Electron magnetic resonance study of gamma-irradiated poly(lactide-co-glycolide) microspheres

Abstract

A series of poly(lactide-co-glycolide) samples of compositions ranging from {75:25} to {65:35} to {50% (65:35):50% (50:50)} were γ-irradiated under ambient conditions in air. The irradiation doses used were 15, 20, 25, and 30 kGy. The generation of radicals resulting from the γ-irradiation was confirmed using EPR. Two major radical species were observed and identified as centered at alkyl and alkyl peroxy groups. The indication from the X-band (9 GHz) frequencies showed that alkyl radicals gave rise to a quartet hyperfine pattern. However, measurements performed at higher W-band frequencies (90 GHz) showed that the X-band spectra are actually a composite profile arising from a series of overlapping individual resonances. Using combined EPR and ENDOR (Electron-Nuclear DOuble Resonance) measurements, an alkyl peroxy radical was identified. For increasing glycolide concentration from 75:25 to the 50:50 blend, there was a factor of 7 increase in the concentration of radicals A and B. Furthermore, both radical species were found to be stable for several weeks after storage at ambient temperature conditions. At elevated temperatures and humidities, radical stability decreased—the decay rate was estimated at ∼3×10 −8 mol K −1. The stability characteristics of the radicals under different conditions are attributed to changes in the morphology of the polymer