Paramagnetic-Resonance Studies of Irradiated High-Density Polyethylene. II. Effect of Irradiation Dose on the Radical Species Trapped at Room Temperature

The previous paper presented a study of the radical species in irradiated Marlex‐50 at low dose levels. This paper extends that study to a level of 4000 megaroentgens. The kind and relative number of each radical trapped at room temperature (RT) depends on the degree and tightness of the crystalline and crosslinked fractions during irradiation. At least two of three distinct species, having different hyperfine structures (six‐, five‐, and a single‐line) are present in varying concentrations at all doses. The six‐ and five‐line radicals are trapped in the crystal, whereas the single‐line radical is trapped in the highly crosslinked medium. During irradiation at RT and liquid‐nitrogen (LN) temperature, the crystal is not as effective in trapping the six‐line radical as in the absence of irradiation. At 640 megaroentgens the six‐line radical does not survive during irradiation at RT, leaving the five‐ and single‐line radicals above this level. The single‐line radical appears at 320 megaroentgens and increases until at 3000 megaroentgens, where crystallinity is zero, it accounts for about 70% of the total. Heating the irradiated samples causes all radicals to disappear; the ``extinction temperatures'' for the six‐, five‐, and single‐line radicals were 90°, 135°, and about 250°C, respectively. The relative effectiveness of the preirradiated annealed samples compared to the nonirradiated Marlex‐50 in trapping radicals at RT was 0.5 for the 20‐megaroentgen, 1.0 for the 320‐megaroentgen, and 1.4 for the 4000‐megaroentgen preirradiated material. There is evidence in the uv spectrum for conjugated double bonds in the highly irradiated samples. It is proposed that the single‐line radical is initially formed adjacent to the conjugated sequence and then becomes trapped within the sequence by resonance. The radical could be represented as $$–\mathrm{CH}=\mathrm{CH}–\mathrm{CH}=\mathrm{CH}–\mathrm{CH}–{\displaystyle {lim}^{\mathrm{\leftarrow ·\to }}}\mathrm{CH}=\mathrm{CH}–\mathrm{CH}=\mathrm{CH}\mathrm{–.}$$ .