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 orie ntation and investigate the coexistence of hexagonal and nematic phase s in highly concentrated PBG/cresol solutions under shear flow. Beyond a threshold concentration, the diffuse nematic lobe in x-ray scatteri ng patterns is accompanied by sharp reflections indicating lateral pac king of molecules in a hexagonal arrangement. The threshold concentrat ion for formation of this hexagonal phase depends on molecular weight, but occurs near 35 wt % PEG and corresponds directly to the onset of region I shear thinning. Applied shear acts to reduce the amount of he xagonal phase and ultimately, at high enough shear rates, return the s olution to a fully nematic state. Relaxation experiments at high shear rates indicate that the final state of the solution is highly sensiti ve to the continued presence of the hexagonal phase during shear. Spec ifically, a much higher final orientation results if the hexagonal pha se-even a very small amount-persists during the preceding shear. We po stulate a nucleation mechanism to explain this behavior, in which regr owth of the hexagonal phase during relaxation is promoted in such a wa y that molecular orientation is favored in a direction parallel to the residual hexagonal phase in the sample at the onset of relaxation. Th is results in increased molecular orientation in both the hexagonal an d nematic phases. Finally, we show examples of erratic relaxation beha vior observed in PBG/cresol at concentrations immediately below the th reshold of hexagonal phase formation. The degree of this erratic behav ior appears to depend on the amount of strain applied before how cessa tion. (C) 1998 The Society of Rheology. [S80148-6055(98)00702-0].