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

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Abstract

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 (P), diffusivity (D), and solubility (S). 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.

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