Nuclear spin relaxation dynamics of ¹³CO₂ sorbed in polyisobutene rubber

Abstract Recently we presented the dynamics of 13 CO 2 molecules sorbed in silicone rubber (PDMS) ascertained from spin relaxation experiments. Results of a similar investigation for 13 CO 2 sorbed in polyisobutene (PIB) are presented in this report. The spin‐lattice and spin‐spin relaxation times as well as nuclear Overhauser enhancements (NOE) were determined as a function of temperature and Larmor frequency. The relaxation mechanisms found to be important for 13 CO 2 /PIB system are intermolecular dipole‐dipole relaxation and chemical shift anisotropy with a minor contribution from spin rotation relaxation. We have determined the parameters which characterize correlation times for 13 CO 2 collisional motion, rotational motion, and translational motions in the PIB. The self‐diffusion coefficient of 5.15 × 10 −8 cm 2 /s obtained from the nuclear magnetic resonance (NMR) data is close to the literature value of the mutual diffusion coefficient of CO 2 in PIB at 300 K obtained from permeability measurements. In contrast to the case of CO 2 /PDMS in which a broad distribution (characterized by a fractional exponential correlation function of the Williams‐Watts type with α = 0.58) is observed, a sharp distribution with a fractional exponent, α, of 0.99 is found for the CO 2 /PIB system. Instead of assuming an Arrhenius type temperature dependence, we used a Williams‐Landel‐Ferry type temperature dependence and found it to be better suited to describe the behavior of this system. PIB is a densely packed “strong” chain polymer which responds gradually to the temperature variation and gas sorption. In contrast PDMS is a relatively loosely packed “fragile” polymer with a propensity to exhibit rapid dynamic responses to the temperature change and gas sorption. © 1993 John Wiley & Sons, Inc.