EPR Simulation of Polyenyl Radicals in Ultrahigh Molecular Weight Polyethylene

Abstract

An irradiated polyethylene sample containing mostly dienyl free radicals was fabricated by using a combination of photo and thermal treatments. This allowed, for the first time, unambiguous observation of the dienyl radical’s EPR signal to be a singlet with eight hyperfine peaks 9 G apart. Accurate simulation parameters for the dienyl radical were determined. While inaccurate for the dienyl radical, the assumption that a smooth singlet can simulate polyenyl radicals holds true for higher-order polyenyl radicals. The line width of the polyenyl singlet is a function of the radical’s location, with the line width decreasing as the surrounding morphology becomes more organized and uniform, as is the case in the crystalline region. The decreasing line width of polyenyl spectra after exposure to air suggests that the rate of oxidatively induced crystallization is more rapid than oxygen diffusion within the polymer.