Multiple Relaxations and Entanglement Behaviors of Polymerized Ionic Liquids with Various Anions/Cations

Abstract

Polymerized ionic liquids (PILs) with anions of bis(trifluoromethylsulfonyl)imide (TFSI^–), hexafluorophosphate (PF₆^–), and tetrafluoroborate (BF₄^–) and cations of poly[1-(4-vinylbenzyl)-3-alkyl-imidazolium] P[VBC_nIM]⁺ with alkyl lengths C₁, C₂, and C₄ were successfully synthesized and characterized. X-ray scattering showed an increase in backbone-to-backbone spacing (d_b) by 0.8 Å per CH₂ added to the alkyl side chain. Rheological and dielectric measurements were used to measure rates of chain relaxation and ion dissociation/association. The glass transition temperatures T_g follow the trend: PC₄-TFSI < PC₂-TFSI < PC₁-TFSI < PC₁-BF₄ < PC₁-PF₆, which correlates well with their dielectric behaviors. However, the fragility m_DR from dielectric relaxation increases with decreasing T_g, which is the opposite of the dependence of fragility m_Rheo from rheology for both our PILs and neutral polymers. The dielectric and rheological relaxations of our PILs are expected to be influenced by both their anion–cation binding energies and their relative free volumes, while for neutral polymers, relaxations are primarily governed by free volume. The increase of fragility of m_DR with decreasing T_g, therefore, suggests that dielectric relaxation is influenced more by anion–cation binding energy than by free volume, while the reverse is true for m_Rheo. The plateau modulus G_N and entanglement molecular weight M_e estimated from rheological measurements agree with predictions of the packing model, using only a small modification of the Flory characteristic ratio C_∞ from that of a neutral polymer. Packing lengths of p = 6.0∼9.3 Å and tube diameters d_t = 11∼17 nm are found, depending on specific cation and anion structures.