A molecular investigation on sorption and diffusion of gas mixture in thermally-rearranged membranes

A well-known method for improving the diffusivity-selectivity, as well as the diffusion coefficient of gas molecules in polymer membranes, involves an increase in free volume that combine with the stiffness of the main chains. Among these, thermally-rearranged polybenzoxazoles (TR-PBOs) have shown outstanding molecular and ionic transport and separation performance, beyond the limits of conventional polymers [?1-4]. Adsorption and diffusion in thermally rearranged polybenzoxazole (TR-PBO) polymer membranes are simulated in single-gas and in CO2/N2 binary mixture focusing at 35 °C and at relative gas concentration corresponding to a fugacity of 5 bar. For this purpose, molecular dynamics simulations are used in a synergistic pairing with Gran Canonical Monte-Carlo approach (GCMC) and Ideal Adsorption Solution Theory (IAST). GCMC simulations are compared with IAST simulations to obtain adsorption isotherms in mixtures. The number of molecules present in the polymer matrix is estimated using GCMC. Molecular dynamics simulations are performed, calculating the Maxwell-Stefan diffusion coefficients. As main results, the evaluated diffusion coefficients of CO2 and N2 are in a satisfactory agreement with the values estimated using available permeability experimental data. CO2 diffusivity calculated in mixture conditions is found to be the same as that in single-gas ones, whereas the N2 diffusivity is slightly higher. These differences are explained in terms of the effect of both the mutual gas diffusion and the competing occupancy of the available free space occupied by the CO2 molecules in mixture. 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 (2007) 254-258 [2]S.Kim, Y.M. Lee, Rigid and microporous polymers for gas separation membranes, Progress Polym. Sci. 43 (2015) 1-32 [3]C.H. Park, E. Tocci, S. Kim, A. Kumar, Y.M. Lee, E. Drioli, A Simulation Study on OH-Containing Polyimide (HPI) and Thermally Rearranged Polybenzoxazoles (TR-PBO): Relationship between Gas Transport Properties and Free Volume Morphology, J. Phys Chem. B 118 (2014) 2746-2757. [4]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), J. Phys. Chem. B 116 (2012) 12864-12877.