Electro-optical and dielectric characterizations of the Goldstone mode relaxation in ferroelectric chiral smectic C liquid crystals.

We report structural (helical pitch), electro-optical (tilt angle and spontaneous polarization) and dielectric (Goldstone mode) investigations of ferroelectric liquid crystals (FLCs) exhibiting the chiral smectic C phase (SmC(*)). All these characterizations were performed on two pure FLCs showing the SmC(*)-SmA-N(*) phase sequence and having small pitch, high spontaneous polarization and a large relaxation frequency. We have determined the Goldstone rotational viscosity and the twist elastic constant in the SmC(*) phase from the helical pitch, tilt angle, polarization, dielectric strength experimental data and from the relaxation frequency of the Goldstone-mode relaxation.

Correlation between dielectric and optical measurements in the smectic-C(*)(alpha) phase.

We present optical and dielectric measurements in the smectic-C(*)(alpha) (SmC(*)(alpha)) phase of three homologues of an alkoxy benzoate series. Two different behaviors are observed depending on the values and the temperature evolution of the azimuthal angle difference alpha between two adjacent layers. For moderate values of alpha, the Goldstone mode is predominant over the whole SmC(*)(alpha) phase.

Experimental observation of multicritical behaviors in pressure-temperature diagrams of pure compounds with twist grain boundary phases.

In order to study critical behaviors of pure compounds anticipated by Renn and Lubensky's theoretical calculations, high-pressure experiments have been performed on six tolan series homologous to the 3-fluoro-4-[(R) or (S)-methylheptyloxy]-4'-(4"-alkoxy-2",3"-difluorobenzoyloxy), which exhibit the twist grain boundary (TGB(A) and TGB(C)) mesophases. Measurements were carried out by thermobarometric analysis. Six pressure-temperature phase diagrams are determined.

Dielectric properties of twist grain boundary phases: influence of the anchoring and the distance between grain boundaries.

The dielectric properties of the twist grain boundaries TGB(A) and TGB(C) of liquid crystal phases differ from the smectic-A and smectic-C phase ones: a theoretical model confirmed by experimental results shows that the Goldstone mode of the TGB(C) phase and the soft mode of the TGB(A) phase are strongly reduced. This behavior is due to elastic strain, which is connected to two parameters: the anchoring at the grain boundaries and the distance between the grain boundaries. It is shown quantitatively that a relatively flexible anchoring in the TGB(A) phase becomes rigid in the TGB(C) one.

Multicritical behaviors and an induced twist grain boundary phase in a binary liquid crystalline mixture.

We report here the composition-temperature phase diagram of a mixture between n=9 and n=16 terms of a chiral tolans series shortly noted nF(2)BTFO1M7. This diagram constitutes an experimental illustration of one of the three theoretical phase diagrams predicted by the Renn and Lubensky's model. The pure compounds n=9 and n=16 exhibit, respectively, the phase sequences Cry-Sm-C*-Sm-A-TGB(A)-N*-BP-I and Cry-Sm-C*-N*-BP-I, where TGB refers to a twist grain boundary phase.