Relaxation dynamics of carbon dioxide sorbed in poly (dimethyl siloxane) rubber

We have probed the dynamics of 13 CO 2 molecules sorbed in poly (dimethyl siloxane) rubber by nuclear magnetic resonance (NMR) relaxation. The spin‐lattice and spin‐spin relaxation times as well as nuclear Overhauser enhancements were determined as functions of temperature at three magnetic field strengths. The relaxation mechanisms found to be important are intermolecular dipole‐dipole relaxation and spin rotation relaxation with a minor contribution from chemical shift anisotropy relaxation. Our analysis shows that the assumption of a single correlation time for each motional mechanism is inconsistent with the NOE data and spin‐spin relaxation times. However, if a distribution of correlation times is assumed, one can interpret all of the experimental data in a reasonable manner. We have quantitatively elucidated the spin relaxation mechanisms and determined the parameters which characterize the distribution times for 13 CO 2 Collisional motions, rotational motions, and translational motions in the silicone rubber. A fractional exponential correlation function of the Williams‐Watts type with a fractional exponent of 0.58 is used in our description of the 13 CO 2 dynamics. This exponent is typical of the distribution of time scales associated with segmental motion in polymeric rubbers. The broad distribution needed to describe the data has implications for the coupling of diffusion to segmental motion and for the relationship between macroscopic permeability and the molecular level character of diffusion.