Nuclear magnetic relaxation of nematic liquid crystals, N-(4-methoxybenzylidene)-4′- n-butylaniline and N-(2-hydroxy-4-methoxybenzylidene)-4′- n-butylaniline

Abstract

Nuclear magnetic resonance linewidth and spin-lattice relaxation time T 1 were measured for 4-methoxybenzylidene-4′- n-butylaniline (MBBA) and N-(2-hydroxy-4-methoxybenzylidene)-4′- n-butylaniline )OHMBBA) in their liquid, nematic, undercooled nematic, and glassy nematic liquid crystal states as well as in the stable crystalline states. In the case of OHMBBA, the motional narrowing occurs in three steps corresponding to excitation of end-group reorientations, self-diffusion, and molecular tumbling. T 1 of OHMBBA in the nematic and undercooled nematic state has two minima at 264 K (24 ± 4 msec) and at 140 K (22.0 ± 0.5 msec); the motions responsible for the minima are the self-diffusion and the end-group reorientations with the activation energies 29 ± 1 J mole −1 and 5.8 ± 0.2 kJ mole −1 respectively. Analogous results were obtained for MBBA. Some measurements were also made for p-azoxyanisole for the sake of comparison. The effect of external magnetic field on the formation of the glassy state was examined.