Dielectric spectroscopy of a binary mixture of liquid crystals showing wide temperature range twisted grain boundary phase with re-entrant cholesteric phase

Correlation between alignment geometries and memory effect in a surface-stabilized ferroelectric liquid crystal

Memory effect in weakly aligned surface stabilized ferroelectric liquid crystal (SSFLC) material has been investigated by electro-optical and dielectric spectroscopy in three configurations of alignment: antiparallel, 90^{∘} twisted, and unaligned planar samples. It has been observed that two types of molecular dynamics exist in antiparallel rubbed cell in which memory effect is observed for longer duration than in other samples. One dielectric relaxation process is near the surface of the electrode and a second is in the bulk of the SSFLC. Both the molecular dynamics contribute in the switching process and affect the memory phenomenon in surface stabilized geometries. However, a single dielectric process is observed in twisted geometry in which the sample is showing shorter memory effect than in antiparallel and this is compared with unaligned samples also having cell thickness less than the pitch value of FLC. In an unaligned sample, a single dielectric process is observed and the smaple does not show memory effect at all. The investigation is significant to understand the anomalies occurring in memory observations in various geometries.

Re-entrant nematic behavior in the 7OCB+9OCB mixtures: evidence for multiple nematic-smectic tricritical points

The metastable phase diagram of the two-component system heptyloxycyanobiphenyl (7OCB)+nonyloxycyanobiphenyl (9OCB) was determined by means of modulated differential scanning calorimetry (MDSC) and optical microscopy measurements. It was experimentally established that the 7OCB+9OCB two-component system exhibits a monotropic re-entrant nematic behavior. A complete quantitative thermodynamic analysis, through Oonk's equal G analysis, was performed, allowing for the calculation of the monotropic re-entrant behavior and the prediction of two tricritical points, one of them experimentally accessible for the SmAd-to-N transition and the other non-experimentally accessible for the RN-to-SmAd transition. From specific-heat measurements, latent heats were obtained for those mixtures displaying a first-order SmAd-to-N transition. Additionally, for some mixtures, the specific-heat critical exponents (alpha), through the second-order SmAd-to-N transition, were obtained. Both batches of data enable us to access to the experimental tricritical temperature for the SmAd-to-N transition.