Successive Excitation of Molecular Motion in a Nematic Liquid Crystalline Compound, HBAB, as Studied by Nuclear Magnetic Resonance

Abstract

Successive excitation of molecular motion was studied, by proton NMR lineshape and relaxation measurements, in relation to the phase transitions of a nematic liquid crystalline compound, p-n-hexyloxybenzylideneamino- p'-benzonitrile (HBAB). The dynamical nature of the intercrystalline phase transition was investigated and the higher-temperature crystalline phase was revealed to be a motionally disordered phase with respect to conformation of the alkoxy chain. The difference between this phase and the general smectic phase in their motional aspects is pointed out. The second rank orientational order parameter in the nematic phase near T _c exhibits a steep descent toward a small critical value of 0.28 at T _c which cannot be explained by Maier-Saupe's mean field theory nor its modifications. A comparison of the behavior of p-n-hexyloxybenzylidene- p'-toluidine (HBT) with HBAB shows that the effect of the terminal substituent on the nematic order parameter is important. Molecular dynamics in the nematic phase is characterized by translational self-diffusion in the low-temperature region and by fluctuation of the order director near T _c. A strong correlation between the self-diffusion and a large amplitude rotational motion is suggested in the isotropic liquid phase.