Abstract
Polymer dense membranes have gas permselective properties. In general, the gas permeability of glassy polymers decreases with increasing gas size. The permselectivity of a large gas molecule to a small gas molecule is always smaller than 1. In contrast, the gas permeability of rubbery polymers increases with increasing gas size. The permselectivity of a large molecule to a small molecule is then always greater than 1. The transport of gases in polymer dense membranes is thought to obey a solution-diffusion mechanism. The total permeability is a product of the solubility and the diffusivity. Hence the permselectivity is a product of the solubility selectivity and the diffusivity selectivity. The solubility selectivity of a large molecule to a small molecule is larger than 1. In addition, the solubility selectivity of glassy polymers is the same as that of rubbery polymers. On the other hand, the diffusivity selectivity of a large molecule to a small molecule is always smaller than 1. Glassy polymers have stronger size-sieving ability compared to rubbery polymers. Therefore the diffusivity selectivity of a large molecule to a small molecule in glassy polymers is much smaller than that in rubbery polymers. The dominant factor to determine the permselectivity is the diffusivity selectivity for glassy polymers and the solubility selectivity for rubbery polymers. Interestingly some highly permeable glassy substituted polyacetylenes show the permeation properties opposite to other glassy polymers. Most glassy substituted polyacetylenes obey the behavior of common glassy polymers. However, some of them show the transport behavior like rubbery polymers. Their gas permeability increases with increasing gas size. Because these polymers have much larger fractional free volume compared to common glassy polymers, they show weak size-sieving ability like rubbery polymers. As a result, the solubility selectivity is dominant relative to the diffusivity selectivity unlike common glassy polymers.
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