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 (ϑL) of 70 K. The lower temperature data for the cold nematic also gave a linear lnf vs T plot with ϑL=47 K for the MBBA glass. The ϑL for the nematic glass was indepentent of molecular orientation. The nematic glass–supercooled nematic transition was determined to be Tg=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 (εL). Above 180 K εL 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.