NMR Studies on the Phase-Resolved Evolution of Cross-Link Densities in Thermo-Oxidatively Aged Elastomer Blends

A knowledge of the distribution of cross-link densities (CDs) in the constituent rubber phases of an elastomer blend is essential to understand its overall service properties, but phase-resolved CD estimations have so far been limited to qualitative assessments. In a first study of its kind, we demonstrate a phase-resolved quantification of the CDs in sulfur-cured blends of natural rubber (NR) and styrene–butadiene rubber (SBR) using solid-state 1H homonuclear dipolar double-quantum (DQ) magic-angle spinning (MAS) NMR spectroscopy. The blends were also subjected to prolonged thermo-oxidative aging to monitor the chemical changes in the two phases. Analyses of the residual dipolar coupling constant (D res), arising due to spatial restrictions by cross-links to molecular motions, as observed in MAS and static 1H DQ experiments, suggest that unaged NR and SBR cross-link to similar extents. A minimum in D res after around 500 h of aging duration is observed in NR, associated with the formation of highly mobile defect fractions, as seen from free induction decay (FID) combined with transverse relaxometry, with further cross-linking up to 1000 h. SBR appears more stable, with a gradual increase in D res over the aging duration. Phase-resolved experiments reveal a somewhat less cross-linked SBR phase in an unaged 50:50 blend. The phase-specific distribution of CDs in the blend phases becomes significant upon aging, which suggests that NR ages more strongly in the blend as compared to the aged single vulcanisates, thus dictating the blend properties.