Influence of a Smectic Phase on Thermodynamical Behaviour of the Nematic to Isotropic Phase Transition

Abstract

From an experimental point of view it does not seem to exist a straightforward relationship between the transition entropy ΔS_NI and the orientational order parameter discontinuity Δη_NI at the nematic-isotropic transition even if, for example, the even-odd effect simultaneously arises on the transition temperature and on these both properties. Although a simple Landau theory (1) predicts a ΔS_NI ~ (Δη)² _NI variation, this proportionality failed in several cases and these experimental discrepancies lead to put forward different explanations:

• recently, within nematic homologous series F. Leenhouts et al. (2) have shown that with long aliphatic chains the difference in population of chain conformations between the nematic and isotropic phase enhances the nematic-isotropic transition entropy without influence on the order parameter η.

• besides this fact, within series exhibiting a nematic-smectic polymorphism ΔS_NI increases anomalously when appears at lower temperature a smectic phase (3,4). The existence of smectic short-range ordering contributes to the N-I transition entropy and does not disturb in the same extent the orientational order (5,6).