A Mössbauer temperature study of a cold nematic liquid crystal: Nematic glass–supercooled nematic

Abstract

A 0.2% (by weight) solution of 1,1′‐diacetylferrocene (DAF) in n‐ (p‐methoxybenzylidene) ‐p‐butylaniline (MBBA) was prepared and studied with the Fe‐57 Mössbauer effect. The Mössbauer parameters were observed for the temperature range 110–300 K. The recoil‐free fraction (f) data showed that the MBBA crystal phase exhibited Debye behavior for the entire temperature range with a Mössbauer–Debye temperature (ϑ_L) of 70 K. The lower temperature data for the cold nematic also gave a linear lnf vs T plot with ϑ_L=47 K for the MBBA glass. The ϑ_L for the nematic glass was indepentent of molecular orientation. The nematic glass–supercooled nematic transition was determined to be T_g=181 K from the deviation of the lnf vs T curve from linear behavior. The nematic glass provided a nonvanishing temperature independent contribution to the Fe‐57 vibrational ellipsoil (ε_L). Above 180 K ε_L became temperature dependent. Line broadening and a reduction of the quadrupole splitting were observed for the supercooled nematic with increasing temperature in the range 220–240 K. We interpret these results in terms of a model for rotational diffusion of the DAF molecule in the MBBA supercooled nematic. The rotational diffusion appears to occur without simultaneous translational diffusion.