Chain Mobility in Crosslinked EPDM Rubbers. Comparison of 1H NMR T2 Relaxometry and Double-Quantum 1H NMR

H-1 NMR transverse relaxation and double-quantum (DQ) build up have been compared for a series of EPDM grades crosslinked with different amounts of sulfur or peroxide. The modulus at 100% elongation of the same EPDM samples correlates linearly to the effective transverse-relaxation and DQ build-up rate, suggesting that both rates are proportional to the total network density caused by chain entanglements and crosslinks. For none of the rates, however, the linear trends of sulfur- and peroxide-crosslinked EPDM coincide. This suggests that sulfur- and peroxide-crosslinked EPDM networks are qualitatively different, which is confirmed by different distributions of residual dipolar coupling constants.