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

The dynamics of oscillatory shear-induced alignment in side-group liquid-crystalline polymers (SGLCPs) are investigated by measuring transmittance 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 nematic, magnetic alignment does not appreciably alter the dynamic moduli. This suggests that, at small strains, distortion of the polymer backbone dominates the linear viscoelastic response so that the moduli are insensitive to macroscopic director alignment. With increasing strain, deviations from linear behavior are manifested by an increase in the dynamic modulus. This strain hardening may be due to the tendency of 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 backbone 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 energy may contribute significantly to stress, resulting in the observed strain hardening. Furthermore, the coupling between the mesogen and backbone 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.