Gas diffusion in Polymer of Intrinsic Microporosity: an experimental and computational approach

Polymers of Intrinsic Microporosity (PIMs) show exceptional gas separation performance due to their unique molecular structure [1]. Their highly rigid and contorted structure causes a high fractional free volume, which in turn confers very high permeability. The rigidity of the fused rings in the backbone is the responsible of their molecular sieve properties. They accomplish what Freeman [2] suggested in 1999 when he stated that high free volume with very stiff chain are necessary to develop high performing membranes, indicating that the selectivity for diffusivity (Dx/Dy) is the key factor to go beyond the Robeson's trade-off. In this work, we compare the gas diffusion through PIMs with the gas diffusion in other polymers commonly used for gas separation membranes. The figure shows the diffusion coefficient of PIM-BTrip-TB in comparison with the rubbery PEBAX®2533 and a well-known high free volume polymer, the Teflon AF2400. The figure demonstrates a high molecular sieve-like [3] size-selectivity for the larger gas molecules due to the extremely stiff polymer structure. The discussion will focus on how this behaviour is related to the PIMs' molecular structure and to the properties of the permeating gas molecules. The structure will be characterized by molecular modelling, while the transport properties will be studied via the experimental analysis of the gas diffusion by the time lag method. The transport properties will then be explained in terms of accessible free volume of the polymer in relationship with the effective size [4] of the permeating species. Acknowledgements: This research received funding from the EU's Seventh Framework Program, GA 608490, project M4CO2. References [1] BD Freeman Macromolecules, 1999, 32, 375 [2] NB McKeown, PM Budd; Macromolecules, 2010, 43, 5163. [3] M Carta et al.; Science, 2013, 339, 303. [4] V Teplyakov, P Meares; Gas Sep Purif, 1990, 4, 66