Membranes composed of Polymers of Intrinsic Microporosity (SBF-PIMs) have potential for commercial gas separation. Here we report a combined simulation and experimental study to investigate the effect on polymer microporosity and gas permeability by placing simple substituents such as methyl, t-butyl and fused benzo groups onto PIMs derived from spirobifluorene (PIM-SBFs). It is shown that methyl or t-butyl substituents both cause a large increase in gas permeabilities with four methyl groups enhancing the concentration of ultramicropores (1.0 nm). Long-term ageing studies (>3.5 years) demonstrate the potential of PIM-SBFs as high-performance membrane materials for gas separations. In particular, the data for the PIM derived from tetramethyl substituted SBF reaches the proposed 2015 Robeson upper bound for O₂/N₂ and, hence, hold promise for the oxygen or nitrogen enrichment of air. Mixed gas permeation measurements for CO₂/CH₄ of the aged PIM-SBFs also demonstrate their potential for natural gas or biogas upgrading.

Polymers of intrinsic microporosity (PIMs) are distinguished from other polymers in how the diffusion coefficients for light gases depend on their effective size, with a stronger size-selective trend for gases larger than He and H ₂ .

A detailed analysis of the basic transport parameters of two triptycene-based polymers of intrinsic microporosity (PIMs), the ultrapermeable PIM-TMN-Trip and the more selective PIM-BTrip, as a function of temperature from 25°C to 55°C, is reported. For both PIMs, high permeability is based on very high diffusion and solubility coefficients. The contribution of these two factors on the overall permeability is affected by the temperature and depends on the pene-trant dimensions. Energetic parameters of permeability, diffusivity and solubility are calculated using Arrhenius-van’t Hoff equations and compared with those of the archetypal PIM-1 and the ultrapermeable but poorly selective poly(trimethylsilylpropyne) (PTMSP). This considers, for the first time, the role of entropic and energetic selectivities in the diffusion process through highly rigid PIMs. This analysis demonstrates how energetic selectivity dominates the gas transport properties of the highly rigid triptycene PIMs and enhances the strong size-sieving character of these ultrapermeable polymers.