Correlation of Structure and Function for CO2 Permeation in Polyphosphazene Membranes Page: 2 of 9
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Correlation of Structure and Function for CO2 Permeation in Polyphosphazene
Membranes
Frederick F. Stewart and Christopher J. Orme
Idaho National Laboratory
P.O. Box 1625
Idaho Falls, ID 83415-2208
Abstract
It has been thought that permeation through polyphosphazene membranes of the
more condensable gases, such as CO2 and H2S, could be enhanced by selection of pendant
groups that exhibit higher affinities for these gases. In this paper, over 20 polyphosphazenes
with a wide array of pendant groups will be discussed in terms of their CO2 transport
properties. From this work, we have concluded that the pendant group chemical
characteristics largely do not play a role in CO2 transport. More important are the physical
characteristics of both the polymer and the gas. For example, permeabilities were found to
correlate well to the glass transition temperature of the polymer, regardless of the polarity of
the pendant group. Thus, segmental chain motion and physical state of the polymer appear to
play a dominant role. This result differs sharply from data taken from liquid transport data that
suggests a strong similarity in the solubility properties between the permeant and the polymer
is required for higher permeation rates.
Introduction
Polyphosphazenes are an intriguing class of polymers because molecular
substitutions can be made onto the phosphorus and nitrogen backbone after polymerization.
Chemical functionality is supplied through selection of the pendant group. In general,
regardless of pendant group, polyphosphazenes embody a high degree of thermal and
chemical stability, although some pendant groups yield more stable polymers as compared to
others. For example, many aryloxyphosphazene formulations are stable at temperatures as
high as 300 - 400 C, while many alkoxy-substituted polymers decompose at lower
temperatures.[1]
The key advantage of polyphosphazene chemistry for detailed studies of transport
mechanisms is that through modification of the pendant groups, polymers with differing
chemical and physical characteristics can be formed without significantly altering either the
polymer backbone structure or the molecular weight. In general, polymer properties are
substantially dictated by choice of pendant group. Water soluble polyether pendant groups
yield water soluble amorphous, flowing polymers. For instance, poly[bis-2-(2-
methoxyethoxy)ethoxyphosphazene] (MEEP) is a flowing elastomer with a glass transition
temperature (Tg) of -81 C.[2] Aromatic components, on the other hand, tend to impart
hydrophobic character onto the phosphazene backbone. For example, poly[bis-
(phenoxy)phosphazene] (PPOP), is a semi-crystalline, hydrophobic, fibrous solid that is only
soluble in organic solvents such as toluene, tetrahydrofuran, and chloroform.[3]
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Stewart, Frederick F. & Orme, Christopher J. Correlation of Structure and Function for CO2 Permeation in Polyphosphazene Membranes, article, October 1, 2005; [Idaho Falls, Idaho]. (https://digital.library.unt.edu/ark:/67531/metadc885904/m1/2/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.