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 Vermette
follow up posts
I am interested in this topic, could you tell me how do you happen to think this problem?
Dear Jiahuan Jiang,
The observed diffusion mechanism is due to a dual sorption mode of CO2 (and also often H2O) with regard to several materials (among which are PDMS and more rigid materials like Polyimide). One part is relative mobile, another part temporarly binds to the polymer chain at adsorption site. The fact that the kinetics of binding are much slower than the kinetic of diffusion, results in the visibility of the phenomenon in the time scale of interest. Please let us know if you need a more rigorous quantification of the phenomenon in laboratory or real-life circumstances.
Kind Regards,
Composite Agency
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 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.
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
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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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