Key factors for tuning hydrolytic degradation of polylactide / zinc oxide nanocomposites

The hydrolytic degradation of thin films of polylactide/surface treated zinc oxide [poly(lactic acid) (PLA)/ZnOs] nanocomposites was investigated in phosphate buffer solution at the temperature of 378 degrees C for more than 10 months. To produce PLA/ZnOs nanocomposites, the previously silanized metal oxide nanofiller has been dispersed into PLA by melt blending using twin screw extruders and the resulting dried pellets were shaped into thin films of about 70 mm thickness. For sake of comparison, pristine PLA was processed and investigated under similar conditions. The evolution of molecular weights of the PLA matrix, as well as of crystallinity and thermal parameters of interest, with the hydrolysis time, has been recorded by size exclusion chromatography (SEC) and differential scanning calorimetry (DSC), respectively. Accordingly, at longer hydrolysis time, the nanocomposites revealed better resistence to the hydrolytic degradation (lower weight loss, smaller decrease of molecular mass, no dramatic increase in dispersity), data that were also associated with the changes in the morphology of specimens over time as evidenced by visual analysis or by microscopy. The results show the possibility to tune the hydrolytic degradation and prolonging the service life of PLA throught the incorporation of a small amount of hydrophobic silanized nanofiller (ZnOs). A bulk degradation mechanism was assumed, whereas the delayed degradation of nanocomposites was ascribed to a slowdown of the water diffusion into PLA matrix thanks to important increases of the crystallinity and especially to the hydrophobic properties of ZnO nanofiller treated with similar to 3 wt-% triethoxycaprylylsilane. Accordingly, the rate of hydrolytic degradation of PLA/ZnOs nanocomposite films can be reduced by increasing the loading of nanofiller and PLA crystallinity.