Solution Self-Assembly of Block Copolymers: Towards Solid-State Electrolyte Applications

Date of Completion

January 2011

Keywords

Chemistry, Physical|Chemistry, Polymer

Degree

Ph.D.

Abstract

Solution self-assembling amphiphilic block copolymers have been widely used in a variety of application areas such as adhesives, rheology modifiers, drug delivery and responsive materials. Understanding the self-assembly of block copolymer solutions which forms micelles in dilute solutions to gels in the semi-dilute regime, is of great importance from the perspective of processing and their applications. The drive of this research is to understand solution self-assembly of block copolymers and further develop novel materials to realize solid state electrolyte applications. ^ In the first study, we have investigated the formation and properties of micelles and gels derived from self-assembly of glassy diblock copolymers in selective solvent, 1-Octanol. The fluid-to-solid transition is through the gel-point where critical gels are formed and critical gel concentration is determined as a function of temperature to get insight in to the mechanism of gelation. Dynamic light scattering and yielding experiments confirmed the formation of attraction gels in these systems. Results from DLS and rheology helped to correlate the structural behavior in dilute solution to macroscopic gelation. The dynamical arrest observed at higher concentrations due to jamming of micelles is a good example of arrested states of soft matter.^ With this scientific understanding from alcohol gels system, we focused on developing gelled electrolytes (or ion-gels) for electrochemical device applications including Li-ion batteries. 1-butyl-3-methylimidazolium tetrafluoroborate, ([BMIM][BF4]) ionic liquid based gels were prepared via self-assembly of triblock copolymer. It is an attractive method which requires small amount of copolymer to form ion-gels of sufficient mechanical strength and higher ionic conductivities that is moderately affected by the structuring soft material. By this methodology, we not only overcome the short-coming of mechanical integrity, handling and leakage problems of ionic liquid but also exploit its unique properties of low vapor pressure, non-flammability, high ionic conductivity and thermal stability in energy applications. At moderately low concentrations (∼ 5 wt%), we demonstrate the formation of strong and thermoreversible ion-gel with high gelation temperature and ionic conductivities similar to bulk [BMIM][BF4]. Tunable gelation temperatures are achieved by simply varying the solvophobic block length which affects their liquid-state processing and device fabrications. These novel ion-gels are promising candidates for a broad spectrum of solid-state electrolyte applications including Li-ion batteries.^

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