POLYLACTIDE STEREOCHEMISTRY - EFFECT ON ENZYMATIC DEGRADABILITY

Polylactide (PLG) with % L repeat unit contents of 100 (PLA-100), 99, 96, 94, 92, 85, 80, 75, 70, 50, and 0 were prepared from mixtures of (D)- and (L)-lactide using stannous octanoate as catalyst. PLA films of these samples were prepared,by solution casting and then annealing at 75 degrees C for 36 h to crystallize. Large changes in the crystalline order for PLA with % L compositions between 85 and 100% were observed by differential scanning calorimetry and wide-angle X-ray scattering measurements. The PLA films were incubated with proteinase K (from Tritirachium album) to determine enzyme-catalyzed rates of both film weight loss and initial surface degradation. The latter rate values were obtained by monitoring pH changes. Proteinase L preferentially degraded (L)-PLA as opposed to (D)-PLA. Optimal rates for both film weight loss and initial surface degradation were found for PLA-92. In addition, a large increase in the film weight loss rate for PLA-92 relative to both PLA-85 and PLA-94 indicates that a critical disruption of the crystalline order resulted from the introduction of 8% D repeat units. Comparison of degradation rates for PLA-100 at percent crystallinities of 57 and 27% showed large increases in both film weight loss and initial surface degradation rates for the lower crystallinity sample. Initial surface degradation rates were unexpectedly high for PLA-75 and PLA-50 relative to PLA-92. In contrast, a rapid decline in film weight loss rate values was found for PLA stereochemical compositions from 92 to 50% L. However, qualitatively, the rates of initial surface degradation and film weight loss measurements are in agreement. Residual PLA-100 from enzyme incubations where film weight loss exceeded 25% showed little change in molecular weight and increased Delta H-f values as was expected for an enzyme-catalyzed film degradation process at the: film surface which occurs preferentially at amorphous domains.