On the Mechanism of Polyenyl Photoconversion in Irradiated Ultrahigh Molecular Weight Polyethylene

Abstract

The mechanism of polyenyl radical creation upon photoirradiation of ultrahigh molecular weight polyethylene (UHMWPE) containing free radicals is shown to be direct photoconversion from lower to higher order polyenyls. Therefore, the total radical concentration remains unchanged during this process. The rate of photoconversion is identical in the presence and absence of oxygen but is suppressed by hydrogen gas. This demonstrates that the conversion does not occur by a linear alkyl radical addition mechanism wherein alkyl radicals migrate to stable polyene unsaturations and polyenyl radicals, thereby increasing their order, as was previously suggested. Monochromatic photoirradiations show that dienyl radicals are created from diene unsaturations, when photoirradiated at 235 nm, and from allyl radicals, when photoirradiated at 275 nm. Additionally, it is shown that polyene unsaturations are not created during this process, and unlike allyl radicals, polyenyl radicals do not photoconvert to alkyl radicals.