DYNAMICS OF FLOW-INDUCED ALIGNMENT OF SIDE-GROUP LIQUID-CRYSTALLINE POLYMERS

The dynamics of oscillatory shear-induced alignment in side-group liqu id-crystalline polymers (SGLCPs) are investigated by measuring transmi ttance and viscoelasticity during and after the alignment process. We compare the dynamic moduli and transmittance of shear-aligned samples with those of magnetically aligned samples. Relative to an unaligned n ematic, magnetic alignment does not appreciably alter the dynamic modu li. This suggests that, at small strains, distortion of the polymer ba ckbone dominates the linear viscoelastic response so that the moduli a re insensitive to macroscopic director alignment. With increasing stra in, deviations from linear behavior are manifested by an increase in t he dynamic modulus. This strain hardening may be due to the tendency o f the mesogens to orient perpendicular to the backbone in the present SGLCP, since the mesogens attached to a given strand may be forced to adopt an intermediate orientation between that dictated by the backbon e segment and that dictated by the local orientation of the director. As strain increases, this competition could reduce the order parameter from its equilibrium value; the associated increase in the free energ y may contribute significantly to stress, resulting in the observed st rain hardening. Furthermore, the coupling between the mesogen and back bone provides a means for the bias in the orientation distribution of the backbone to bias the orientation of the mesogens, which may drive the observed flow-induced alignment of SGLCPs.