Microinjection molded PLA parts with modulated degradation rates

In this work, an original and versatile system to dynamically control the local temperature of the cavity surfaces in microinjection molding was adopted to obtain 200µm thick PLA parts having a given distribution of crystalline and amorphous zones. The system allows to selectively change temperature of specific regions of the mold surface. This makes it possible not only to allow the complete filling of the mold even with thin cavities and highly viscous polymers, thanks to a high surface temperature during filling, but also to obtain, in the same microinjected part, regions presenting a higher degree of crystallinity and completely amorphous regions. The local morphology of the parts was investigated with different techniques (DSC, WAXS, microscopy) to understand the effect of the peculiar thermal history on the material. Given the well-known higher resistance of PLA crystalline regions to hydrolysis, the same molded parts naturally presents region with a faster degradation rate and regions with slower degradation rates. The degradation rates were assessed by immersing the samples in water at 58°C and measuring by GPC the molecular mass of samples taken from different regions of the parts. The preliminary results obtained are encouraging toward the possibility of effectively obtaining parts having selected regions with time modulated degradation rates.