Abstract
When functionalized by the solid-state sulfonation process, the amorphous regions of the semi-crystalline syndiotactic-polystyrene (sPS) become hydrophilic, and thus can conduct protons upon membrane hydration, which increases the interest in this material as a potential candidate for applications with proton exchange membranes. The resistance of sulfonated sPS to oxidative decomposition can be improved by doping the membrane with fullerenes. In previous work, we have described the morphology in hydrated sulfonated sPS films doped with fullerenes on different length scales as determined by small-angle neutron scattering (SANS) and the structural changes in such membranes as a function of the degree of hydration and temperature. In the current work, we report on the relationship between the morphology of hydrated domains as obtained by SANS and the proton conductivity in sulfonated sPS-fullerene composite membranes at different temperature and relative humidity (RH) conditions. Based on this combined experimental approach, clear evidence for the formation and evolution of the hydrated domains in functionalized sPS membranes has been provided and a better understanding of the hydration and conductivity pathways in this material has been obtained.
Highlights
Polymer electrolyte materials (PEM) for fuel cells applications (PEMFC) are characterized by a nanoscale phase separation into hydrophilic domains and hydrophobic regions, which is a combination that enables a high proton conductivity and provides a good chemical and mechanical stability, membrane durability
The preparation of an sulfonated syndiotactic polystyrene (s-sPS) membrane should start from the δ-form, which enables a homogeneous sulfonation of only the amorphous regions and can be subsequently transformed into the thermodynamically stable β-form by high-temperature annealing procedures [17]
The sPS-based membranes were characterized via UV-Vis, thermo-gravimetric analysis (TGA), prompt-gamma neutron activation analysis (PGAA), optical microscopy, and WAXD prior to their investigation by small-angle neutron scattering (SANS) and the conductivity measurements
Summary
Polymer electrolyte materials (PEM) for fuel cells applications (PEMFC) are characterized by a nanoscale phase separation into hydrophilic domains and hydrophobic regions, which is a combination that enables a high proton conductivity and provides a good chemical and mechanical stability, membrane durability. Several crystalline phases with polymer chains arranged in either the trans-planar or helical conformation were identified and characterized [9,10,11], with the δ- and ε-phases, which are formed by crystallization from solution, representing co-crystals of s-PS with low molecular mass guest molecules (clathrates) This property offers the possibility to load different guests molecules in the cavities formed between the helices of the sPS in the crystalline regions by using the guest-exchange process [12,13], which makes the sPS suitable for different possible applications, such as fluorescent materials (with chromophore guest molecules), optical memories (with photo-reactive guest molecules), non-linear optical materials (with polar guests), and chiro-optical memories (with chiral guest molecules) [14]. The preparation of an s-sPS membrane should start from the δ-form (clathrate with guest molecules), which enables a homogeneous sulfonation of only the amorphous regions and can be subsequently transformed into the thermodynamically stable β-form by high-temperature annealing procedures [17]
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