Increasing numbers of technologies are based on the idea of harnessing charge transport for useful processes such as energy harvesting, actuation, and sensing. Although much progress exists based on perfluorosulfonated platforms, new and more complicated parameters arise as applications require multifunctionality in engineering materials. For instance, in addition to using lightweight, durable, stable, high-conductivity proton-exchange membranes in a fuel cell, it may be desirable to have the energy source bear load and operate as a structural installment. The most prevalent class of materials potentially capable of such a tradeoff is rubbery “salt-in-polymer” electrolytes that employ polymer segmental motion for transport of free ions from co-dissolved salts.

Another promising material class is conductive ionic glasses that demonstrate a high dielectric constant such that ion hopping through lattice defects provides sufficient conductivity in the glassy state. The multifunctional requirement starkly reflects the difficulty of using materials that display low ambient temperature modulus (especially when hydrated), including PEO and Nafion, or consideration of the significant decrease in ionic conductivity with the removal of the hydrating diluent/electrolyte. An overall approach to achieve both goals is to decouple ion transport from the segmental motion of the polymer through macromolecules designed to incorporate polar, bulky, irregular, rigid monomers into the backbone.

Very few investigations exist concerning the effect of branching on decoupled ion transport in glassy, low-humidity, single-ion conductors. The chosen matrix of materials includes linear and long-chain-branched polysulfone phosphine oxides (LCB-PSF-PO). The use of polar B2 and B3 monomers will aid in the complexation of mobile ions from the imbibed salt within the matrix for charge stabilization and a decrease in the activation energy for ion hopping. The investigation of decoupled ion transport in such materials is also interesting for reasons of its implications in the electromechanical transducer field.

Increases in ion transport without diluents or charges bound to the backbone are likely to augment ionic polymer transducer (IPT) performance through additive increases in ionic conductivity when introduced into ionomer composition and construction of the IPT composite. IPTs also do not require the same modulus levels as multifunctional batteries and fuel cells, thus providing a potential application for high-conductivity composites that fall short of the structural target.

Previous systems demonstrated with PEO that three to four etheric oxygens serve to complex a single monovalent cation. Inclusion of branching into the system anticipates the difficult balance between establishing rigidity in the material while increasing intermolecular and intramolecular-free volume for the purpose of complexation. To further extend the free volume effect, all chosen monomers are bulky in addition to their rigid nature, placing at least four aromatic rings in the backbone of each repeat unit. The monomers were recrystallized for purity before examining their characteristic spectra in comparison to the final polymers with Fourier transform infrared spectroscopy (FTIR). The final polymers are characterized with nuclear magnetic resonance (NMR) spectroscopy, size exclusion chromatography (SEC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). Films cast in the presence of Li-Tf and lithium bis(trifluoromethane)sulfonimide (Li-Bim) were also characterized for their ionic conductivity with electrical impedance spectroscopy (EIS).

Development of molecular weight in the linear system is more easily achieved, almost by two times the molecular weight of the branched polysulfones. To qualify as long-chain branches, each branch must have an individual molecular weight greater than that of the entanglement molecular weight for the linear polymer. Although the degree of branching is not yet confirmed from NMR, the multi-modal peaks in SEC are typical of branched polymer elution. Due to the low level of total branching associated with long-chain branches, the polydispersity index is approximately equal to the linear sample and does not show the high values normally observed for hyperbranched systems.

This work was done by James F. Snyder of the Air Force Research Laboratory’s Weapons and Materials Research Directorate; and Andrew J. Duncan, Donald J. Leo, and Timothy E. Long of Virginia Polytechnic Institute and State University. For more information, download the Technical Support Package (free white paper) at  under the Materials category. ARL-0114

This Brief includes a Technical Support Package (TSP).
Synthesis of Long-Chain-Branched (LCB) Polysulfones for Multifunctional Transport Membranes

(reference ARL-0114) is currently available for download from the TSP library.

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