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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2018

Month Degree Awarded

September

First Advisor

E. Bryan Coughlin

Subject Categories

Physical Chemistry | Polymer Chemistry

Abstract

Structure-property correlations in charged polymers is an interesting facet of polymer science. Understanding the effects of intermolecular forces on the morphologies of polymers can lead to the design of membranes with desired structures to improve properties, for example ion conductivity. In random, comb-shaped polycations, competing intermolecular forces result in two different short-range orderings. Side-chain steric repulsion results in backbone-backbone morphology characterized by periodic spacing between polymer backbones. However, dipole - dipole attraction in these polycations can facilitate the formation of ionomer cluster morphology characterized by a spacing between clustered dipoles. Although both of these short-range orderings have disparate origins, their similar dimensions when characterized by X-ray scattering can lead to a misattribution of one morphology for the other. To investigate this interplay between side-chain sterics and dipole-dipole attraction in polycations, random copolymers, and terpolymer of poly(4-vinylpyridine) (P4VP), polyisoprene (PI), and polystyrene (PS) were synthesized and fully quaternized with 1-alkylhalides. X-ray scattering show that in samples having 2 carbons on its pendant side-chain dipole-dipole attraction facilitates the formation of ionomer cluster morphology. Whereas samples with 4, or more carbons, on their pendant side-chains were dominated by side-chain sterics resulting in backbone-backbone morphology. Copolymers with polyisoprene, having flexible backbones, favored the formation of ionomer cluster morphology. An “In-Line” Dipole Model was developed to predict the separation between polymer backbones at which both ionomer cluster and backbone-backbone morphologies could coexist. The pendant polyisoprene units in the random copolymers of the fully quaternized P4VP and PI were crosslinked into robust anion exchange membranes (AEMs). Ionic conductivities for AEMs with coexistent morphologies were exceptionally high. To utilize these highly conducting AEM morphologies for fuel cell applications, stable quaternary ammonium monomers were designed, synthesized, and characterized. The monomers, norbonenepropoxy-6-azonia-spiro(5,5)undecane, and norbonenehexoxy-6-azonia-spiro(5,5)undecane, were readily polymerized into solvent processable AEMs. Random and block copolymerization of the stable quaternary ammonium monomers with norbornenemethylbenzylether were performed. The resultant copolymers were solvent processed into flexible anion conducting membranes. In the random copolymers, the competition between electrostatics and sterics facilitated the formation of coexistent morphologies resulting in high ionic conductivities in these membranes. In the block copolymers, electrostatics facilitated the formation of a continuous ionic phase even at low ionic volume fractions. This percolated phase in the block copolymers resulted in excellent bromide conductivity.

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