Determination of scission and crosslinking in gamma irradiated butyl rubber

Structural changes in butyl rubber (a copolymer of isobutylene and 1.3% isoprene) after γ irradiation in vacuum at 303 and 398 K have been determined by ¹³C-NMR spectroscopy. A variety of new chain-end and crosslink structures have been identified and estimates made of the yields of main-chain scission, G(S), and crosslinking, G(X). Significant dependence of the yields on dose and temperature have been observed. Values of G(S) and G(X) have also been derived from dose dependence of the molecular weight averages $\text{M}$_n, $\text{M}$_w and $\text{M}$_z determined by GPC after irradiation at 77, 195, 303, 348 and 398 K, and are in good agreement with the values derived from the NMR spectra. Radicals formed on isobutylene units by H loss from methyl and methylene groups, and allyl radicals on isoprene units have been observed by ESR spectroscopy, and mechanisms proposed for crosslinking by the formation of H and Y links at the isoprene units.