Title: The synthesis, characterization, and electrochemical analysis of structured polymer electrolytes having strong ionic interactions
Description: Polymer electrolytes, ionic monomers catenated into a macromolecule, have received a considerable attention in the past decade as they combine the mechanical benefits of flexible chain with the ionic properties of liquid electrolytes. For this reason, they have been widely accepted as potential ion conducting membrane candidates for electrolyzers, energy storage devices, and desalination applications. In efforts to improve the efficiency of polymer electrolyte separators a block copolymer paradigm has been employed. This material’s strategy allows for a spatial separation of the components that control the mechanical and electrochemical properties and thus enables independent engineering of each. Charge – neutral block copolymers (CN-BCPs), a diblock copolymer containing a polyelectrolyte block, attempt to leverage this paradigm, however, to date, the impact ions have on the CN-BCP self-assembly is still an open question. In this dissertation, we are devoted to uncovering the fundamental impact of ionic interactions on CN-BCP self-assembly in bulk and thin-films.

First, we survey the literature and compile a list of thoroughly investigated CN-BCPs (derived from imidazolium, quaternary ammonium, and phosphonium motifs) with known dielectric properties. The ionic interaction strength for each CN-BCP was determine using the polyelectrolyte block’s static dielectric constant (ε_r) and the definition of the Bjerrum length (l_B=e^2⁄(4πε_0 ε_r k_b T)). The CN-BCPs studied in the literature, copolymers containing polyelectrolytes blocks with a high ε_r, display a morphology diagram akin to what is expected from the traditional block copolymer self-assembly. However, using a suite of experiment characterization techniques we show that CN-BCPs that contain a polyelectrolyte block with a low static dielectric constant (ε_r=2.5) produce an asymmetric morphology diagram. In this case, we find morphologies that would assemble the polyelectrolyte block into a discrete phase are suppressed if not completely absent. To explain this usual result, we invoke a simple free energy argument, geometric analysis, and a packing parameter model derived from small molecule surfactant principles to rationalize the asymmetry in the morphology diagram on a basis of long-range ionic correlations.

Utilizing the packing parameter model we were able to quantitively capture the morphology diagrams of all known CN-BCPs using a single interaction energy parameter. Next, we explore the impact of film thickness on CN-BCP self-assembly via solvent vapor annealing thin-films. A complimentary CN-BCP morphology diagram is constructed for a constant film thickness (h=40 nm) and compared to the bulk phase behavior. Similar to the bulk morphology mapping, we found an asymmetric thin-film morphology diagram with only a singular ordered morphology – cylinders. The influences of film confinement are discussed and utilized to reversibly switch the CN-BCP domain orientation on demand. Finally, we use in-situ grazing incidence small angle x-ray scattering to explore the CN-BCP thin-film structural evolution during the solvent vapor annealing process. We find quantitively different processing pathways when nonselective or selective solvents are used to vapor anneal CN-BCPs. A wide range of annealing solvents spanning a ε_r=4.8-32.7 range was chosen to vapor anneal CN-BCP thin films and a relationship between CN-BCP periodicity (d) and ε_r was determined.

We find d~1⁄√ε relationship suggesting the CN-BCP periodicity can be engineer by choice of annealing solvent. Additionally, in this dissertation, we explore the broader impact of morphology on the ion transport through polymer electrolyte membranes. In this vein, we propose structure – property relationships that will enable a rational design of single-ion conducting separators. Using in-house electrochemical techniques, nanoindentation, and gravimetric analysis we explore the relationship between ionic selectivity, quantified through the counterion transference number (t_c^m), the ionic conductivity (κ), and the separator hardness with respect to separator water volume fraction ( ϕ_(H_2 O)). We find t_c^m increases with increasing separator swollen-state charge density which is consistent with a donnan repulsion perspective. Additionally, as ϕ_(H_2 O) increases we find κ increases by orders of magnitude indicting that hydration plays an intimate role in improving ion diffusion in polymer electrolytes.

Finally, we present a summary of the permselectivity, Ψ^m=(〖(t〗_c^m-t_c^aqu))⁄((1-t_c^aqu)) , where t_c^aquis equivalent to the aqueous transference number, to κ for a series of polymer electrolytes characterized in literature. We find that separators derived from polymers with rigid backbones (e.g. poly(phenyl sulfone) and poly(2,6-dimethyl-1,4-phenylene oxide)) tend to have a small tradeoff between Ψ^m and κ yet cannot access ideal counterion selectivity (Ψ^m=1) when κ→0. As the backbone flexibility increases the Ψ^m- κ tradeoff is more dramatic yet a Ψ^m=1 when κ→0 is recovered suggesting the importance of molecular packing in this regime. We thus conclude that designing separators with an ideal Ψ^m- κ relationship will require a simultaneous manipulation of molecular architecture, separator hydration state, and morphology.