Dielectric studies of liquid crystals under high pressure: Static permittivity and dielectric relaxation in the nematic phase of pentylcyanobiphenyl (5CB)

Dielectric studies of a thermotropic-liquid-crystalline substance [pentylcyanobiphenyl (5CB)] under high pressure are reported. Measurements of a complex dielectric permittivity were carried out in the pressure range 0.1--160 MPa, the frequency range 1 kHz--13 MHz, and the temperature range 287--343 K. Static permittivities measured at constant pressures are governed by the [2S(T)+1]/T factor. An increase in permittivity with pressure at a constant temperature is interpreted as a result of an increasing molecular ordering and of a breaking of the dipole-dipole correlations between neighboring molecules. The relaxation time ${\mathrm{\ensuremath{\tau}}}_{\mathrm{\ensuremath{\parallel}}}$ characterizing molecular rotations around the short axes in the nematic phase is analyzed as a function of temperature and pressure, yielding an activation enthalpy of approximately 60 kJ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$, which decreases with increasing pressure, and an activation volume of about 60 ${\mathrm{cm}}^{3}$ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$, decreasing with increasing temperature. An activation energy in isochoric conditions was evaluated to be 37 kJ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$ at ${\mathit{V}}_{\mathit{m}}$=247 ${\mathrm{cm}}^{3}$ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$, 25 kJ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$ at 243 ${\mathrm{cm}}^{3}$ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$. A relation between the different activation parameters is discussed.