Correlation of crosslink densities using solid state NMR and conventional techniques in peroxide-crosslinked EPDM rubber

Peroxide-crosslinked ethylene propylene diene terpolymer (EPDM) elastomer samples were prepared with various types and concentrations of peroxide, with and without co-agent. The variation of crosslink density, the spatial distribution of the crosslinks in the network and the presence of network defects in EPDM rubber were investigated on a molecular level using proton low-field solid-state double-quantum (DQ) NMR spectroscopy. The results reveal that types and concentrations of peroxides do not affect the overall rather inhomogeneous spatial distribution of the crosslinks, but do affect significantly the average crosslink density. The introduction of a co-agent leads to higher crosslink densities and increased levels of spatial crosslinking inhomogeneity as well as non-elastic defects. Apparent crosslink densities as measured by DQ NMR, solvent swelling, linear and non-linear stress-strain measurements (Mooney-Rivlin analysis), and the curemeter, are correlated and discussed. While most of the results can be interpreted consistently and in agreement with the usual assumptions concerning the effect of (trapped) entanglements, a previously unnoticed, decisively non-linear correlation of the NMR results with the linear-regime modulus indicates a large influence of the substantial crosslinking inhomogeneities On the mechanical behaviour. Also, the data suggest a rather efficient trapping of entanglements, possibly because of reduced chain slippage due to the bulky norbornene comonomers. (C) 2014 Elsevier Ltd. All rights reserved.