Georgia Institute of Technology Materials Science and Engineering

Committee Members: 

Prof. Vladimir V. Tsukruk, Advisor, MSE

Prof. Alexei P. Sokolov, ORNL/University of Tennessee, CHEM/PHYS

Prof. Blair K. Brettman, ChBE/MSE

Prof. Zhiqun Lin, MSE

Prof. Alberto Fernandez-Nieves, PHYS

Prof. Paul S. Russo, MSE

"Branched Polymer Electrolytes: Responsive Nanomaterials for Controlled Ion Mobility" Abstract: 

Polymers containing ionic groups such as polyelectrolytes and polymerized ionic liquids are promising candidates for the design of organized ionically conductive media due to their controlled morphology, robust chemical and thermal stability, and single-ion conductivity. However, while polymerization of ionic groups affords electrolytes a greater degree of dimensional control, the effect of nonlinear chain architecture remains mostly an unexplored consideration, despite the unique functional group densities, chain conformations, counterion condensation, and dynamics of branched polymers.

First, the stimuli-responsive interfacial assembly and tunable morphologies of star-shaped polyelectrolyte block-copolymers and polymerized ionic liquids in monolayers and multicomponent systems are examined. In the former case, a dual-responsive star-graft block-quarterpolymer with variable arm number, arm length, and grafting density are integrated into hydrogen-bonded multilayer films and their morphologies were evaluated in different environments using surface probe microscopy and neutron reflectivity. The results point toward the amphiphilicity endowed by the star-graft architecture as the chief factor controlling the temperature and pH-induced conformational changes which lead to the diverse star-like clustering at the molecular scale. Likewise, the surface organization of linear and star-shaped polymerized ionic liquids in monolayers and multilayers is compared under variable adsorption conditions for polymers with the different branching architectures. Both studies demonstrate how polyelectrolytes and polymerized ionic liquids with branched architecture assemble into multilayer films with variable porosity, thickness, and textured morphologies featuring compartmentalized internal morphologies that are remarkably distinct from traditional multilayer systems.

The second part of this work focuses on the ion transport in polyelectrolytes comprised of star and hyperbranched polymerized ionic liquids. Long-chain arms were found to exhibit more sluggish and elastic dynamics at longer timescales while the glass transition temperature, rates of segmental relaxation, ion disassociation, and dc conductivity were similar regardless of the polymer architecture and arm length. But when polymerized ionic liquids are branched on a smaller scale, such as in the ionic liquid tethered macromolecules consisting of both POSS and hyperbranched polyester cores, considerable shifts in the glass transition temperatures and conductivities were observed.

This ability to control the ion mobility in polymerized ionic liquids near the Tg is critical for the development of solid-state electrolytes in which it is desirable to have high conductivities in the near glassy state. Overall, this dissertation provides an initial view of branched polymer electrolytes as uniquely versatile nanomaterials in the assembly of multifunctional polymer electrolytes with tunable morphologies and controlled ion transport properties.