Diffusion in thermotropic liquid crystals

Abstract

Diffusion coefficients in mesophases of thermotropic liquid crystals have been measured to a considerable extent only during the last twelve years. Some theories on diffusion in mesophases have been developed too. Measurements in nematic, smectic A, smectic B, and smectic C mesophases have been carried out by mass transport techniques, detected by radio-tracers or optically, by NMR spin-echo techniques, and by quasi-elastic neutron scattering. The diffusion is anisotropic in most of the cases. In nematics self-diffusion parallel to the molecular director ( D ‖) is somewhat faster than perpendicular ( D ⊥). Both diffusion coefficients show about the same activation energy. Impurity diffusion (small molecules dissolved in mesophases) demonstrates the same behaviour with less anisotropy. In smectics A and C the diffusion coefficients D ‖ and D ⊥ can be nearly equal. The anisotropy is now shown in their respective activation energies with E ‖> E ⊥. This effect is more pronounced in impurity than in self-diffusion. In addition impurity diffusion shows a strong anisotropy of the diffusion coefficients ( D ⊥⪢ D ‖). In smectics B the activation energies E ‖ and E ⊥ of self-diffusion seem again to be equal and the diffusion coefficients show a small anisotropy ( D ⊥≥ D ‖).