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 resona nce (ESR), and gel permeation chromatography (GPC) measurements. PLA film d oped with N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) was irradiated at 77 K using UV light (lambda (c) = 356 nm) by which the PLA matrix itself c annot be directly excited. After photoirradiation, a new broad absorption b and appeared over the original spectrum due to TMPD+., which was produced b y two-photon ionization. The ESR spectrum of the irradiated sample indicate d the presence of the TMPD+. radical and main-chain scission radical of PLA . During the thermal annealing at 0 degreesC, the latter radical changed to another radical species by dehydrogenation of the alpha hydrogen of the PL A main chain. TMPD+. was extremely stable at room temperature for 7 d. Howe ver, by thermal annealing at 40 degreesC, all the radicals decayed due to t he enhanced molecular motions near T-g of PLA (58.7 degreesC). Spectral sim ulation for the obtained ESR spectra revealed the relative amounts of four radicals: TMPD+., a main-chain scission radical, a main-chain tertiary radi cal, and an unknown radical. The last one was tentatively assigned to the P LA radical anion because of its short decay time. GPC measurements clearly indicated a decrease in the molecular weight of PLA after irradiation. (C) 2001 John Wiley & Sons, Inc.