Solid-state 1H NMR study on chemical cross-links, chain entanglements, and network heterogeneity in peroxide-cured EPDM rubbers

Peroxide-cross-linked ethylene-propylene-(diene) rubbers, EP(D)M, have complex network structures with various types of chemical cross-links as well as both temporary and trapped chain entanglements. To obtain a detailed picture of the respective contributions of these different types of cross-links and entanglements to the total network density, solid-state H-1 NMR relaxometry and spectroscopy have been applied to a series of peroxide-cured EPDM with systematic variation of the peroxide and diene contents. The effective Hahn-echo decay time T-2 reflects the volume-average network density, which correlates well to mechanical properties, such as torque and modulus, and linearly depends on the initial peroxide content for the compositions probed. The slope reflects the peroxide-induced chemical cross-links. The chain-entanglement density that is estimated from the intercept agrees with published neutron scattering values. The observed differences in cross-link density between EPDM an EPM are consistent with the double bond conversion estimated from magic angle spinning H-1 NMR and Raman spectra. By comparing T-2 relaxation times in solid and swollen EPDM, we can distinguish between the contributions of temporary and trapped entanglements. The shapes of the Hahn-echo decays suggest strong network heterogeneity, which has also been probed by use of double-quantum-filtered T-2 relaxometry.