Enzymatic Degradability of Poly(lactide):  Effects of Chain Stereochemistry and Material Crystallinity

Polylactide (PLA) stereocopolymers with ( l ) repeat unit contents of 75%, 80%, 82%, 85%, 90%, 91%, 92%, 94%, and 95% were prepared from mixtures of ( l )-/( d )-lactide and ( l )-/ meso -lactide. Compression molding of these products gave amorphous films which, for ( l ) contents ≥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%.