Permeation of gases

The study of the permeation of gases is the simplest case both theoretically and experimentally and is therefore treated first. 

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The permeabUity of ceUular polymers to gases and vapors depends on the fraction of open ceUs as weU as the polymer-phase composition and state. The presence of open ceUs in a foam allows gases and vapors to permeate the ceU stmcture by diffusion and convection dow, yielding very large permeation rates. In closed-ceUed foams the permeation of gases or vapors is governed by composition of the polymer phase, gas composition, density, and ceUular stmcture of the foam (194,199,215,218,219). 

Fig. 36. Mechanisms for permeation of gases through porous and dense gas-separation membranes.

Rogers, C.E. 1985, Permeation of Gases Vapours in Polymers. In Polymer Permeability, Comyn, J., Ed. Elsevier Applied Science LTD London, England pp. 11-73. 

Permeation of gases, liquids, and vapors through a polymeric film can be looked at as a three-step process as follows (1) the rate of dissolution of the small molecules in the polymer, (2) the rate of diffusion of the small molecules in the polymer film in accordance with the concentration gradient, and (3) the energies of the smaller molecules on the opposite side of the polymer. 

From the above table it is seen that the coated filler accommodates more carbon dioxide leading to blistering in the seal, which is known as explosive decompression. It becomes evident that rubber compounds that have low permeation rates are chemically resisting the permeation of gases and are the best to minimize blistering due to the cavity or vacuoles in the rubber or polymer matrix. 

Permeation of gases in glassy polymers can also be described in terms of the dual sorption model. One diffusion coefficient (Do) is used for the portion of the gas dissolved in the polymer according to the Henry s law expression and a second, somewhat larger, diffusion coefficient (DH) for the portion of the gas contained in the excess free volume. The Fick s law expression for flux through the membrane has the form... 

Permeation of gases, water vapor and volatile organic compounds... 

P. Mercea, L. Muresan, V. Mecea, D. Silipas, and I. Ursu, Permeation of gases through poly(ethyleneterephthalate) membranes metallized with palladium, J, Membr, ScL i5 19 (1988). 

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