title: 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 gamma-irradiated under ambient conditions in air. The irradiation doses used were 15, 20, 25, and 30 kGy. The generation of radicals resulting from the gamma-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 approximately 3x10(-8) mol K(-1). The stability characteristics of the radicals under different conditions are attributed to changes in the morphology of the polymer.
authors.affiliation: AstraZeneca, Silk Road Business Park, Charterway, Macclesfield, Cheshire SK10 2NA, UK. mike.claybourn@astrazeneca.com
authors.forename: Mike
authors.initials: M
authors.lastname: Claybourn
authors.forename: Helen
authors.initials: H
authors.lastname: Gray
authors.forename: Damien M
authors.initials: DM
authors.lastname: Murphy
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Purnell
authors.forename: Christopher C
authors.initials: CC
authors.lastname: Rowlands
mesh_terms: D005720:Gamma Rays; D006016:Glycolates / Q000032:analysis* / Q000528:radiation effects*; D019344:Lactic Acid; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008863:Microspheres; D011100:Polyglycolic Acid; D000077182:Polylactic Acid-Polyglycolic Acid Copolymer
chemical_list: D006016:Glycolates; D000077182:Polylactic Acid-Polyglycolic Acid Copolymer; D011100:Polyglycolic Acid; D019344:Lactic Acid