A dual‐mode interpretation of spin relaxation for 13CO2 sorbed in polycarbonate

Abstract

AbstractCarbon‐13 spin‐lattice relaxation, spin‐spin relaxation and nuclear Overhauser enhancements (NOE) are reported for 13CO2 sorbed in glassy polycarbonate. These spin relaxation parameters were determined as a function of temperature and Larmor frequency. A quantitative interpretation is presented based on a two‐site, rapid exchange model. The model is related to the dual‐mode model developed to interpret sorption and permeability data for gases sorbed in glassy polymers. This model proposes the existence of two types of sorbed gas molecules: dissolved molecules and Langmuir sorbed molecules. The nuclear magnetic resonance (NMR) interpretation supports the conceptual features of the dualmode model. Gas molecules in the dissolved site have rotational, collisional, and translational dynamics typical of low molecular liquids. Langmuir sorbed gas molecules have much slower rotational and translational motions. The activation energy for motions in the dissolved site is about 1 kJ while the activation energy for the Langmuir site is about 10 kJ. The relative values of these activation energies are again consistent with the dual‐mode picture. The translational diffusion constants determined from the NMR data range from 10−7 to 10−13 cm2 s−1 and are consistent with the apparent diffusion constant from permeability measurements of 3x10−8 cm2 s−1. A relation between the permeability value and the NMR values is available from a theoretical description of the dualmode model in terms of a random walk on a lattice containing two sites.