Effect of thermal treatment on sterility, molecular and mechanical properties of various polylactides. 2. Poly(L/D-lactide) and poly(L/DL-lactide).
Pins for orthopaedic applications injection-moulded from as-supplied poly(L/D-lactide) 95/5% and poly(L/DL-lactide) 95/5%, raw as-supplied polymers or raw methanol-extracted polymers were heat-treated for a predetermined time at 135 degrees C under moisture-free argon. The sterility, molecular weight, polydispersity, mechanical properties, and crystallinity of the heat-treated samples were evaluated. All the samples heat treated under argon were sterile after 2 h exposure to heat. The heat-treated pins and raw as-supplied polymers showed a continuous decrease in molecular weight over the entire 50 h of heating, the decrease being more substantial for poly(L/D-lactide) than poly(L/DL-lactide). Raw methanol-extracted polymers showed an increase of molecular weight after 2 h of heat treatment, followed by a gradual decrease of molecular weight up to 50 h. A drop in the bending strength, an increase in the bending moduli, and no change in the shear strength was observed for polylactide pins during the first 5 h of thermal treatment. For both the polymers, there was a progressive increase in crystallinity over time of thermal treatment, and practically no change in the melting temperature.