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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.

Multiple computational and experimental techniques are used to understand the nanoscale morphology and water/proton transport properties in a series of sulfonated Diels–Alder poly­(phenylene) (SDAPP) membranes over a wide range of temperature, hydration, and sulfonation conditions. New synthetic methods allow us to sulfonate the SDAPP membranes to much higher ion exchange capacity levels than has been previously possible. Nanoscale phase separation between the hydrophobic polymer backbone and the hydrophilic water/sulfonic acid groups was observed for all membranes studied. We find good agreement between structure factors calculated from atomistic molecular dynamics (MD) simulations and those measured by X-ray scattering. With increasing hydration, the scattering ionomer peak in SDAPP is found to decrease in intensity. This intensity decrease is shown to be due to a reduction of scattering contrast between the water and polymer and is not indicative of any loss of nanoscale phase separation. Both MD simulations and density functional theory (DFT) calculations show that as hydration levels are increased, the nanostructure morphology in SDAPP evolves from isolated ionic domains to fully percolated water networks containing progressively weaker hydrogen bond strengths. The conductivity of the membranes is measured by electrical impedance spectroscopy and the equivalent proton conductivity calculated from pulsed-field-gradient (PFG) NMR diffusometry measurements of the hydration waters. Comparison of the measured and calculated conductivity reveals that in SDAPP the proton conduction mechanism evolves from being dominated by vehicular transport at low hydration and sulfonation levels to including a significant contribution from the Grötthuss mechanism (also known as structural diffusion) at higher hydration and sulfonation levels. The observed increase in conductivity reflects the impact that changing hydration and sulfonation have on the morphology and hydrogen bond network and ultimately on the membrane performance.

We performed atomistic simulations on a series of sulfonated polyphenylenes systematically varying the degree of sulfonation and water content to determine their effect on the nanoscale structure, particularly for the hydrophilic domains formed by the ionic groups and water molecules. We found that the local structure around the ionic groups depended on the sulfonation and hydration levels, with the sulfonate groups and hydronium ions less strongly coupled at higher water contents. In addition, we characterized the morphology of the ionic domains employing two complementary clustering algorithms. At low sulfonation and hydration levels, clusters were more elongated in shape and poorly connected throughout the system. As the degree of sulfonation and water content were increased, the clusters became more spherical, and a fully percolated ionic domain was formed. These structural details have important implications for ion transport.

We present atomistic simulations of a single PNIPAM-alkyl copolymer surfactant in aqueous solution at temperatures below and above the LCST of PNIPAM. We compare properties of the surfactant with pure PNIPAM oligomers of similar lengths, such as the radius of gyration and solvent accessible surface area, to determine the differences in their structures and transition behavior. We also explore changes in polymer-polymer and polymer-water interactions, including hydrogen bond formation. The expected behavior is observed in the pure PNIPAM oligomers, where the backbone folds onto itself above the LCST in order to shield the hydrophobic groups from water. The surfactant, on the other hand, does not show much conformational change as a function of temperature, but instead folds to bring the hydrophobic alkyl tail and PNIPAM headgroup together at all temperatures. The atomic detail available from these simulations offers important insight into understanding how the transition behavior is changed in PNIPAM-based systems.

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Listed below are the simulation details of each sample set of PIMs including: the simulation box length, the total number of united atoms in the box, and the weight average molecular weight of the polymer chains within the simulated sample. Also shown are the generalized amber force field atom definition, the Transferable Potential for Phase Equilibria type definition and parameters, and the ab initio calculated partial charge for each united atom type in each polymer. Molecular simulation details of each sample set-Polymer (sample no.) Box length (Å) No. of UA atoms M w (g/mol) PIM-1 (1) 44.14 3745 5401 PIM-1 (2) 45.22 3885 5347 PIM-1 (3) 44.06 3710 6029 PIM-1 (4) 44.43 3745 4385 PIM-1 (5) 45.01 3815 4955 sPIM-1 (1) 52.87 6055 11589 sPIM-1 (2) 49.27 5005 20802 sPIM-1 (3) 51.59 5355 11932 sPIM-1 (4) 53.17 6090 17781 sPIM-1 (5) 52.18 5845 16713 sPIM-0 (1) 49.09 4393 15411 sPIM-0 (2) 48.42 4278 12766 sPIM-0 (3) 48.86 4370 9454 sPIM-0 (4) 48.69 4416 15241 sPIM-0 (5) 47.62 4255 11069 soPIM-1 (1) 54.71 7439 14417 soPIM-1 (2) 51.04 6149 25877 soPIM-1 (3) 52.95 6579 14842 soPIM-1 (4) 54.93 7482 22118 soPIM-1 (5) 56.75 8600 17983 soPIM-0 (1) 49.86 5157 18342 soPIM-0 (2) 48.86 5022 15194 soPIM-0 (3) 49.14 5130 11252 soPIM-0 (4) 49.38 5184 18140 soPIM-0 (5) 50.03 4995 13174

The design of a new class of materials, called organic molecules of intrinsic microporosity (OMIMs), incorporates awkward, concave shapes to prevent efficient packing of molecules, resulting in microporosity. This work presents predictive molecular simulations and experimental wide-angle X-ray scattering (WAXS) for a series of biphenyl-core OMIMs with varying end-group geometries. Development of the utilized simulation protocol was based on comparison of several simulation methods to WAXS patterns. In addition, examination of the simulated structures has facilitated the assignment of WAXS features to specific intra- and intermolecular distances, making this a useful tool for characterizing the packing behavior of this class of materials. Analysis of the simulations suggested that OMIMs had greater microporosity when the molecules were the most shape-persistent, which required rigid structures and bulky end groups. The simulation protocol presented here allows for predictive, presynthesis screening of OMIMs and similar complex molecules to enhance understanding of their structures and aid in future design efforts.