The effect of shear on thermotropic liquid crystalline polymers

We present and review optical and rheological data on a number of sheared thermotropic liquid crystal polymers LCPs. Optical observations were carried out by using a shearing apparatus that was designed for these and additional in situ X-ray measurements. X-ray data on sheared LCPs are also presented together with orientation relaxation measurements made on the cessation of flow. To reconcile the optical, X-ray and rheological data, a numerical scheme is constructed that can qualitatively, and in some cases quantitatively predict observed behaviour. Essential elements of the model are concerned with a local molecular anisotropy, a molecular correlation coefficient, and a local defect structure. The model shows, among other predictions, that stress relaxation is essentially decoupled from both orientation and optical relaxation. In addition the presence of a defect texture influences orientation aspects more than the rheology.

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