Novel glass-forming liquid crystals V. Nematic and chiral-nematic systems with an elevated glass transition temperature

The formerly implemented molecular design concept behind glass-forming liquid crystals (gLCs) was generalized by increasing the volume of the non-mesogenic central core, with an attendant increase in the number of nematic pendants, using 5-hydroxyisophthalic acid as the bridging unit. New nematic gLCs were synthesized and characterized, showing an elevation in T_g by 30 to 40°C with no definite trend in T_c over the benzene, cis, cis-cyclohexane, and exo, endo-bicyclo[2.2.2]oct-7-ene base cores. The exo, exo-configured gLC showed a higher T_g and a higher T _c than the exo, endo-counterpart. Morphological characterization with X-ray diffractometry revealed the non-crystalline nature of pristine samples and the morphological stability of thermally processed gLC films against recrystallization for six months. Nematic gLC films were prepared for characterization by FTIR linear dichroism, resulting in an orientational order parameter in the range 0.52 to 0.63. A chiral-nematic gLC derived from exo, exo-bicyclo[2.2.2.]oct-7-ene also showed an elevation in T_g by 10 to 20°C over the cyclohexane-based systems reported previously. With (S)-(-)-1-phenylethylamine as the chiral moiety, the left-handed, chiral-nematic gLC film yielded a selective reflection band centred around 375 nm. Tunability of selective reflection from the UV to visible region was demonstrated by mixing the chiral-nematic and nematic gLCs at varying ratios.