Block copolymer-based ion gels as solid polymer electrolytes

Abstract

Block copolymer-based (BCP) ion gels are a class of interesting solid polymer electrolytes (SPEs) in electrochemical applications. This thesis aims to systematically study the mechanical and electrical properties of BCP-based ion gels formed by the selfassembly of ABA triblock copolymers in an ionic liquid, and find ways to enhance the properties of the gels, in order to formulate optimal designs in terms of the triblock for applications to electrochemical devices. Two particular target applications are organic transistors and electrochemical capacitors, due to the very large specific capacitance (on the order of F/cm2) of these electrolytes and therefore low voltage operation and potentially desirable energy storage. To study the effect of the BCP on the properties of ion gels, BCPs with different midblocks and end-block lengths were prepared, and the viscoelastic and electrical properties of the ion gels were investigated over large composition and temperature ranges. The gels were formed by the self-assembly of poly(styrene-b-methyl methacrylate-b-styrene) (SMS) and poly(styrene-b-ethylene oxide-b-styrene) (SOS) in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]). The end-blocks associate into cross-links, while the midblocks are well-solvated by this ionic liquid. In terms of viscoelastic properties, it was found that the plateau modulus of the gels depends primarily on concentration and the molecular weight of the mid-block, while high temperature behavior is controlled by the length of the end-blocks. A body-centered cubic (BCC) structure was observed at elevated temperatures only for gels with short end-blocks due to end-block pull-out from the cross-linking cores, while the relaxation of the end-blocks are within the cores for gels with long end-blocks. In terms of electrical properties, the double-layer capacitance of the gels was found to be fairly insensitive to polymer content and identity, whereas the ionic conductivity varies significantly especially at polymer concentrations of more than 20 wt%. It was also found that the presence of the end-blocks primarily obstructs the ion paths without much effect on ion number density. In terms of materials design, a flexible, low molecular weight mid-block is desirable. Generally, there is a trade-off between ionic conductivity and shear modulus for this type of gels. To enhance the mechanical properties of the gels, a novel ion gel based on poly[(styrene-r-vinylbenzyl azide)-b-ethylene oxide-b-(styrene-r-vinylbenzyl azide)] (SOS-N3) with chemically cross-linkable end-blocks was prepared. The gel with 10 wt% polymer goes through two transitions as temperature increases: solid (physically crosslinked network) --> liquid --> solid (chemically cross-linked network). The modulus and ionic conductivity was found to remain fairly constant after chemical cross-linking, while the toughness is more than 8 times higher. This demonstrates a promising approach to improve the mechanical properties of a moderately dilute gel without interfering with the high ionic conductivity. Overall, BCP-based ion gels are promising SPEs for transistor and capacitor applications. Through judicious selection of the triblocks, the properties of the gels can be tuned to fulfill different requirements.