(6ds) Solvent-Based Control over Nanostructure of Midblock Sulfonated Block Copolymers

Block copolymers containing ionic pendant groups, also referred to as block ionomers, have become increasingly popular due to their potential application as fuel cell and water desalination membranes, as well as components in photovoltaic devices and polymeric actuators. Block ionomers hold promise in these contemporary technologies because of their inherent ability to form separate ionic and nonpolar microdomains at nanoscale dimensions. Segregation of ionic and nonpolar segments enables simultaneous water, or ion, transport in conjunction with mechanical robustness. The ionic and nonpolar blocks are, however, highly incompatible, which can lead to long-term metastable morphologies. While thermal annealing is typically used to refine block copolymer self-assembly, most block ionomers possess inaccessible glass transition temperatures, thereby making thermal annealing ineffective. The development of morphological and property design paradigms in block ionomers represents a main challenge preventing widespread use. Here, we present a facile means by which to alter the morphological behavior of midblock-sulfonated pentablock ionomers through the use of solvent-based processing. A combination of transmission electron microscopy and microtomography (TEM/T) and synchrotron small-angle X-ray scattering (SAXS) are used to probe nanostructural features present in bulk films (~100 μm thick) produced from a variety of different preparation strategies. Results indicate that solution-cast morphologies are solvent-templated and that subsequent vapor annealing can either modify geometric parameters within one morphology or convert any morphology to the anticipated equilibrium nanostructure, depending on the selectivity of the solvent. To the best of our knowledge, these results provide the first evidence of morphological control/refinement in block ionomers of commercially relevant molecular weight.