Effect of polymer structure on ion transport in an anhydrous proton conducting electrolyte.

摘要

I have studied the role that polymer structure plays on the ion-transport properties in a H+ conducting polymer electrolyte. In this dissertation, I have explored H+ transport in an anhydrous H+ conducting poly(ethylene glycol) polymer.;Initially, I investigated the mechanism of anhydrous H+ conductivity in our sol-gel based MePEGn polymer. In this study, our proton conducting electrolytes are composed of mixtures of MePEG nSO3H acid and sol-gel based MePEGn polymers. These solutions display anhydrous proton conductivity reaching a maximum value of 1.38 x 10-5 S/cm at 55°C with a 1.32 M mixture of MePEG16SO3H dissolved in the MePEG12 polymer. In addition, I have correlated the molar equivalent conductivity of H + with the volume fraction of PEG in the mixture. This result indicates that conductivity in these solutions of acid and polymer is a function of the PEG content, strongly suggesting the dominance of a Grotthus mechanism of conductivity in this system. Moreover, I have shown a lack of dependence of ionic and equivalent conductivity on the size of the MePEG nSO3H acid, indicating little or no contribution from the vehicle mechanism to the overall ionic conductivity.;Secondly, I have characterized the structure of one of the sol-gel based MePEGn polymers, using 29 Si NMR spectroscopy and gel-permeation chromatography. These studies indicate that the MePEG3 polymer is composed of several different structures giving a distribution of molecular weights and silicon resonances. In this study, I showed that the sol-gel prepared MePEG3 polymer is primarily composed of incompletely condensed T6 silsesquioxane (POSS-type) clusters. These incompletely condensed T8 clusters are seen in the 29Si NMR spectra at chemical shifts between --62 and --70 ppm. In addition, the minority composition of the MePEG3 polymer contains a small amount of completely condensed T6 silsesquioxane clusters, observed at --55.5 ppm, and T 2 dimers from --48 to --50 ppm. End-group analysis showed the presence of 0.67 uncondensed Si-OH groups per silicon atom in the MePEG 3 polymer, supporting the presence of incompletely condensed or ladder-type structures. Moreover, the completely condensed POSS cluster (MePEG3) 8T8 was synthesized for study as a model compound, showing 29Si NMR peaks between --65.5 and --69 ppm. Furthermore, "Q"-type silicon species are observed in the 29Si NMR spectra of the MePEG3 monomer and MePEG3 polymer at approximately --80 to --90 and --101 ppm. I determined that these "Q"-type structures are generated from a coupling of triethoxysilane in the hydrosilation step of the monomer synthesis.;Finally, I have studied the effects of changing the structure of the MePEGn polymers. Here, I have specifically adjusted the viscosity and free volume in the polymer, and studied the effects of these changes on the H+ conductivity and fluidity. These studies indicate that both free volume and viscosity are involved in conductivity in our polymer-acid mixture. In addition, I have shown that high viscosity MePEGn polymers show almost same ionic conductivity as less viscous MePEGn polymer systems. Here, the effect of this change in viscosity is offset by a simultaneous change in the free volume of the MePEGn polymer.