Ionic Purity and Connectivity of Proton-Conducting Channels in Fluorous-Ionic Diblock Copolymers

Abstract

Diblock copolymers of sulfonated poly([vinylidene difluoride-co-hexafluoropropylene]-b-styrene) [P(VDF-co-HFP)-b-SPS] were prepared for the purpose of studying the role of ionic purity and connectivity on proton exchange membranes. Block ratios were controlled to provide membranes with different morphologies. Within each membrane series, the ion content was controlled either by varying the length of the fully sulfonated polystyrene (PS) block, or by varying the degree of sulfonation of a fixed PS block. For a given ion exchange capacity (IEC), water uptake and proton conductivity were shown to be significantly influenced by the degree of sulfonation of the PS block and, thus, the ionic purity of the “ion-rich” channels. Fully sulfonated membranes with 6–17 vol % PS possessed disordered ionic clusters (6–20 nm in diameter). Although these membranes show relatively high water sorption at low IEC ranges, their water sorption and proton conductivity are less sensitive to changes in IEC. This reduced their tendency for acid dilution at high ion contents, allowing for a continuous increase in proton conductivity over a greater range of IEC. Partially sulfonated membranes with 35, 45, and 50 vol % PS displayed a lamellar morphology. Water content was found to be lower in the partially sulfonated membranes due to the influence of the nonsulfonated PS host matrix surrounding the ionic aggregates of polystyrenesulfonic acid (PSSA). For a given IEC, partially sulfonated membranes with lower PS contents required relatively high degree of sulfonation, resulting in enhanced connectivity of ionic PSSA aggregates, which, in turn, led to greater water uptake and proton conductivity. This work concludes with the finding that the design of polymers that form well-defined “proton-conducting channels”, as determined by electron microscopy, do not necessarily translate to high proton conductivity membranes, as it is the connectivity of the protogenic aggregates within these channels that has the strongest influence on the membrane transport properties.