Thermal behavior,decomposition mechanism by TG/MS/FTIR technique and theoretical study of some symmetric and asymmetric bent-core liquid crystals based on 2,7-dihydroxynaphthalene

Journal of Thermal Analysis and Calorimetry - The thermal stability of some symmetric and asymmetric bent-core liquid crystals derived from 2,7-dihydroxynaphthalene as well as the degradation...     

Liquid crystalline polymers: 14. Synthesis and thermal behaviour of some polyethers containing azo-mesogens

The paper presents a study on the relationship between the structure of macromolecular chain and its capacity to generate a mesophase, when mesogens with an azobenzene structure are implied. The polymers have been synthesized by phase transfer catalysis starting from 1,9-dichlorononane and different bisphenols: diphenyl-4,4^′-bis[(azo-4-)phenol], 4,4^′-dihydroxyazobenzene, 4,4^′-dihydroxydiphenyl, bisphenol A and 4,4^′-dihydroxybenzophenone. The polymers have been characterized by ¹H-NMR spectroscopy, DSC calorimetry, optical microscopy in polarized light and thermogravimetrical analysis. Theoretical conformational studies, using molecular simulations have also been performed. Due to their particular geometry, bis-(azobenzene) units are better mesogenic groups as compared with the azobenzene ones. The highly aromatic structure makes impossible the samples isotropisation, as the degradation processes starting advance. For these polymers, under UV irradiation, due to the presence of two azo groups in each mesogen unit, strong conformational modifications are expected. The replacement of the bis-(azobenzene) moieties with azobenzene ones reduces the transition temperatures, making possible the samples isotropisation.