High gas permeability in open-structure membranes |
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Authors: | Guangxiang?Wu Catherine?L?Bothe?Almquist Email author" target="_blank">Sun-Tak?HwangEmail author |
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Affiliation: | (1) Department of Chemical and Materials Engineering, University of Cincinnati, OH 45221-0012, Cincinnati, USA;(2) Dept. of Chemistry, Purdue University, IN 47907, West Lafayette, USA;(3) Paper Science and Engineering, Miami University, OH 45056, Oxford, USA |
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Abstract: | For most polymeric membranes, the gas permeability coefficient (P) is often interpreted as the product of diffusivity (D)
and solubility (S) of a penetrant gas in the polymer (P=D S). The basic assumption is that molecular diffusion is primarily
responsible for mass transport in the membrane permeation process. However, for some open structure membranes, such as poly(1-trimethylsilyl-1-propyne)
PTMSP] or poly(dimethylsiloxane) PDMS], the high permeabilities of some gases yield much higher diffusivities when calculated
from the above relationship (P=D S) than when calculated by using the direct kinetic measurement of diffusivity. It is hypothesized
that this discrepancy is due to the convective transport of gas molecules through such open structured polymers. In most cases,
the convective contribution to mass transport through membranes is negligible. However, for polymer membranes with high free
volume, such as PTMSP, whose free volume fraction is 20 to 25%, the convective term may dominate the permeation flux. In this
study, a non-equilibrium thermodynamic formalism is employed to properly treat the diffusion term and convective term that
constitute the Nernst-Planck equation. The current analysis indicates that the total permeation flux, which consists of a
diffusion term and a convective term, agrees well with the experimental data for several permeation systems: pure components
propane and n-butane/PTMSP, pure gas hydrogen/PTMSP, and mixed gas hydrogen/PTMSP. Also, the permeation systems of a nonporous
rubbery membrane, PDMS, and eight organophosphorus compounds were included in the study. It is recommended that the proposed
model be validated by using other polymers with high free volumes and high permeabilities of gases and vapors, such as poly(1-trimethylgermyl-1-propyne)
PTMGeP] and poly(4-methyl-2-pentyne) PMP].
This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University. |
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Keywords: | Permeation Flux Diffusion Flux Convective (Bulk) Flux Large Permeability Poly(1-trimethylsilyl-1-propyne) Poly(dimethylsiloxane) |
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