Proton-Conducting Sulfonated Ionomers by Chemical Modification and Atom Transfer Radical Polymerization

The cornerstone in this dissertation is made up by three individual assessments of the diversity in the macromolecular landscape that can be obtained by applying relatively few efficient chemical tools. The intention is to gain deeper knowledge on the chemical tuning of proton exchange membranes (PEM) and thereby optimizing their properties.

Equally important has been the evolution of model systems that are part of a bigger research perspective as well as the application of unconventional strategies within the field. In the design of amphiphilic polymers, a commercially available polysulfone (PSU), Udel, is chosen as backbone due to its mechanical and thermal properties.

Sulfonic acid functionalized, dendronised side chains are attached by click chemistry in the study of hydrocarbon structures with highly flexible spacers. Various degrees of sulfonation (DS) are used in the perspectivation of the influence of ion exchange capacity (IEC) on water sorption and proton conductivity.

There appears to be a narrow IEC-window where the water percolation increases tremendously from being very low to where severe swelling occurs, and the proton conductivity proportionally with it. In another model study of hydrocarbon macromolecular architectures, PSU with postsulfonated polystyrene (PS) grafts are investigated.

Here, IEC is controlled through the degree of substitution, the graft length and DS. The grafting is performed with atom transfer radical polymerization (ATRP). The third assessment is dedicated to a partially fluorinated system that is based on a poly(vinylidene fluoride) (PVDF)-containing backbone with fully sulfonated PS grafts.

To counteract the dimensional change upon water contact that is a result of the increased IEC, the ionomer is blended with a high molecular weight PVDF, which contributes to the conservation of mechanical stability. The morphology of these blends is affected by the PVDF content. At 25 vol% ionomer macro-phase-separation occurs, while a 40 vol% ionomer content on top of the macro-phase-separation develops a repetitive patten of ion-rich domains in primarily PVDF-containing areas.

The blends are highly humidity sensitive, yet, despite lower absolute conductivities than Nafion, they display a reduced dependence on both humidity and temperature. Under fully humidified conditions the blends perform superior to fully sulfonated graft copolymer analogues. The combination of a high degree of control by ATRP and click chemistry enables a wide selection of polymer structures with the handles: degree of substitution (DS), polymerization and sulfonation, and blending.