NMR Study of Orientational Order in the 7OCB Nematic Liquid Crystal

Abstract

Liquid crystals are anisotropic materials whose properties vary with the degree of alignment along a common direction known as a director. The nematic phase with the lowest ordering of all the liquid crystal phases is characterized by the state in which the molecules have no positional order, but tend to align along the director. Thus, the orientational order-parameter is an important quantity for determining the capability of a nematic liquid crystal because it is a measure of the tendency of molecules to point along the director. The nematic liquid crystal 4-heptyloxyl-4’cyanobipheny (7OCB) undergoes a first-order phase transition from an isotropic liquid (iso) phase to a nematic (N) phase at 347 K [1, 2]. The orientational ordering and the nematic phase transition of 7OCB were studied by several experiments [1–6]. In a measurement of the dielectric permittivity, Rzoska et al. determined the critical exponent β for the orientational order-parameter to be 0.25 from the temperature dependence of the dielectric strength, indicating a tricritical behavior of the nematic phase transition [1]. In addition, the other permittivity quantities were found to show changed characteristics for this behavior around the nematic phase transition. The aim of the present work is to examine such behavior of the nematic phase transition in 7OCB. Therefore, we analyzed the chemical shift of C NMR spectra in the N phase and determined the temperature dependence of the orientational order-parameter to examine the nematic phase transition. In this paper, obtained results were reported along with the procedure for the spectral analysis.