Interactions and Morphology of Triblock Copolymer - Ionic Liquid Mixtures and Applications for Gel Polymer Electrolytes
Date of Award
Open Access Dissertation
Doctor of Philosophy (PhD)
Polymer Science and Engineering
Russell P. Thomas
James J. Watkins
Room temperature ionic liquids (ILs) are a unique class of solvents which are characterized by non-volatility, non-flammability, electrochemical stability and high ionic conductivity. These properties are highly desirable for ion-conducting electrolytes, and much work has focused on realizing their application in practical devices. In addition, hydrophilic and ionophilic polymers are generally miscible with ILs. The miscibility of ILs with ion-coordinating polymers makes ILs effective plasticizers for gel polymer electrolytes. Due to their unique properties, ILs present a means to realize the next generation of energy storage technology. In this dissertation, the fundamental interactions between poly(ethylene oxide) (PEO) and a variety of room temperature ILs were investigated. ILs with acidic protons were demonstrated to form a stronger interaction with PEO than ILs without such protons, suggesting that hydrogen bonding plays a dominant role for PEO miscibility with ILs. The hydrogen bonding interaction is selective for the PEO block of a PEO-b-PPO-b-PEO block copolymer (BCP). Therefore, blending these copolymers with the strongly interacting IL 1-butyl-3-methylimidazolium hexafluorophosphate ([BMI][PF6]) induced microphase separation into a well-ordered structure, whereas the neat copolymer is phase mixed. At sufficient quantities, the interaction between [BMI][PF6] and PEO suppresses PEO crystallinity entirely. In addition, the induced microphase separation may prove beneficial for ion conduction. Therefore, microphase separated copolymer/IL blends were investigated as potential gel polymer electrolytes. Cross-linkable block copolymers which microphase separate when blended with [BMI][PF6] were synthesized by modifying PPO-b-PEO-b-PPO copolymers with methacrylate end-groups. Cross-linking these copolymers while swollen with an IL generates ion gels with high ionic conductivities. The copolymer/IL blends vary from a well-ordered, strongly microphase separated state to a poorly ordered and weakly microphase separated state, depending upon the molecular weight. Stronger microphase separation results in higher mechanical strength upon cross-linking. However, this does not greatly affect ion conductivity. Nor is conductivity affected by forming gels from cross-linked PEO homopolymers when compared to BCPs. It was found that BCPs can be beneficial in producing gel electrolytes by allowing sequestration of phase selective cross-linkers away from the conducting block. Cross-linker molecules that are selective for the PPO blocks can be used to increase the mechanical strength of the gels with only a small effect on the conductivity. When cross-linkers that partition to the mixed PEO/IL block are used, the conductivity decreases by nearly a factor of 2. These studies show how ILs interact with PEO and how gel polymer electrolytes can be constructed with the IL [BMI][PF6]. While BCPs cannot directly be used to increase ion conductivity, they do allow for greater mechanical strength without sacrificing conductivity. This suggests many new approaches that may be used to simultaneously achieve high ionic conductivity and mechanical strength in solid and gel polymer electrolytes.
Miranda, Daniel F., "Interactions and Morphology of Triblock Copolymer - Ionic Liquid Mixtures and Applications for Gel Polymer Electrolytes" (2012). Open Access Dissertations. 617.