A Mössbauer temperature study of two cold smectic (B and H) liquid crystals

Abstract

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.

Glass Transition Dynamics and Crystallization Kinetics in the Smectic Liquid Crystal 4-n-Butyloxybenzylidene-4′-n′-octylaniline (BBOA)

“…Real part ε′( T ) and imaginary part ε″( T ) of the complex dielectric permittivity of BBOA at ν = 3.4 Hz measured on fast cooling (open symbols) and on subsequent slow heating (solid symbols). The vertical dashed lines mark the glass transition temperature, T g = 175 K, determined by Mossbauer spectroscopy. , Other discernible phase transition temperatures are T CrB = 225 K, T CrA = 250 K, and T Cr = 270 K.…”

“…Upon slow cooling, an additional phase transition from the SmB hex to the SmB Cr phase is observed at 287 K, which corresponds to a change in the type of stacking sequences of molecular layers. On the basis of Mossbauer spectroscopy, Uhrich et al 12,13 found that the SmB Cr phase vitrifies at 175 K. The molecular motions of BBOA in smectic phases were studied by NMR and quasielastic neutron scattering. 20,21 In this article, we report a detailed study of molecular dynamics of BBOA molecules in various equilibrium phases, in the glass transition region as well as during crystallization, by means of broadband dielectric spectroscopy (BDS).…”

“…Although the vitrification of crystal-like smectic phases, in which molecules are arranged within the layers (smectic B, E, F, G, H, etc. ), − is not evident, a few examples of the glasses of smectic phases have been reported: the glass of smectic B in 4- n -butyloxybenzylidene-4′- n ′-octylaniline (BBOA), the glass of smectic H, the glass of smectic E, , and the glass of smectic G. , Contrary to “conventional” glasses considered as amorphous materials, the glassy state of the crystal-like phase is characterized by orientational disorder of the molecules, which otherwise possess translational order. In previous work, we showed that in the vitreous state of the SmE phase, the smectic layers are preserved, but the translational order of molecules within the layers is slightly disturbed. , We found an analogy between the glass of a smectic E phase and the glassy state in plastic crystalline phases .…”

“…Upon slow cooling, an additional phase transition from the SmB hex to the SmB Cr phase is observed at 287 K, which corresponds to a change in the type of stacking sequences of molecular layers. On the basis of Mössbauer spectroscopy, Uhrich et al , found that the SmB Cr phase vitrifies at 175 K. The molecular motions of BBOA in smectic phases were studied by NMR and quasielastic neutron scattering. , …”

Moessbauer spectroscopy

“…Measurements on FeSn03-particles in the liquid crystal EBBA by Bhide and Kandpal (42) show anomalous line broadening around the transition to the isotropic liquid phase and line broadening due to Brownian motion in the nematic and liquid phases. LaPrice and Uhrich (178) suggest a rotational diffusion model for diacetylferrocene to explain their recoilless fraction data for two liquid crystalline materials.…”