Discotic liquid crystals Physical parameters of some 2,3,7,8,12,13-hexa(alkanoyloxy)truxenes. Observation of a re-entrant isotropic phase in a pure disc-like mesogen

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Abstract
The synthesis of a new discotic mesogen, 2,3,7,8,12,13-hexa(octadecanoyloxy)-truxene is reported. This new compound has been studied using polarization microscopy, differential scanning calorimetry, small angle X-ray scattering and adiabatic calorimetry. The results are compared with those of two other truxene derivatives from the same series: 2,3,7,8,12,13-hexa(decanoyloxy)truxene and 2,3,7,8,12,13-hexa(tetradecanoyloxy)truxene. Refractive index measurements have been performed on these three truxene derivatives as a function of temperature. A re-entrant isotropic phase has been observed in 2,3,7,8,12,13-hexa(octadecanoyloxy)truxene. This is the first report of such a phase in a pure mesogen. We also report measurements of the ratio of the Frank constants for splay and bend in the nematic regime of this new mesogen as a function of temperature using the magnetic Frederiks transition technique.

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