ESR studies of photosensitized degradation of poly(L‐lactic acid) via photoionization of dopant

The photosensitized degradation of poly(L ‐lactic acid) (PLA) via an anionic reaction process was studied using spectrophotometry, electron spin resonance (ESR), and gel permeation chromatography (GPC) measurements. PLA film doped with N,N,N′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) was irradiated at 77 K using UV light (λ_c = 356 nm) by which the PLA matrix itself cannot be directly excited. After photoirradiation, a new broad absorption band appeared over the original spectrum due to TMPD⁺ ·, which was produced by two‐photon ionization. The ESR spectrum of the irradiated sample indicated the presence of the TMPD⁺ · radical and main‐chain scission radical of PLA. During the thermal annealing at 0 °C, the latter radical changed to another radical species by dehydrogenation of the alpha hydrogen of the PLA main chain. TMPD⁺ · was extremely stable at room temperature for 7 d. However, by thermal annealing at 40 °C, all the radicals decayed due to the enhanced molecular motions near T_g of PLA (58.7 °C). Spectral simulation for the obtained ESR spectra revealed the relative amounts of four radicals: TMPD⁺ ·, a main‐chain scission radical, a main‐chain tertiary radical, and an unknown radical. The last one was tentatively assigned to the PLA radical anion because of its short decay time. GPC measurements clearly indicated a decrease in the molecular weight of PLA after irradiation. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 706–714, 2001