Liquid crystal polymers

The lattice theory of liquids consisting of rodlike molecules is presented and discussed with emphasis on polymers exhibiting nematic or cholesteric liquid crystallinity. Steric repulsions between the solute particles are principally responsible for order in lyotropic liquid-crystalline systems. In the case of rigid rods, the axial ratio of the particles governs the concentration at which separation of a nematic or cholesteric phase sets in. For semi-rigid chains such as those of cellulose and its derivatives, the axial ratio of the Kuhn segment is the relevant parameter. These and other predictions of the lattice theory are confirmed by numerous experiments. Liquid crystallinity may be promoted by orientation-dependent intermolecular attractions between extended chain molecules. Such forces originate in the anisotropy of the polarizabilities of groups, e.g., phenylene, in the main chain. They may be especially important in thermotropic melts and concentrated solutions.

Molecular theory of liquid crystals

This review is be concerned primarily with the synthesis and properties of such polymers which show thermotropic behaviour.

Liquid crystal polymers with flexible spacers in the main chain

The historical development of both main-chain (M.C.) and side-chain (S.C.) liquid crystalline polymers (LCPs) is presented. Synthetic routes and characterization techniques for LCPs are introduced. Various approaches to reduce the melting points of M.C. LCPs and to increase the mesogenic order of S.C. LCPs are reviewed and evaluated. The criteria to form a liquid crystalline phase are then discussed. Comparisons among thermotropic, lyotropic, and low-molecular-weight liquid crystals are also made. It is concluded that the preparation of LCPs has been well developed by the industry, but that the understanding of the formation of LC domains has not been completely understood. The theoretical explanations for some unexpected results are still not available. These anomalies are detailed in this work. Future research activity en LCPs for plastics engineers is also mentioned in this article.

The Recent Developments of Thermotropic Liquid Crystalline Polymers

Properties of polymer liquid crystals: choosing molecular structures and blending

Several different series of rigid and flexible polyesters with main chain liquid crystalline units were prepared and their properties were examined in relation to their structures. The first group of polymers were rigid aromatic copolyesters with mesogenic groups based on either chloro or methyl hydroquinone terephthalate units combined with varying amounts of different types of bisphenol terephthalate units The bisphenol comonomers used contained the structure: 






in which X was none, C(CH3)2, CH2, O, S, and So2. It was observed that the bisphenols with the bulkier X group were more efficient in destroying thermotropic liquid crystallinity of the resulting copolymers. The second group of polymers studied were flexible polyesters consisting of various types of mesogenic units which were connected together by different lengths of polymethylene flexible spacers. The liquid crystalline behaviours of these polymers, particularly their transition temperatures, were correlated with their structures. A brief review of previous studies on the synthesis of thermotropic liquid crystalline polyesters is included.

Thermotropic Liquid Crystalline Polyesters with Rigid or Flexible Spacer Groups

Two closely related series of polyesters that contain mesogenic units interconnected by flexible spacers along the main chain were prepared and characterized for their liquid crystal properties. All of these polymers showed theotropic behavior, which was examined by DSC, hot-stage microscopy on a polarizing microscope, small-angle light and wide-angle x-ray scattering methods, and visual observation of stir-opalescence of the polymer melts. The effect of the length of the flexible spacer and the nature of the substituent, which is on the central aromatic ring of the mesogenic unit, on the stability and molecular order of the mesophase for each of the polymers was investigated and the results are discussed on the basis of the thermodynamic data obtained.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pol.1981.170190804

J.-I. Jin

Papers

Thermotropic Liquid Crystalline Polyesters with Rigid or Flexible Spacer Groups

Liquid crystal polymers. IV. Thermotropic polyesters with flexible spacers in the main chain