This used to be the Title for Frequently Asked Questions

(and now it's just a title for a lowly chart)

Generalized Rules for Gas Permeability

Here is a reproduction of that chart that I mentioned, originally written up by Freidlander and Rickles.

Essentially, it lists principal variables and how they effect gas permeation rate.

    Temperature
    1. Arrhenius-type effect - for permanent and other small size gases
    2. Activation Energy is function of temperature for CO2 and H20 above glass temperature
    3. Arrhenius-type effect foes not hold for organics

    Pressure, Concentration
    1. For permanent gases and CO2, independent of pressure
    2. For most other gases, permeability is a function of pressure

    Penetrant nature
    1. Permeability is a decreasing function of molecular diameter
    2. Mutual compatibility and ease of condensation increases permeability

    Plasticizing effect
    1. Ability to plasticize increases permeability

    Polymer structure
    1. Crystalling region acts as an impermeable phase so rates are proportional to volume fraction of amorphous phase
    2. Thermal and mechanical working history is important
    3. Solvent conditioning is important
    4. Cross linking reduces diffusion rates

    Grafting
    1. Grafting greatly affects permeability

    Irradiation
    1. Irradiation causes cross-linking, which reduces diffusion

    Film Thickness
    1. Usually has no effect, but may be important for very thin films or for films with large concentration gradients.