The effects of the crystallinity and polymorphism of PLLA and PDLA–PLLA 50:50 blends on the free volume and transport properties have been studied. In the case of PDLA–PLLA 50:50 the increase on crystallinity promotes a process of densification and a reduction in free volume fraction and rigid amorphous fraction (RAF), contrary to PLLA in which the enlargement of a rigid amorphous fraction led to a dedensification. This result offers a unique opportunity to analyze separately the opposite influence in the transport properties of the crystallization and free volume associated with RAF. Overall, these findings provide a better understanding on the relationship between crystallinity and transport properties and would explain also the controversial data reported in the literature.

The effectiveness of tributyl citrate (TbC) as a plasticizer for polylactide and polyhydroxybutyrate was analysed in order to improve the ductility of these polymers and make them good candidates for food packaging applications. Although the thermal and mechanical properties have been widely studied in the literature, the effect of the plasticizer on free volume and transport properties has not been deeply analysed. The free volume was characterized using positron annihilation lifetime spectroscopy observing its linear increase with TbC content. The permeability to water vapour, oxygen and carbon dioxide was determined and the obtained results were related to the changes in glass transition temperature, level of crystallinity of the samples and free volume. This work would allow a better understanding of the effect of the plasticizer on the barrier and mechanical properties of polymers allowing the development of competitive materials for packaging applications. © 2018 Society of Chemical Industry

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