Dielectric studies of liquid crystals under high pressure II. Low frequency relaxation process in 4-n-pentyl-4′-cyanobiphenyl in relation to theories of the nematic state

Abstract

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

Pressure Dependence of the Dielectric Anisotropy of 5CB and 6CB

“…Because of this effects, together with a systematic decrease with pressure of the activation parameters characterizing the molecular reorientations around the short axes, it was concluded that a relatively low pressure breaks the dipole-dipole correlations in the nematic phase of 5CB. This statement was then sup ported by further studies of nCB compounds [5,[7][8][9].…”

“…Former h.p. dielectric measure ments of four nCB homologues (n = 5 -h 8) have shown that the pressure distinctly influences the static dielec tric constant e0 M [4,5,[7][8][9]. On the other hand, the effect was considerably weaker for a similar substance, 6CHBT, in which such correlations are relatively weak [6].…”

“…A few years ago, a systematic h.p. studies of LCs, especially on regards the phase transitions and dielectric relaxation, have been undertaken at the University of Bochum [3][4][5][6][7][8][9][10]. The high pressure chamber used, however, allowed for the measurement of the parallel component of the dielectric permittivity only.…”

Dielectric studies of liquid crystals under high pressure. IV. Static permittivity and low frequency relaxation process in 4-n-hexyl-4′-cyanobiphenyl (6CB)

“…of 4-n-pentyl-4-cyanobiphenyl(5CB) [ 1,2] and 4-(trans-4-nhexylcyclohexy1)isothiocyanatobenzene (6CHBT) [3]. Both compounds have similar molecular dimensions and possess strong dipole moments directed roughly along the main molecular axes.…”

Nematic Potential and Order Parameter Determined from Dielectric Measurements

“…for some nCBs [2][3][4] and 7PCH [5], but the authors point out that many corrections are necessary, partially due to experimental flaws [3,4]. In spite of these shortcomings we have used the S data for the analysis of the high-pressure behaviour of the nematic potential q (calculated from the retardation factors) under various conditions [1,6,7], In particular, we examined the proportionality between q and S, which is an important assumption in the Maier-Saupe theory [8]. In this analysis both quantities, q and S, were taken from different experiments.…”