Proton Magnetic Resonance in Natural Rubber: Comparison with Dielectric Measurements

Proton magnetic resonance studies have been made of cured natural rubber containing up to 31 per cent combined sulfur. Line shapes and spin-lattice relaxation times were observed at temperatures between −190° and 180° C. In the line shape studies, the main effect of the combined sulfur was upon the temperature at which the line width is narrowed by the molecular segmental motions. The temperature at the onset of the major line width narrowing increases from −50° C for raw rubber to about 10° C for 31 per cent combined sulfur. A relation was found between the dependence upon sulfur content of the line width changes and the second-order transition temperature. The proton spin-lattice relaxation time T1 was measured for three cured samples in the temperature range above the motional narrowing region. The T1 vs. temperature curves agree qualitatively with the theory of Kubo and Tomita. The general features of the T1 curves are consistent with a distribution of correlation times which broadens with increasing sulfur content. Energies of activation were computed from these results and they are compared with the dielectric relaxation data obtained previously. The effect of a distribution of correlation time is discussed.