Results of dielectric studies under elevated pressure of selected liquid crystalline materials are reviewed. The tensor components of the static permittivity of two nematics, 60CB and 8PCH, measured at p = 1 atm as a function of temperature, and at T = constant as a function of external pressure, are compared. They show striking similarities, if the ranges of the nematic phase at the isobaric and isothermal conditions are normalized. The Maier and Meier equations are applied for discussing the observed behaviors. The dielectric relaxation processes in many substances exhibiting the nematic, smectic A, smectic C and smectic (crystalline) E phases were studied in the past in our group. The low frequency relaxation times characterizing the molecular rotations around the short axes were determined at the isothermal, isobaric and isochoric conditions. This allowed us to calculate three activation parameters: activation volume, activation enthalpy and activation energy, which are interrelated, In particular, it was found that the isobaric activation enthalpy is twice as large as the isochoric activation energy, independently of the phase studied. This indicates the role of steric constraints against the molecular rotations around the short axes.
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