Transient molecular orientation and rheology in flow aligning thermotropicliquid crystalline polymers

Quantitative measurements of molecular orientation and rheology are reporte d for various transient shear flows of a nematic semiflexible copolyether. Unlike the case of lyotropic liquid crystalline polymers (LCPs), whose stru cture and rheology in shear are dominated by director tumbling, this materi al exhibits flow aligning behavior. The observed behavior is quite similar to that seen in a copolyester that we have recently studied [Ugaz and Burgh ardt (1998)], suggesting that flow aligning dynamics may predominate in mai n-chain thermotropes that incorporate significant chain flexibility. Since the flow aligning regime has received little attention in previous attempts to model the rheology of textured, polydomain LCPs, we attempt to determin e whether available models are capable of predicting the orientation and st ress response of this class of LCP. We first examine the predictions of the polydomain Ericksen model, an adaptation of Ericksen's transversely isotro pic fluid model which accounts for the polydomain distribution of director orientation while neglecting distortional elasticity. This simple model cap tures a number of qualitative and quantitative features associated with the evolution of orientation and stress during shear flow inception, but canno t cope with reversing flows. To consider the possible role of distortional elasticity in the re-orientation dynamics upon reversal, we evaluate the me soscopically averaged domain theory of Larson and Doi [Larson and Doi (1991 )], which incorporates a phenomenological description of distortional elast ic effects. To date, their approach to account for polydomain structure has only been applied to describe tumbling LCPs. We find that it captures the qualitative transient orientation response to flow reversals, but is less s uccessful in describing the evolution of stresses. This is linked to the de coupling approximation adopted during the model's development. Finally, a m odified polydomain Ericksen model is introduced that provides some of the b enefits of the Larson-Doi model while offering more realistic stress predic tions. (C) 2001 The Society of Rheology.