Electronic Theses and Dissertations

Date of Award

2011

Document Type

Dissertation

Degree Name

Ph.D. in Chemistry

First Advisor

Jason E. Ritchie

Second Advisor

Richard Raspet

Third Advisor

Amala Dass

Abstract

This dissertation explores the relationship between ionic conductivity, viscosity, and thermal properties of several MePEG and MePPG based copolymer electrolytes. In particular two main copolymer modifications have been investigated namely copolymerization with "bulky groups" and cross polymerization of MePEG and MePPG monomers. The modifications were made to vary the fractional free volume of the MePEG polymers. All of the copolymers obeyed the Doolittle equation. For both the bulky copolymers and MePEG/MePPG copolymers, increases in the FFV corresponded to increases in the viscosity and decreases in FFV correspond to decreases in viscosity. The FFV and H+ conductivity for these copolymers were not correlated per the Forsythe equation. When the Vf,ether was substituted for FFV in the Forsythe equation, a strong correlation was observed indicating that the mechanism of proton conduction is dependent on the amount of ether units in the material. The viscosity and ionic conductivity for each individual copolymer electrolyte were correlated following the Walden rule. The ? values for all of the copolymer electrolytes were between 0.3 and 0.6 indicating that forces besides viscosity impede the ionic conductivity. These forces can include polymer rigidity, dissociation constant and the blocking of H+ channels. Most of the copolymer electrolytes had similar ? values which indicates that the same forces are affecting all of the copolymer electrolytes. The Cp showed a correlation to the activation energy for viscosity only for the MePPG3/MePPG2 copolymers. There was no correlation observed for all of the other copolymer electrolyte series. The correlation for the MePPG3/MePPG2 copolymers indicates that the molecular rearrangement in the material is dependent on the intermolecular forces present. There was no correlation observed between Cp and the activation energy for H+ conductivity for neither the high or low acid concentration MePEG/MePPG copolymers nor the low acid concentration bulky copolymers. The high acid concentration bulky copolymer electrolytes showed correlations for the copolymer series. The correlation increased as the polarity of the bulky groups increased with the exception the MePEG/Ph 2Si copolymers had the highest correlation. The correlations seen indicate that the bulky groups actually impede the H+ conductivity by altering the intermolecular forces in the materials. Lastly, the acid dissociation constant for the MePEG 7SO3H acid was measured. The acid dissociation was implicated as a contributing force that impedes ionic conductivity by the Walden plots. It was found that the pKa of the MePEG7SO3H decreased as the fraction of MePEG7OH increased in the binary solvent system. This corresponded to the acid weakening due to the fact that the pK a was actually further away from the pKa of the protonated solvent than in aqueous media. The mean activity coefficient also increased as the fraction of MePEG7SO3H increased. The results of the acid dissociation experiments further support the Grotthus mechanism as the prominent mechanism for H+ conductivity for PEG based polymer electrolytes.

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Chemistry Commons

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