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Structure-Property Relations in Polymers for Gas Separations

Lyman Handy Colloquium SeriesPresentsBenny D. FreemanUniversity of Texas at AustinAbstract:This presentation will discuss structural features important in the use of polymers as rate-controlling membranes for gas separations. In particular, materials having desirable combinations of high permeability and high selectivity based upon solubility selectivity (e.g., butane removal from natural gas, CO2 separation from H2 or N2) or diffusivity selectivity (e.g., CO2 removal from natural gas) will be presented. For example, cross-linked poly(ethylene oxide) (XLPEO) polymers, which are flexible, rubbery polymers identified as promising materials to remove polar and acid gases, such as CO2, from mixtures with light gases, such as H2. One member of this family of materials was reported to have a CO2 permeability coefficient of approximately 500 Barrer and a CO2/H2 mixed gas selectivity of 30 at -20C.1 Such materials achieve high selectivity based upon their high solubility selectivity favoring CO2 transport. Prepared by cross-linking low molecular weight poly(ethylene glycol) diacrylate with other poly(ethylene oxide) acrylates, XLPEO polymers exhibit good separation properties thanks to ethylene oxide group interaction with CO2 and suppression of crystallinity normally found in high molecular weight, linear poly(ethylene oxide).
Polymers can also be tailored to achieve high selectivity based upon high diffusivity selectivity. In this case, highly rigid, glassy polymers with proper free volume element size and size distribution are desirable. Polyimides with ortho-position functional groups may be solution-processed to form conventional films and membranes. Such materials can undergo thermal rearrangement to form highly rigid benzoxazole or benzithiazole structures having very high permeability coefficients and high selectivity. For example, one member of this family was prepared having a CO2 permeability coefficient of 1610 Barrer and a CO2/CH4 selectivity, under mixed gas conditions, of 42-46, depending on the partial pressure of CO2 in the mixture.2 These thermally rearranged (TR) polymers are insoluble in common solvents, giving them good chemical stability, and highly thermally stable, which are important attributes for membranes that would be used in chemically or thermally aggressive environments.
The overarching message from this presentation is that polymers can be exquisitely tuned to have favourable permeation properties. Materials may be designed to achieve high selectivity by being more soluble to one molecule than another or by having a strong ability to sieve gas molecules based on minute differences in gas molecule size. In both cases, the structure of the polymer may be optimized to permit rapid permeation.

Location: James H. Zumberge Hall Of Science (ZHS) - 159

Audiences: Everyone Is Invited

Contact: Petra Pearce Sapir