T. Saleesung, D. Reichert, K. Saalwächter, C. Sirisinha.
Correlation of crosslink densities using solid state NMR and
conventional techniques in peroxide-crosslinked EPDM rubber.
Polymer 56, 309-3017 (2015).
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.