Interaction Potential in Nematogenic 6CHBT

Abstract

Pressure-volume-temperature (PVT) measurements were carried out on the nematic liquid crystal 4(trans-4′-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT). In combination with previous dielectric relaxation measurements at elevated pressure and new measurements extending to GHz frequencies, the characteristics of the anisotropic interaction potential were determined. The thermodynamic potential parameter, Γ, which measures the variation of the interaction energy with volume, equals 5.03 ( 0.06, with a barrier height equal to ∼7 kJ/mol. Thus, there is a low potential barrier to reorientations of the 6CHBT molecule about its short axis; however, the retarding potential is strongly volume-dependent. The longitudinal reorientational times determined for various thermodynamic conditions superpose using a scaling exponent equal to Γ within the experimental error. It then follows, as found recently for other liquid crystals, that this relaxation time must be constant along the pressure-dependent clearing line. Thus, the control parameter (e.g., Gibbs free energy) governing the competition between the anisotropic energy and the entropy must also govern the rotational dynamics in the ordered phase of this liquid crystal.

Volumetric, dielectric, calorimetric and X-ray studies of smectogenic 10PBO8 at atmospheric and elevated pressures

“…This is close to the reported for other smectogens, whereas the N-Iso transition is usually associated with larger values of (see Table 1). For comparison, included in Table 1 are the potential parameter γ derived from thermodynamic scaling of the longitudinal relaxation times [2,16,17,19,20,[35][36][37][38].…”

Low frequency relaxation in liquid crystals in relation to structural relaxation in glass-formers

“…Fig. 5 shows examples of the volume-temperature plots at constant τ || and constant P for two LC substances in the nematic phase: pentyl-cyanobiphenyl (5CB) [22] and 4(trans-4′-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT) [23]. The obtained expansivity coefficients are listed in Table 1, together with the data for other LCs.…”

Volumetric study ofn-octyloxy-cyanobiphenyl (8OCB)

“…The former is considerably larger, as found for other LC materials [2,[5][6][7][8][9][10][11][12]24,25]. Figure 6 compares the two quantities for the N phase of 8OCB and 8CB [26].…”

“…It has been shown that the scaling relation, Equation (2), originally proposed for glass-forming liquids and polymers ( [23,29] and references therein), is applicable to the longitudinal relaxation times in various types of LCs [5][6][7][8][9][10][11][12]. As shown in Figure 8 (lower curve), the values of τ in Figures 4 and 5 superpose to form a master curve when plotted versus the scaling variable TV γ , with the value γ = 4.2 the same as determined for 8CB [8].…”

“…Email: ufurban@cyf-kr.edu.pl of the interaction parameter γ characterising the steepness of the intermolecular potential [5][6][7][8][9][10][11][12]. The possible equivalence of the two potential parameters, Γ and γ , was discussed in refs.…”