Polymer electrolyte membrane (PEM) fuel cells have the potential to provide cleaner and more efficient energy. Often, current PEMs for these fuel cells are limited in terms of their operating conditions, especially with regard to humidity and temperature. By using different polymers than have been traditionally used, more versatile operating conditions may be achieved. The structural details of current PEMs is often not studied in great detail. By performing a fundamental and in depth study of the structure of potential polymer membrane materials, more insight can be gained in terms of achieving good performance through material design. A multi-polymer system has been created and analyzed in terms of the relationship between ionic conduction properties and nanometer scale structure.

In the current work, aromatic polyimide (PI) and poly (ethylene glycol) (PEG) hybrid membranes have been synthesized for fuel cell PEM applications. Because a variety of properties are needed for an effective PEM, multiple polymers often need to be used. Also, these multi-polymer systems often lead to phase separation and self-assembly that can greatly impact material properties. PIs are known to be very stable, while PEG has been known to provide proton conduction properties under certain conditions. The goal is to exploit the drastically different properties of these polymers and understand how the structure of these materials relates to the conductivity. The polyamic acid precursors to these polymer systems were synthesized by a classical, random, one pot poly-condensation method that has been shown to work for many different types of polyamic acids. The PEG was used in pre-polymerized form and incorporated in the polyamic acids through chemical bonding to achieve structural self-assembly on the nanometer length scale. These polymer systems were converted to PI-PEG systems through thermal imidization. Composition and morphology of these hybrid systems was varied to create a family of materials.

The polyamic acid precursors and PIs were characterized using FTIR. TGA was used to assess thermal properties. Morphology was determined by DSC. Small angle x-ray scattering was used to determine the polymer structure on a length scale between 0.5 and 80 nanometers, because it has been shown that features on this length scale can often impact conduction properties. Oftentimes understanding how structure can improve ion transport is important in fuel cell membranes. Since liquid electrolytes are often needed to provide any significant conduction properties for polymers, inorganic acid or ionic liquids were incorporated into the films. The films were then analyzed for ionic conductivity properties using electrochemical impedance spectroscopy and cyclic voltammetry. Effects of temperature were studied with regards to the conductivity. These multi-polymer systems were analyzed in order to determine composition-structure-property relationships for fuel cell membrane applications.

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