THE INFLUENCE OF MOLECULAR-MOTION ON CROSS-POLARIZATION IN CROSS-LINKED ELASTOMERS

The effect of molecular motion on the heteronuclear cross-polarization rate for the case of the spin-lock procedure was investigated. In app lying heteronuclear solid state NMR techniques to mobile elastomer sys tems the influence of molecular motion cannot be neglected. Starting i n the slow motion regime a strong collision model was used for predict ing changes of the cross-polarization rate in the dipolar spectral den sity function of abundant spins. The dipolar correlation time and henc e the cross-polarization rate is found to scale with the inverse of th e correlation time of the molecular motion. The same behavior is obtai ned using a second approach valid in the intermediate molecular motion regime. This is based on the effect of the motion on the homonuclear and heteronuclear van-Vleck moments and leads to a linear dependence o f the cross-polarization rate on the correlation time of molecular mot ion. This dependence was verified experimentally by H-1-C-13 high-reso lution cross-polarization measurements on sulfur cross-linked elastome r systems. C-13 rotating frame spin-lattice relaxation rate measuremen ts were used to corroborate these data and the approximations used to evaluate the influence of molecular motion on cross-polarization rates . The dependence of these rates on the cross-link density of the elast omer network is analyzed and it is shown that they scale with the cros s-link density. The correlation of the H-1-C-13 cross-polarization rat es with the dynamic storage moduli was demonstrated.