A Mössbauer temperature study of a cold nematic liquid crystal: Nematic glass-supercooled nematicWe have used the Fe-57 Mossbauer effect (ME) to study solutions (0.2% by weight) of 1,1'diacetylferrocene (DAF) in two liquid crystalline materials, 4-n -butoxybenzylidene-4' on' -octylaniline (BBOA or 40,8) and 4-n -hexoxybenzylidene-4' -n'-propylaniline (HBPA or 60,3). The ME parameters were determined for the temperature range 100--300 K in the crystalline solid phase of both systems and in the cold smectic B phase of BBOA and the cold smectic H phase of HBPA. Here "cold" means the smectic glass and the supercooled smectic phase. The Inf (recoil-free intensity) versus T plots exhibited linear Debye behavior for the solid phases of each system over the entire temperature range. The low temperature portion of the cold smectic Inf versus T data also exhibited Debye behavior for both systems. In both instances the smectic glass gave lower Mossbauer-Debye temperatures (Od than the corresponding crystalline phase. For the smectic H glass of HBPA, 01. = 46 K and is independent of molecular orientation. For the smectic B glass of BBOA, 01. = 64 K for preferred molecular alignment parallel to the ME gamma direction and Of. = 49 K for perpendicular alignment. The smectic glass-supercooled smectic transition was determined to be Tg = 175 K for BBOA and Tg = 190 K for HBPA from the deviation of the Inf versus T data from linear behavior. Above T g , the deviation from linear Debye behavior followed a (T -Tg)2 dependence. For DAF in HBPA, the intermolecular contribution to the Fe-57 vibrational ellipsoid (Ed was isotropic. For DAF in BBOA, EI. declined linearly with increasing temperature below T g • For 220--245 K line broadening and quadrupole relaxation were observed for the supercooled smectic H solution of HBPA. These results are accounted for using a model for rotational diffusion of the DAF molecule. The rotational diffusion apparently occurs without simultaneous translational diffusion.
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.
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