A mössbauer study of anisotropic molecular diffusion in the supercooled nematic liquid crystalline phase

Abstract

The effect of the anisotropic diffusion of the Mössbauer probe 1,1'-diacetyl ferrocene (DAF) dissolved in the cold liquid crystals p-methoxybenzylidence- p-butylaniline and an eutectic mixture of cyano biphenyls and terphenyls, commercially known as E8, on the Mössbauer line broadening has been studied using 57Fe Mössbauer spectroscopy in the temperature range 80–300 K. The Mössbauer spectral lines begin to broaden as one approaches the glass transition temperature (∼199 K for MBBA and ∼ 175 K for E8). The anisotropy in the line broadening is due to the easier transport of the DAF molecules in the direction parallel to the liquid crystal molecular direction than in the perpendicular direction. The diffusion coefficients in the directions parallel ( D ⌈) and perpendicular ( D ⊥) to the liquid crystal molecular direction have been calculated assuming a continuous diffusion model. It has been found that D ⌈> D ⊥. The disappearance and subsequent reappearance of the Mössbauer effect above the glass transition temperature is shown to be due to the nematic liquid crystal glass converting itself into supercooled liquid and then the recrystallization of the supercooled liquid crystal. The anisotropic diffusive line broadening in the supercooled liquid crystalline phase is associated with the anisotropy of the Lamb-Mössbauer factor.