OSCILLATORY SHEAR ALIGNMENT OF A LIQUID-CRYSTALLINE POLYMER

The alignment of a semiflexible main-chain liquid crystalline polymer by oscillatory and steady shear is investigated in the nematic and smectic states by rheometry and X-ray scattering. Steady shear of the nematic produces alignment in the now direction, whereas dynamic shear produces planar alignment parallel to the now and neutral directions. In the smectic state, alignment of the layer normal along the neutral direction was predominant - the result of various frequencies and strain amplitudes. The predominance of this alignment arises from the high degree of interlayer connectivity characteristic of main-chain liquid crystals. More surprisingly, for certain alignment procedures (those with strain amplitude less than 5% and moderate to low frequency), alignment of the layer normal is approximately parallel to the now direction. Alignment parallel to the velocity-gradient direction was found only when another smectic form, having less interlayer connectivity, was induced by the deformation process. This study demonstrates a rich relationship between applied shear now, phase behavior, and the molecular orientation.