The structure of the smectic phases of terephthal-bis-(Butylaniline) studied by electron spin resonance spectroscopy

Electron spin resonance (E.S.R.) measurements at X-band (3.3 kG) of the spin probe 17/3-hydroxy-4,4'-dimethylspiro[5a-androstane-3,2'-oxa-zolidin]-3'-yloxyl dissolved in the smectic phases (A, C and B) of the mesogen 4,4'-terephthal-bis(butylaniline) (TBBA) are reported. Two types of samples were studied in the dependence on temperature, sample orientation and strength of the magnetic field: (i) glass plate sandwiches with less than Ol mm of the mesogen, and (ii) 4 mm i.d. cylindrical glass tube. The sandwich samples were prepared as monodomains with the smectic layers parallel to the glass plates. In these samples the orientation of the layers and of the director is fixed and cannot be reoriented even in a magnetic field of 21 kG. From the angular dependence of the spectrum, the tilt angle, the order parameter and their temperature dependence were determined. In the cylindrical samples the original orientation of the smectic layers is preserved, and up to about 3 kG the director's orientation with respect to the sample is almost unaffected in all smectic phases. On subjecting the sample to higher fields (~ 20 kG) the director in the smectic C phase will re-align so as to minimize the magnetic energy, subject to the fixed tilt angle. The new alignment is preserved when the strength of the magnetic field is reduced. No such re-alignment is observed in the smectic B phase even after subjecting the sample to a field of 21 kG, in contrast to previous deuterium N.M.R. measurements at 14 kG. It is suggested that in this phase the director is pulled by strong fields but relaxes to its original direction when the magnetic field is brought back to 3.3 kG.