Estimation of gas permeability

Given the relationship, P = D S, the permeability can be calculated from knowledge of the solubility of the gas in polymer and its diffusion coefficient. 

Due to the very low solubility of normal gases in polymers the S values for various polymers do not differ very strongly from one another. The sorption coefficient S (298) at 25 °C can be estimated quite well in an amorphous polymer in its rubbery state with help from the following formula (van Krevelen, 1990)  

For amorphous polymers in their glassy states, the following expressions analogous to Eqs. (9-17) and (9-18) are used  

Using these expressions the solubility of a gas at a given temperature, S(T), in an amorphous polymer can be estimated  

If the degree of crystallinity in the polymer is designated with xc and Sa is designated as the solubility of the gas at 25 °C in the amorphous polymer, then Eq. (9-21a) can be used for an approximation of the solubility in a semi-crystalline polymer  

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