Diffusion of Gases through Polymers and Metals

In Steps 1 and 3, the gas is assumed to be in instantaneous equilibrium with the neighboring solid matrix. The solid-phase concentration C is related to the gas pressure by a linear relation, given as 

External gas pressures and internal solid-phase concentration in the diffusion of gases through polymers. 

A dual approach is also used in the formulation of the diffusional rate laws. In the first version. Pick s law is used with concentrations as defined above i.e., we have 

The second and preferred version uses a pressure gradient and leads to the rate law 

Here P is the so-called permeability, which is most commonly expressed in units of cm (STP) x cm/cm s or cm (STP) x cm/cm s Pa. Several other additional units are still in current use and the translation from one set of units to another is a frequent necessity. To ease this task, we have provided in Table 3.5 a listing of e most commonly required conversion factors. 

At the high-pressure interface, the gas dissolves or condenses in the solid matrix. 

Following dissolution or condensation, the gas diffuses along a solid-phase concentration gradient dC/dx in accordance with Fick s law to the low-pressure interface. 

On arrival at the low-pressure gas-solid boundary, the dissolved gas is desorbed or released to the gaseous medium. 

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