Raman spectroscopic study of phase transitions and configurations in ester ferroelectric liquid crystals

摘要

Raman spectroscopy was examined for ester ferroelectric liquid crystal D-4-(2-methylbutoxy)phenyl 4-decyloxybenzoate (MBOPDOB) to study configuration between s-cis and s-trans isomers and phase transitions between adjacent phases. Temperature behaviors were investigated for three low-frequency Raman modes at 15, 49 and 93 cm~(-1). The 15 and 49 cm~(-1) Raman lines vanish at crystal-Sm C~* point and their temperature-cycle measurement gives 4 ℃ width of thermal hysteresis from crystal to Sm C~*, which indicates first-order transition. Quasielastic scattering near the crystal-Sm C* point below 30 cm~(-1) shows a critical slowing-down phenomenon relative to a long orientational relaxation in the Sm C* phase. The second-order property of the Sm C~*-Sm A transition were proved by the facts that the 93 cm~(-1) Raman line and spontaneous polarization go smoothly through the point, in addition a very small heat of the transition was recorded. Two conformations of COO group characterized by C=O stretch mode at 1708cm~(-1)and C-O-C asymmetric stretch at 1177 cm~(-1), the S-trans and S-cis isomers were found to coexist in the crystal phase. The S-trans isomer are transformed to the S-cis during the crystal-Sm C~* transition and in the Sm A and isotropic phases. The larger dipole of the S-cis conformer is in favor of the formation of the ferroelectric phase.