Diffusion Process of a Liquid Crystal Probed by the Transient Grating Method

Translational diffusion of MBBA (N-(4-methoxybenzylidene)-4-n-butylaniline) in the nematic and isotropic phases is investigated by the transient grating (TG) method. The origin of the TG signal due to the population grating is attributed to the spatial variation of the order parameter and, as a result, the time dependence of the signal reflects the diffusion of the cis form of MBBA, not of the trans form. In the nematic phase, the diffusion constant parallel to the director orientation, D||, is always larger than the perpendicular one, D⊥. The obtained values are 2.2—2.5 times larger than previously reported ones measured by using a photosensitive probe molecule in the liquid crystal. The anomaly previously reported near the nematic–isotropic phase transition temperature is not observed, and the diffusion constant in the isotropic phase is close to the average value of the both components, 1/3(D + 2D⊥). The experimental values are also compared with the ones calculated from two theoretical expressions by Franklin, and Chu and Moroi.