Novel Glass-Forming Liquid Crystals. 6. High-Temperature Glassy Nematics

Abstract

To furnish new insight into structure−property relationships governing glassy nematic liquid crystals, novel material systems were synthesized and characterized. Highly ordered, solid films were also prepared via spin-coating for the determination of optical birefringence. It was found that a linear nematogen, such as the cyanoterphenyl group, contributes to an elevated glass transition temperature and a wide nematic fluid temperature range with a mixed or an extended core structure. However, an angular nematogen, such as the cyanophenylnaphthyl group, is more versatile in inducing glass formation with a broader range of core structures. Morphologically stable, glassy nematics with a Tg close to 130 °C, a Tc close to 350 °C, a nematic fluid temperature range of 265 °C, and an optical birefringence of 0.44 accomplished in this study represent significant improvements over existing materials. A stereochemical modification of the bicyclooctene ring from an exo-, exo- to an endo-, and exo-configuration, fluorination of the cyanophenylnaphthyl group, and a longer spacer to nematic pendants were found to depress Tc to a much greater extent than Tg. An extension in the core structure accompanied by an increased number of nematic pendants per core was found to elevate Tg, but no consistent effect on Tc was observed.