Photosensitized Reaction of Poly(L-lactic Acid) via Two-Photon Ionization of Dopant
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Photosensitized Reaction of Poly(L-lactic Acid) via Two-Photon Ionization of Dopant Wataru Sakai, Kosuke Wakabayashi, Hiroshi Nishinaka, Hiroki Kawamoto, Motoi Kinoshita and Naoto Tsutsumi Department of Polymer Science & Engineering, Kyoto Institute of Technology, Matsugasaki Sakyo-ku, Kyoto 606-8585 JAPAN ABSTRACT We here studied photosensitized radical reaction of poly(L-lactic acid) (PLA) using spectrophotometry, electron spin resonance (ESR), gel permeation chromatography (GPC) measurements, and semiempirical molecular orbital calculation. PLA doped with N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD) was irradiated at 77 K using UV light. After the irradiation, we observed a new broad absorption band that is assigned to TMPD radical cation produced by two-photon ionization. ESR spectrum of the irradiated sample also indicated presence of main chain scission radical of PLA as well as the TMPD radical cation. By increasing the irradiation time, the ESR spectrum changed by another radical species which is produced by dehydrogenation of PLA main chain. Spectral simulation for the obtained ESR spectra revealed changes of relative amounts. The GPC measurements clearly indicated decrease in molecular weight of PLA after the irradiation. The semiempirical molecular orbital calculation was used to confirm the reaction scheme of radical anion decomposition. INTRODUCTION Photodegradation of polymer materials is not only a negative phenomenon but also an affirmative reaction for some applications. For example, photoresist materials have been exceedingly developed for these decades and surface modifications are done by photoreaction to functionalize the polymer materials. Commonly, the photodegradation of polymer is caused by direct photoexcitation of polymer material. However, some organic compounds doped in the polymer matrices can also trigger subsequent photoreactions by being selectively photoexcited. Such a photosensitized reaction has a large potential for development of novel photofunctional polymers. For these several years, we have been studying the photosensitized reaction of aliphatic linear polyester, poly(lactic acid) (PLA), using N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD) as the photosensitizer [1,2]. Quantitative electron spin resonance (ESR) spectroscopic analysis has revealed the reaction process as follows (Scheme 1); when the PLA doped with TMPD is irradiated by UV light, a free electron is ejected from TMPD via photoionization (1) [3], and captured by the PLA matrix producing a radical ester anion (2) [4]. This radical anion B immediately produces a main chain scission radical C through decomposition at RCOO−−R’ bond (3), and the main chain radical succeedingly produces a tertiary main chain radical D by dehydrogenation (4). In this degradation process, the ester radical anion plays a key role. Photochemical studies for the low molecular anionic aliphatic carboxylic esters have been already investigated by several groups using different systems, such as Na atom contact [5], aqueous photochemical sy
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