Chemical Structure of the Permeant Molecule

The size of the permeant molecule, as well as the chemical affinity between the permeant and the polymer, is an important determinant of permeability. Polymers can act as molecular sieves, allowing some molecules to pass through rapidly while retarding the passage of others. This is the principle used industrially in polymer membrane separation of gas blends. 

The size of a permeant molecule Is related to its molecular weight, but this is an inaccurate measure. The van der Waals diameter is a good measure for isotropic molecules, but many permeants of interest, such as n-alkanes, are strongly anisotropic. How best to compare the sizes of isotropic and anisotropic molecules is an issue that has not been resolved. 

Van Krevelen [1] has presented an empirical relationship between relative values of the permeability of a polymer to various gases, which has been found to have reasonable accuracy for a broad range of polymers. Values are shown in Table 14.6. 

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