VOLUMETRIC AND DIELECTRIC STUDIES ON 4-N-PENTYL-4'-CYANOBIPHENYL (5CB) UNDER HIGH-PRESSURE

The results of measurements of the specific volume V-m versus temperat ure T and pressure p are presented together with the new results of hi gh pressure dielectric studies of 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic and isotropic phase. The knowledge of the equation of sta te enables us to compare changes under isothermal, isobaric, and isoch oric conditions within the nematic phase of 5CB. The data on the press ure dependence of the nematic-isotropic phase transition T-NI and corr esponding molar volume V-NI 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 orde r 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-analy zed. The energy barrier that opposes the molecular rotation around the short axis in the nematic phase of 5CB can be calculated from the tem perature dependence of the relaxation time. Comparison of the isochori c and isobaric activation energies shows that about 50% of the molecul ar rotation is affected by volume effects.