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

Abstract

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%.