Probing small network differences in sulfur-cured rubber compounds by combining nuclear magnetic resonance and swelling methods

We studied the network structure of elastomers based on polyisoprene, polybutadiene, and their blends in the narrow range of formulations compatible with actual use in the automotive tyre industry. Cross-link density (CLD) was quantified comparatively by low-field ¹H multiple quantum time domain nuclear magnetic resonance (MQ TD-NMR) and by equilibrium swelling technique. The robustness and agreement of the two methods was demonstrated in measuring minute alterations of unfilled vulcanizates beyond the optimum cure time. Comparison with samples where polysulfidic bonds were selectively cleaved also demonstrates that the length of the sulfur chain constituting the cross-link does not significantly affect residual dipolar coupling. Kraus, Lorenz, and Parks correction for filler restriction on swelling is validated by MQ TD-NMR, which also allows extracting information on cross-link distribution not provided by swelling measurement. Cross-link distributions in the blends were demonstrated to be significantly different from the weighted average of the pure samples, while average proton-proton residual dipolar coupling values correspond.