Evaluating sustainable materials for membrane separations through molecular simulations: the case of Polyxydroxyalkanoates (PHA)

Polyxydroxyalkanoates (PHA) are a family of linear optically active semi-crystalline polyesters produced by bacterial fermentation, known for their overall sustainability, including biodegradability and biocompatibility. PHA are also characterized by thermoplasticity and good mechanical properties, comparable to those of commercially relevant standard polymers. The gas transport properties of these materials are still scarcely characterized experimentally, and their determination is complicated by a number of uncertainty sources, such as a time-dependent degree of crystallinity. In this study we aim at evaluating the physicochemical and transport properties of such materials with molecular simulations, to gain information about their applicability in the membrane gas separation field. In order to draw correlation between the molecular structure and the performance of these materials, three homopolymers and two copolymers of the PHA family were considered: • poly(3-hydroxybutyrate) (P3HB); • poly(3-hydroxyvalerate) (P3HV); • poly(4-hydroxybutyrate) (P4HB); • poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV); • poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (PHBB). Molecular models of each material were simulated using Molecular Dynamics (MD), obtaining amorphous density and solubility parameter values, that were successfully validated with experimental data found in literature. The simulated values of radius of gyration, accessible free volume, density, cohesive energy and elastic modulus in the different copolymers were correlated to their chemical composition. Sorption and diffusion in the polymers were then analysed for three gases, O2, CH4 and CO2, by means of Grand Canonical Monte Carlo (GCMC) and MD simulations. The results were compared with experimental values, obtained through permeation tests at different temperatures, performed on PHBV with 8% of 3-hydroxyvalerate monomers purchased from Merck-Sigma.