Static and dynamic dielectric properties of strongly polar liquids in the vicinity of first order and weakly first order phase transitions

Abstract

The paper presents the results of measurements of the linear dielectric properties of the compounds from the homologous series of alkylcyanobiphenyls (C(n)H(2n+1)PhPhCN, nCB) in the vicinity of the first order transition (from the isotropic liquid to the crystalline phase) of nonmesogenic nCB's (n=2-4) and the weakly first order transition (from the isotropic liquid to the nematic phase) of 5CB. The experimental method for the separation of the critical part of the static permittivity derivative and the activation energy for rotation of the mesogenic molecules, in the vicinity of weakly first order phase transition, is proposed. It is shown that the critical temperature dependence of the permittivity and the activation energy can be described with a function of (T-T*)(-alpha) type, with the same values of the temperature of virtual transition of the second order (T*) and the critical exponent (alpha).

Anomalous Dielectric Relaxation in Binary Mixtures of Mesogenic Solvent/Non-Mesogenic Solute

“…Then, the value of the activation energy cannot be determined in a simple way as the slope of ln τ (T ) on T −1 dependence. For such situations, we have recently proposed [39] the procedure of the activation energy E A determination by calculating the (15), that derivative leads directly to the activation energy at a given temperature, so allowing one to analyze how E A evolves with T . As we discussed in previous papers [39,40], the activation energy should be, in general, temperature dependent and the tenet of its constancy has no physical basis.…”

“…For such situations, we have recently proposed [39] the procedure of the activation energy E A determination by calculating the (15), that derivative leads directly to the activation energy at a given temperature, so allowing one to analyze how E A evolves with T . As we discussed in previous papers [39,40], the activation energy should be, in general, temperature dependent and the tenet of its constancy has no physical basis. Of course, the proposed procedure of the determination of E A (T ) dependence requires very accurate values of the relaxation time obtained for an appropriate step of the temperature, particularly in the vicinity of the phase transition.…”

Static dielectric pretransitional effects in thermotropic liquid crystals

“…The dielectric permittivity shows no pretransitional effects in vicinity of the first order transition from the isotropic liquids to the crystalline (I-Cr) phase [4]. As shown in Fig.…”

“…The effect is much more pronounced in the permittivity derivative [ Fig. 2(b)], which shows a critical-like behaviour as temperature approaches to the IN phase transition [4]. It is worthy to notice that several degrees before the transition to the nematic phase, the permittivity derivative of 5CB changes its sign to the positive one.…”

“…The prenematic effects which appear in the linear dielectric properties as a peculiar temperature behaviour of the static permittivity [2][3][4][5][6] and the activation energy for molecular rotation derived from the dielectric relaxation time [4,5] or as the anomalously slowing down rotational diffusion [7], manifest themselves especially in the case of strongly polar mesogenic liquids. This paper presents a review of recent experimental results on the static dielectric behaviour of the mesogenic compounds of different polarity in the vicinity of the I-N phase transition.…”