Radical Processes Induced by Ionizing Radiation

I 23 Electron Paramagnetic Resonance Spectroscopy and forward Recoil Spectrometry 

From the trend of concentration-dose dependence, conclusions can be drawn regarding the relative rates of radical generation and disappearance due to recombination. If the two rates are comparable, a linear dependence would be expected (cf. PA6 and LLDPE data), whereas in the case of recombination being faster than generation, a downward deviation from linearity would occur (cf. HOPE and LDPE data). For all examined blends, however, an upward deviation from linearity was observed, which meant that the process of radical formation (chain scission) was predominant. Based on quantitative determinations for aged samples, it has been concluded that radicals decay more rapidly in blends than in pure polymers, and this can be attributed to a higher chain mobility and facilitated 

Ali [89] reported the results of a study on a LDPE blend with an ionomer resin based on ethylene-methacrylic acid copolymer (Surlyn 1857 10 wt%), which also contained citric acid (1 wt%) as a blowing agent. The blend, as well as the reference samples of pure LDPE and LDPE/citric acid, were e-beam-irradiated at room temperature with a dose of 40kGy, and the EPR spectra were recorded at different time intervals after irradiation. The septet corresponding to allyl radicals was a dominant spectral component in the initial spectra, the total radical concentration being about 10% higher in the presence of the ionomer due to a contribution of Surlyn-derived radicals. The latter were more stable, such that in the blend the EPR signal was still observed three days after irradiation, whereas in the reference samples the radicals had decayed completely within 3.5 h. 

Finally, mention must be made of a study on PS-PP blends in which PP was a minority component (20wt%). When the blends were yirradiated at low doses ( 70kGy) [95], EPR data relating to the concentration of trapped radicals indicated that in these systems the PP was protected against a strong oxidative degradation (chain scission). 

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