Morphology of thermotropic longitudinal polymer liquid crystals

Among the several hundreds of PLCs reported in papers and patents, thermotropic longitudinal PLCs [1] comprise a very important category. Longitudinal PLCs, formerly called main-chain PLCs, are those whose backbones consist of rod-like mesogens connected parallel to the backbone. Most thermotropic longitudinal PLCs are aromatic LC 

Mechanical and Thermophysical Properties of Polymer Liquid Crystals Edited by W. Brostow 

The term hierarchical structure was first described for aramid by Dobb et al. [4] and for injection molded thermotropic liquid crystalline copolyester by Weng et al. [5]. It has been found that no matter which LC polyester is concerned, wholly aromatic or semi-aromatic ones. 

Practical polymer processing operations, such as extrusion and injection, are complex processes highly dependent on the conditions of shear, extensional stress and temperature. It has now been realized that shear stress and extensional stress have different effects on the orientation of PLGs. Nematic PLCs have a polydomain texture and each domain consists of mesogens with the same local orientation. The directions of these domains are randomly aligned while in a quiescent state. Only if a stress is applied are the domains oriented in one direction. As Ide and Ophir [6] and Viola et ah [7] have pointed out, shear stress is related to rotational motion (torque), and its application will result in a tumbling flow of PLC domains. Only once the shear stress has reached a critical value will it break down the domains and lead to a uniform molecular orientation. In contrast, extensional stress tends to orient the domains in one direction without breaking down the domains even if the stress is low. Viola et ah [7] also considered that shear flow induces sheet-like textures while extensional flow induces fiber-like textures. Therefore there will be differences in the hierarchical and fibrillar structures acquired in different fibrication processes. 

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MORPHOLOGY OF THERMOTROPIC LONGITUDINAL POLYMER LIQUID CRYSTALS... 

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