Chiral smectic-A and smectic-C phases with de Vries characteristics

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Abstract

Infrared and dielectric spectroscopic techniques are used to investigate the characteristics of two chiral smectics, namely, 1,1,3,3,5,5,5-heptamethyltrisiloxane 1-[4-(undecyl-1-oxy)-4-biphenyl(S,S)-2-chloro-3-methylpentanoate] (MSi3MR11) and tricarbosilane-hexyloxy-benzoic acid (S)-4-(1-methyl-hexyloxy)-3-nitrobiphenyl-4-yl ester (W599). The orientational features and the field dependencies of the apparent tilt angle and
the dichroic ratio for homogeneous planar-aligned samples were calculated from the absorbance profiles obtained at different temperatures especially in the smectic-A* phase of these liquid crystals. The dichroic ratios of the C-C
phenyl ring stretching vibrations were considered for the determination of the tilt angle at different temperatures and different voltages. The low values of the order parameter obtained with and without an electric field applied
across the cell in the Sm-A∗ phase for both smectics are consistent with the de Vries concept. The generalized Langevin-Debye model introduced in the literature for explaining the electro-optical response has been applied
to the results from infrared spectroscopy. The results show that the dipole moment of the tilt-correlated domain diverges as the transition temperature from Sm-A∗ to Sm-C∗ is approached. The Debye-Langevin model is
found to be extremely effective in confirming some of the conclusions of the de Vries chiral smectics and gives additional results on the order parameter and the dichroic ratio as a function of the field across the cell. Dielectric spectroscopy finds large dipolar fluctuations in the Sm-A∗ phase for both compounds and again these confirm
their de Vries behavior.
Original languageEnglish
Article number062704
Number of pages9
JournalPhysical Review E
Volume95
Issue number6
DOIs
Publication statusPublished - 21 Jun 2017

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