Structures in Sulfur-crosslinked EPDM Analyzed by Hahn Echo Method

Sulfur-crosslinked EPDMs were examined by 1H pulsed NMR. Spin-spin relaxation time (T2) was measured by Hahn echo method. The decay curve was resolved into two components, T2n and T2nn, which were arising from network molecules and non-network molecules, respectively. For unfilled EPDMs prepared at varying sulfur content, the higher order structures analyzed by Hahn echo method were almost the same as those analyzed for peroxide-crosslinked EPDMs. The reciprocal of Ten measured at 150°C for unfilled EPDMs increased linearly with increase in apparent crosslinking density (νs) values determined by modified Flory-Rehner equation. For EPDMs filled with carbon black (CB), the crosslinking density of rubber phase (νR) was calculated from the relationship between νs and 1/T2n for unfilled EPDMs. The crosslinking density related to CB (νF) was obtained from the difference between the νs and νR. The νF values showed good correlation with the tensile stress at 100% elongation, independent of the kinds of cure agents.