Plasticized polylactide for food packaging applications : the effect of the molecular weight of the plasticizer on the free volume and transport properties )

Nowadays the main strategy to solve the problem of plastic waste is to use biodegradable polymers. Among them, polylactide has attracted a great attention. However in order to be suitable for packaging applications its ductility must be improved. With this aim, polylactide has been blended with polyethylene glycol (PEG). More exactly, three different polyethylene glycols have been used and the effect of the molecular weight on the physical properties has been analysed. The addition of the plasticizer reduces the glass transition temperature, especially the polyethylene glycol with the highest molecular weight, and the crystallinity level is increased. The changes in the free volume fraction are small but with high plasticizer content the free volume fraction increases considerably. Regarding the barrier character, the oxygen permeability is reduced or maintained, whereas carbon dioxide and water permeability increase with the addition of the plasticizer. It is worthy to note that among the blends containing different plasticizers, the system containing the plasticizer with the lowest molecular weight shows the highest permeability values. In the case of systems containing PEG 1500 and PEG 4600 the changes are subtle and apart from the molecular weight, other factors such as the glass transition temperature, the degree of crystallinity and the free volume influence the transport properties.