No abstract

The promise of ultrapermeable polymers, such as poly(trimethylsilylpropyne) (PTMSP), for reducing the size and increasing the efficiency of membranes for gas separations remains unfulfilled due to their poor selectivity. We report an ultrapermeable polymer of intrinsic microporosity (PIM-TMN-Trip) that is substantially more selective than PTMSP. From molecular simulations and experimental measurement we find that the inefficient packing of the two-dimensional (2D) chains of PIM-TMN-Trip generates a high concentration of both small (<0.7 nm) and large (0.7-1.0 nm) micropores, the former enhancing selectivity and the latter permeability. Gas permeability data for PIM-TMN-Trip surpass the 2008 Robeson upper bounds for O/N, H/N, CO/N, H/CH and CO/CH, with the potential for biogas purification and carbon capture demonstrated for relevant gas mixtures. Comparisons between PIM-TMN-Trip and structurally similar polymers with three-dimensional (3D) contorted chains confirm that its additional intrinsic microporosity is generated from the awkward packing of its 2D polymer chains in a 3D amorphous solid. This strategy of shape-directed packing of chains of microporous polymers may be applied to other rigid polymers for gas separations.

Polymers of intrinsic microporosity, or PIMs, are characterized by rigid and nonlinear or nonplanar backbones that inhibit space efficient packing, thus creating microporosity. PIM-1 has been well studied by both simulations and experiments and is compared in this work to two different PIM-1-like polymers, PIM-1c and PIM-1n. A detailed method for the generation of representative structures, including charge assignment from ab initio calculations, is presented along with simulated characterization of the pore size distributions, surface areas, structure factors, and methane adsorption isotherms. Simulated scattering for PIM-1c and PIM-1n show similar characteristic peaks as PIM-1, suggesting similar conformations. Adsorption isotherms of methane in PIM-1, PIM-1c, and PIM-1n were also predicted and compared to experimental data for PIM-1.

This review concentrates on the advances of atomistic molecular simulations to design and evaluate amorphous microporous polymeric materials for CO capture and separations. A description of atomistic molecular simulations is provided, including simulation techniques, structural generation approaches, relaxation and equilibration methodologies, and considerations needed for validation of simulated samples. The review provides general guidelines and a comprehensive update of the recent literature (since 2007) to promote the acceleration of the discovery and screening of amorphous microporous polymers for CO capture and separation processes.

Methyl or t-butyl substituted spirobifluorenes greatly enhance PIM permeability.