Thermally rearranged polymeric membranes in gas separation

Environmental considerations like water stress, global warming, fossil fuel depletion, environmental pollution gave rise to the concept of sustainable growth and to related strategies such as process intensification, reuse of water and solvents or CO2 capture and storage. Membranes have a key role to play in the new technologies and in the separation operations associated with these strategies. Since gas separation applications, using polymer membranes becomes highlighted due to developing highly porous polymer membranes with high gas transport performance, polymer materials with high rigidity are withdrawing the focus of researchers' attentions due to their high free volume and ensuing improved gas transport performance [1-3]. Conventionally, glassy polymers with rigid chains promote diffusivity selectivity - i.e. the preferential transport of lighter gas molecules of smaller kinetic diameters over that of larger molecules - whereas high gas permeability relies on large inter-chain spacing [4]. However, new porous rigid polymer groups such as thermal rearranged polymers are showing an opposite trend: the improved rigidity results in higher free volume, higher permeability and lower selectivity, compared to the conventional rigid polymer groups such as polyimides [1]. Accordingly, it is still unclear how rigidity affects the gas transport properties. To address this issue, we performed quantum mechanics (QM) and molecular dynamics (MD) simulation of thermal rearranged polybenzoxazole (TR-PBO) and its parent precursor, hydroxyl-containing polyimide (HPI). TR-PBO is one of the highest permeable polymer membranes for gas separation exceeding the upper-bound limitation of conventional polymeric membranes [1]. We first elucidated how the difference in the chemical structures of HPI and TR-PBO affects their main chain rigidity. Next, we demonstrated the relationship between the rigidity and the free volume morphology, and compared the amount and morphology of free volume in HPI and TR-PBO with their transport performances. Consequently, we could integrate our simulation results with the experimental findings already reported in the literature yielding insightful indications and finally establish the role of molecular structure on the perm-selective behavior of the high performance rigid polymer membranes. 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, Science, 318, 254 (2007). 2. M. Carta, R. Malpass-Evans, M. Croad, Y. Rogan, J. C. Jansen, P. Bernardo, F. Bazzarelli, N. B. McKeown, Science, 339, 303 (2013). 3. M. D. Guiver, Y. M. Lee, Science, 339, 284 (2013). 4. B. D. Freeman, Macromolecules, 32, 375 (1999).