Permeability glassy polymer membrane

J.Y. Park and D.R. Paul, Correlation and Prediction of Gas Permeability in Glassy Polymer Membrane Materials via a Modified Free Volume Based Group Contribution Method, J. Membr. Sci. 125, 29 (1997). 

The above-mentioned inverse selectivity/permeability relationship of polymers has been summarized by Robeson by means of log-log plots of the overall selectivity versus the permeability coefficient, where A is considered to be the more rapidly permeating gas. These plots were made for a variety of binary gas mixtures from the list He, H2, O2, N2, C02, and CH4, and for a large number of rubbery and glassy polymer membranes. Such representations, shown in Fig. 8 and Fig. 9 are often referred to as upper bound plots (Robeson, 1991). The upper bound lines clearly show the inverse selectivity/permeability relationship of polymer membranes. While these plots were prepared in 1991, only small advances have been made to push the upper bound higher since that time. 

Park, J. Y., and Paul, D. R. (1997). Correlation and prediction of gas permeability in glassy polymer membrane materials via a modified free volume based group contribution method, J. Membrane Sci. 125, 23. 

When the interaction between one penetrant and the polymer is not affected by the presence of another penetrant, the pure-component permeabilities of the two penetrants in the mixture can be used in Equation 7. For rubbery polymers at low penetrant partial pressures, this assumption of independent-permeation appears satisfactory (19-20). It does not, however, appear to hold in general for glassy polymer membranes (12,13,21-25). Moreover, it also has been shown that plasticization of both rubbery (26) and glassy (27) polymers can occur at higher penetrant activities. 

It is known that glassy polymer membranes can have a considerable size-sieving character, reflected mainly in the diffusive term of the transport equation. Many studies have therefore attempted to correlate the diffusion coefficient and the membrane permeability with the size of the penetrant molecules, for instance expressed in terms of the kinetic diameter, Lennard-Jones diameter or critical volume [40]. Since the transport takes place through the available free volume in the material, a correlation between the free volume fraction and transport properties should also exist. Through the years, authors have proposed different equations to correlate transport and FFV, starting with the historical model of Cohen and Turnbull for self diffusion [41], later adapted by Fujita for polymer systans [42]. Park and Paul adopted a somewhat simpler form of this equation to correlate the permeability coefficient with fractional free volume [43] ... 

Poly(4-methyl-2-pentyne) [PMP] is a glassy, disubstituted, purely hydrocarbon-based polyacetylene. PMP has a density of only 0.78 g/cm and a high fractional free volume of 0.28. The polymer has very high hydrocarbon permeabilities for example, the /i-butane permeability of PMP in a mixture of 2 mol% n-butane in methane is 7,500 X lO l cm3(STP) cm/cm2 s cmHg at 25 C. In contrast to conventional, low-free-volume glassy polymer membranes, PMP is significantly more permeable to n-butane than to methane in gas mixtures. In this paper, we present the gas permeation properties of PMP in mixtures of -butane with methane. The mixed-gas permeation and physical aging properties of PMP are compared to those of poly(l-trimethylsilyl-l-propyne), the most permeable polymer known. 

Poly(substituted acetylene)s such as PTMSP and PMP, amorphous fluoro-polymers like Teflon AF and Hyflon AD, polymers with intrinsic microporosity, and thermally rearranged (TR) polymers are the candidate polymers for highly permeable glassy polymer membranes. The high free volume in glassy polymers contributes to enhanced diffusion and permeation of small gas molecules. The gas permeation performances of these highly permeable polymers even surpass upper bounds for CO2/N2, CO2/CH4 and H2/CO2 separations. 

Fig. 38. Permeability as a function of molar volume for a mbbery and glassy polymer, illustrating the different balance between sorption and diffusion in these polymer types. The mbbery membrane is highly permeable the permeability increases rapidly with increasing permeant size because sorption dominates. The glassy membrane is much less permeable the permeability decreases with increasing permeant size because diffusion dominates (84).

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