Local structure determination in the amorphous polymer electrolyte: Poly(propylene oxide)-zinc bromide

• Main Author: Fishburn, James R
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1996
• Subjects: Polymers
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9619386

Polymer electrolytes are neutral polymers that are complexed with salts. They may show promise for use in energy storage devices. However, the conductivity in polymer electrolytes is several orders of magnitude low for use in this application. A major hindrance to improving the conductivity is a lack of understanding of the conduction mechanism. The local structure in the amorphous polymer electrolyte, poly(propylene oxide)-zinc bromide ( (PPO) $\sb{\rm x}$ZnBr$\sb2$), was investigated to obtain a microscopic snapshot of the ionic structure in the conducting amorphous phase. This would provide insight into the conduction mechanism through elucidation of polymer-ion and ion-ion interactions. A combination of differential anomalous x-ray scattering (DAS) and extended x-ray absorption fine structure (EXAFS) were used to determine the structure around the Zn and Br ions in (PPO) $\sb{\rm x}$ZnSr$\sb2$, where x = 6,16. On average, two bromines and two oxygens were coordinated with each Zn. This represented a mixture of species described by the formula ZnBr$\sb{\rm 4-Y}$(O)$\sb{\rm Y}\sp{\rm Y-2}$, of which the majority were the tetrahedral species Zn(O)$\sb4\sp{2+}$ and ZnBr$\sb4\sp{2-}.$ The nature of the ionic groups implied that the conductivity in (PPO) $\sb{\rm x}$ZnBr$\sb2$ would not be high enough to be useful for many applications. This work was also important for the novel application of DAS for investigating polymer electrolytes. The accuracy of DAS in calculating coordination numbers was increased. Two approximations were tested for a weighting factor used when calculating coordination numbers from the peak areas in the differential Radial Distribution Functions (dRDF). A k-peak approximation was more accurate than a k-averaged approximation. Finally, Quasi-Elastic Neutron Scattering (QENS) was used to study the local motions of the polymer chains and to correlate them to the structure determined by DAS and EXAFS. Only preliminary results were obtained and a more complete analysis is needed before any conclusions about the chain motions can be made.