Tailoring gas separation performance in plastic crystal membranes with polymer additives

Abstract

Membrane-based technologies offer a sustainable and energy efficient alternative to conventional CO₂ separation methods, yet their performance is often limited by permeability–selectivity trade-offs. This study addresses this challenge by exploring the gas separation performance of composite membranes based on the organic ionic plastic crystal, 3-ethyl-3-methyloxazolidinium bis(fluorosulfonyl)imide ([C₂moxa][FSI]) blended with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and functional additives including polytetrafluoroethylene (PTFE), polydimethylsiloxane (PDMS), polyethersulfone (PES), and alumina. These additives improved the gas separation performance in different ways. The addition of PDMS yielded the highest CO₂ permeability (328 barrer), while PTFE-enhanced membranes achieved a record CO₂/N₂ selectivity (415), surpassing the Robeson upper bound. Positron annihilation lifetime spectroscopy (PALS) and pulsed-field gradient NMR (PFG-NMR) revealed that improvements in gas transport are governed by additive-induced increases in fractional free volume and enhanced ion mobility. The calculated inverse Haven ratios further elucidated the effect of the decoupling of ion dynamics in the high-performing membranes. These results demonstrate the synergistic effects of both polymer and inorganic additives in tuning solubility and diffusivity, offering a platform for next-generation OIPC-based membranes for carbon capture or other applications.