Abstract
Summary: 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 (T~g~, T~cc~, and T~m~, 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 T~g~, and annealing‐induced formation of crystallite nuclei should have lowered T~cc~ and increased T~m~. The exceptional large decreases in T~cc~ and T~m~ of UV‐irradiated PLLA‐A films and in T~g~ of UV‐irradiated PLLA‐C films at high TMPD contents are attributable to the large decrease in molecular weight, whereas the exceptional decrease in T~m~ 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.