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

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 Troger'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 129 Xe 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.

Aging of polymers of intrinsic microporosity tracked by methanol vapour permeation

“…The measurements were carried out in two different modes: continuous mode (one experimental run lasted more than 650 hours) and momentary mode (numerous short-term experimental runs, each of which lasted about 6 hours). Moreover, as the measurements were conducted using MeOH vapours, long-term effects similar to that of the so called MeOH treatment [6] can be expected and were, indeed, also observed in this work.…”

“…Thin flat membrane samples were prepared by the solvent evaporation method from 2 -3 wt.% polymer solutions in chloroform and dried in partially covered Petri-dishes under ambient temperature and pressure for at least 12 h. In order to remove the residual solvent (for details see Appendix A.2 for PIM-1 and the literature [6] for PIM-EA-TB) and to rejuvenate the polymer structure, the as-cast films were treated with liquid MeOH. The treatment was performed by immersing the membrane overnight in MeOH and then allowing it to dry for 1 h under ambient conditions.…”

“…Thanks to their high permeabilities and moderate selectivities, PIMs exhibit very good perm-selective properties [1]. Recently, several PIMs have overcome Robeson's 2008 upper bound [2] for certain pairs of gases [3][4][5][6]. This achievement makes PIMs potentially very attractive for industrial use.…”

“…For example, their transient sorption kinetics cannot be described by the commonly used Fick's second law of diffusion. The second problem is the aging phenomenon [8], which causes a significant decrease of PIMs permeability over time [3,6,[9][10][11][12][13].…”

“…PIM-EA-TB was chosen because of its unique properties, especially for its gas separation ability [5]. To the best of our knowledge, the aging of PIM-EA-TB has not yet been systematically studied, with a few basic details described in the literature [5,6].…”