Gas Permeability of Hexaphenylbenzene Based Polymers of Intrinsic Microporosity

The synthesis and characterization of a series of novel hexaphenylbenzene (HPB) based polymers of intrinsic microporosity (PIM-HPBs) containing methyl, bromine, and nitrile substituents are reported. The successful formation of thin films from these polymers allowed the evaluation of the influence of the substituents on intrinsic microporosity and gas permeability. Analysis by the time-lag method also yielded information about gas diffusion coefficients and, indirectly, the gas solubility. The gas permeability varies as a function of the polarity of the substituents and shows a significant increase after treatment of the samples with methanol, especially for films cast from THF as the solvent. This enhancement, which is mostly due to an increase in the diffusion coefficient, is only partially lost upon aging of the membranes for 5 months. Measurements at different feed pressures confirm the typical dual mode sorption behavior, with increasing diffusivity and decreasing permeability and solubility as a function of the feed pressure

Enhancing the Gas Permeability of Tröger’s Base Derived Polyimides of Intrinsic Microporosity

A series of four novel Tröger’s base (TB) derived polyimides of intrinsic microporosity (PIM–TB–PI) is reported. The TB diamine monomer (4MTBDA) possesses four methyl groups in order to restrict rotation about the C–N imide bonds in the resulting polymers. The polymers possess apparent BET (Brunauer, Emmett, and Teller) surface areas between 584 and 739 m² g^–1, complete solubility in chloroform, excellent molecular mass, high inherent viscosity and good film-forming properties. Gas permeability measurements demonstrate enhanced performance over previously reported polyimide-based Tröger’s base (TB) polymers confirming the benefit of the additional methyl groups within the TB diamine monomer. Notably, a polyimide derived from 4MTBDA and pyromellitic anhydride (PMDA) demonstrates gas permeability data above the 2008 upper bounds for important gas pairs such as O₂/N₂, H₂/N₂, and H₂/CH₄

Highly Permeable Benzotriptycene-Based Polymer of Intrinsic Microporosity

A novel polymer of intrinsic microporosity (PIM) was prepared from a diaminobenzotriptycene monomer using a polymerization reaction based on Tröger’s base formation. The polymer (PIM-BTrip-TB) demonstrated an apparent Brunauer, Emmet, and Teller (BET) surface area of 870 m² g^–1, good solubility in chloroform, excellent molecular mass, high inherent viscosity and provided robust thin films for gas permeability measurements. The polymer is highly permeable (e.g., <i>P</i>H₂ = 9980; <i>P</i>O₂ = 3290 Barrer) with moderate selectivity (e.g., <i>P</i>H₂/<i>P</i>N₂ = 11.0; <i>P</i>O₂/<i>P</i>N₂ = 3.6) so that its data lie over the 2008 Robeson upper bounds for the H₂/N₂, O₂/N₂, and H₂/CH₄ gas pairs and on the upper bound for CO₂/CH₄. On aging, the polymer demonstrates a drop in permeability, which is typical for ultrapermeable polymers, but with a significant increase in gas selectivities (e.g., <i>P</i>O₂ = 1170 Barrer; <i>P</i>O₂/<i>P</i>N₂ = 5.4)

Polymers of Intrinsic Microporosity Containing Tröger Base for CO₂ Capture

Properties of four polymers of intrinsic microporosity containing Tröger’s base units were assessed for CO₂ capture experimentally and computationally. Structural properties included average pore size, pore size distribution, surface area, and accessible pore volume, whereas thermodynamic properties focused on density, CO₂ sorption isotherms, and enthalpies of adsorption. It was found that the shape of the contortion site plays a more important role than the polymer density when assessing the capacity of the material, and that the presence of a Tröger base unit only slightly affects the amount adsorbed at low pressures, but it does not have any significant influence on the enthalpy of adsorption fingerprint. A comparison of the materials studied with those reported in the literature allowed us to propose a set of guidelines for the design of polymers for CO₂ capture applications

Molecular Modeling and Gas Permeation Properties of a Polymer of Intrinsic Microporosity Composed of Ethanoanthracene and Tröger’s Base Units

Polymers of intrinsic microporosity (PIMs) are receiving increasing attention from the membrane community because of their high gas and vapor permeability. Recently a novel ethanoanthracene-based PIM synthesized by Tröger’s base formation (PIM-EA-TB) was reported to have exceptional transport properties, behaving as a polymer molecular sieve membrane. In the present work, an extensive investigation of the structural, mechanical, and transport properties of this polymer, both by experimental analysis and by molecular simulation, offers deep insight into the behavior of this polymer and gives an explanation for its remarkable performance as a membrane material. Transport properties were determined by the barometric time-lag method, by the volumetric method with gas chromatographic or mass spectrometric gas analysis, and by gravimetric sorption measurements, yielding all basic transport parameters, permeability (<i>P</i>), diffusivity (<i>D</i>), and solubility (<i>S</i>). Upon alcohol treatment, PIM-EA-TB exhibited a much stronger permeability increase than archetypal “benchmark” polymer PIM-1, with performance above the Robeson upper bound for several gas pairs. This is in part due to an extremely high gas solubility in PIM-EA-TB, higher than in PIM-1. The experimental data were supported by extensive modeling studies of the polymer structure and the spatial arrangement of its free volume. Modeling confirms that the high gas permeability must be attributed to the large fractional free volume of the polymer. The simulated free volume size distribution in PIM-EA-TB is in agreement with the average experimental free volume elements size determined by PALS and ¹²⁹Xe NMR analysis. The modeled spatial arrangement of the free volume revealed a slightly lower interconnectivity of the FV elements in PIM-EA-TB compared to PIM-1. Along with its higher chain rigidity, determined by analysis of the torsion angles in the polymer model, this was identified as the main reason for its stronger size sieving behavior and relatively high permselectivity. A number of peculiarities in the behavior of PIMs will also be discussed here, explaining discrepancies between results published in the literature by different laboratories, the effect of their thermomechanical history, aging, or conditioning, and the influence of the measurement technique and of the experimental conditions on the results. This makes this study of inestimable value for unifying the results of different experimental techniques and fully understanding the transport properties