Facilitated transport membranes (FTMs) make use of a reversible reaction between the target solute and chemical sites in the membrane, termed “carriers”, to improve the permselectivity for the solute. Polyvinylamine (PVAm) is the most studied polymeric amine for CO₂ separation FTMs due to its high density of amine carriers and excellent film forming properties. PVAm prepared by partial hydrolysis of poly(N-vinylformamide) (PNVf) is a random copolymer of vinylamine and N-vinylformamide repeat units. Since CO₂ transport through the membrane is enhanced by reaction with the amine carriers, a higher concentration of carrier sites should lead to improved CO₂ permeability, but this is not the case.
PVAm with a range of amine contents was prepared from a single batch of PNVf. Bulk films of these PVAm’s were then permeation tested under the following conditions: 14/86 CO₂/N₂ mixed gas, 95% relative humidity, 60 °C. The PVAm with 52 mol% amine content had the highest CO₂ permeability and selectivity, while further increasing amine content reduced CO₂ permeability while retaining selectivity. Contrary to prior reports which attribute such trends to increases in PVAm crystallinity, XRD and DMA experiments showed the PVAm films were amorphous across the composition range. Instead, our rheology experiments strongly suggest that the CO₂-amine reaction products lead to a reduction in polymer chain mobility, which in turn reduces gas diffusivity in the film. This effect becomes more significant as amine content in the polymer increases.

Effect of PVAm content on the CO<sub>2</sub> permeability of films tested with 14/86 CO<sub>2</sub>/N<sub>2</sub> mixed gas at 95% relative humdity and 60 °C.

Effect of PVAm content on the CO₂ permeability of films tested with 14/86 CO₂/N₂ mixed gas at 95% relative humdity and 60 °C.

James S. Baker

Research Chemist, NETL- Battelle Memorial Institute

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