(619e) Simulation of Propylene/ Propane Plasticization in 6FDA-Based Polyimide Membranes

The permeation properties of hydrocarbon gases through polymeric membranes are of interest for the development of membrane processes for separation of olefin/paraffin mixtures, such as propylene (C₃H₆)/propane (C₃H₈), in the petrochemical industry. Propylene is largely a byproduct of ethylene production, which is the largest volume organic produced in the petrochemical industry. Furthermore, propylene is produced in propane dehydrogenation, and natural gas to olefins and other olefin conversion processes. While rubbery polymers have very low selectivity for propylene/propane, glassy polyimides (PIs) and polyimide-co-polypyrrolones based on 6FDA (4,4’-(hexafluorisopropylidene)dipthalic anhydride) have been reported to exhibit the best performance [1]. However, a major issue for these polyimides is the plasticization of the membrane during the permeation of these condensable hydrocarbons.

Propylene and propane, both being condensable, tend to plasticize polymeric membranes, even at partial pressures as low as 2 bars. There are several experimental studies investigating plasticization effect of propane and propylene in polyimides reported in literature [1-4]. The observed plasticization pressure among these studies was between 2 and 5 atm. In order to suppress plasticization, cross-linking and thermal treatment have been used. On the other hand, there are no molecular simulation studies reported in the literature that focus on propane and propylene-induced plasticization in polyimides.

In this study, molecular simulation techniques are used to estimate the degree of plasticization of 4,4-hexafluoroisopropylidene-diphthalic anhydride (6FDA)-based PI membranes induced by sorption of propane (C₃H₈) and propylene (C₃H₆). The diamines used to build the polyimide structure are 2,4,6-trimethyl-m-phenylene diamine (DAM), 2,5-dimethyl-p-phenylenediamine (DPX), and 4,4-oxydianiline (ODA).  To the best of our knowledge, this is the first simulation study examining propane/propylene plasticization in 6FDA based PIs. The sorption simulations are carried out in the Grand Canonical Monte Carlo (GCMC) ensemble. To reproduce the hydrocarbon-induced plasticization effect, sorption-relaxation cycles are applied until C₃H₆ or C₃H₈ concentration converges. The increase in the FFV of the resulting polymer structure is considered as the extent of plasticization. The change in the sorption selectivity is determined for both pure gases and binary C₃H₆/C₃H₈ mixtures for % 50/50 feed composition. The glass transition temperatures (Tg) of the polymers are estimated by observing the volume change of the simulation cells during cooling down from 760 K to 380 K. Rotational time autocorrelation functions of the unit vectors defining to the distance between the diamine and dianhydride moities are also calculated in order to analyze changes in the local dynamics of the polymers due to hydrocarbon sorption. Our recent work [5] on the CO₂-sorption induced plasticization of these polymers have shown that the CO₂ sorption capacity at 35ºC and 10 bars depend on the FFV of the swollen PI, yielding similar CO₂ concentrations for similar FFV values. Thus, particular interactions between CO₂ and sorption sites have limited differential contribution on sorption capacity. However, no such relationship was obtained for propane and propylene at 25ºC and 1.13 bar due to changing site preferences, indicating the strength of sorbate-site interactions is significant on sorption capacity, as well as the FFV.

[1]    Das, M., Koros, W.J., J. Memb. Sci., 365, 399-408, 2010.

[2]    Staudt-Bickel, C., Koros, W. J., J. Memb. Sci., 170, 205, 2000.

[3]    Tanaka, K., Taguchi, A., Hao, J. Q., Kita, H., Okamoto, K., J. Memb. Sci., 121, 197, 1996.

[4]    Jiang, L. Y., Chung, T. S., Rajagopalan, R., Chem. Eng. Sci.,63, 204-216, 2008.

[5]    Velioglu, S., Ahunbay, M. G., Tantekin-Ersolmaz, S.B., J. Memb. Sci., 417, 217-227, 2012.