Permeation of permanent gases

In amorphous or rubbery polymers, a permanent gas forms only a very dilute solution in the polymer. Consequently, there is no distortion of the polymer matrix by the gas and the diffusion coefficient is independent of the amount of 

In a homogeneous substrate the diffusivity and permeability are isotropic. However, the polymer may have been orientated during processing. If that should be the case, permeation is greater along the orientation dimension than perpendicular to it. 

It has been found that polymers with bulky groups on the backbone or in the side chain have lower permeabilities than sterically less hindered materials. 

In both the homogeneous amorphous phase and the crystal defects, the gas diffusivity is a function of molecular size. Obviously, the smaller molecules have a greater diffusivity than the larger ones. However, the critical size factor, or selectivity, is different in the two phases. 

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Figure 1. Temperature dependence of (a) Permeance and (b) Flux for permeation of permanent gases and light hydrocarbons through silicalite membranes.

Of all the proportioning principles already known from the production of calibration gas from permanent gas, only the permeation method can be used because the diffusion process is not bound to the gaseous state and even for the dosage of permanent gases the permeation tubes contain two-phase mixtures. This method, as well as the mixed gas cylinders, are limited to the trace domain because the tubes only contain insignificant amounts. 

If a gas at two different pressures is separated by a membxane or a film, then the gas will permeate through the membrane until the pressures on each side are equal. If the pressure difference, Ap, is kept constant, then, according to Henry s law, which holds for the simplest case of permanent gases, a concentration difference for the gas on each side of the membrane is formed according to... 

Until now we have considered the simplest case of more or less ideal permeation behaviour Henry s law for sorption (sorbed penetrant randomly dispersed within the polymer) and Fick s first law for diffusion (diffusion coefficient independent of the concentration of the sorbed penetrant). This ideal behaviour is observed in practice only when "permanent gases" are the penetrants and if the gas pressure is nearly atmospheric. In this case there are no strong polymer-penetrant interactions and no specific interactions between the penetrant molecules. 

For the separation of gas mixtures (permanent gases and/or condensable vapors) where the feed and permeate streams are both gas phase, the driving force across the membrane is the partial pressure difference. The membrane is typically a dense film and the transport mechanism is sorption-diffusion. The dual-mode transport model is typically used with polymer materials that are below their glass transition temperature. 

Several recent studies report gas permeation, sorption, and free volume characteristics of these two amorphous fluoropolymers. Nemser and Roman (1991 Nemser, 1993) reported permeability properties of Teflon AF polymer membranes prepared by a melt-press method. Similar to PTMSP, these polymers are permeable to permanent gases. For example, oxygen... 

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