Sulfur-33 Nuclear Magnetic Resonance: Application to the characterization of vulcanized rubbers

Sulfur vulcanization is a widely used crosslinking process of elastomers in the rubber industry, but the involved chemical mechanisms and the structure of the cross-linked materials are still poorly understood. Nuclear Magnetic Resonance (NMR) spectroscopy, which allows selectively probing the sulfur chemical environments, can provide new information about the local structure of the cross-linked material. However, due to its intrinsic properties, the observation of the NMR active isotope of sulfur (³³S) is challenging in solid materials and requires the use of a specific methodology. In this work, we show that the use of very high magnetic fields and convenient NMR methods allows studying the chemical environment of sulfur in solid materials. First principle computations of the NMR parameters have been performed and compared to experimental results. This comparison shows that the computations lead to a reliable prediction of the NMR parameters and can be used to assign the observed NMR signals to a chemical structure. The NMR parameters characteristic of sulfur atoms involved in crosslinks have been computed from structural models. For such sulfur local environments, extremely large quadrupolar coupling constants (C_Q > 40 MHz) and thus ultra-broad resonances are expected. The NMR detection limit of sulfur environments giving rise to such very broad lines has been investigated through the ³³S NMR study of elemental sulfur. In the case of vulcanized rubbers, the results of this work suggest that the NMR observation of the distinct sulfur chemical environments present in the cross-linked networks requires the use of both ultra-high magnetic field and very low temperature. (author)