I want to confirm the alkyl radicals of LDPE By EPR and FTIR.
You should try using EPR/ESR spectroscopy or a combination of this technique with radical capturing additives, such as TEMPO.
Check in attached file article "Modification of microstructures and physical properties of ultra high molecular weight polyethylene by electron beam irradiation",Y.C. Nho, S.M. Lee, H.H. Song ,
FTIR at 1712 cm-1
Be carefull, the life time of alkyl radicals in LDPE is very short, especially in amorphous phase, by crosslinking or reaction with oxygen.
So I think you can follow crosslinking by SEC and reaction with oxygen with perxide titration. But it is quite complicated to be quantitave, and EPR/ESR method (you already tested) is the more an "absolute" method.
One more (indirect) technique is thermoluminescence, where irradiation is done at cryogenic temperature and the luminescence intensity is measured during linear heating, showing maxima around relaxation transitions where recombination accelerates. You may alos measure thermally stimulated current after cryogenic irradation - it also gives information on de-trapping. Both methods inlcude ot only radcials but trapped electrons as well.
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