In Reply to: CO2 solubility in Polycarbonate posted by Kathy Fagan on October 31, 2006 at 20:42:57:

Dear Kathy Fagan,

In the initial syringe situation the partial pressure of carbon dioxide CO2 is approximately 0.65 bar and N2 is 0.35 bar. Realize that the atmospheric outside N2 pressure is 0.78 bar: the chemical potential of the outside nitrogen N2 is higher than in the syringe (the gas mixture in the syringe can be assumed to be ideal). There will be a netto flux of nitrogen N2 gas and oxygen O2 gas into the syringe, and a flux of carbon dioxide CO2 out of the syringe in the first stage. Therefore the relative concentration of CO2 gas drops down fast.

When the Nitrogen and Oxygen approach equilibrium - the partial pressure of Nitrogen in the syringe becomes around 0.78 bar and Oxygen approaches 0.21 bar, the Carbon Dioxide relative concentration drops more slowly as the partial pressure of Carbon Dioxide gas is getting lower and the "dilution" effect by Nitrogen and Oxygen cancels out.

A good polymeric material gives a sufficient time lag before the mass transfer situation as described will occur. Furthermore Carbon Dioxide gas has a high solubility in Polycarbonate which is making the loss situation worse than when you would for example use a good grade of PET (highly crystalline).

The phenomenon can be solved by use of the figures in the gas diffusion section and the appropriate (Maxwell-Stefan) mass balances.