Differential Thermal Analysis (DTA) and Dielectric Studies of 4 -(trans-4-Heptyl-Cyclohexyl)-benzonitrile (7PCH) under High Pressure

Abstract

With the aid of DTA the phase diagram of 4-(trans-4-heptyl-cyclohexyl)-benzonitrile, 7PCH, was redetermined. Dielectric studies on the static and complex permittivity have been performed on 7PCH in the pressure range 0.1 -220 MPa, the frequency range 1 kHz-13 MHz and the temperature range 290-380 K. For the slow relaxation process in the nematic phase, characterized by the relaxa­tion time tII , we have calculated the activation volume, Δ ≠VII, of about 80 to 60 cm3/mol (decreasing with increasing temperature), and the activation enthalpy, Δ ≠HII, of approximately 70 kJ/mol. The isochoric activation energy, Δ ≠UII, amounts to about 36 kJ/mol. The pressure dependence of Δ ≠HII is less pronounced compared with previous findings for 5PCH and 7CB. Estimates of t0 (extrapo­lated from the relaxation time in the isotropic phase) allow the calculation of the retardation factor gII = tII/t0 that is analysed in terms of theories of Martin-Meier-Saupe and Coffey in order to evalu­ate the nematic potential q. Using data for the order parameter 5 from NMR studies it was possible to determine the strength of the interaction potential in the nematic phase and to check the relation q~S which is valid at p = const., but not at V = const. The results are compared with recent findings for other homologous series.

Dielectric Behaviour of 1-(4-Isothiocyanatophenyl)-2-(4-hexyl- bicyclo[2,2,2]octane-1)ethane in Nematic and Isotropic Phases

“…The knowledge of their dielectric properties is substantial on the one hand for technical reasons, because in the electrooptical liquid crystal displaydevices the threshold voltage, the multiplexability and other important parameters depend on the anisotropy of the electric permittivity Ae = £| ( -e x , on the other hand because dielectric measurements in static fields provide information about the relation between the dielectric anisotropy of the mesogenic substance and its molecular structure, the dipole-dipole correlation and also the symmetry of both the molecule and the liquid crystalline phase [1]. The dynamic behaviour of the molecules is investigated through measurements of the complex electric permittivity, e* -e' -i e", as a function of frequency, temperature [1] and pressure [2][3][4][5]. These studies are a valuable source of information about the molecular movements in both the mesomorphic and isotropic phases.…”

Activation volume of selected liquid crystals in the density scaling regime

“…For this reason, the dielectric relaxation spectra yield solely the low frequency relaxation time connected with the flip-flop molecular motions in a given LC phase (N for 7CB, 7PCH, 8PCH and 8OCB, Cr E for 6BT and 8BT). It should be added that the compounds with the CN group show a tendency to forming antiparallel doublets in the LC phases 50 The experimental dielectric and pVT data examined by us herein have been earlier measured and reported for the nematic phase of 7CB 51 52 , 7PCH 53 54 , 8PCH 55 56 , and 8OCB 18 57 as well as the crystalline E phase of 6BT 58 59 and 8BT 60 61 .…”

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

“…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.…”