Relationships between Structures and Liquid Crystalline and Optical Properties in Combined-type Copolyisophthalates Having Nitro- and Methoxyazobenzene Units and Aliphatic Spacers in the Side Chain

Abstract

ABSTRACT:Combined-type thermotropic liquid crystalline (LC) homo-and copolyisophthalates having nitroand methoxyazobenzene units and three aliphatic spacers (hexamethylene, octamethylene and decamethylene) in the side chain and biphenyl unit in the main chain were prepared by melt polycondensation and relationships between polymer structures and LC and optical properties were discussed. Differential scanning calorimetry (DSC) measurements, optical texture observations using polarizing microscopy and powder X-Ray analyses indicated that the homopolyisophtalates with the nitroazobenzene unit form highly ordered smectic E phase and the other homo-and copolymers show less ordered smectic B or smectic C phase. The temperature ranges of smectic LC phases in the polymers decreased with increasing the methoxyazobenzene content. In these polymers, specific interaction such as electron donor-acceptor interaction between the nitro-and the methoxyazobenzene units plays no role in the formation and the stabilization of smectic LC phases. All the polymers displayed UV-vis absorption peak maxima on the basis of the azobenzenes or the biphenyl in solutions and in films, but no fluorescent properties were found, probably due to intraand intermolecular quenching. [DOI 10.1295/polymj.36.607] KEY WORDS Combined-type Polyisophthalate / Melt Polycondensation / Liquid Crystalline Property / Optical Property / Azobenzene / Biphenyl / Differential Scanning Calorimetry / Smectic Phase / It is well known that there are three types of polymers in thermotropic liquid crystalline (LC) polymers: (1) main-chain polymers, (2) side-chain polymers and (3) combined-type polymers.1 The combined-type polymers, which have calamitic mesogens such as biphenyl, diphenyls and azobenzenes both in the side and the main chains, tend to have broader LC temperature ranges of mesophases than the parent main-chain and side-chain polymers and to show nematic phase together with smectic phases owing to the synergy effect of mesogens in the side and the main chains.2 Researches on the combined-type LC polymers are still not adequate, 3 although a number of literatures on chemical and physical properties of main-chain and side-chain LC polymers have been reported. 4,5 On the other hand, side-chain azobenzene-containing polymers have attracted much attention because of potentials as polymers for photonics applications 7-16 in addition to LC properties. 4,17 The azobenzene derivatives have been known for displaying no photoluminescence (PL), but it has been found that the azobenzene derivatives 18 and poly(p-phenylenevinylene)s (PPV) 19 having side-chain methoxyazobenzene unit emit PL light, having very low quantum yields. 18 Our previous paper 20 has described that novel combined-type semi-rigid homopolyisophthalates having biphenyl or 2,5-diphenyl-1,3,4-thiadiazole (DTD) unit in the main chains and azobenzene moieties in the side chains are synthesized by melt polycondensation and that most of them form stable smectic LC (B, C and E) phases without showing nema...