Kyle E. Hart

Polymatic : a generalized simulated polymerization algorithm for amorphous polymers

This work presents a generalized structure generation methodology for amorphous polymers by a simulated polymerization technique and 21-step molecular dynamics equilibration, which is particularly effective for high-Tg polymers. The essential framework and parameters of the techniques and algorithms are described in detail, and example input scripts are provided for use with the freely available Polymatic simulated polymerization code and LAMMPS molecular dynamics package. The capabilities of the methods are examined through application to six linear, glassy polymers ranging in functionality, polarity, and rigidity. Validation of the methodology is provided by comparison of the simulations and experiments for a variety of structural, adsorption, and thermal properties, all of which showed excellent agreement with available experimental data.

Analysis of force fields and BET theory for polymers of intrinsic microporosity

A detailed force field analysis for polymers of intrinsic microporosity (PIMs) was carried out in this study. The generalised amber force field (GAFF) with united atom transferable potential for phase equilibria (TraPPE-UA), and the atomistic polymer consistent force field were evaluated. Analysis carried out with PIM-1 showed that the use of GAFF for bonded interactions and TraPPE-UA for non-bonded interactions yielded a simulated sample that compared best with available experimental data (wide-angle X-ray scattering and nitrogen adsorption at 77 K). In addition, Brunauer–Emmett–Teller surface areas, calculated from simulated nitrogen isotherms as pseudo-experimental data, showed that this common method failed to measure the geometric surface area of this type of material. These findings are expected to facilitate the predictive screening of different PIM functionalities.

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.

Predictive simulations of the structural and adsorptive properties for PIM-1 variations

Despite the sizeable and growing body of research on polymers of intrinsic microporosity (PIMs), a greater understanding of the relationship between the monomer, polymer–polymer and polymer–gas interaction is of significant interest. Methane (CH4), carbon dioxide (CO2), oxygen (O2) and nitrogen (N2) adsorption isotherms at 20°C and up to 20 bar obtained from grand canonical Monte Carlo simulations are presented for PIM-1, PIM-1c, PIM-1n and PIM-1f. The new proposed structure, PIM-1f, is presented and characterised by geometric accessible surface area, pore size distribution, radial distribution function, X-ray scattering and gas adsorption isotherms. PIM-1f increased the geometric surface area when compared with PIM-1; however, the higher system density in combination with the lack of strong adsorption sites yielded the least effective adsorbent for the gases analysed in this study. The gas solubility and ideal solubility selectivity values are also presented and compared with available experimental data for all gases and several gas mixtures illustrating that PIM-1c is the most effective functionality studied for adsorbing these four gases. The conclusions made here are projected to facilitate the design of a material that combines the higher surface area of PIM-1f with the high adsorption capacity of PIM-1c, which will improve the performance of future PIMs.

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 m(2)/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.

In Silico Determination of Gas Permeabilities by Non-Equilibrium Molecular Dynamics: CO2 and He through PIM-1

We study the permeation dynamics of helium and carbon dioxide through an atomistically detailed model of a polymer of intrinsic microporosity, PIM-1, via non-equilibrium molecular dynamics (NEMD) simulations. This work presents the first explicit molecular modeling of gas permeation through a high free-volume polymer sample, and it demonstrates how permeability and solubility can be obtained coherently from a single simulation. Solubilities in particular can be obtained to a very high degree of confidence and within experimental inaccuracies. Furthermore, the simulations make it possible to obtain very specific information on the diffusion dynamics of penetrant molecules and yield detailed maps of gas occupancy, which are akin to a digital tomographic scan of the polymer network. In addition to determining permeability and solubility directly from NEMD simulations, the results shed light on the permeation mechanism of the penetrant gases, suggesting that the relative openness of the microporous topology promotes the anomalous diffusion of penetrant gases, which entails a deviation from the pore hopping mechanism usually observed in gas diffusion in polymers.

Morphology and molecular bridging in comb- and star-shaped diblock copolymers

Block copolymers spontaneously self-assemble into nanostructured morphologies with industrially attractive properties; however, the relationships between polymer architecture and self-assembled morphology are difficult to tailor for copolymers with increased conformational restrictions. Using Dissipative Particle Dynamics, the self-assembled morphology of comb- and star-shaped diblock copolymers was simulated as a function of the number of arms, arm length, weight fraction, and A-B incompatibility. As the number of arms on the star, or grafting points for the comb, was increased from three to four to six, the ability to self-assemble into ordered morphologies was restricted. The molecular bridging between adjacent ordered domains was observed for both comb- and star-shaped copolymers, which was found to be enhanced with increasing number of arms. This study illustrates that comb- and star-shaped copolymers are viable alternatives for applications that would benefit from highly bridged nanostructural domains.

Modeling Amorphous Microporous Polymers for CO2 Capture and Separations

This review concentrates on the advances of atomistic molecular simulations to design and evaluate amorphous microporous polymeric materials for CO2 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 CO2 capture and separation processes.

Ionic-Functionalized Polymers of Intrinsic Microporosity for Gas Separation Applications

Ionic-functionalized microporous materials are attractive for energy-efficient gas adsorption and separation processes and have shown promising results in gas mixtures at pressure ranges and compositions that are relevant for industrial applications. In this work, we studied the influence of different counterions (Li+, Na+, K+, Rb+, and Mg2+) on the porosity, carbon dioxide (CO2) gas adsorption, and selectivity in ionic-functionalized PIM-1 (IonomIMs), a polymer belonging to the class of linear and amorphous microporous polymers known as polymers of intrinsic microporosity (PIMs). It was found that an increase in the concentration of ionic groups led to a decrease in the free volume, resulting in a less porous polymer framework, and Mg2+-functionalized IonomIMs exhibited a relatively larger porosity compared to other IonomIMs. The CO2 adsorption capacity was affected by the different counterions for IonomIM-1, and a higher loading capacity for pure CO2 was observed for Mg2+. Furthermore, the IonomIMs showed an enhanced CO2 selectivity in CO2/CH4 and CO2/N2 gas mixtures at conditions used in pressure swing adsorption and vacuum swing adsorption applications. It was also observed that the concentration of ionic groups plays a vital role in changing the CO2 gas adsorption and selectivity.

The synthesis, chain-packing simulation and long-term gas permeability of highly selective spirobifluorene-based polymers of intrinsic microporosity

Membranes composed of Polymers of Intrinsic Microporosity (SBF-PIMs) have potential for commercial gas separation. Here we report a combined simulation and experimental study to investigate the effect on polymer microporosity and gas permeability by placing simple substituents such as methyl, t-butyl and fused benzo groups onto PIMs derived from spirobifluorene (PIM-SBFs). It is shown that methyl or t-butyl substituents both cause a large increase in gas permeabilities with four methyl groups enhancing the concentration of ultramicropores ( 1.0 nm). Long-term ageing studies (>3.5 years) demonstrate the potential of PIM-SBFs as high-performance membrane materials for gas separations. In particular, the data for the PIM derived from tetramethyl substituted SBF reaches the proposed 2015 Robeson upper bound for O2/N2 and, hence, hold promise for the oxygen or nitrogen enrichment of air. Mixed gas permeation measurements for CO2/CH4 of the aged PIM-SBFs also demonstrate their potential for natural gas or biogas upgrading.