Elastic free-standing RTIL composite membranes for CO2/N2 separation based on sphere-forming triblock/diblock copolymer blends

• Novel free-standing RTIL composite membranes show excellent CO2 separation performance exceeding the 2008 Robeson upper bound.
• The membranes are based on thermally processible melt-blends of sphere-forming diblock and triblock copolymer that absorb very large quantities of RTIL (>94 wt%).
• The unique nanostructure imparts excellent mechanical elasticity under both compression and tension withstanding transmembrane pressure testing to 400 kPa.
• Membranes showed minimal hysteresis under cyclic mechanical loading, and no detectable fatigue in performance over 28 days in operation.

Solvent-free, melt-state self-assembly of sphere-forming polystyrene-b-poly(ethylene oxide) diblock copolymer/polystyrene-b-poly(ethylene oxide)-b-polystyrene triblock copolymer (SO/SOS) blends was used to produce free-standing, room-temperature ionic liquid (RTIL) composite membranes with excellent mechanical properties and CO2/N2 separation performance. Membranes were prepared from vitrified SO/SOS diblock/triblock melts by swelling to greater than 94 wt% 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) ionic liquid. These free-standing composite membranes were evaluated for their CO2/N2 separation performance over a range of transmembrane pressures (26-413 kPa), with results traversing the 2008 Robeson upper bound. Even with such high loadings of neat [EMIM][TFSI], these membranes exhibit the mechanical properties of solid elastomers, evident by the ultimate tensile strength of 250 kPa and the compressive modulus at 40% strain of 348 kPa for the membranes with 46% triblock copolymer content (SOS46). Cyclic loading and unloading in both tension and compression revealed reversible elasticity, with no mechanical property degradation or permanent hysteresis detected. For each composition studied, membrane failure occurred at uniaxial extension ratios of 3.8. The CO2 permeability performance of these membranes was stable for 28 days of testing with no detectable RTIL leakage. Transmembrane pressures exceeding 400 kPa could be accommodated, with cyclic application of transmembrane pressure differentials at 230 kPa producing neither plastic deformation nor diminished performance in gas separations. Such behavior is intimately tied to the highly distensible and elastic nature of the melt-assembled diblock/triblock network underpinning this RTIL-based composite membrane design.

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