Impacts of material composition on the structural complexity and conductivity of polyimide-poly (ethylene glycol) materials for proton exchange membrane applications [electronic resource]
Keywords: Chemical Engineering.

The topic of energy has been a crucial and exciting research subject in recent years, primarily due to global energy demands continuing to rise from improved standards of living in rapidly growing countries. In addition to the potentially finite supply of many current energy sources, these sources often cause negative environmental impacts; thus, alternative energy sources and conversion devices are needed to help meet the future demand and ensure sustainability. Fuel cells represent one type of device that can convert alternatively sourced energy and provide energy storage. One class of fuel cells, polymer electrolyte membrane fuel cells, or proton exchange membrane fuel cells, are capable of converting completely carbon-free or carbon-neutral energy from alternative fuel sources and also possess high power density. Often, current polymer electrolyte membranes for fuel cells show decreased performance under certain operating conditions. By incorporating different polymers than have been traditionally used, more versatile operating conditions and better efficiencies may be achieved. The current work performed a fundamental and in-depth study of the composition, structure, and properties of poly (ethylene glycol) (PEG) and aromatic polyimide systems. The chemical compounds were synthesized via either Williamson-Ether chemistry or polycondensation reactions followed by thermal imidization. Liquid electrolytes were incorporated into the polymer films through soaking to achieve ionic conductivity. The properties of the polymers were characterized using a variety of techniques. Nuclear magnetic resonance and Fourier transform infrared spectroscopy were used to verify the success of the chemical syntheses. Thermal stability of the polymers was evaluated using thermal gravimetric analysis. Differential scanning calorimetry revealed the morphology of the polymers, while the nanometer-level structure was characterized using small-angle x-ray scattering. Finally, conductivity was measured through electrochemical impedance spectroscopy and cyclic voltammetry. Protocol development and optimal material choice were tested by varying the method of polymer combination as well as exploring changes in processing and the presence of inorganic additives. Some preliminary results revealed that the method of PEG incorporation could impact the nanometer level structure of these PEG-PI systems. Chemical linking of the PEG to the polyimide in a copolymer architecture led to attractive material stability, morphology, and structure. Other studies revealed that different humidity levels and variations in liquid electrolyte dopant had profound effects on the conductivity. Once optimal testing procedures and materials were determined, the composition of the optimal polymers was varied systematically to create a family of materials that was structurally evaluated and tested for conductivity. Changes in relative ratio of the two polymer components, PEG molecular weight, choice of aromatic diamine, choice of aromatic dianhydride, synthesis solvent, type of ionic liquid electrolyte, and number of synthesis steps included the primary material variations explored. The analysis revealed that different material changes had varying impacts on structure and conductivity, some of which were quite significant. The most significant impact on conductivity was seen by varying the weight percent of PEG. Variations in PEG molecular weight and aromatic diamine had relatively moderate impacts on conductivity. In contrast, changes related to the aromatic dianhydride, ionic liquid electrolyte, and number of synthesis steps had a relatively minimal effect on conductivity. The choice of solvent did not have as clear of a relationship to conductivity as some of the other material variations. The composition and structure of the polymers can have substantial influence on the conductivity of these materials, and the current work provides insight into how to engineer polymers with optimal conductivity for polymer electrolyte membrane applications.