solution is highly sensitive to the continued presence of the hexagonal phase during shear. Specifically, a much higher final orientation results if the hexagonal phase - even a very small amount - persists during the preceding shear. We postulate a nucleation mechanism to explain this behavior, in which regrowth of the hexagonal phase during relaxation is promoted in such a way that molecular orientation is favored in a direction parallel to the residual hexagonal phase in the sample at the onset of relaxation. This results in increased molecular orientation in both the hexagonal and nematic phases. Finally, we show examples of erratic relaxation behavior observed in PBG/cresol at concentrations immediately below the threshold of hexagonal phase formation. The degree of this erratic behavior appears to depend on the amount of strain applied before flow cessation.