NEUTRON QUASI-ELASTIC SCATTERING STUDY OF ROTATIONAL MOTIONS IN THE SMECTIC C, H AND VI PHASES OF TEREPHTAL-BIS-BUTYL-ANILINE (TBBA)

Abstract

A study of rotational motions in the smectic C, H and VI phases of TBBA by means of incoherent neutron quasi-elastic scattering (NQES) is presented. The data are analyzed in terms of the elastic incoherent structure factor (EISF). To interpret its dependence versus neutron momentum transfer and temperature, the simple, uniaxial rotational model used in previous work [2] is refined by including additional, rapid (time scale : 10-11 s) fluctuations of the body long axis. The true gyration radii (rather than an average value) of the various protons are now considered. It is found that this model describes satisfactorily the data for all temperatures between 152 °C (Sm C) and 94°C (supercooled SmH). The order parameter, defined by S1 = < cos λ >, characterizing the axis fluctuations, is found to vary from 0.5-0.6 at 152 °C to ≈ 0.95 at 94 °C. The average reorientational correlation time changes from about 1 to 3 x 10-11 s in this temperature range. At the SmH-SmVI transition, there is complete quenching (on the same time scale) of these fluctuations and the onset of orientational ordering around this axis. The possible influence on this analysis of the body isomerisation is discussed. The thermal history of the sample is shown to be an important factor for obtaining reproducible results. The calculation of the EISF of various rotational models discussed is also presented