Molecular Transport Behavior of CO₂ in Ionic Polyimides and Ionic Liquid Composite Membrane Materials

Abstract

Ionic polyimides (i-PI) are a new class of polymer materials that are very promising for CO2 capture membranes, and recent experimental studies have demonstrated their enhanced separation performance with the addition of imidazolium-based ionic liquids (ILs). However, there is very little known about the molecular-level interactions in these systems, which give rise to interesting gas adsorption and diffusion characteristics. In this study, we use a combination of Monte Carlo and molecular dynamics simulations to analyze the equilibrium and transport properties of CO2 molecules in the i-PI and i-PI + IL composite materials. The addition of several different common ILs are modeled, which have a plasticization effect on the i-PI, lowering the glass transition temperature (T g). The solubility of CO2 strongly correlates with the T g, but the diffusion demonstrates more unpredictable behavior. At low concentrations, the IL has a blocking effect, leading to reduced diffusion rates. However, as the IL surpasses a threshold value, the relationship is inverted and the IL has a facilitating effect on the gas transport. This behavior is attributed to the simultaneous contributions of the increased i-PI plasticization at higher IL concentrations (facilitating gas hopping rates from cavity-to-cavity) and the increased IL continuity throughout the system, enabling more favorable transport pathways for CO2 diffusion.

Sorption‐induced polymer rearrangement: approaches from molecular modeling

“…Relatedly, for the combined GCMC/MD technique the aim is to use GCMC simulations to predict loading and NPT MD simulations to account for the adsorbate-induced polymer chain dynamics, swelling and plasticization associated with that loading. Approximately a dozen works have used the MC/MD approach to incorporate polymer dynamics and make predictions of adsorption processes with adsorbent polyimides, [44][45][46][47][48] composites with ionic liquids, 49 vinyl polymers, 50,51 PIMs 52,53 and cellulose materials. 54 The workflow of the MC/MD procedure begins with a GCMC simulation on a static equilibrated 'fresh' polymeric framework, which results in the uptake of an initial concentration of adsorbate.…”

Facilitated Transport Membranes With Ionic Liquids for CO2 Separations

“…Most recent examples include ionic polyimides that incorporate imidazolium-based ILs (Mittenthal et al, 2017 ). Szala-Bilnik et al ( 2019 , 2020 ) studied the impact of the anion in ionic polyimide–IL composite membranes, where the imidazolium functionality is present in both the polymer backbone and the plasticizer. They showed that the ion mobility in pure ILs does not translate to cationic membranes, due to ion coordination with the fixed cation.…”

Solubility Behavior of CO2 in Ionic Liquids Based on Ionic Polarity Index Analyses

“…), there are several polymer platforms that can leverage the benefits of ILs. These include supported IL membranes (SILMs), , polymerized ionic liquids (PILs), and other hybrid/composite materials such as ionic polyimides. − As computer resources have increased dramatically over the last 10–20 years, rational screening approaches for IL design have emerged. Based on a variety of computational approaches (and underlying assumptions), some well-known screening techniques include the Quantum Structure–Property Relationship (QSTR), Volume-Based Thermodynamic (VBT) or Volume-Based Approach (VBA), − COnductor-like Screening MOdel for Realistic Solvation (COSMO-RS), , and quantum chemical calculations of single ion pairs, , which have been discussed in a recent review …”