Viscoelastic and Glass-Forming Behavior of Imidazolium-Based Poly(Ionic Liquids) with Bulky Pendant Groups

Abstract When ionic liquid (IL) monomers are chemically bonded into a polymer, either as pendant groups or within the main chains, the resulting poly(ionic liquids) (PILs) retain the key properties of ILs while offering the mechanical integrity and processability of polymers. Their dynamics and rheological behaviors often differ from neutral polymers due to factors such as ionic interactions, charge density, and morphology. Here, a new class of PILs composed of imidazole-bis(trifluoromethyl-sulfonyl)imide cation–anion pairs with especially bulky side chains containing aromatic, sulfonamide, and aliphatic building blocks were synthesized to investigate the effects of bulky side groups on viscoelastic and glass-forming behaviors. Being unentangled with a degree of polymerization in the 40–50 range and low dispersity, they exhibit unusually broad transitions from segmental to terminal relaxation, spanning 7 decades of frequency, behaving as viscoelastic fluids. Structural variations, including methyl, phenyl, and tolyl side-chain endcaps, exhibited a tunable array of glass transition, fragility, and relaxation time. Extended backbone conformation stabilized by the ionic nature of PILs, combined with dipolar/quadrupole inter- and intrachain interactions, and steric effects of bulky side groups dominate the viscoelastic spectra, offering an opportunity to create distinctive viscoelastic properties. As intermediate glass formers, their low fragility relative to most linear polymers can be traced to low B-fitting parameter values in the Vogel–Fulcher–Tammann relation.