Characterization of Poly(propylenimine) and Poly(N-methylpropylenimine) as Polymer Electrolytes with N,N'-dimethylethylenediamine and N,N'-dimethylpropylenimine as Model Compounds

Abstract

Many factors including light weight, flexible form factor, processing ease and environmental friendliness make solid polymer electrolytes (SPEs) desirable materials for use in lithium ion battery systems. However, consistently low room temperature conductivities have precluded the realization of a true solid polymer battery. Despite nearly forty years of extensive study, fundamental interactions occurring within SPEs are not thoroughly understood. Knowledge gained from poly(ethyleneoxide), the most well-characterized SPE, has spurred the quest for understanding of other systems.

This dissertation examines complexes of poly(propylenimine), PPI and poly (methylpropylenimine), PMPI with lithium triflate with the goal of identifying the primary interactions taking place in the systems and determining how these interactions impact ionic conductivity. The polymers are compared against their better-characterized homologs poly(ethylenimine), PEI and poly(methyl-ethylenimine), PMEI. Vibrational spectroscopy was used to probe interactions between cations, anions and the polymers, while differential scanning calorimmetry and ac impedance measurements were used to characterize the thermal and ionic conductivity behaviors of the systems. Conductivities of both PPI and PMPI were determined to be insufficient for use in battery systems, but interesting results in terms of ionic associations were obtained. Additionally, significant amounts of hydrogen bonding were shown to be present in PPI and the PPI:salt complex.

Two small molecule model compounds, N,N‘–dimethylethylenimine and N,N‘–dimethylpropylenimine, were utilized to meet the second goal of this research. This objective set included: establishing what types of the hydrogen bonding interactions occur within the model compounds and their salt complexes, identifying the spectral signatures of those interactions and determining how the cation, anion and polymer interactions change in the presence of salt. Dilution of the compounds and their salt complexes in carbon tetrachloride aided significantly in discerning the various hydrogen bonding interactions by eliminating, to a great extent, the intermolecular interactions. This allowed for the identification of six unique NH species based on spectral evidence. Unexpectedly, dilution of the salt complexes also resulted in the collapse of an apparent network involving cation, anion and polymer interactions.