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anomalous diffusion of CO2 in membrane materials

Posting by jonathan vermette on April 28, 2008 at 18:56:50.

I'm studying carbon dioxide (CO2) diffusion through PDMS (poly dimethyl siloxane polymer, silicone rubber) thin membrane (0.02''). The feed pressure "Po" of carbon dioxide is held constant over the diffusion time.

For small values of Po (5 Torr), the flux of CO2 passing through the membrane in function of time as a Fickian diffusion. The probleme occurs when I increase the Po where I observe a strange diffusion behavior.

At the beginning of the plot, the flux of CO2 shows a normal increase. At maximal flux, where the steedy state is suppose to occur, the curve suddenly begin to drop and after is stabilizing at a steedy state. It look's like a Gaussien curve but the two end point (flat line) are not at the same "y" values. The second point is higher than the first one.

I did a research on non-Fickian diffusion and permeation but I didn't found anything like that (although there is a lot of literature on carbon dioxide, water and carbonic acid in polymer pipeline coatings and plastics linings of steel and composite pipelines).

Is there someone who can help me please!

Jonathan Vermette

          follow up posts
    On 04/29/2008 composite agency posts: Dear Jonathan Vermette,

    In this case the diffusion coefficient seemingly decreases, with an increase of concentration of carbon dioxide in the elastomer membrane. This is the opposite of what normally happens with plasticizing and swelling: an increase of the diffusivity (or decrease of the mutual friction coefficient) as function of the permeant concentration.

    From various rubbers this is known to happen with molecules like H2O and CO2. These molecules cause a better ordening and dipole-dipole / hydrogen bonding in the chain structure of these rubbers.

    Mind that filler particles do also influence the diffusion rate and the solubility of small molecules in elastomers and rubbers. From carbon fillers it is known that they cause accumulation of molecules such as helium, hydrogen, nitrogen, carbon dioxide and water around their internal surface. This diffusion with simultaneous accumulation, causes a decrease in overall diffusion rate and an increase in solubility.

    Maybe you could provide us with some more specific information, such as the laboratory test results - carbon dioxide pressure build-up as a function of time, partial pressure and thickness of poly dimethyl siloxane polymer.

    By the way: mind that for carbon dioxide and flue gas scrubbing or transport Sulfonated Polyether Etherketone (S - PEEK) is also an option (probably an issue because of global warming, world wide reduction of carbon dioxide exhaust). For pipeline coating purposes an epoxy would probably be a good solution.

    Regards
    [responses: 1]

      On 05/02/2008 jonanathan vermette posts: Hi!

      See the graph:

      For the pressure and the thickness:

      5 torr = 5 mm Hg
      2"(inch) = 508 micro meter

      The membrane surface is 10,21 cm^2

      We test three membranes of differents thickness (280, 508 and 970 micro meter). The same behavior occurs with differents thickness. But thicker is the membrane higher the feed pressure must be to see the strange behavior.

      I think that CO2 alters the structure of the polymer but in this case, I don't understand why there is a flux fall after the higher flux is achieved. If it was cristallization, the steady-state should not have been lowered instead of the present behavior ? 'cause cristallization should lower the polymeric chains mobility and then the gaz diffusion ?

      In other hand, if plastizisation occur (as we think) the steady-state should have been higher but not modified (falling. Maybe, the CO2 effect is high for a short period of time and the the carbon dioxide distribute equally in the membrane causing the flux to fall we are observing. Is this explanation correct ???

      Jonathan Vermette
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      [responses: 0]


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