Dynamics and the mesomorphic properties of a novel antiferroelectric liquid crystalline thiobenzoate MHPSBO10: thermal, optical and dielectric spectroscopy study.
The complementary studies of the mesomorphic properties of a novel antiferroelectric liquid crystal (AFLC) (S)-2-octile 4-S-(4'decyloxybiphenyl-4-tiocarboxy)benzoate, known under MHPSBO10 acronym have been undertaken. The polymorphism has been complementary studied in details by Differential Scanning Calorimetry (DSC), Transmitted Light Intensity (TLI) and Polarization Microscopy (POM). The switching characteristics along with multiple macroscopic parameters describing the mesomorphic properties were determined by using electro-optic measurements, both upon cooling and heating. Frequency domain dielectric spectroscopy (DS), covering a wide frequency range, has been applied to characterize the molecular motions. Several collective modes, including the low frequency processes in the condensed hexatic phase were detected, analyzed in details and followed with the temperature. The presented studies deliver a wide report of the phase transitions, molecular dynamics and the macroscopic properties of the novel antiferroelectric thiobenzoate.
On the temperature dependence of the tilt and spontaneous polarisation in high tilt antiferroelectric liquid crystals
The temperature-dependent physical properties of four related liquid crystalline materials are reported. The materials differ chemically only in the degree of fluorination of one of the core aromatic rings and all materials exhibit both chiral smectic-C (ferroelectric) and antiferroelectric liquid crystal phases. The steric tilt angles, calculated from the layer spacing, are relatively low, saturating at a maximum of 25°, while the optical tilt angles are much higher, achieving values of 42.5°. Most obviously, the tilt angle and spontaneous polarisation exhibit very different temperature dependencies, the former appearing to be discontinuous at the orthogonal to tilted transition, while the latter follows a simple power law. It is noted that this behaviour has been observed in other similar materials, but that it is not in agreement with the simple coupling between spontaneous polarisation and tilt that is often assumed in ferroelectric liquid crystals. A theoretical explanation is given for the different temperature dependence of these two physical quantities, suggesting that the biaxial order parameter, not directly determined by the tilt angle, is important in these systems.
BI-MESOGENIC ORGANOSILOXANE LIQUID CRYSTAL MATERIALS EXHIBITING ANTIFERROELECTRIC PHASES
The first antiferroelectric low molar mass organosiloxane liquid crystal materials are presented. The molecules are bi-mesogens consisting of two chiral mesogenic units attached, via methylene spacers, symmetrically to a siloxane group. Three series of compounds with different laterally substituted halogens on the phenyl ring nearest to the chiral centre have been studied. For siloxane groups containing three silicon atoms all of the materials exhibit antiferroelectric phases over a 60 C temperature range. The tilt angle in the SmC* A is, within experimental uncertainties, independent of temperature and close to 45 degrees for the three series with a spontaneous polarization greater than 110nC cm -2.