Paramagnetic-Resonance Studies of Irradiated High-Density Polyethylene. I. Radical Species and the Effect of Environment on Their Behavior

Abstract

Linear polyethylene (Marlex‐50) was irradiated at different temperatures with 800‐kv (peak) electrons. It was examined for paramagnetic resonance at +25° and —196°C to determine the radical species and their postirradiation behavior as well as that of the crystalline trapping medium. At low doses the spectrum is composed of two radical species which decay at different rates at room temperature. The predominant radical decays to zero in about five days; its six‐line hyperfine structure is attributed to –CH₂–CH–CH₂–. The fast decay supports a previous suggestion that the polymer radicals are formed in pairs on adjacent chains. The other radical has a basic five‐line spectrum with additional ``very fine'' structure. It lasts for months at room temperature. The behavior of the ``very fine'' structure on cooling to liquid‐nitrogen (LN) temperature and the initial low concentration of the radical suggest its probable structure to be $–{\mathrm{CH}}_2–{\mathrm{H}}_2\mathrm{C}\mathrm{·|·}{\mathrm{CH}}_2–{\mathrm{CH}}_2–$ . The relative numbers of each radical trapped at room temperature depend on the rigidity of the crystal and, therefore, on temperature during irradiation. The six‐line radical pairs exist as several groups decaying at different rates at room temperature. The half‐life of the fastest decaying group was 10 sec, that of the slowest, 25 hr. The fastest group comprises pairs in which the two radicals are formed nearly opposite each other on adjacent chains. They cause spin‐spin line broadening in the time‐zero LN spectrum. The decay rate of the radical pair is determined by the rigidity of the crystal. An apparent transition point for the crystal exists at —70°C, below which there is only a slight decay of the closest‐spaced radicals. The total number of radicals produced and trapped by 40 megaroentgens at LN temperature was 6.2×10¹⁹/g which corresponded to a G(total radicals) = 3.0.