A DUAL-MODE INTERPRETATION OF NUCLEAR-SPIN RELAXATION FOR (CO2)-C-13 SORBED IN POLYSTYRENE

Recently we presented a dual-mode interpretation of the nuclear spin relaxation for (CO2)-C-13 sorbed in polycarbonate [E. J. Cain, W.-Y. Wen, A. A. Jones, P. T. Inglefield, B. J. Cauley, and J. T. Bendler, Journal of Polymer Science, Part B, Polymer Physics, Vol. 29, p. 1009 (1991)]. We have made similar investigations on spin relaxation for (CO2)-C-13 sorbed in polystyrene and are presenting the results in this report. C-13 spin-lattice relaxation, spin-spin relaxation, and nuclear Overhauser enhancements (NOE) are reported for (CO2)-C-13 sorbed in glassy polystyrene. These spin relaxation parameters were determined as a function of temperature and Larmor frequency. A quantitative interpretation is based on a two-site, rapid exchange model, which is consistent with the NMR results. Gas molecules in the dissolved site have rotational, collisional, and translational dynamics typical of low molecular weight liquids. Sorbed molecules in the Langmuir site have much slower rotational and translational motions. The activation energy for motions in the dissolved site is about 7.5 kJ/mol, while that for the Langmuir site is about 15.5 kJ/mol. The translational diffusion constants derived from the NMR data range from 10(-7) to 10(-9) cm2/s and are consistent with the apparent diffusion constant from the permeability measurements of 4.9 X 10(-8) cm2/s at 35-degrees-C. A relation between the permeability value and the NMR values is available from a theoretical description of the dual-mode model in terms of a random walk on a lattice containing two sites. As a modification of our previous lattice model (E. J. Cain et al., ibid.), the temperature dependence of the fraction of Langmuir sites in the lattice is taken into account. Parameter values characterizing the motion of CO2 in polystyrene are found to be similar to the corresponding ones in polycarbonate.