Synthesis, Morphology, and Ion Conduction of Polyphosphazene Ammonium Iodide Ionomers

Anion conducting polyphosphazene ionomer analogues of poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) were synthesized and their iodide transport properties studied. Polymer bound cations were quaternized with either short alkyl or short ether oxygen chains. X-ray scattering reveals a low q peak near 4 nm(-1) arising from the backbonebackbone spacing between polyphosphazene chains, an ion-related peak at 8 nm(-1), and a peak at 15 nm(-1) corresponding primarily to the amorphous halo of the PEO side chains. Because of the short spacing of the intermediate q peak, the ions are proposed to exist mostly in isolated ion pairs or small aggregates. First-principles calculations combined with dielectric spectroscopy suggest that less than 10% of the ions are in isolated pairs while the remainder participate in quadrupoles or other small aggregates. These ionomers display high values for the high frequency dielectric constant, epsilon(infinity) (highest value epsilon infinity = 11), due to atomic polarization of the iodide anion. These MEEP-based ionomers have room temperature dc conductivity of order 10(-6) S cm(-1) and show potential for application in iodide conducting solar cells if the segmental mobility could be increased.