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 (λc = 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 Tg 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.