Morphological and rheological responses to shear start-up and flow reversal of thermotropic liquid-crystalline polymers

Novel rheo-optical methods have been used to directly observe morphology ev olution, during shear start-up and reversal flow, in semiflexible main-chai n thermotropic liquid-crystalline polymers (TLCPs). Using a specially desig ned microrheometer allowing for simultaneous transient optical and mechanic al observations, we observed band formation upon reversal of flow direction . It was seen that this band formation causes asymmetric light diffraction in H-V small-angle light scattering mode, indicating a tilted arrangement o f the regularly spaced bands. Also conducted were shear start-up and flow r eversal experiments using a cone-and-plate rheometer under the same thermal and deformation histories as those in rheo-optical experiments for polymer s of differing spacer lengths at equal temperature difference below the nem atic-to-isotropic transition temperature (T-NI) It was observed that, durin g both shear startup and flow reversal, the first normal stress difference N-1(+)(gamma, t) exhibits a large overshoot followed by an oscillatory deca y, while shear stress o(+)(gamma, t) exhibits a large overshoot followed by a monotonic decay. It was found that the higher the applied shear rate, th e larger the overshoot of N-1(+)(gamma, t) and sigma(+)(gamma, t), and the longer the persistence of oscillations in N-1(+)(gamma, t). Similarity was found between the ratio N-1(+)(gamma, t)/ sigma(+)(gamma, t) and flow biref ringence Delta n(+)(gamma, t) during shear start-up and flow reversal of th e TLCPs investigated in this study. Further, we found that the ratio sigma( +)(t, gamma)/sigma scales with gamma t but the ratio N-1(+)(gamma, t) /N-1 does not, where sigma denotes shear stress at steady state and N-1 denotes first normal stress difference at steady state.