Structure-property relationships of PEMs using fluorous-ionic copolymers

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Proton exchange membranes
Fuel cells
Structure-property relationships
Phase separation
Proton conductivity

Proton exchange membranes (PEMs) are a key component in PEM fuel cells, serving as both a fuel separator and an electrolyte. The goal of this thesis work is to investigate structure-property relationships in PEMs. Specifically, the role of polymer microstructure on membrane morphology and physicochemical properties is examined. This is achieved by the design, synthesis and characterization of model polymers with controlled chain architectures and chemical composition, leading to membranes with controlled nanophase-separated morphologies, from which the influence of morphology upon proton transport and other membrane properties is investigated. Two classes of model polymer systems were devised and studied: diblock copolymers of sulfonated poly([vinylidene difluoride-co-hexafluoropropylene]-b-styrene) [P(VDF-co-HFP)-b-SPS]; and graft copolymers of sulfonated poly([vinylidene difluoride-co-chlorotrifluoropropylene]-g-styrene) [P(VDF-co-CTFE)-g-SPS]. These model polymer systems are of interest due to chemical dissimilarity between the hydrophobic fluoropolymer segments and the hydrophilic sulfonated polystyrene segments, which promote phase separation into ionic and non-ionic domains. In addition, controlled radical polymerization techniques were employed to grow the polystyrene segments, which provide high degrees of structural control. Macromolecular structural parameters, such as block ratio, graft length and degree of sulfonation, were systematically varied to determine the effects of polymer microstructure on morphology and proton conductivity. One of the key findings obtained from this work is that block ionomers, whether linear or graft structure, with a lower content of the acid-bearing constituent block (i.e., polystyrene block) gave enhanced proton conductivity at a given ion content. This is attributed to the relatively high degree of sulfonation required and therefore, closer spatial proximity between sulfonic acid groups, which allows for the formation of purer and more percolated ionic aggregates within the proton-conducting domains. Additionally, direct comparison between the diblock and the graft copolymers revealed that the formation of smaller-scale ionic domains is preferable for PEMs because of reduced water swelling which mitigates acid dilution at high ion contents. Furthermore, membranes with smaller-scale ionic domains provided enhanced water retention and proton conduction under low humidity and high temperature conditions. The knowledge gained from this thesis work provides useful insights into aspects of membrane design and preferred structures.

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Steven Holdcroft
Science: Department of Chemistry
Thesis type: 
(Thesis) Ph.D.