Measuring S, D, and

The permeability P of a gas through a polymer can be measured directly by determining the transport of mass through a membrane per unit time. The sorption constant S can be measured by placing a saturated sample into an environment, which allows the sample to desorb and measure the loss of weight. As shown in Fig. 2.63, it is common to plot the ratio of concentration of absorbed substance c t) to saturation coefficient Coo with respect to the root of time. 

The diffusion coefficient D is determined using sorption curves as the one shown in Fig. 2.63. Using the slope of the curve, a, we can compute the diffusion coefficient as 

Another method uses the lag time, to, from the beginning of the permeation process until the equilibrium permeation has occurred, as shown in Fig. 2.64. Here, the diffusion coefficient is calculated using 

Permeation of nitrogen through polyethylene films of various densities. 

The most important techniques used to determine gas permeability of polymers are the ISO 2556, DIN 53 380 and ASTM D 1434 standard tests. 

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