The network structure of peroxide-crosslinked ethylene-propylene-diene rubber (EPDM) was examined by dynamic mechanical analysis and ¹H pulsed NMR. The loss tangent (tanδ) hardly changed with the increase of the crosslinking density (ν_e) in the glass transition region (<0°C). However, in the higher temperature region, a broad tanδ peak was observed for less crosslinked systems. This relaxation was thought to be connected with the entanglement of long dangling chain ends. The tanδ values at 150°C decreased with the increase of ν_e. The signal of spin-spin relaxation time (T₂) was resolved into three components. The three components respectively correspond to the molecules surrounding crosslinking points, molecules between crosslinking points, and non-network molecules such as dangling chains and end-chains. The increase of ν_e reduced the molecular mobility of each T₂ component, and decreased the fraction of component with the longest T₂ consisting of dangling chains. The fraction of each T₂ component showed good correlation with tanδ values at 150°C independent of the kinds of fillers.

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