Fundamental Aspects and promising Attributes of Midblock‑Sulfonated Block Ionomers

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Abstract

Block copolymers continue to capture the attention of the academic and industrial worlds due largely to their fascinating ability to spontaneously self-assemble into a wide variety of "soft" nanostructures that are ideally suited for a broad range of diverse nanotechnologies. The development of thermoplastic elastomers (TPEs), such as triblock copolymers with glassy endblocks and a rubbery midblock, also endows these materials with elastic network-forming characteristics, and selective solvation of the rubbery midblock results in thermoplastic elastomer gels (TPEGs) with remarkable mechanical properties for dielectric elastomers, shape-memory systems, and flextronics. While most block copolymers are inherently nonpolar, targeted functionalization of block copolymers can permit these materials to be used in polar environments. Sulfonation of block copolymers, for example, yields materials that possess amphiphilic properties for new applications such as desalination membranes and fuel cells. Combination of TPEs with a sulfonated midblock produces a unique TPEG that is capable of forming a physical hydrogel. We have recently demonstrated that these materials are competitive candidates for electroactive media and photovoltaic devices, and these achievements will be presented. Unfortunately, the inherently high incompatibilities and glass transition temperatures of such block ionomers effectively prevent the use of thermal annealing, routinely employed to refine the morphologies of nonionic block copolymers. An alternative approach is therefore required to promote morphological equilibration in block ionomers. This presentation likewise explores the morphological characteristics of midblock-sulfonated pentablock ionomers (SBIs) differing in their degree of sulfonation (DOS) and cast from solvents differing in polarity, followed by solvent-vapor annealing (SVA). Transmission electron microscopy confirms that films deposited from different solvent systems form nonequilibrium morphologies due to solvent-templated self-assembly and drying. A series of SVA tests performed with solvents varying in polarity reveals that exposing cast films to the vapor of a polar solvent constitutes the most effective SVA protocol, yielding the anticipated equilibrium morphology. That is, three SBI grades subjected to SVA order into alternating lamellae wherein the increase in DOS is accompanied by an increase in lamellar periodicity, as measured by synchrotron small-angle X-ray scattering. I shall also demonstrate how discrete ion-rich microdomains undergo a phase transformation and become continuous in the presence of water.