Thermotropic liquid crystalline polymers—A new type of mesomorphous macromolecular system

Abstract

SYNOPSIS A new approach to the synthesis of thermotropic liquid-crystalline (LC) polymers is described based on the polymerization of monomers containing various mesogenic groups attached to the backbone chain by flexible polymethylene spacer groups. Dozens of new LC polymers are described, including derivatives of polymethacryloyl L-lysine, cholesteric esters of long-chain aminocarboxylic acids, and other aromatic polymers. Phase transition temperatures, heats of fusion, and x-ray structural analysis were evaluated. Polymers exhibiting spontaneous optical anisotropy in the glasslike, elastic, and fluid states are described, and schemes of molecular packing in the LC phase are proposed. We show the possibility of realizing smectic, nematic, and cholesteric types of structures in these thermotropic LC polymers. An LC structure for various comblike copolymers not having mesogenic groups at every monomeric unit was detected. The role of specific interactions in side groups and their mobility in allowing for the LC structure are discussed, and the necessity of having particular conformations of the macromolecules is shown to provide the LC structure in films made from various solutions. Low-molecular-weight liquid-crystalline (LC) substances are subjects paid continuing and careful attention by organic chemists, crystallographers, and other representatives of the chemical disciplines due to their unique complex of properties. structure, and practical applications. The situation is different in polymer chemistry; the first publications concerning the synthesis of thermotropic LC polymers were in the early 1970s. Today they number about 100 and increase every year, reflecting the interest of synthesis chemists, physical chemists, and physicists in such unusual, for classical polymer chemistry, substances. Experimental results obtained in the University of Moscow during last five to six years will be reviewed in this paper. These experiments led us to formulate a new principle of thermotropic LC polymer synthesis [ 1-71. Polymer systems which provide LC properties and have structures analogous to the “classical” organic liquid crystals could be divided schematically into three groups: