Investigations on 6FDA/BPDA-DAM polymer melt properties and CO2 adsorption using molecular dynamics simulations

Abstract

With the increasing demand for developing alternative technologies for carbon capture, polymeric membranes have been widely researched as potential CO2 separation and capture media. 6FDA/BPDA-DAM is an excellent precursor polymer for creating high quality carbon molecular sieve membranes through pyrolysis. The polymer, as well as the derived carbon molecular sieve membranes, have both shown excellent selectivity and solubility towards CO2 gas molecules owing to a distribution of ultra-micropores and micro-pores. Molecular modelling has evolved as a powerful method to test the selectivity of the materials as well as develop novel materials for gas separations. In this work, we use molecular dynamics simulations, to develop a molecular model for 6FDA/BPDA-DAM polymer using a modified Dreiding force-field, to better understand the statistics and the structure of the pores in a melt configuration. A compression-decompression step, followed by repeated annealing cycles are performed to equilibrate the polymer melt. Our calculated bulk properties like density (1.328 g/cc), glass transition temperature (Tg = 698.7K), and average fractional free volume (FFV = 0.123) are in quantitative agreement with available experimental data. We use a combination of grand canonical Monte Carlo (GCMC) and isothermal isobaric (NPT) molecular dynamics simulations in order to obtain the CO2 adsorption isotherm of the polymer matrix, which include matrix swelling effects. Solvent accessible surface area of different atom types, which form the pore interiors, reveal that the exposed oxygen and methyl carbon atoms have increased binding affinity towards CO2. The simulated isotherms predict a higher loading of CO2 when compared with the experimental data and we discuss possible origins for this deviation.