The density of chemical crosslinks and chain entanglements in unfilled EPDM vulcanizates as studied with low resolution, solid state 1H NMR

The network density in unfilled EPDM vulcanizates was studied by the following methods: low resolution, proton T-2 NMR relaxation, Mooney-Rivlin analysis of stress-strain curves, and equilibrium swelling experiments. For the vulcanizates as such, the NMR method measures the total network density, which is composed of chemical crosslinks (CC) and of temporary (EN/TE) and trapped chain entanglements (EN/TR). The estimated molar mass of EPDM chains between apparent chain entanglements (EN) is 1900 +/- 200 g/mol. For partially swollen samples, the NMR method determines the network density composed of CC and EN/TR. The CC density is obtained from the total density of network junctions and the measured entanglement density assuming that CC and chain entanglements are decoupled and additive. The contribution of CC, EN/TE and EN/TR to the total network density is estimated.