Biodegradable materials from grafting of modified PLA onto starch nanocrystals

PLA was grafted onto starch nanoparticles using a novel synthetic strategy consisting of three reaction steps. The first step was aimed to protect the hydroxyl groups of PLA by benzoylation (PLABz), the second one involved the activation of carboxyl groups using thionyl chloride and the last reaction was the grafting of the modified PLA onto the starch nanoparticles (PLASTARCH). The thermal behavior of the composite obtained by this method was very different from that displayed by the physical mixture of PLA and the starch nanoparticles (PLA-NC blend). The benzoylation step that leads to PLABz produces an increase of the molecular mobility, resulting in lower glass transition temperature, Tg, than that of the original PLA; a change that was observed in the DSC thermograms of the samples. On the other hand, the Tg of the PLASTARCH was similar to that of the PLA as a consequence of two opposite effects acting simultaneously: a free volume increase due to the presence of benzoyl groups and a confinement of the polymer chain, originating from the grafting onto NC. The material obtained by chemical modification (PLASTARCH) has a degradation temperature slightly lower than that of PLA, which does not affect its potential use in the packaging industry.