X-ray diffraction, optical birefringence, dielectric and phase transition properties of the long homologous series of nematogens 4-(trans -4′- n -alkylcyclohexyl) isothiocyanatobenzenes

Abstract

The temperature dependence of density, refractive indices, apparent molecular length and intermolecular distance of 11CHBT, are reported, together with the static dielectric permittivities, ε∥, ε┴, ε _iso of 11CHBT and 12CHBT; the effect of chain length on various physical properties of the CHBT series is also discussed. All the temperatures T _m, T _NI and T _NCr show an odd-even (OE) effect, but transition enthalpies ΔH _m and ΔH _c show no such behaviour. T _m, T _NI and T _NCr and ΔH _m are found to increase with chain length, while ΔH _c shows a maxima at n = 6. At the common reduced temperature T* = 0.98 T_NI, the density of the compounds are found to decrease with increasing chain length while the polarizability parameters α_e, α_o and α increase. However, the volume expansion coefficient, polarizability anisotropy and normalized polarizability anisotropy show a distinct OE effect. Similarly, n _e decreases smoothly with chain length but n _o and Δn show an OE effect like ε┴ and Δε. The ratio ln ⟨P ₂⟩ /ln ⟨ P ₄⟩ remains closer to Faber theory than MS theory. Effective dipole moments are found to be approximately constant in the series; antiparallel association is observed in all the members. Kirkwood correlation factors g┴ and Δg are found to exhibit an OE effect like ε∥ and ε∥, but g∥ and ε∥ show no such effect. The effects of substitution in the core and end polar group on these properties are also discussed.