NMR relaxation dispersion of vulcanized natural rubber.

The dependence of the 1H spin-lattice relaxation time on the magnetic field strength has been determined for linear and cross-linked polyisoprene for Larmor frequencies between 5 kHz and 20 MHz. Universal power-law relations are found for all temperatures and cross-link densities under investigation and are compared to published results of rotating-frame experiments on similar natural rubber samples. The shape of the individual dispersion functions can be superposed into a master curve using appropriate shift factors. While addition of filler particles even at large weight fractions has only a minor effect on the relaxation times, uniaxial deformation and swelling are demonstrated to alter the molecular dynamics significantly.