Retarding hydrolytic degradation of polylactic acid: Effect of induced crystallinity and graphene addition

The combination of elevated temperature and humidity leads to rapid degradation of polylactic acid (PLA) because of hydrolysis. Consequently, PLA, which is a bio-derived and biodegradable polymer, is not currently used for durable applications since properties cannot always be maintained over time. In this work, the ability of polymer crystals to reduce the rate of degradation during accelerated aging tests was studied. Also examined was the influence of addition of 2 wt % graphene nanoplatelets to act as moisture transport barriers in the polymer. PLA samples were immersed in aqueous media of different pH or exposed to 100% relative humidity at 50 degrees C for different lengths of time to study the hydrolytic degradation behavior. In addition to monitoring the loss in mass of the samples, the values of crystallinity, melt viscosity, and mechanical properties, among others, were measured as functions of aging time using techniques such as DSC and rheometry. It was found that both crystallization and graphene addition are able to slow down the rate of degradation at short times, but significant degradation of PLA still occurs at long times. This is because PLA crystallites and graphene nanoplatelets can only reduce, but not eliminate, moisture diffusion into the polymer sample. Between the use of nanoplatelets and crystals, though, the former approach may be the better choice since enhanced crystallization tends to make PLA brittle. (c) 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44166.