Impacts of polymer-polymer interactions and interfaces on the structure and conductivity of PEG-containing polyimides doped with ionic liquid

- PEG-containing polyimides were synthesized and have sufficient thermal stability.
- Amorphous morphology was found for the random PEG-PI copolymers.
- PEG-PI structure can be varied by changing PEG weight percent or molecular weight.
- Doping with ionic liquids can cause significant structural changes to the PEG-PIs.
- Conductivity of IL-doped PEG-PIs is tuned through PEG content or molecular weight.

Polymer electrolyte membrane (PEM) fuel cells can provide alternatively sourced energy, but current PEMs lose performance under certain conditions. The current work examines composition, structure and properties of poly (ethylene glycol)-aromatic polyimide-ionic liquid systems for PEM applications, as poly (ethylene glycol) (PEG) is an ion conductor and polyimides are stable. To evaluate polymer interactions, different PEG concentrations (0-50% by weight) and different PEG molecular weights (990-6000 g/mole) were examined. Characterization techniques included Fourier transform infrared spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, small-angle x-ray scattering, electrochemical impedance spectroscopy and cyclic voltammetry. By increasing the PEG amount, PEG domains and polymer flexibility are increased, which increases conductivity by two to three orders of magnitude. By increasing PEG molecular weight, PEG segmental motion and PEG-polyimide interface quality are decreased, which decreases conductivity by a factor of two. The maximum conductivity was 64 mS/cm at 80 °C and 70 %RH.

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