Modeling gas sorption in glassy HAB-6FDA and in its thermally rearranged homologues
Michele Galizia, Kevin A. Stevens, Zachary P. Smith, Donald R. Paul, Benny D. Freeman
J.McKetta Department of Chemical Engineering, 200 E. Dean Keeton Street, 78712 Austin,
and Center for Energy and Environmental Resources, 10100 Burnet Road, 78758 Austin,
The University of Texas at Austin, Texas (USA)
Due to their outstanding transport properties and unusual combination of high gas permeability and selectivity, thermally rearranged polymers have attracted a great interest in membrane science, especially for CO2/CH4 separation . These polymers, first reported by Park et al. , are formed upon conversion of solvent-soluble polyimides, via thermal rearrangement, to insoluble polybenzoxazoles. The resulting materials show separation properties that, in some cases, surpass the upper bound defined by Robeson in 2008 .
Many research efforts have been devoted in the last decade to the fundamental understanding of gas transport properties of these polymers. In this work, for the first time, we attempted to analyze the sorption properties of TR-polymers from a theoretical point of view, by using the Non-equilibrium Lattice Fluid theory. Unfortunately, the characteristic lattice fluid parameters are not available in the literature for the pristine polyimide HAB-6FDA, nor for the corresponding thermally rearranged samples. These parameters are usually determined by fitting the polymer PVT data in the rubbery region to the Sanchez-Lacombe equation of state. However, thermodynamic data in the rubbery state are not accessible for these high performance polymers, thus the scaling parameters T*, p* and r* were estimated by using an alternative approach first reported by Galizia et al. . Interestingly, the model is able to describe with good precision the sorption isotherms of hydrogen, nitrogen, methane and carbon dioxide in both the pristine polyimide and corresponding thermally rearranged samples.
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 M. Galizia et al., J. Membr. Sci. 2012 405-406 201-211