Shear-induced alignment of smectic side group liquid crystalline polymers

Large amplitude oscillatory shear (LAOS) is frequently capable of generating macroscopic alignment from an initially random orientation distribution in ordered polymer fluids. Side-group liquid crystalline polymers are of special interest in that the flow field may couple differently to the polymer backbone and the mesogen ordering. We report combined rheological and in situ X-ray scattering investigations of LAOS-induced alignment in smectic side-group LCPs. Synchrotron X-ray scattering is used to study orientation development using a rotating disk shear cell in which orientation is tracked within the flow-vorticity (1-3) plane. In all cases, we find that shear promotes anisotropic orientation states in which the lamellar normal tends to align along the vorticity direction of the shear flow ("perpendicular" alignment). We examine the effects of shear strain amplitude and polymer backbone molecular weight on the ability of LAOS to induce alignment. Rheological measurements of the dynamic moduli reveal that large amplitude shearing in the smectic phase causes a notable decrease in the modulus. X-ray and rheological data demonstrate that increasing strain promotes higher degrees of orientation, while increasing molecular weight impedes development of smectic alignment.