SOME REFLECTIONS ON THE DIRECTION OF THE STRESS-INDUCED MACROSCOPIC ORIENTATION OF THE MESOGENIC GROUPS IN SIDE-CHAIN LIQUID-CRYSTALLINE POLYMERS - A NEW MECHANISM

We propose a mechanism that can effectively determine the stress-induced orientation direction of the mesogenic groups in side-chain liquid crystalline polymers. The mechanism emphasizes the role of the ansiotropic shapes of the liquid crystalline domains and their direct coupling with the stress field. Examples are given to show that only the coupling effects between the polymer backbone and the mesogenic groups through the flexible spacer cannot explain all experimental observations. We suggest that, analogous to the smectic layers, the nematic domains could be anisotropic in shape. When a mechanical field is applied the nematic domains could directly be coupled to the stress held and, owing to the intermolecular cooperativity, respond to it by aligning their long axes preferentially along the field direction; consequently, the orientation of the mesogenic groups relative to the long axes of the domains prior to the mechanical alignment determines the macroscopic orientation direction of the mesogenic groups. We show that this mechanism accounts for a wide range of phenomena observed so far and can be corroborated by many complex, often apparently conflicting, experimental results. The mechanism also allows some predictions to be made.