Polymer ultrapermeability from the inefficient packing of 2D chains

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 O2/N2, H2/N2, CO2/N2, H2/CH4 and CO2/CH4, 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

pysimm: A python package for simulation of molecular systems

In this work, we present pysimm, a python package designed to facilitate structure generation, simulation, and modification of molecular systems. pysimm provides a collection of simulation tools and smooth integration with highly optimized third party software. Abstraction layers enable a standardized methodology to assign various force field models to molecular systems and perform simple simulations. These features have allowed pysimm to aid the rapid development of new applications specifically in the area of amorphous polymer simulations. Keywords: Amorphous polymers, Molecular simulation, Pytho

Ionomers of Intrinsic Microporosity: In Silico Development of Ionic-Functionalized Gas-Separation Membranes

This work presents the predictive molecular simulations of a functionalized polymer of intrinsic microporosity (PIM) with an ionic backbone (carboxylate) and extra-framework counterions (Na⁺) for CO₂ gas storage and separation applications. The CO₂-philic carboxylate-functionalized polymers are predicted to contain similar degrees of free volume to PIM-1, with Brunauer–Emmett–Teller (BET) surface areas from 510 to 890 m²/g, depending on concentration of ionic groups from 100% to 17%. As a result of ionic groups enhancing the CO₂ enthalpy of adsorption (to 42–50 kJ/mol), the uptake of the proposed polymers at 293 K exceeded 1.7 mmol/g at 10 kPa and 3.3 mmol/g at 100 kPa for the polymers containing 100% and 50% ionic functional groups, respectively. In addition, CO₂/CH₄ and CO₂/N₂ mixed-gas separation performance was evaluated under several industrially relevant conditions, where the IonomIMs are shown to increase both the working capacity and selection performance in certain pressure swing applications (e.g., natural gas separations). These simulations reveal that intrinsically microporous ionomers show great potential as the future of energy-efficient gas-separation polymeric materials

Update 0.2 to “pysimm: A python package for simulation of molecular systems”

An update to the pysimm Python molecular simulation API is presented. A major part of the update is the implementation of a new interface with CASSANDRA — a modern, versatile Monte Carlo molecular simulation program. Several significant improvements in the LAMMPS communication module that allow better and more versatile simulation setup are reported as well. An example of an application implementing iterative CASSANDRA–LAMMPS interaction is illustrated. Keywords: Amorphous polymers, Molecular dynamics simulations, Monte Carlo simulation

Formation of Microporosity in Hyper-Cross-Linked Polymers

Molecular simulations of poly­(styrene-<i>co</i>-vinylbenzyl chloride) hyper-cross-linked polymers (HCPs) are prepared using a “virtual synthesis” approach with vinylbenzyl chloride contents ranging from 25 to 100%. The trends in porosity from the simulations are in good agreement with experimental data, where surface areas increase with the degree of cross-linking. In addition to studying the final structures, an advantage of using a simulated polymerization is the ability to examine the evolution of porosity throughout the virtual synthesis. Measures of the surface areas and pore size distributions indicate a gradual formation of pores in the swollen state. Additionally, the extent of pore collapse on moving from the swollen to dried states is shown to depend heavily on the degree of cross-linking. This unique perspective gained from the simulations provides important insight in order to gain a better understanding of the structure–property relationships in HCPs

Porosity and Ring Formation in Conjugated Microporous Polymers

Pyrene-based conjugated microporous polymers (CMPs) have been shown to exhibit significant microporosity and strong luminescence, but their structures are not well understood due to their insolubility and amorphous nature. Here, a series of pyrene-based CMPs with varying monomer compositions is studied using molecular simulations. The results are in good agreement with available experimental BET surface areas and powder X-ray diffraction data. As the monomer composition is adjusted to increase the degree of cross-linking, greater porosity is formed. Additionally, with high cross-linking degrees, the formation of 3-, 4-, 5-, and 6-monomer rings are found to be more prevalent. The increase in strained rings within the network structures correlates with shifts in optical spectra due to the increased conjugation. Together with experimental and other computational results, the simulations here enable a better understanding of the structure–property relationships in pyrene-based CMPs