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

Abstract

Dielectric studies of 4-n-hexyl-4-cyanobiphenyl (6CB) were performed in the pressure range 0.1-150 MPa, the frequency range 1 kHz-13 MHz and the temperature range 290-327K. The temperature and pressure dependencies of the static permittivity E~, , and the relaxation time 7,, are analysed and compared with analogous data obtained recently for 5CB and 6CHBT (parts 1-111 of this series). It was shown that in the nematic phases of both 4-n-alkyl-4'-cyanobiphenyls the dipole-dipole correlations are broken by relatively low pressure. IntroductionIn previous parts of this series, we have presented the results of dielectric studies under high pressure (h.p.) 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. On the other hand, they are representatives of two homologous series which show considerably different degrees of molecular association in the nematic phase. It is well established that the alkylcyanobiphenyl (nCB) molecules exhibit strong dipole-dipole correlations in the liquid crystalline phases ([I 11 and references therein), whereas in the isothiocyanatobenzene (nCHBT) compounds, such correlations either do not exist or are very weak ([3] and references therein).These different behaviours of molecules are reflected in the dielectric properties at atmospheric as well as at elevated pressures. The static permittivity of 5CB exhibits markedly stronger dependence on pressure (at T= constant) than in the case of 6CHBT [3]. In addition, activation parameters characterizing the molecular reorientations around the short axes in the nematic phase of 5CB (activation enthalpy, energy and volume) decrease with increasing pressure [ 11, whereas they are practically independent of pressure in the case of 6CHBT [3]. These features of the dielectric behaviour allowed us to conclude that relatively low pressure causes breaking of the dipole-dipole correlations in the nematic phase of 5CB.The aim of the present study was to check whether the above mentioned phenomena can also be observed in the next member of the nCB homologous series,

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.…”