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The results of acoustic investigations of the viscous properties of a nematic liquid crystal mixture with wide temperature range at varying PVT-state parameters are presented. The original ultrasonic set-up was elaborated to provide the determination of some dissipative parameters including the ratio of twist viscosity c 1 to magnetic susceptibility anisotropy Dv as a function of pressure (P) and temperature (T) at constant volume ( V ). It is found that the ratio c 1 =Dv exponentially decreases with increasing temperature or with decreasing pressure. This agrees with the free volume theory. According to our calculation free volume increases with temperature or decreasing pressure. The energies of activation (E v , E p ) of the twist viscosity at constant volume and pressure are calculated. The dependence of Leslie viscosity coefficient a 1 on temperature is determined too.

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The relaxation properties of nematic liquid crystals by the pulsed method of a fixed distance in a rotating magnetic field are investigated. The adequacy of the solution of the equation of motion of the nematic director in the conditions of asynchronous motion of the director and the magnetic field induction vector was experimentally confirmed. The values of the rotational viscosity coefficient and the orientation relaxation time that characterize the parameters of devices with a liquid-crystal working medium in a wide range of pressures and temperatures are calculated. In the framework of the free volume theory, the dependence of the rotational viscosity coefficient on temperature and pressure is analyzed.