The Dependence of Volumetric Properties on the Molecular Composition of Nematic Liquid Crystals

Abstract

Density studies on various terminally polar and non-polar nematic liquid crystals, measured as a function of temperature, are reported. The molar volumes were fitted to model equations by a non-linear numerical least squares fit. These equations contain non-linear terms of the form c _I (T - T ^I _c)^βI for T > T ^I _c and - C _N (.T ^N _c - T)^βN for T < T ^N _c. It is shown that the coefficients C _N and c_I are correlated with the extrapolated volume discontinuity ΔV _c,99 at the nematic-isotropic phase transition temperature T _c. Furthermore, for all liquid crystals investigated there exists a correlation between ΔV _c.99 and the order parameter S _zz (t _r = 0.99) as determined by infrared dichroism. Our density measurements suggest that C _N and C _I depend on the molecular structure of the liquid crystal and are not universal for the nematic-isotropic phase transition. Assuming universal exponents β_N and β₁, we evaluate the average values β_N = 0.388 ⁺ ₋ ^0.096 _0.085 and β_I = 0.135⁺ ₋ ^0.065 _0.045.