Role of Chain Polarity on Ion and Polymer Dynamics: Molecular Volume-Based Analysis of the Dielectric Constant for Polymerized Norbornene-Based Ionic Liquids

Abstract

Norbornene-based polymerized ionic liquids (PILs) were systematically prepared to understand the role of PIL architecture and linker on the ionic conductivity (σDC), segmental dynamics, and dielectric constant. In PILs with two- or one-armed imidazolium (Im⁺)-bis(trifluoromethanesulfonyl)imide (Tf₂N–) pendants, their dynamics and polarity were tuned by incorporating either oxyethylene [(OCH₂CH₂)ₓ = (OE)ₓ, x = 1 or 3] or alkylene [(CH₂)₂] moieties as linkers between the norbornene and imidazolium cation moieties and ethyleneoxy [(CH₂CH₂O)y = (EO)y, y = 2 or 3] terminal units on the imidazolium. All PILs exhibit three dipolar relaxations; the local chain β motions of the PIL pendants were observed below the glass transition temperature (Tg), while above Tg, the segmental α and slower ionic rearranging α₂ relaxations were observed. Unlike the alkylene linker, the OE linker incorporation speeds up the α and α₂ relaxations (consistent with the decrease in Tg), increases the relaxation strengths [leading to an increase in the static and Coulombic dielectric constants (eₛ and eC)], imparts a higher number density of simultaneously conducting ions (p), and boosts their mobility (μ). This is directly reflected in the OE-containing PILs having 10 times higher ionic conductivities (σDC) than the PIL with the alkylene linker. The Tg, eₛ, and σDC are significantly influenced by the repeat unit molecular volume (Vₘ), polarizability volume (Vₚ), and Keesom volume (VK), suggesting that the OE linker not only facilitates ion dissociation of Im⁺Tf₂N– but also promotes segmental motion, thereby leading to a strong correlation of ionic conductivity with the dielectric constant and glass transition temperature.