Experimental measurement of gas permeability

The measurement of sorption, diffusion and permeability coefficients takes place as a rule using one of three methods sorption of the gases in the polymer, permeation through a membrane (film or sheet) into a sealed container or permeation through a membrane into a gas stream. As far as possible sorption methods should be used together with permeation methods that are specific for the measured gas/polymer system in order to uncover any possible anomalies or errors in the measurements by comparison of results. 

Sorption The method allows a direct measurement of S from the equilibrium between gas and polymer. The volume of gas taken up by the polymer can be measured gravimetrically, manometrically or using a gas specific detector. Headspace gas chromatography is an useful technique for this. The S values then come from  

Where me is the amount of gas absorbed in the volume V of the polymer at equilibrium and pc is the corresponding partial pressure of the gas being studied. In order to check if S stays constant, one measures the me value for several pressures pe at the same temperature. S comes then from the slope of the straight line me = f(pe). Deviations from linearity of the line means deviations from Henry s law. 

Permeation in a sealed container The amount of gas mt found in the container s chamber as a result of permeation through the film is plotted as a function of time t (Fig. 9-1). The partial pressure p] of the gas permeating into the film is held constant. Normally the chamber volume is so large that even upon reaching steady state permeation the partial pressure p2 of the permeated gas remains negligibly small (p2 Pi)- 

A simple method is to measure the increase in pressure (p2) over time in an initially evacuated chamber. Another possibility is the use of sorbents in the chamber so that the partial pressure (p2) is kept practically at zero. The amount of permeating gas can be gravimetrically determined with the help of the sorbent. It is also possible to use 

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