Comprehensive multiphase NMR spectroscopy: A new analytical method to study the effect of biodiesel blends on the structure of commercial rubbers

Biodiesel has been incorporated into the energy matrix in several countries, but its compatibility with some materials used in automotive engines is of growing concern. In the present study, comprehensive multiphase (CMP) NMR is applied to understand the different phases in fuel-rubber systems. CMP NMR is a novel technology that integrates all the hardware from solution-, gel- and solid-state into a single NMR probe, permitting all phases to be studied in samples in their natural unaltered state. Transverse relaxation experiments in combination with inverse diffusion editing permit the increasing mobility of the rubbers chains with biodiesel exposure to be monitored. Conversely diffusion editing and RADE experiments highlight the more rigid domains. In summary NR and NBR showed the absence of highly rigid domains after exposure to biodiesel whereas SBR and EPDM especially better retained their structural integrity. 13C editing protocols and 13C-1H HSQC confirmed the increasing gel-like properties of NR, NBR and SBR with exposure to biodiesel. However, variable contact time experiments showed that biodiesel penetrates even the most resilient EPDM pores causing relaxation of the polymer chains and demonstrates that NMR is sensitive enough to highlight even the very slight swelling of the EPDM. Through these results it is possible to observe that the elastomer that exhibits the lowest susceptibility to biodiesel in molecular terms was EPDM, followed by SBR, NR, and NBR. It was also observed that biodiesel molecules were present in the EPDM structure despite the literature reporting its general resilience to biodiesel.