(733a) Sub-Ambient Temperature Gas Separation Performance of Phosphazene Membranes

Stewart, F. F., Idaho National Laboratory
Orme, C. J., Idaho National Laboratory

The trade-off in membrane performance between permeability and selectivity is well-known. Processes that increase permeability often result in decreases in selectivity. Understanding the interaction between these two fundamental characteristics is critical to the development of more effective membrane separation processes. In this paper, several polyphosphazene membranes will be discussed in terms of their permeability, selectivity, and their physical and chemical characteristics. Polyphosphazenes are materials that have a backbone comprised of alternating phosphorus and nitrogen atoms. Each phosphorus atom is pentavalent, which leaves two points through which pendant groups can be attached. The pendant groups largely dictate the properties of the polymers. For example, the most direct influence of the pendant groups is observed through the glass transition temperature. For example, linear organic pendant groups such as alkyl and perfluorinated alkyl impart a rubbery character to the polymers as observed in a low glass transition temperature. Likewise, bulkier groups such as substituted phenols give the materials higher glass transition temperatures, an in some cases, varying degrees of crystallinity.

In this paper, several new polyphosphazenes with varying glass transition temperatures will be discussed. Chemical and physical characterization data will be presented as well as gas permeability measurements ranging in temperature from -30 ºC to 75 ºC. Additionally, ideal separation factors will be discussed facilitating a discussion of the relative effects of temperature on each parameter. In general, for the polyphosphazenes to be reported in this paper, permeability decreases and the selectivity increases as the temperature is lowered. Depending on the structure, the relative magnitudes of each parameter change can vary resulting in either improvement or a reduction in performance. Data will be presented in a Robeson-type permeability-separation factor plot that shows an overall improvement in membrane performance as the temperature is decreased where loss in permeability is more than compensated for by an increase in selectivity. Gases studied include CO2, H2, and CH4.