Novel polyimide ionomers: CO2 plasticization, morphology, and ion distribution

Highly Propylene-Selective Mixed-Matrix Membranes by in Situ Metal-Organic Framework Formation Using a Polymer-Modification Strategy

Despite the potential of C₃H₆/C₃H₈ separation, there have been no industrial applications of zeolitic-imidazole framework-8 (ZIF-8) mixed-matrix membranes (MMMs) because of the moderate separation performances and several challenging processing issues. Herein, we present a new paradigm of MMM fabrication, named polymer-modification-enabled in situ metal-organic framework formation (PMMOF), enabling in situ formation of ZIF-8 fillers inside the 4,4-(hexafluoroisopropylidene)diphthalic anhydride 2,4,6-trimethyl-1,3-phenylenediamine polymer. PMMOF consists of four steps including hydrolysis of a polymer, ion-exchange, ligand treatment, and imidization. Each step was thoroughly analyzed and important processing parameters were identified, enabling the structural control of MMMs by PMMOF. The binary C₃H₆/C₃H₈ separation performance of the MMMs showed much higher separation factors than conventionally prepared MMMs at similar filler loadings, satisfying the commercial C₃H₆/C₃H₈ separation performance criteria. PMMOF was successfully applied for other MOFs, demonstrating that the process could be general. Finally, as a proof of concept, asymmetric mixed-matrix hollow fiber membranes (i.d. of 0.45 mm and o.d. of 0.63 mm) with ultrathin selective skin layers were prepared by PMMOF, showing C₃H₆ permeance of 2.17 GPU and C₃H₆/C₃H₈ separation factor of ∼20.

Structural, Thermal, and Gas-Transport Properties of Fe3+ Ion-Exchanged Nafion Membranes

The physical and gas-transport properties of Fe³⁺ cross-linked Nafion membranes were examined. Wide-angle X-ray diffraction results revealed a lower crystallinity for Nafion Fe³⁺ but showed essentially no changes in the average chain spacing upon cation exchange of Nafion H⁺. Raman and Fourier transform infrared spectroscopy techniques qualitatively measured the strength of the ionic bond between the Fe³⁺ cations and sulfonate anions. Thermal gravimetric analysis indicated that the incorporation of Fe³⁺ adversely affected the thermal stability of Nafion due to the catalytic decomposition of perfluoroalkylether side chains. Gas sorption isotherms of Nafion Fe³⁺ measured at 35 °C up to 20 atm exhibited a linear sorption uptake for O₂, N₂, and CH₄ following Henry's law and slight concave behavior for CO₂. Pure-gas permeation results showed reduced gas permeability but higher permselectivities compared to Nafion H⁺ with αN₂/CH₄ = 4.0, αCO₂/CH₄ = 35, and αHe/CH₄ = 733 attributable to the strong physical cross-linking effect of Fe³⁺ that caused chain stiffening with enhanced size-sieving behavior. Gas mixture permeation experiments using 1:1 molar CO₂/CH₄ feed demonstrated reduced CO₂ plasticization for Nafion Fe³⁺. At 10 atm CO₂ partial pressure, CO₂/CH₄ selectivity decreased to 28 from the pure-gas value of 35, which was a significant improvement compared to the performance of a Nafion H⁺ membrane.

Characterization of maleimide dimers in photo-cross-linked copolyimide films

Copolyimide membranes are established materials for the separation of gaseous and liquid mixtures. Cross-linking of the polymer strands improves the physical and chemical stability. The photo-cross-linking of a 6FDA-ODA/6FDA-DABA 4 : 1 copolyimide membrane containing maleimide side groups as linker was investigated by FTIR spectroscopy. IR absorption spectra of the copolyimide backbone, 3-hydroxypropyldimethyl maleimide and the copolyimide functionalized with 3-hydroxypropyldimethyl maleimide were measured before and after different irradiation times and compared to each other. For band assignment a normal mode analysis was performed. The backbone of the polymer and the maleimide linker can be well distinguished due to their different spectral band positions. Only the films containing a maleimide moiety perform a photoreaction, the polymer backbone does not interfere. Based on the difference spectra and the results of the DFT calculations it was shown that the trans- and the cis-cycloadduct as well as the previously suggested 2-2'-adduct without a cyclobutane ring are formed upon UV irradiation. Evidence for an oxetane-like photoproduct was not found. Different time constants for the increase of the product bands were observed. The cycloadduct accumulates with a shorter time constant (τ = 2 to 5 min) than the 2-2'-adduct (τ = 75 min). The yield of the photo-cross-linking reaction was determined by spectral deconvolution and kinetic fitting of several marker bands. For the copolyimide synthesized in this work, a maximum value of 6% was reached. The stiffness of the copolyimide backbone inhibits further photo-cross-linking.