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2 Polish Journal of Chemistry

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Retardation of Molecular Rotations Around the Short Axes at the Transition from the Isotropic to Different Liquid Crystalline Phases

Urban S., Gestblom B., Dąbrowski R.

Polish Journal of Chemistry

2002

Vol. 76 / nr 2-3

263-271

Results of the dielectric relaxation studies of several homologous series and some single compounds exhibiting different phase sequences (isotropic - nematic, isotropic - smectic A, C, B and E) are presented. Relaxation times characterizing the molecular rotations around the short axes in the isotropic and a given liquid crystalline (LC) phases were determined. Two quantities, the so-called retardation factor gLC-Is = _LC/_Is and the activation enthalpy are analyzed. In spite of drastic changes of the g-factors in the case of the isotropic - crystal-like smectic phase transitions (Sm B and E) in comparison with the isotropic - liquid-like phases (N, Sm A, Sm C) transitions the activation enthalpy does not depend practically on the LC phase. This will be discussed as resulting from the coupling of the rotational and translational motions.

Volumetric and dielectric studies on 4-n-pentyl-4'-cyanobiphenyl (5CB) under high pressure

Urban S., Wurflinger A., Busing D., Bruckert T., Sandmann M., Gestblom B.

Polish Journal of Chemistry

1998

Vol. 72, nr 2

241-250

The results of measurements of the specific volume Vm versus temperature T and pressurep are presented together with the new results of high pressure dielectric studies of 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic and isotropic phase. The knowledge of the equation of state enables us to compare changes under isothermal, isobaric, and isochoric conditions within the nematic phase of 5CB. The data on the pressure dependence of the nematic-isotropic phase transition TNI and corresponding molar volume VNI allow us to determine the exponent gamma, characterizing the volume dependence of the molecular field potential. The relationships between: (i) the dielectric anisotropy and the order parameter, (ii) the longitudinal relaxation time and the retardation factor, (iii) the nematic potential and the order parameter, (iv) the variations of the activation parameters with p, V, and T, are re-analyzed. The energy barrierthat opposes the molecular rotation around the short axis in the nematic phase of 5CB can be calculated from the temperature dependence of the relation time. Comparison of the isochoric and isobaric activation energies shows that about 50% of the molecular rotation is affected by volume effects.