Dielectric studies of liquid crystals under high pressure: Static permittivity and dielectric relaxation in the nematic phase of pentylcyanobiphenyl (5CB)

Abstract

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

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

“…The factor (2 S(p) + 1)T, normalized to the permittivity at high pressure, differed much from that at low pres sures. Especially close to the N-I transition the per mittivity decreased more strongly than according to (4). 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.…”

“…dielectric studies of 5CB [4], (1) was used for the analysis of the relationship between the measured permittivities (which were obtained for an unperfectly oriented sample, however) and the or der parameter known form optical measurements [4,13]. Assuming other quantities to be constant and neglecting the induced polarization, the approximate relation eoii -1 oc (2S + 1)/F (4) was discussed.…”

“…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.…”

“…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].…”

“…In [4] we have discussed the change of the static dielectric constant e0 y with increasing pressure at con stant temperature in the nematic phase of 5CB using the well known Maier-Meier equations [11] relating the principal permittivity components with the dipole moment of the LC molecule and the order parameter S. However, some simplifications had to be done when theoretical predictions were confronted with the ex perimental results. The use of the dielectric an isotropy, Ae = e01 | -£01, makes such a confrontation much more straightforward.…”