Nuclear spin relaxation and a model for molecular reorientation in supercooled liquids and glasses

Abstract

Precise measurements are reported of the proton magnetic resonance Bloch decay shapes over the line narrowing region for liquid glycerol and liquid isobutyl bromide. Measurements are also given for the proton spin-lattice relaxation time for isobutyl bromide over a wide temperature range including the glassy region. These results and earlier results for the proton spin-lattice relaxation in glycerol including the glassy state are interpreted in terms of Glarum's defect-diffusion model which corresponds to a non-exponential correlation function for molecular reorientation. This results in a very striking agreement between molecular reorientational correlation times as deduced from nuclear magnetic resonance studies and the dielectric relaxation studies, which contrasts with earlier analyses using an exponential correlation function. By combination of a time-independent and a defect-diffusion molecular reorientation process it is shown that a plausible interpretation of the magnetic resonance results can be given including those for the temperature region in which the materials are glassy. Glarum's model has been extended to three dimensions and it is shown that essentially similar results are obtained as for the moderately successful one-dimensional analysis. The extra parameter, the effective radius of a defect for relaxation of orientation of a molecule, is not as helpful as was hoped. The three-dimensional analysis introduces several complications and several new parameters which make comparison with experiment more difficult and show the need for considerably more accurate experimental data. The defect-diffusion model remains nevertheless as a plausible mechanism for molecular reorientation in viscous liquids and possibly in organic glasses.