High-Loading Poly(ethylene glycol)-Blended Poly(acrylic acid) Membranes for CO₂ Separation

Poly(ethylene glycol) (PEG) is an amorphous material of interest owing to its high CO2 affinity and potential usage in CO2 separation applications. However, amorphous PEG often has a low molecular weight, making it challenging to form into the membrane. The crystalline high average molar mass poly(ethylene oxide) (PEO) cannot exhibit CO2 separation characteristics. Thus, it is crucial to employ low molecular weight PEG in high molecular weight polymers to increase the CO2 affinity for CO2 separation membranes. In this work, poly(acrylic acid) (PAA)/PEG blend membranes with a PEG-rich phase were simply fabricated by physical mixing with an ethanol solvent. The carbonyl group of the PAA and the hydroxyl group of the PEG formed a hydrogen bond. Furthermore, the thermal stability, glass transition temperature, and surface hydrophilicity of PAA/PEG blend membranes with various PEG concentrations were further characterized. The PAA/PEG(1:9) blend membrane exhibited an improved CO2 permeability of 51 Barrer with high selectivities relative to the other gas species (H2, N2, and CH4; CO2/H2 = 6, CO2/N2 = 63, CO2/CH4 = 21) at 35 °C and 150 psi owing to the enhanced CO2 affinity with the amorphous PEG-rich phase. These PAA/PEG blend membrane permeation characteristics indicate a promising prospect for CO2 capture applications

Functional Bisphenol-Based Additive for Controlled Gas Transport and Suppressed Plasticization of Polyimide Membranes

This paper describes a rationally designed, small-molecule additive that alters the physical properties of polyimide membranes. The additive molecule has a bisphenol-based core that can sufficiently interact with the polyimide backbone and a compatibilizer that ensures uniform distribution on a molecular level. Upon postcasting heat treatment, in situ morphing of the additive is caused to induce additional noncovalent interactions, reinforcing the matrices and affecting their gas transport properties. By incorporating 1 wt % small-molecule additives, the polymer chain packing structure of the polymer matrix was noticeably modified. This feature resulted in an improvement in gas selectivity for gas pairs with large kinetic size differences, such as CO2 or H2 discrimination from CH4 or N2. Moreover, the CO2 plasticization phenomenon was dramatically suppressed due to the formation of a noncovalent-induced rigid polymer chain arrangement. This design concept could be advanced by employing a variety of core and compatibilizer units or adapting it to other external stimuli, allowing stimuli-responsive additives for gas separation membranes

Ionic Triptycene Additive-Blended Poly(2,6-dimethyl-1,4-phenylene oxide)-Based Anion Exchange Membranes for Water Electrolyzers

Triptycene is a popular molecule with a bulky and rigid molecular structure and is three-dimensionally connected by three benzene rings. Owing to this, triptycene has been employed in complicated synthetic processes to form an inefficient polymer chain packing structure with high free volume. However, it is difficult to control the polymerization degree and increase its mechanical properties because of the three-dimensional contorted bulky structure of the triptycene moiety. In this study, we simply incorporated the triptycene molecule without a complex polymerization process. We fabricated brominated poly­(2,6-dimethyl-1,4-phenylene oxide) (BPPO) and chloromethylated triptycene (triptycene-Cl) to form uniformly blended membranes with various small triptycene molecule additive weight loadings. Based on the BPPO matrix with appropriate bromination degree, the ionic triptycene additive provides an additional quaternary ammonium group and free volume for efficient hydroxide ion transport. Additionally, the dimensional stability of the dual-quaternized BPPO/triptycene-Cl blended membrane was effectively controlled because of the increased free volume, despite the high water uptake. As a result, the dual-quaternized BPPO/triptycene-Cl blended membrane with high triptycene-Cl weight loading shows a significantly enhanced hydroxide ion conductivity and reduced activation energy estimated within the water electrolyzer operation temperature range. The dual-quaternized BPPO/triptycene-Cl blended membrane with 20% triptycene-Cl exhibits a significantly lower and more stable area-specific resistance as well as higher water electrolyzer cell performance when compared to those of the commercial FAA-3-50 membrane

Hollow Heteropoly Acid-Functionalized ZIF Composite Membrane for Proton Exchange Membrane Fuel Cells

Heteropoly acids (HPAs) have been used in perfluorinated sulfonic acid polymers such as Nafion or Aquivion to form organic/inorganic composite membranes with improved proton conductivity and water management ability. However, the HPA has a low BET surface area with water-soluble characteristics, which prevents enhancement in the number of proton-transferable sites and accelerates HPA leaching while operating the proton exchange membrane fuel cells (PEMFCs). The HPA was functionalized on zeolite imidazolate framework-67 (ZIF-67) nanoparticles to address these drawbacks. Incorporating it into the MOF made it water insoluble and enhanced the internal surface area, leading to a good proton conductor. Using a synthetic approach, we were able to form HPA-functionalized ZIF-67 (HZF), which can be optimized with simple compositional modifications and whose HPA content is controllable. The HZF nanoparticles exhibited a hollow structure that formed an HPA–ZIF shell layer because the dissociated cobalt ion and 2-methylimidazole diffused from the core side to the surface layer to interact with the HPA. The HZF/Aquivion composite membranes exhibited excellent mechanical properties and good resistance to the polymer chain swelling phenomenon. The electrochemical properties of the HZF/Aquivion composite membranes with various HZFs were characterized to determine the optimal HPA content in the HZF nanoparticles. The 3 wt % hollow HZF/Aquivion composite membrane with the appropriate HPA content exhibited higher proton conductivities than the pure Aquivion membrane, measuring 0.14 S/cm at 25 °C and 100% RH and 0.09 S/cm at 80 °C and 30% RH. This result indicates that the hollow HZF/Aquivion composite membrane can provide efficient proton transfer and water management ability, suggesting a good strategy for the PEMFC operation

Ionic Cross-Linked MOF-Polymer Mixed-Matrix Membranes for Suppressing Interfacial Defects and Plasticization Behavior

To address the plasticization phenomenon and MOF-polymer interfacial defects, we report the synthesis of ionic cross-linked MOF MMMs from a dual brominated polymer and MOF components by using N,N′-dimethylpiperazine as the cross-linker. We synthesized brominated MIL-101­(Cr) nanoparticles by using mixed linkers and prepared brominated polyimide (6FDA-DAM-Br) to form ionic cross-linked MMMs. The gas permeation properties of the polyimide, ionic cross-linked MOF-polymer MMMs, and non-cross-linked MOF-polymer MMMs with various MOF weight loadings were investigated systematically to effectively understand the effects of MOF weight loading and ionic cross-linking. The ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly enhanced gas permeability with an H2 permeability of 1640 Barrer and CO2 permeability of 1981 Barrer and slightly decreased H2/CH4, H2/N2, CO2/CH4 and CO2/N2 selectivities of 16.9, 15.4, 20.5, and 18.6, respectively. The H2 and CO2 permeabilities are approximately 2–3 fold higher than those of the pure polyimide (6FDA-DAM) membrane. Moreover, the ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly increased resistance to plasticization. This is because the brominated MOF incorporation boosted molecular transport and polymer chain rigidity, and ionic cross-linking further reduced the number of interfacial defects and polymer chain mobility