(731e) Effect of Cross-Linking on CO2-Induced Plasticization Resistance of Polyimides Containing DABA Diamine - a Molecular Simulation Study

Polyimides have been widely used in membrane based gas separation due to their outstanding separation properties and high processability. Their application can be further expanded for CO2 capture in natural gas purification, coal gasification, and flue gas treatment processes. The main concern in these applications is the plasticization of the polyimide membranes at high CO2 partial pressure. Plasticization can lead to a decrease in membrane selectivity due to the loss in size discrimination ability and unpredictable membrane properties.

Recently, to investigate structure-plasticization relationship, we have focused on three sulfonated copolyimides, 6FDA/BTDA-pBAPS (3:1), 6FDA-pBAPS/DABA (3:1), and 6FDA-pBAPS/mPDA (3:1) by using all-atomistic approach [1]. The selection of the copolyimide structures was based on our previous study [2] in which the permeability coefficients of H2, O2, He, CO2, N2, and CH4 gases for more than 2200 possible co-polyimide structures were estimated by the group contribution method of Alentiev et al. [3]. It is well-demonstrated in the literature that carboxylic acid group containing DABA diamine can be used to crosslink the polymer chain and enables h-bonding. Decarboxylation-induced thermal cross-linking is one of the approaches for crosslinking of polyimides that occurs at elevated temperatures (∼15 °C above glass transition temperature) between DABA diamines leading to excellent plasticization resistance for CO₂. As a result of cross-linking, selectivity can be enhanced one step further with a slight decrease in permeability by improving stability of the membrane. Therefore, in our recent study [1], 6FDA-pBAPS/DABA copolyimide was proposed for further studies to explore its cross-linking capacity.

In this work, we aimed to reveal the effect of cross-linking on CO2-induced plasticization behavior of 6FDA-pBAPS/DABA (1:1) copolyimide by carrying out Molecular Dynamic (MD) and Monte Carlo (MC) simulations. All molecular interactions were modelled using Polymer Consistent Force Field (PCFF) in which quantum-level charge calculations were also incorporated. 6FDA-pBAPS/DABA (1:1) membranes with different crosslinking ratio (from none to 100 % crosslinking) were tested for the effect on swelling by tracking the change of chain flexibility and fractional free volume. Spesifically, plasticization behavior was estimated using a novel FFV approach [1] which depends on CO2 accesible free volume while CO2 is present in the membrane.

The characteristic properties of simulated membranes matched well with the experimental values. The plasticization resistance with increasing crosslinking ratio was evident in the polyimides, however there was a dramatic decrease in adsorption coefficients for 6FDA-pBAPS/DABA (1:1) with the increase of croslinking ratio, which might be somewhat discouraging for gas separation processes.

References:

1. Balcik M., Ahunbay M.G., “Prediction of CO2-induced plasticization pressure in polyimides via atomistic simulations”, J. Memb. Sci., 547, 146-155, (2017)
2. S. Velioğlu, S.B. Tantekin-Ersolmaz, “Prediction of gas permeability coefficients of copolyimides by group contribution methods”, J. Memb. Sci., 480, 47–63, (2015).
3. Alentiev, A. Y.; Loza, K. A.; Yampolski, Y. "Development of the methods for prediction of gas permeation parameters of glassy polymers: polyimides as alternating co-polymers", J. Memb. Sci., 167, 91-106 (2000).