An NMR study of segmental motion in poly(isobutylene) and the relationship to translational diffusion of sorbed CO₂

The spin‐lattice relaxation times as well as nuclear Overhauser enhancements (NOE) are determined for the methylene carbon of polyisobutylene (PIB). Three different correlation functions are used to treat the relaxation data to describe the local motion of the PIB backbone rubber. The first is the Hall‐Helfand function combined with restricted anisotropic rotational diffusion, the second is the Dejean‐Lauprêtre‐Monnerie function, and the third is the Williams‐Watts function combined with restricted anisotropic rotational diffusion. Spin‐lattice relaxation times are also determined on PIB under pressure of CO₂ gas to check for changes in segmental motion in the presence of this penetrant. All the models give a satisfactory description for the local dynamics of the backbone carbon of the polymer chain, yielding comparable values for the time scale of conformational changes, apparent activation energies and distribution of correlation times. In addition, a proton line shape experiment is performed on bulk PIB as a function of temperature. The results are interpreted in terms of two motional components in the polymer chains: fast and slow ones. Applying a revised version of Rössler's formalism to our data, an average energy barrier is found which is consistent with the apparent activation energies. The apparent activation energy obtained for PIB, from the nuclear magnetic resonance (NMR) measurements, about 35 kJ, is slightly higher than the apparent activation energy for the translational diffusion of CO₂ in PIB from NMR data. The distribution of correlation times and the correlation times themselves for segmental motion of the PIB and translational diffusion of CO₂ in PIB are very similar, indicating a close link between the two motional processes. The distribution of correlation times is close to a single exponential in PIB consistent with Angell characterization of this material as a strong liquid. © 1994 John Wiley & Sons, Inc.