EPR study of persistent free radicals in cross-linked EPDM rubbers

Mouna Zachary, Susana Camara, Adrian C. Whitwood, Bruce C. Gilbert, Martin van Duin, Robert J. Meier, Victor Chechik

Research output: Contribution to journalArticlepeer-review


Cross-linking of ethylene propylene diene monomer (EPDM) rubbers containing different amounts of dicyclopentadiene (DCPD), 5-ethylidene-2-norbornene (ENB) and 5-vinyl-2-norbornene (VNB) dienes was examined by EPR spectroscopy. The cross-linking was initiated by thermal decomposition of dicumyl peroxide at 440 K. The concentration of free radicals increased towards the end of the cross-linking process before reaching a maximum and decaying to zero. This is explained by the spatial confinement of the radicals in the cross-linked rubber, which leads to increased life time and, hence, higher radical concentration at a time when most peroxide has decomposed. The EPR spectra showed the presence of two components: a well-resolved spectrum overlapping a broad line. Both components are assigned to allyl radicals possessing different mobility. The more mobile component is assigned to allyl radicals along the EPDM chains, whereas the immobilised allyl radicals are formed in the cross-links. The stability of the allyl radicals decreases in the order DCPD > ENB > VNB. EPDMs containing two dienes show more persistent radicals than their single-diene counterparts. The most persistent radicals are observed for highly cross-linked (e.g., 28% ENB) or mixed diene EPDMs (e.g., 2.2% DCPD-4.4% ENB); the EPR spectra of free radicals in these systems can be observed for several hours. (C) 2008 Elsevier Ltd. All rights reserved.

Original languageEnglish
Pages (from-to)2099-2107
Number of pages9
JournalEuropean Polymer Journal
Issue number7
Publication statusPublished - Jul 2008


  • cross-linking
  • radical polymerization
  • rubber

Cite this