Physical aging in glassy polymer gas separation membranes: mechanistic insights into filler mitigation strategies

Physical aging in high-free-volume glassy polymers progressively reduces gas permeability, limiting the long-term performance of gas separation membranes. The incorporation of nanoporous fillers into mixed-matrix membranes (MMMs) and thin-film nanocomposite (TFN) membranes has emerged as a promising strategy to mitigate aging while preserving high gas transport performance. This perspective outlines the fundamental characteristics of physical aging, including its underlying mechanisms, recovery approaches, and key factors governing its progression, followed by a review of the techniques used to quantify aging and their associated limitations. The central focus of this perspective is a mechanistic evaluation of how nanoporous fillers suppress physical aging, moving beyond the conventional free-volume spacer concept to structural pillaring effects, interfacial rigidification through chain adsorption, and direct suppression of segmental chain mobility. Evidence across diverse polymer systems indicates that aging resistance is governed by filler properties rather than matrix chemistry. The analysis also highlights the coupled influence of filler loading and selective-layer thickness in TFC membranes on aging behaviour. A critical inconsistency in current reporting practices is identified, where variations in observation time, membrane format and performance metrics prevent meaningful cross-study comparison. To address this issue, minimum reporting standards are proposed, with aging-corrected performance as the primary benchmark for filler effectiveness evaluation. Broader adoption of broadband dielectric spectroscopy is also advocated to directly probe chain dynamics, while nanoporous engineering of gutter layers in TFC membranes is identified as a promising yet underexplored pathway toward durable membrane performance.