Structures and properties of liquid-crystalline polymers based on laterally attached oligo p-phenylenes

The structures and properties of liquid-crystalline polymers containing laterally attached p-terphenyl and p-pentaphenyl have been studied. In contrast to their mesogenic groups, that is, p-terphenyl and p-pentaphenyl, the polymers have much lower crystallinity and also lower nematic-to-isotropic transition temperatures. The significant depression in crystallinity can be attributed to flexible chain segments laterally attached to the oligo p-phenylene rods, which prevent close packing of the rods and thus disrupt the crystallization. The destabilization of the liquid-crystalline phase is due to the diluting effect of the flexible polymer backbones; that is, the concentration of the mesogenic groups is reduced. The polymer containing p-pentaphenyl can still exhibit good solubility in common solvents and emit light at about 402 nm in the solvent tetra-hydrofuran. In the solid state, the emission redshifts to 418 nm, which is fairly close to the blue-light emission. An interdigitated packing structure of mesogenic groups has been proposed to represent the structure of the polymer in the oriented state.