Correlation of Structure and Function for CO2 Permeation in Polyphosphazene Membranes

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 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 degrees Celsius, while many alkoxy-substituted polymers decompose at lower temperatures. It has been thought that permeation 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 chemical characteristics of the pendant group largely do not play a role in CO2 or permanent gas transport. More important are the physical characteristics of the polymer. 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 more 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