Sorption and Transport of Small Gas Molecules in Thermally Rearranged Polymer Membranes: Experimental and Simulation Aspects

Membrane gas separation processes have been considered as environmentally sustainable processes with various benefits including low energy consumption and compact process design. Membrane processes for CO2 recovery have been mainly applied for the natural gas processing, landfill biogas recovery, and enhanced oil recovery. More recently, CO2 separation as a greenhouse gas treatment has been considered against global warming. Among various CO2 separation technology membrane processes using highly permeable membranes have been demonstrated the great potential to reduce the separation cost with low energy consumption [1]. Thermally Rearranged (TR) polymer membranes have been studied as extraordinarily permeable membrane materials with high free volume elements. TR-polymer membranes exhibits extraordinary CO2 permeability with appropriate selectivity [2]. Thermal rearrangement of ortho-functional polyimides is a favorable and irreversible process to present thermally and chemically stable polymers with rigid chains. The thermal conversion into stiff, rigid heteroaromatic rings cause a gradual increase of free volume elements as well as change of polymer structure as a function of thermal rearrangement temperature resulting in an outstanding increase in gas permeability. Solution-diffusion model is widely used to elucidate gas transport mechanism in polymeric membranes where the permeability coefficient is a product of a diffusion coefficient and a solubility coefficient [3]. Gas diffusion through the membrane matrix is related with the size of free volume in polymers and the size of gas penetrant. Gas solubility is related with sorption property where gas molecules are sorbed in free volume matrix. Solubility is a term of chemical affinity between a polymer and a penetrant and it gives the mount of sorbed penetrant on membrane under equilibrium state. Solution-diffusion model is well defined to understand gas transport mechanism of polymer membranes. In this study, both experimental and simulation approaches were investigated to characterize the gas transport properties of TR-polymer membranes [4,5]. Experimentally, permeation and diffusion coefficient is usually determined from time-lag method and solubility coefficient is determined using pressure decay method which can describe sorption of gases in a wide range of pressure [4]. Gas diffusion and sorption characteristics can also be determined from a molecular dynamics simulation method from the calculation of mean square displacement [6] and adsorption isotherm. Independent contribution of gas diffusion and sorption on gas permeation has been investigated for gas permeation. The effect of thermal conversion ratio on the gas transport mechanism has also been studied. References [1] Enrico Drioli, Lidietta Giorno, Membrane Operations, Wiley-VCH Verlag GmbH & Co. KgaA (2009) [2] H.B. Park, C.H. Jung, Y.M. Lee, A.J. Hill, S.J. Pas, S.T. Mudie, E. Van Wagner, B.D. Freeman, D.J. Cookson, Polymers with cavities tuned for fast selective transport of small molecules and ions, Science 318, 254-258 (2007) [3] M. Mulder, Basic principles of membrane technology, Kluwer Academic Publishers (1996) [4] S. Kim, S.H. Han, Y.M. Lee, Sorption and transport of small gas molecules in thermally rearranged (TR) polybenzoxazole membranes based on 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF) and 4,4'-hexafluoroisopropylidene diphthalic anhydride (6FDA), Journal of Membrane Science, in press (2013) [5] C.H. Park, E. Tocci, Y.M. Lee, E. Drioli, Thermal Treatment Effect on the Structure and Property Change between Hydroxy-Containing Polyimides (HPIs) and Thermally Rearranged Polybenzoxazole (TR-PBO), Journal of Physical Chemistry B 116, 12864-12877 (2012) [6] E. Tocci, D. Hofmann, D. Paul, N. Russo, E. Drioli, A molecular simulation study on gas diffusion in a dense poly(ether-ether-ketone) membrane, Polymer 42, 521-533 (2001)