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The physical behaviour, especially the dielectric properties, of two series of isomeric compounds formed by banana-shaped molecules is reported. The feature of all liquid crystalline substances investigated is the formation of a ferroelectric short range order. In one layer of the SmCPA phase the spontaneous polarisation shows in one direction and in the following in the opposite resulting in an antiferroelectric behaviour. It is demonstrated that the molecular origin of the polarization is connected with the perpendicular dipole moments. From the two relaxation modes seen the first results from ferroelectric units and the second from the reorientation of the molecules about the long axes. The collective character of both motions is demonstrated. The same mechanisms appear in the columnar phase, but they behave in a different way due to the different structure. Strong changes of the dielectric properties are observed at the transition from the isotropic in the liquid crystalline phase, but astonishingly not between both mesophases. [ABSTRACT FROM AUTHOR]

A new series of five-ring banana-shaped mesogens derived from isophthalaldehyde was synthesized and investigated. Some of the compounds exhibit a B 2 -B 4 dimorphism. The mesophases were characterized alternatively by optical methods, X-ray studies, and dielectric and electro-optical measurements. [ABSTRACT FROM AUTHOR]

Dielectric measurements on a sample consisting of banana-shaped molecules were carried out in a frequency range between 0.1 Hz and 10 MHz. The sample exhibited B[sub 2] and B[sub 4] phases. As usual, two ranges of relaxation were detected in the B[sub 2] phase, the fast reorientation about the long axes of the molecules and a slow collective process. In the B[sub 4] phase, only one dielectric active process at low frequencies was found; this does not differ from the low frequency relaxation of the B[sub 2] modification. This relaxation is probably related to the dynamics of superstructures. Crystallization could be observed after keeping the sample for a longer time at higher temperature. Thus, it was possible to differentiate clearly between the crystalline and the B[sub 4] phases. AFM investigations prove the existence of focal-conic domains and periodic superstructures in the B[sub 4] phase; then do not appear in the crystalline state. For this reason the B[sub 4] phase is regarded as different from a classical crystalline phase. [ABSTRACT FROM AUTHOR]

Dielectric measurements on the antiferroelectric smectic C (SmCPA) and isotropic (I) phases of a substance formed by bent-shaped molecules were performed in the frequency range from 0.1 Hz to 0.1 GHz. Three absorption ranges, corresponding to a slow collective process, the reorientation of the molecular core around the long axes and the fast dynamics of the terminal groups, were detected in the SmCPA phase, whereas only two ranges, related to the two last processes, appear in the isotropic state. At the I-SmCPA phase transition the process of molecular reorientation around the long axes changes stepwise with respect to the absorption band intensity and the relaxation time, while the activation energy remains practically unchanged. The activation energy for the dynamics of the terminal groups increases at the I-SmCPA transition. [ABSTRACT FROM AUTHOR]