New sustainable routes for gas separation membranes: The properties of poly(hydroxybutyrate-co-hydroxyvalerate) cast from green solvents

The gas separation performance of biopolymers is still scarcely characterized, mostly because of their poor thermomechanical properties and high crystallinity which is associated to low permeability. In this work we characterize the gas transport in a poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) random copolymer, that has a relatively low crystallinity and good mechanical properties, combined with a renewable origin, biodegradability and biocompatibility. In the study we also compared several solvents for membrane casting, with different toxicity levels. We found that dimethyl carbonate allows the production of polymer films with transport properties similar to those obtained with the more toxic CHCl3, and it leads also to stable crystallinity of the samples over time. PHBV films show a size-sieving gas separation behaviour, as the permeability decreases significantly with the gas kinetic diameter. However, the strong energetic interactions of CO2 with the polymer matrix, confirmed by the Flory-Huggins model, induce a marked solubility-driven CO2/N2 and CO2/CH4 selectivity, which could make the material potentially interesting for CO2 removal processes.