H-2-NMR STUDY OF MOLECULAR ORDER AND DYNAMICS IN MIXTURES OF 80BCAB AND NTPCHB

H-2-NMR measurements have been performed on pure 8OBCAB-d2 and on its mixture with compounds of the nTPCHB series. The aim of this investigation was to achieve an insight into the unusual mixing properties of these compounds, which have been associated to the formation of molecular pairs in the smectic A phase. By quadrupolar splittings measurements, performed at various temperatures, ordering information on the various mixtures was obtained. The Zeeman (T(1Z)) and the quadrupolar (T(1Q)) relaxation times were simultaneously measured by the Jeener-Broekaert experiment; then the temperature dependence of the individual spectral densities J(omega) and J(2omega) was analysed. The fitting of these parameters has been obtained using Nordio's rotational diffusion model. Comparison of all the mixtures at the same reduced temperature (T/T(NI)) showed a connection between the presence of the nematic re-entrant behaviour and the slowing down of both D(parallel-to) and D(perpendicular-to) elements of the rotational diffusion tensor.