1966 年 22 巻 2 号 p. 84-89
Reaction mechanisms of photodegradation of cellulose have been proposed under consideration of the experimental facts obtained in report (1) to (VI). The degradation mechanisms are different according to wavelength of light used in irradiation experiments, i.e., (1) on irradiation by λ 1850A, the photochemically primary dissociation occurs in cellulose by its strong absorption of light of this wavelength, the reaction being independent of the presence of oxygen. At the comparatively early period of exposure, direct scission of glucosidic linkage predominates but on prolonged exposure, the formation of 2, 3- dialdehyde by splitting off H-atoms from secondary carbon atoms of the molecule occurs, the latter being easily degradated by alkali medium such as cuam solution. (2) On the other hand, coexistence of oxygen is indispensable to the photodegradation by λ 2537 A irradiation. In this case, absorption matter of λ 2537A is oxygen molecule (Herzberg band), because cellulose is transparent against visible and ultraviolet light having a wavelength longer than λ 2000 A. The main initial step of this reaction is as follows: the excited oxygen molecule thus formed abstracts a hydrogen atom from 1-carbon atom of pyranose ring to form cellulose radical and this reacts with oxygen and other cellulose molecule to produce cellulose peroxide as can be seen in the case of autoxidation of ordinary hydrocarbons. This peroxide is then decomposed photochemically and at the same time, the chain is broken into two chain fragments, accompanying the formation of δ-lactone as the end group of one fragment. The end group of the other fragment has the same configuration as ordinary cellulose molecule. The lactone end group is further hydrolyzed photochemically under the trace of water to form hydroxy-carboxilic acid end group.
The quantum yields of each wavelength are calculated and the following values are obtained: γ (1850A)=1×10-2, γ(2537A)=0.5×10-2. Both values are far smaller than unity. The cause of these small values may be attributed mainly to the cristalinity of cellulose fine structure, i.e., in the case of λ 1850A irradiation, recombination of two dissociated chain fragments occurs in crystal region according to Franck-Rabinowitch theory, while in the case of λ 2537A irradiation, diffusion of oxygen molecule, necessary for photoxidation, into this region are difficult and in consequence, photoxidative degradation is inhibited. In both cases, role of crystal region reduces the quantum yield.