Effect of Rubber Polarity on Cluster Formation in Rubbers Cross-Linked with Diels-Alder Chemistry.

Diels-Alder chemistry has been used for the thermoreversible cross-linking of furan-functionalized ethylene/propylene (EPM) and ethylene/vinyl acetate (EVM) rubbers. Both furan-functionalized elastomers were successfully cross-linked with bismaleimide to yield products with a similar cross-link density. NMR relaxometry and SAXS measurements both show that the apolar EPM--furan precursor contains phase-separated polar clusters and that cross-linking with polar bismaleimide occurs in these clusters. The heterogeneously cross-linked network of EPM--furan contrasts with the homogeneous network in the polar EVM--furan. The heterogeneous character of the cross-links in EPM--furan results in a relatively high Young's modulus, whereas the more uniform cross-linking in EVM--furan results in a higher tensile strength and elongation at break.