Sorption and Transport Behavior of Small Gas Molecules In TR-Polymers

Thermally Rearranged (TR) polymer membranes are extraordinarily permeable membrane with high free volume elements. TR-polymers have been studies as gas separation membranes exhibiting especially high CO₂ permeability with relevant selectivity. Moreover, TR-polymers can be formed to hollow fiber membrane to prepare gas separation membrane module, which can be studied for CO₂ separation such as natural gas sweetening and CCS from coal-fired power plant [1,2]. TR-polymers are prepared from polyimides containing ortho-hydroxyl group (HPI) by thermal rearrangement process. Conventional TR-Polybenzoxazoles (TR-PBO) are prepared from hydroxyl group containing polyimide (HPI) at around 450°C. Various thermal treatment temperature and condition can affect on membrane performance of TR membranes which is related with thermal conversion ratio from HPI to TR-polymers. During thermal rearrangement process, free volume element is dramatically increased and gas permeability increased as well. Thus, it is important to explain a substantial increase in gas permeability with gas transport mechanism in TR-polymer membranes [3].

To investigate gas transport properties of TR-polymers, solution-diffusion mechanism has been introduced where the permeability coefficient is the product of a diffusion coefficient and solubility coefficient [4]. Due to gas transport properties through the polymeric membranes are related with physical properties of membranes and gas molecules where gas diffusion and sorption properties are applied to explain the gas transport. Gas diffusion through the membrane is related with the size of free volume matrix in polymers and the size of gas molecules. 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 better undetstand gas permeation property and to undetstand gas transport mechanism in polymeric membrane matrix.

To investigate transport mechanism of TR polymer membranes, solubility coefficients and diffusion coefficients need to be studied. Diffusion coefficient is usually determined from time-lag method and solubility coefficient is determined using pressure decay method which can describe sorption property at wide range of pressure. In this presentation, solublility coefficients of small gas molecules have been investigated to understand gas transport mechanism of a series of TR-polymer membranes with various monomer compositions. This study concludes that diffusion controls the permeation for this TR polymer membranes for small gas molecules investigated.       

References

[1] 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)

[2] S. Kim, S.H. Han, Y.M. Lee, Thermally rearranged (TR) polybenzoxazole hollow fiber membranes for CO₂ capture, Journal of Membrane Science, Article available online (2012)

[3] H.B. Park, S.H. Han, C.H. Jung, Y.M. Lee, A.J. Hill, Thermally rearranged (TR) polymer membranes for CO₂ separation, Journal of Membrane Science 359, 11-24 (2010)

[4] M. Mulder, Basic principles of membrane technology, Kluwer Academic Publishers (1996)