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 liqu id crystals (gLCs) was generalized by increasing the volume of the non-meso genic central core, with an attendant increase in the number of nematic pen dants, using 5-hydroxyisophthalic acid as the bridging unit. New nematic gL Cs were synthesized and characterized, showing an elevation in T-g by 30 to 40 degrees C with no definite trend in T-c over the benzene, cis,cis-cyclo hexane, and exo,endo-bicyclo [2.2.2] oct-7-ene base cores. The exo,exo-conf igured gLC showed a higher T-g and a higher T-c than the exo,endo-counterpa rt. Morphological characterization with X-ray diffractometry revealed the n on-crystalline nature of pristine samples and the morphological stability o f thermally processed gLC films against recrystallization for six months. N ematic gLC films were prepared for characterization by FTIR linear dichrois m, 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 s howed an elevation in T-g by 10 to 20 degrees 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 reflec tion band centred around 375 nm. Tunability of selective reflection from th e UV to visible region was demonstrated by mixing the chiral-nematic and ne matic gLCs at varying ratios.