Vitrified chiral-nematic liquid crystalline films for selective reflectionand circular polarization

Nematic and left-handed chiral-nematic liquid crystals comprising methoxybi phenylbenzoate and (S)-(-)-1-phenylethylamine pendants to a cyclohexane cor e were synthesized and characterized. Although pristine samples were found to be polycrystalline, thermal quenching following heating to and annealing at elevated temperatures permitted the molecular orders characteristic of liquid crystalline mesomorphism to be frozen in the glassy state. Left at r oom temperature for 6 months, the vitrified liquid crystalline films showed no evidence of recrystallization. An orientational order parameter of 0.65 was determined with linear dichroism of a vitrified nematic film doped wit h Exalite 428 at a mole fraction of 0.0025. Birefringence dispersion of a b lank vitrified nematic film was determined using a phase-difference method complemented by Abbe refractometry. A series of vitrified chiral-nematic fi lms were prepared to demonstrate selective reflection and circular polariza tion with a spectral region tunable from blue to the infrared region by var ying the chemical composition. The experimentally measured circular polariz ation spectra were found to agree with the Good-Karali theory in which all four system parameters were determined a priori: optical birefringence, ave rage refractive index, selective reflection wavelength, and film thickness.