Thermal and ion transport properties of hydrophilic and hydrophobic polymerized styrenic imidazolium ionic liquids

Polymerized ionic liquids (PILs) are a platform for fundamental studies of structure-property relationships in single ion conductors, with potential applications in energy storage and conversion. The synthesis, thermal properties, and ionic conductivities of homologous, narrow dispersity styrenic PILs are described. Hydrophilic poly(4-vinylbenzyl alkylimidazolium chloride) (PVBn(alkyl)ImCl) homopolymers with constant average degrees of polymerization were synthesized by post-synthetic functionalization of a poly(4-vinylbenzyl chloride) (M_n = 15.9 kg/mol, M_w/M_n = 1.34) master batch with N-alkylimidazoles (alkyl = -CH₃ (Me), -C ₄H₉ (Bu), and -C₆H₁₃ (Hex)). The chloride counterions of PVBnHexImCl were exhaustively metathesized with PFa ₄^-, PFa₆^-, and bis(trifluoromethanesulfonyl)imide (TFSI^-) to yield a series of hydrophobic PILs. Thermogravimetric analyses indicate that PVBn(alkyl)ImCl homopolymers are unstable above 220 °C, whereas the hydrophobic PILs remain stable up to 290 °C. The glass transition temperatures (T_g) decrease with both increasing alkyl side-chain length and increasing counterion size, exemplified by T_g = 9 °C for PVBnHexImTFSI. Hydrophilic PILs exhibit high ionic conductivities (as high as ∼0.10 S cm^-1) that depend on the relative humidity, water uptake, and the PIL side chain length. The hydrophobic PILs exhibit lower conductivities (up to ∼5 × 10^-4 S cm^-1) that depend predominantly on the polymer T_g, however, counterion size and symmetry also contribute.