(715f) Effect of Metal Ions in Polymer Electrolytes on Gas Transport Characteristics

Metal-organic frameworks (MOFs) have been dispersed in polymers to improve gas separation performance, and favorable interactions between the MOFs (usually metal centers) and polymers are preferred to improve interfacial compatibility and eliminate any non-selective voids. However, there lacks an understanding of the effect of polymer-metal ion interactions on gas separation properties. In this study, we investigate the effect of metal ions dissociated in amorphous cross-linked poly(ethylene oxide) (XLPEO) on physical properties and gas transport characteristics. Polymer electrolytes were prepared by photo-polymerization of solutions containing poly(ethylene glycol) diacrylate (PEGDA), poly(ethylene glycol) methyl ether acrylate (PEGMEA), and different salts such as LiClO₄, Ni(BF₄)₂, and Cu(BF₄)₂. The salts can be fully dissociated, and the Li⁺, Ni²⁺, and Cu²⁺ ions form conjugations with the ethylene oxide in XLPEO, as validated by FTIR and XRD. Increasing the salt content increases the density (following the additive model) and glass transition temperature (following the Gordon-Taylor equation) except for Cu(BF₄)₂. For instance, adding 17 wt.% Ni(BF₄)₂ to PEGDA-co-PEGMEA increases the Tg from -65 °C in pure polymer to -33 °C. Gas permeation experiments at 35 °C reveal that gas permeability and selectivity decrease by adding more salt to the polymer network. As an example, adding 17 wt.% LiClO₄ to XLPEO results in decreasing CO₂ permeability from 720 to 20 Barrer and CO₂/N₂ selectivity from 48 to 31 because of the competing effect for the polar ether to interact with metal ions, instead of CO₂. The effect of the salt loading on the gas permeability can be described using the Vogel-Fulcher-Tammann (VFT) equation. The effect of the salt loading on gas solubility and diffusivity is investigated. The elucidation of the effect of the polymer and metal ion interactions on gas transport properties may provide guidance in designing mixed matrix materials containing MOFs.