Real-time 1-2 plane SAXS measurements of molecular orientation in sheared liquid crystalline polymers

We report studies on the average molecular orientation state in steady and transient shear flow of two lyotropic liquid crystalline polymers: poly(ben zyl glutamate) [PBG] and hydroxypropylcellulose [HPC], both in m-cresol sol ution. An annular cone and plate X-ray shear cell is used to probe molecula r orientation in the "1-2" plane, allowing simultaneous measurements of the degree of anisotropy and the average orientation angle relative to the flo w direction. In steady shear flow, molecular orientation increases with she ar rate in both materials. Comparisons with separate measurements in the "1 -3" plane indicate that both materials exhibit a macroscopically biaxial or ientation distribution function. The orientation angle is always small and exhibits a sign change from positive to negative values with increasing she ar rate. In transient flows, anisotropy and orientation angle both exhibit damped oscillations that scale with shear strain. The Larson-Doi polydomain model is in qualitative agreement with data collected on PBG in flow rever sal, but only after an initial transient response seen in the experiments o ver the first several strain units following the reversal. Following step-i ncrease and step-decrease of shear rate, the Larson-Doi model makes qualita tively correct predictions of anisotropy but qualitatively incorrect predic tions of the transient average orientation angle.