Solid-State Carbon-13 NMR Studies of Vulcanized Elastomers. VI. Relaxation in Sulfur-Vulcanized Natural Rubber
Abstract Molecular motions in sulfur-vulcanized NR are investigated by solid state 13C NMR relaxation techniques. Since the high-resolution spectra of crosslinked samples exhibit overlapping in the aliphatic region, a combined application of variable temperature spin-lattice relaxation measurements and the computer simulation of the overlapped spectral region is used in order to detect resonance signals. Motional restrictions introduced by crosslinks are investigated from the temperature dependence of nT1 relaxation times for individual carbons. The V-curves for all polyisoprene signals and for the detectable signals arising from the network units exhibit a similar general trend with increased curing time: curve broadening, shift of the minima to higher temperatures, and increase of the T1 min values. All the backbone carbons show quantitatively similar effects of vulcanization on the spin-lattice relaxation. At shorter curing times, motional restrictions for the methyl side group are more pronounced compared to the main-chain carbons. The experimental data suggest that the isotropic motion is strongly affected by the crosslinking. Librational motion is less affected, while the change in rotational motion has no significant influence on the relaxation curve. Although experimental data cover a relatively narrow temperature range, not sufficiently wide for a more accurate quantitative analysis, the results indicate that concepts of plural correlation times and a distribution of correlation times are applicable.