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 evolution, during shear start-up and reversal flow, in semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs). Using a specially designed microrheometer allowing for simultaneous transient optical and mechanical 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 of the regularly spaced bands. Also conducted were shear start-up and flow reversal experiments using a cone-and-plate rheometer under the same thermal and deformation histories as those in rheo-optical experiments for polymers of differing spacer lengths at equal temperature difference below the nematic-to-isotropic transition temperature ( T NI ). It was observed that, during both shear start-up and flow reversal, the first normal stress difference N 1 + (γ̇, t ) exhibits a large overshoot followed by an oscillatory decay, while shear stress σ + (γ̇, t ) exhibits a large overshoot followed by a monotonic decay. It was found that the higher the applied shear rate, the larger the overshoot of N 1 + (γ̇, t ) and σ + (γ̇, t ), and the longer the persistence of oscillations in N 1 + (γ̇, t ). Similarity was found between the ratio N 1 + (γ̇, t )/σ + (γ̇, t ) and flow birefringence Δ n + (γ̇, t ) during shear start-up and flow reversal of the TLCPs investigated in this study. Further, we found that the ratio σ + ( t,γ̇)/σ scales with γ̇ t but the ratio N 1 + (γ̇, t ) / N 1 does not, where σ denotes shear stress at steady state and N 1 denotes first normal stress difference at steady state.