Comparing ammonium and phosphonium polymerized ionic liquids: Thermal analysis, conductivity, and morphology

Conventional free radical polymerization and anion metathesis of ammonium and phosphonium styrenics successfully generates high-molecular-weight polymerized ionic liquids (PILs). Phosphonium polyelectrolytes containing Cl^- counterions display significantly higher thermal stabilities (>370 °C) compared with ammonium analogs (<220 °C). Anion exchange to BF₄ ^-, TfO^-, and Tf₂N^- improves the thermal stability of all the PILs and depresses their T _g. Impedance-spectroscopy-probed ionic conductivities of PILs containing Tf₂N^-, and phosphonium PILs exhibit higher values than ammonium analogs. Phosphonium PILs displayed many advantages over ammonium PILs for emerging applications that demand higher thermal stabilities and ionic conductivities. A large library of ammonium- and phosphonium- containing polymerized ionic liquids (PILs) enables a thorough structure-property analysis. The alkyl substituent length and counterion directly impact the thermal properties, wherein longer alkyl substituent lengths and bulkier counterions depress the glass transition temperature. Ultimately, phosphonium PILs exhibit superior thermal properties and ionic conductivities compared with ammonium analogs.