title: The influence of molecular motion on cross-polarization in cross-linked elastomers.
abstract: The effect of molecular motion on the heteronuclear cross-polarization rate for the case of the spin-lock procedure was investigated. In applying heteronuclear solid state NMR techniques to mobile elastomer systems the influence of molecular motion cannot be neglected. Starting in the slow motion regime a strong collision model was used for predicting changes of the cross-polarization rate in the dipolar spectral density function of abundant spins. The dipolar correlation time and hence the cross-polarization rate is found to scale with the inverse of the correlation time of the molecular motion. The same behavior is obtained 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 of the cross-polarization rate on the correlation time of molecular motion. This dependence was verified experimentally by 1H-13C high-resolution cross-polarization measurements on sulfur cross-linked elastomer systems. 13C rotating frame spin-lattice relaxation rate measurements 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 elastomer network is analyzed and it is shown that they scale with the cross-link density. The correlation of the 1H-13C cross-polarization rates with the dynamic storage moduli was demonstrated.
authors.affiliation: RWTH-Aachen, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Fülber
authors.forename: D E
authors.initials: DE
authors.lastname: Demco
authors.forename: B
authors.initials: B
authors.lastname: Blümich
mesh_terms: D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003432:Cross-Linking Reagents / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008432:Mathematical Computing; D012408:Rubber / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D003432:Cross-Linking Reagents; D012408:Rubber