Adverse Effect of Polyelectrolyte Complexation on the CO₂ Permeability of Polyvinylamine Copolymers

Polyvinylamine (PVAm) is one of the most studied polymers for facilitated transport membranes (FTMs) in CO₂ separation applications. In this study, poly­(N-vinylformamide) (PNVf) was hydrolyzed to prepare a range of PNVf-PVAm copolymer compositions, and the effect of the PVAm fraction on the CO₂ permeability and CO₂/N₂ selectivity of these polymers was examined. The most permeable film was the polymer containing 52 mol % PVAm units with a CO₂ permeability of 353 Barrer and CO₂/N₂ selectivity of 96 when tested at 60 °C with humidified 14/86 CO₂/N₂ mixed gas. Despite containing a higher loading of amines to act as CO₂ carriers, higher PVAm contents yielded significantly lower CO₂ permeability, without a loss of selectivity. This trend was not due to polymer crystallization since none of the PVAm films exhibited crystallinity in X-ray diffraction analysis. The loss of gas permeability at high PVAm content was instead due to strong polyelectrolyte interactions between polymer chains formed when PVAm reacts with CO₂. Since only the PVAm units are ionized upon reaction with CO₂, with the PNVf units remaining neutral, the density of these ionic effects increased as PVAm content increased, leading to increased resistance to gas permeation.