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~~~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 mol ', which decreases with increasing pressure, and an activation volume of about 60 cm' mol ', decreasing with increasing temperature. An activation energy in isochoric conditions was evaluated to be 37 kJmol ' at V =247 cm'mol ', 25 kJmol ' at 243 cm'mol '. A relation between the different activation parameters is discussed.PACS number(s): 61.30.Eb, 64.70.Md, 77.40.+i

Differential thermal analysis (DTA) and dielectric measurements have been performed on 2,2-dimethyl- 1-propanol (neopentanol) up to 200 MPa. Neopentanol exhibits at least one orientationally disordered (ODIC) phase (solid I) that transforms at lower temperatures to a non-plastic phase (solid II). There is evidence of a further ODIC phase denoted as solid I'. The pressure dependence of the phase transitions and the dielectric behaviour up to frequencies of 13 MHz are described. Activation enthalpies and volumes are derived from the dielectric relaxation time and compared with results for other alcohols

ConclusionsThe photochemical generation of peroxy radicals enables the direct determination of rate coefficients of the reaction of these radicals with alcohols.According to Howard, Schwalm and Ingold [ 3 ] secondary peroxy radicals are about 3-5 times more reactive in hydrogen abstraction reactions than tertiary ones. Comparing our results with those of Kroo et al.[lS], who investigdted the reactions of 2-cyano-2-propyl peroxy radicals with I-phenyl ethanol and benzyl alcohol as well, we obtain a factor o f 5 -5.5 at T = 55 C. Our values, however, seem to be somewhat higher than the rate coefficient of the reaction between a-hydroxyl-benzylperoxy radicals and benzyl alcohol measured by Howard and Korcek [8].The temperature dependence of the reaction between 1-phenylethyl peroxy radicals and 1-phenyl ethanol is in very good agreement with the data of Hajdu et al. Oxid. Commun. 6, 371 (1984).J. Chem. Kinet. 18, 31 (1986).( 1 988

The results of high pressure dielectric studies of 4-n-pentyl-4'-cyanobiphenyl (5CB) are analysed in terms of theories of the nematic state. The retardation factor g , , = t ,,/to and the effective, single-particle potential of mean torque were calculated at the nematic-isotropic transition temperature T,, and along the isothermal, isobaric and isochoric paths within the nematic phase of 5CB. The potential of mean torque is compared with the order parameter known for the same conditions. The values of parameter y relating the potential to the volume is discussed. IntroductionHigh pressure studies of thermotropic liquid crystals have been performed for many compounds with the aim of obtaining information about the (p, T ) phase diagram (see for example [l-31) or the order parameters at constant volume in the nematic phase [4-71. There is a lack of experimental data which could be related to other parameters characterizing the nematic state, for example the potential of mean torque. Such information can be deduced from studies of the dielectric relaxation process connected with the reorientation of molecules around their short axes. Recently we have performed high pressure dielectric studies of 4-n-pentyl-4-cyanobiphenyl (5CB). In Part I [8] the results obtained were analysed in terms of molecular associations (static permittivity) and molecular reorientation around the short axes (relaxation time t ,,) in the nematic phase of 5CB. It was concluded that relatively low pressure markedly influences the monomer-dimer equilibrium involving the breaking of the antiparallel dipoledipole correlations of molecules.Here we discuss the results obtained in terms of theories of the nematic state [9-181. Many of them were worked out within the molecular field approximation. We shall use the notions introduced in the Maier-Saupe theory [9] as it has been extended to explain the dielectric properties of the nematic phase [19-241. We realize, of course, that this theory, as well as any other molecular field theories, should not be used for quantitative comparisons with the experiments. Nevertheless, it seems worthwhile to do that in order to check the real discrepancies between the predictions of the theory