Enzymatic degradability of poly(lactide): Effects of chain stereochemistry and material crystallinity

Polylactide (PLA) stereocopolymers with (L) repeat unit contents of 75%, 89%, 82%, 85%, 90%, 91%, 92%, 94%, and 95% were prepared from mixtures of (L)-/(n)-lactide and (L)-/meso-lactide. Compression molding of these products gave amorphous films which, for (L) contents greater than or equal to 90%, were also annealed above T-g to crystallize. Analyses by differential scanning calorimetry and wide angle X-ray scattering gave information on the crystalline order of PLA films. For identical (L) contents, stereocopolymers of (L)-/(D)-lactide had higher crystallinities than those from (L)-/meso-lactide. PLA films were incubated with proteinase K (from Tritirachium album), and the enzyme-catalyzed film weight loss rates were measured. Film crystallinity, chain stereochemical composition, and repeat unit sequence distribution were analyzed as independent variables affecting film enzymatic hydrolysis. Amorphous films from (L)/(D)-lactide copolymerizations with (L) compositions ranging from 80% to 95% exhibited film weight loss rates that were almost identical. Also, amorphous PLA films prepared from (L)-/meso-lactide copolymers for (L) contents of 80-95% showed a similar invariability in weight loss rates. It was concluded that proteinase K has a high degree of tolerance for (D) repeat units. Amorphous PLA films from (L)-lactide/meso-lactide copolymerizations had weight loss rates which were about 43% slower than amorphous PLA films from (L)-/(D)-lactide copolymerizations. These results were analyzed considering differences in chain stereosequence distributions. Proteinase K showed an extraordinarily high sensitivity to film crystalline order. For example, the decrease in the film weight loss rate due to crystalline order for a 95% (L) (L)-/(D)-lactide stereocopolymer was 93%.