Nuclear Magnetic Resonance Study of Liquid Crystal Transitions

Publisher Website
Google Scholar
Abstract
The spin‐lattice relaxation time (T1) and the transverse relaxation time (T2) are determined, by the spinecho nuclear magnetic resonance method, for the isotropic‐liquid state of the liquid‐crystal compounds p‐azoxyanisole (PAA), cholesteryl benzoate, cholesteryl n‐butyrate, and cholesteryl propionate. The T1 values are 1.2 sec for PAA, and 0.06 to 0.15 sec for the cholesteryl compounds. The T2 values are 0.02 to 0.03 sec for the cholesteryl compounds, and at least 0.05 sec for PAA. The results are consistent with those for other isotropic liquids. As the temperature decreases toward the transition to the anisotropic phase, a pretransitional decrease of T2 is observed for the cholesteryl compounds, and a similar decrease of T1 for PAA. An explanation in terms of Frenkel's theory of heterophase fluctuations is discussed. Thermal expansion curves and interphase surface tension for PAA are also considered, and hysteresis‐like behavior of the phase transition.