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<p>Microporous polyimides (PIM-PIs) have emerged as promising high-performance membranes for gas separation. However, achieving an optimal balance between permeability and selectivity remains a major challenge. In this study, we designed and synthesized a series of PIM-PIs by combining rigid dianhydrides 9-bis(trifluoromethyl)-2,3,6,7-xanthenetetracarboxylic dianhydride (6FCDA) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with contorted diamines, including 9,9-bis(4-aminophenyl)fluorene (FDA), 9,9′-spirobifluorene-2,2′-diamine (SBFDA), and 3,3,3′,3′-tetramethyl-1,1′-spirobiindane-5,5′-diamine-6,6′-diol (TSDA), to systematically elucidate the relationship between hierarchical microstructure and gas transport behavior. Comprehensive characterization revealed that the 6FCDA-based polymers exhibited a higher microporosity (<i>V</i><sub>micro</sub>/<i>V</i><sub>total</sub> up to 54.7%) and fractional free volume compared to their 6FDA counterparts. Gas permeation measurements showed that the 6FCDA/SBFDA membrane delivered a CO<sub>2</sub> permeability of 386 Barrer and CO<sub>2</sub>/CH<sub>4</sub> selectivity of 30.2, exceeding the 2008 Robeson upper bound. Structure-property correlation analyses indicated that diffusion selectivity predominantly governed gas separation performance, with rigid, spirocyclic architectures suppressing chain packing to generate sub-5 Å micropores, as further validated by molecular simulations. The optimized 6FCDA/FDA membrane achieved a BET surface area of 423 m<sup>2</sup>·g<sup>−1</sup>, while maintaining excellent mechanical strength and high thermal stability. This work establishes an effective monomer design strategy to overcome the permeability-selectivity trade-off through backbone rigidification, thereby advancing PIM-PIs for practical applications in natural gas purification and carbon capture.</p>

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Molecularly Engineered Contorted Polyimides: Unraveling the Role of Backbone Rigidity in Gas Separation

  • 期刊论文

  • 发表日期：

    2026年2月2日

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  • Hong-Qin Zhao,
  • Bing-Yu Zou,
  • Bing-Xi He,
  • Wei-Feng Peng,
  • Lu-Hao Qiu,
  • Feng Bao,
  • Ming-Jun Huang,
  • Huan-Yu Lei

摘要

Microporous polyimides (PIM-PIs) have emerged as promising high-performance membranes for gas separation. However, achieving an optimal balance between permeability and selectivity remains a major challenge. In this study, we designed and synthesized a series of PIM-PIs by combining rigid dianhydrides 9-bis(trifluoromethyl)-2,3,6,7-xanthenetetracarboxylic dianhydride (6FCDA) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with contorted diamines, including 9,9-bis(4-aminophenyl)fluorene (FDA), 9,9′-spirobifluorene-2,2′-diamine (SBFDA), and 3,3,3′,3′-tetramethyl-1,1′-spirobiindane-5,5′-diamine-6,6′-diol (TSDA), to systematically elucidate the relationship between hierarchical microstructure and gas transport behavior. Comprehensive characterization revealed that the 6FCDA-based polymers exhibited a higher microporosity (Vmicro/Vtotal up to 54.7%) and fractional free volume compared to their 6FDA counterparts. Gas permeation measurements showed that the 6FCDA/SBFDA membrane delivered a CO2 permeability of 386 Barrer and CO2/CH4 selectivity of 30.2, exceeding the 2008 Robeson upper bound. Structure-property correlation analyses indicated that diffusion selectivity predominantly governed gas separation performance, with rigid, spirocyclic architectures suppressing chain packing to generate sub-5 Å micropores, as further validated by molecular simulations. The optimized 6FCDA/FDA membrane achieved a BET surface area of 423 m2·g−1, while maintaining excellent mechanical strength and high thermal stability. This work establishes an effective monomer design strategy to overcome the permeability-selectivity trade-off through backbone rigidification, thereby advancing PIM-PIs for practical applications in natural gas purification and carbon capture.