Institute on Membrane Technology, CNR-ITM, Via P. Bucci 17/C, 87036 Rende (CS), Italy
Institute of Sustainable Processes, University of Valladolid, Dr. Mergelina s/n., 47011 Valladolid, Spain
University of Edinburgh Joseph Black Building David Brewster Road Edinburgh EH9 3FJ
In the search for more efficient gas separation membranes, blends offer a compromise between costly high-performance polymers and low-cost commercial polymers. Here, blends of the polymer of intrinsic microporosity, AO-PIM-1, and commercial Matrimid® 5218 polyimide are used to prepare dense films by solution casting. The morphology of the pure polymers and their blends with 20, 40, 60 and 80 wt% of AO-PIM-1 in Matrimid® are studied by scanning electron microscopy (SEM), and their pure gas permeability is studied as a function of the blend composition with H2, He, O2, N2, CH4 and CO2. The polymers were found only partially miscible and a two-phase structure was formed with large domains of each polymer. When necessary, the films were coated with a thin silicone layer to heal possible pinhole defects. Even small amounts of Matrimid® in AO-PIM-1 resulted in an unexpectedly strong decrease in the permeability of the PIM, whereas a small amount of the PIM led to a modest increase in permeability of Matrimid®. Due to the two-hase structure, the Maxwell model was more suitable to describe the gas permeability as a function of the blend composition than the model for miscible blends. At low Matrimid® concentrations in AO-PIM-1, all models fail to describe the experimental data due to an unexpectedly strong depression of the permeability of the PIM by Matrimid®. Time lag measurements reveal that the changes in permeability as a function of the blend composition are mostly due to changes in the diffusion coefficient.