Diffusion Coefficient Polymers Water

anomalous diffusion of co2 in membrane materials

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 polymer membrane in function of time as a Fickian diffusion. The problem 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 in plastics and polymers, but I didn't found anything like that (although there is a lot of literature on the anomalous diffusion of Water in PDMS elastomers and rubbers).

Is there someone who can help me please!

Jonathan Dear Jonathan,

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 Water 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.

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