31 October 2001 Raman spectroscopic study of phase transitions and configurations in ester ferroelectric liquid crystals
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Abstract
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 degree(s)C 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 equals O stretch mode at 1708 cm-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.
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Zhen Yin, Ping Zhang, Ming-Sheng Zhang, "Raman spectroscopic study of phase transitions and configurations in ester ferroelectric liquid crystals", Proc. SPIE 4469, Raman Spectroscopy and Light Scattering Technologies in Materials Science, (31 October 2001); doi: 10.1117/12.447378; https://doi.org/10.1117/12.447378
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