Junctions and Strands: Breaking Property Tradeoffs in Polymer Networks and Composite Polymer Electrolytes

Abstract

This dissertation first examines the mechanics of polymer networks, specifically material toughness and the nature of material fracture. Polymer networks, which include tire rubber, windmill turbine blades, tissue engineering scaffolds, polymer electrolytes (vide infra) and many other materials, possess a useful lifetime typically limited by a fracture event. Thus, methods of controlling toughness (the resistance of a material to tearing) without compromising composition or other properties would dramatically affect waste generation and energy use in the myriad applications in which polymer networks are employed. Toughness in rubbery polymer networks derives from the length and density of the polymer strands; thus, it is generally inversely related to stiffness, as captured in the classic Lake-Thomas theory. This inverse relationship has been perturbed through incorporation of forceresponsive molecules (mechanophores) that may either toughen or weaken the material depending on network construction and topology. The first part of this thesis identifies a new class of mechanophores called tetrafunctional cyclobutanes (TCBs), which can be used to either toughen or weaken a network of single topology without substantial change of network composition, even in dilute gels which are difficult to toughen by other methods. TCBs are then used to identify the mechanisms of mechanophore toughening or weakening in other networks, through a proposed topological metric called network strand continuity (NSC). We show that TCB substituents control the regio- and chemo-selectivity of the cyclobutane core under stress, and this molecular-level selectivity is responsible for network toughening or weakening on the macroscale. These effects can be predicted based on knowledge of activation energetics of the junction guided by NSC. Subsequently, effects of other network structure parameters on the magnitude of toughening or weakening are considered and the molecular design of second-generation highly active TCBs is described. The second part of this dissertation concerns the design of microporous polymer electrolytes and the applications of their composites and gels in batteries. Polymer electrolytes are a highly anticipated alternative to the liquid electrolytes currently in use, which are toxic, flammable, and incompatible with next-generation battery chemistries. Previous polymer electrolytes exhibit inadequate conductivity and a severe tradeoff between conductivity and mechanical properties. These challenges are accentuated in single-ion conductors, which are theorized to have the strongest rate capability. A new class of single-ion conducting polymer electrolytes that mimics the conduction mechanism of ceramic electrolytes to achieve strong mechanical properties, high conductivity, processability, stability, and recyclability is described. These polymers constitute the first regular microporous polyanions, and the most dissociative microporous polyanions to date. These polymers, alongside other rigid (but not microporous) polysulfonimides enable strong conductivity performance when coupled with suitable dopant (here succinonitrile) in low weight fractions. Low molecular weight controls flexible polymer analogs show inferior mechanical and conductivity properties. In fact, microporous composites outperform even liquid analogs. These composites show best-in-class combinations of mechanical and conductivity properties, and can even conduct divalent cations like Zn(II), a challenging but energy-dense battery metal. Simulations show that polymer-succinonitrile interactions enable fast conduction at the pore edge which results in synergistic behavior.