Photodegradation of Poly(L‐lactic acid): Effects of Photosensitizer

Abstract

AbstractSummary: Amorphous and crystallized poly(L‐lactic acid) (PLLA‐A and PLLA‐C, respectively) films with different contents of N,N,N′,N′‐tetramethyl‐1,4‐phenylenediamine (TMPD) as a photosensitizer were prepared, and the effects of the addition of TMPD on the photodegradation of PLLA films were investigated. It was found that the addition of TMPD effectively enhanced the photodegradation of PLLA films and thereby decreased their molecular weight of PLLA films regardless of their crystallinity, and that PLLA films with different molecular weights can be prepared by the addition of different amounts of TMPD and subsequent UV irradiation. Too high contents of TMPD however caused the brittleness of PLLA films due to a large decrease in molecular weight. The PLLA chains in crystalline regions as well as those in amorphous regions are photodegradable even at an early stage, in marked contrast to their hydrolytic degradation, where the chains in the amorphous regions are selectively degraded. The basic changes in glass transition, cold crystallization, and melting temperatures (Tg, Tcc, and Tm, respectively) of PLLA films during UV irradiation can be ascribed to low‐temperature annealing effects; i.e., annealing‐induced stabilization in chain packing should have elevated Tg, and annealing‐induced formation of crystallite nuclei should have lowered Tcc and increased Tm. The exceptional large decreases in Tcc and Tm of UV‐irradiated PLLA‐A films and in Tg of UV‐irradiated PLLA‐C films at high TMPD contents are attributable to the large decrease in molecular weight, whereas the exceptional decrease in Tm of PLLA‐C films at high TMPD contents can be due to the folding surface structural change of crystalline regions or to the lattice disorder caused by molecular structural changes.$\overline M _{\rm n}$ of PLLA‐A films before UV irradiation and after UV irradiation for 60 h as a function of TMPD content.magnified image$\overline M _{\rm n}$ of PLLA‐A films before UV irradiation and after UV irradiation for 60 h as a function of TMPD content.