Hundreds of polymers have been evaluated as membrane materials for gas separations, but fewer than 10 have made it into current commercial applications, mainly due to the effects of physical aging and plasticization. Efforts to overcome these two problems are a significant focus in gas separation membrane research, in conjunction with improving membrane separation performance to surpass the Robeson upper bounds of selectivity versus permeability for commercially important gas pairs. While there has been extensive research, ranging from manipulating the chemistry of existing polymers (e.g., thermally rearranged or cross-linked polyimides) to synthesizing new polymers such as polymers of intrinsic microporosity (PIMs), there have been three major oversights that this review addresses: (1) the need to compare the approaches to achieving the best performance in order to identify their effectiveness in improving gas transport properties and in mitigating aging, (2) a common standardized aging protocol that allows rapid determination of the success (or not) of these approaches, and (3) standard techniques that can be used to characterize aging and plasticization across all studies to enable them to be robustly and equally compared. In this review, we also provide our perspectives on a few key aspects of research related to high free volume polymer membranes: (1) the importance of Robeson plots for membrane aging studies, (2) eliminating thermal history, (3) measurement and reporting of gas permeability and aging rate, (4) aging and storing conditions, and (5) promising approaches to mitigate aging.

Three structurally diverse polymers of intrinsic microporosity reversibly adsorb significant quantities of hydrogen (1.4–1.7 % by mass at 77 K) and represent the first examples of a new type of purely organic hydrogen storage material, which can be tailored to meet the specific requirements of hydrogen physisorption.

The concept of free volume is useful for explaining aspects of the chain mobility and permeability of polymers, even though its precise definition is subject to debate. Polymers that trap a large amount of interconnected free volume in the glassy state behave in many respects like microporous materials and potentially find application in membrane separations and heterogeneous catalysis. The development is outlined of a new type of polymer, for which the molecular structure contains sites of contortion (e.g. spiro-centres) within a rigid backbone (e.g. ladder polymer). These polymers of intrinsic microporosity (PIMs) include both insoluble network polymers and soluble non-network polymers that may be processed into membranes or other useful forms. Experimental methods are discussed for elucidating the free volume or micropore distribution, and the behaviour of PIMs is compared with that of the ultrapermeable polymer poly(1-trimethylsilyl-1-propyne).

For gas separation membranes, materials are required that offer high permeability as well as good selectivity for a desired separation. For glassy polymers, the gas transport properties depend on the amount and distribution of free volume and on chain mobility. The most highly permeable polymers have rigid, twisted macromolecular backbones that give rise to microvoids. Examples include substituted polyacetylenes, perfluoropolymers, addition-type polynorbornene, polymers of intrinsic microporosity (PIMs) and some polyimides. High permeability membranes may also be produced by thermal rearrangement of precursor polymers.

Organic–organic composite membranes are prepared by in situ crystallization of cage molecules in a polymer of intrinsic microporosity. This allows a direct one‐step route to mixed‐matrix membranes, starting with a homogeneous molecular solution. Extremely high gas permeabilities are achieved, even after ageing for more than a year, coupled with good selectivity for applications such as CO₂ recovery.