(396ab) Molecular Simulation of CO2 Sorption in Thermally Rearranged (TR) Polymers

Sorption induced volume swelling and plasticization is a serious concern limiting the use of polyimides as membrane materials for separations involving CO₂, H₂S, and condensable hydrocarbons. Thermally rearranged (TR) aromatic polyimides has recently been presented as a new class of membrane materials for gas separation. TR polymers are based on soluble aromatic polyimides with ortho-positioned functional groups; exposure of the polyimides to thermal rearrangement (TR) around 400°C leads to fully aromatic, insoluble polybenzoxazoles (PBOs) with exceptional thermal and chemical resistance characteristics. Modification of the polyimide chain during the rearrangement produces fundamental changes in molecular connectivity and conformation that alter chain packing, resulting in a narrow free volume distribution and hourglass shaped cavities [1]. These polymers show outstanding physical properties and high separation performance that exceeds the trade-off relationships for many gas pairs. Additionally, these polymers appear to be resistant to CO₂-induced plasticization, which is of great importance for separation of gas streams containing high levels of CO₂.

Research on TR polymers indicated no evidence of plasticization up to CO₂partial pressures of 20 atm [1-3]. Considering the difficulty of high pressure separation measurements, complementing experimental studies with molecular simulation predictions may be a powerful tool to understand the structure of and transport mechanisms in these promising materials. Although TR polymers are a hot topic in the membrane based gas separation area, there are only two simulation studies in the literature. Jiang et al. [2] determined the cavity size distribution and transport properties of six recently synthesized TR polymers and their precursors whereas Park et al. [4] analyzed the structure of TR polymers and their precursors. They carried out their simulations for a broad range of temperatures in order to clarify the effect of temperature.

In this study, molecular simulation techniques were used to estimate the degree of plasticization of 4,4-hexafluoroisopropylidene-diphthalic anhydride (6FDA)-2,2´-bis(3-amino-4-hydroxyphenyl)-hexa-fluoropropane (bisAPAF) membrane and its TR polymer (thermally rearranged polybenzoxazole) induced by CO₂ sorption. The structural properties, such as glass transition temperature (T_g), FFV and its distribution, d-spacing, radius of gyration, and cohesive energy density, along with sorption isotherms were investigated to understand the dynamics of TR polymer and its precursor chains. The simulations were compared to the experimental data available in the literature. The sorption simulations were carried out in the Grand Canonical Monte Carlo (GCMC) ensemble. To reproduce the CO₂-induced plasticization effect, sorption-relaxation cycles were applied until the CO₂ concentration converges. The increase in the fractional free volume (FFV) of the resulting polymer structure was considered as the extent of plasticization. Radial distribution functions (RDFs) were used for an in-depth analysis of interactions between CO₂and TR polymer or its precursor in order to obtain an insight to plasticization resistant structure of TR polymers. The oxygen in the hydroxyl groups of the diamine and the oxygen in the imide group were the most preferential sorption sites in 6FDA-bisAPAF, whereas they were not in its TR polymer.

[1]  Park, H. B.,  Jung, J. H., Lee, Y. M., Hill, A. J., Pas, S. J., Mudie, S. T., Van Wagner, E., Freeman, B. D., Cookson, D. J., Science, 318, 254, 2007.

[2]  Jiang, Y., Willmore, F. T., Sanders, D., Smith, Z. P., Ribeiro, C. P., Doherty, C. M., Thornton, A. W., Hill, Freeman, B. D., Sanchez, I. C., Polymer, 52, 2244-2254, 2011.

[3]  Sanders, D. F., Smith, Z. P., Ribeiro, C. P., Guo, R., McGrath, J. E., Paul, D. R., Freeman, B. D., J. Memb. Sci.,409-410, 232-241, 2012.

[4]  Park, C. H., Tocci, E., Lee, Y. M., Drioli, E., J. Phys. Chem. Part B, 116, 12864-12877, 2012.