Molecular Alignment of Polymer Liquid Crystals in Shear Flows. 2. Transient Flow Behavior in Poly(benzyl glutamate) Solutions

Flow birefringence is used to study the degree of molecular orientation in transient shear flows of liquid crystalline solutions of poly(benzyl glutamate) (PBG). Experiments are confined to the low shear rate regime in which it is known that such solutions exhibit director tumbling. Our optical results exhibit characteristics common to PBG solutions, including oscillatory responses that scale with shear strain and relaxation processes that scale inversely with the previously applied shear rate. The measured degree of orientation is compared with the transient predictions of the linear Larson and Doi tumbling polydomain model [Larson, R. G.; Doi, M. J. Rheol. 1991, 35, 539], which is known to qualitatively reproduce transient stress behavior well. Our results reveal some shortcomings of the model regarding its predictions of molecular orientation. Our most startling observation is a significant increase in molecular orientation upon cessation of shear flow. This increase in orientation provides a straightforward explanation for the gradual decrease in dynamic moduli that is observed following cessation of shear flow. This result is in direct contradiction with conclusions drawn by Asada and co-workers on similar PBG solutions [Asada, T.; Onagi, S.; Yanase, H. Polym. Eng. Sci. 1984, 24, 355]. We attribute this discrepancy to ambiguity in birefringence measurements resulting from multiple retardation orders; we have employed a spectrographic birefringence technique that eliminates this source of uncertainty. This work is an extension of an accompanying paper on steady-state flow behavior.