Relationship between gas separation properties and chemical structure in a series of aromatic polyimides

Gas permeabilities and permselectivities for various gas pairs [ He/CH,, CO,/CH,, N. H,, and 0,/N,]are reported for a series of aromatic polyimides synthesized in our laboratory. Gas solubility and diffusivity data are also reported for some of the polyimides. Systematic variations in chemical structure were found to lead to significant changes in permeabilities and selectivities. Further analysis shows that the permeability and selectivity changes in these cases are primarily due to the alteration of diffusivity factors. Generally, increases in permeability are attended by losses in permselectivity; however, some of the polyimides in the present series of materials were found to deviate very favorably from this typical behavior. Specifically, materials were discovered having simultaneously higher permeabilities and permselectivities than commercial polymers cur- rently being used as membrane materials. The relationship between gas separation properties and chemical structure is discussed in terms of intrasegmental mobility and intersegmental packing of the constituent polymers. The experimental data and their physical implications suggest the possibility that many new polymers having both high permeability and selectivity can be designed by tailoring the intrasegmental mobility and intersegmental packing of membrane polymers.