Polymerized Ionic Liquids with Enhanced Static Dielectric Constants

Dielectric spectroscopy was used to determine the static dielectric constants (εs) of imidazolium acrylates and methacrylates and their ionomers, with different imidazolium pendant structures containing a combination of alkylene [(CH2)n, n = 5 or 10] and ethyleneoxy [(CH2CH2O)n, n = 4 or 7.3 (the average of a mixture of n = 1 to 20)] units as spacers between the backbone and the imidazolium cation. All monomers and polymers exhibited two dipolar relaxations, assigned to the usual segmental motion (α) associated with the glass transition and a lower frequency relaxation (α2), attributed to ions rearranging. From the analysis of the static dielectric constants using the Kirkwood g correlation factor, the dipoles in conventional (smaller) ionic liquids prefer antiparallel alignment (g ≈ 0.1), lowering εs values (≤30), because their polarizability volumes Vp strongly overlap, whereas the dipoles in the larger ionic liquid monomers display g of order unity and 50 ≤ εs ≤ 110. A longer spacer leads to higher static dielectric constant, owing to a significant increase of the relaxation strength of the α2 process, which is directly reflected through an unanticipated increase of the static dielectric constant with ionic liquid molecular volume Vm. The glass transition temperature of polymerized imidazolium ionic liquids with various counterions is also shown to simply be a monotonically decreasing function of Vm. Furthermore, the ionomers consistently exhibit 1.5–2.3 times higher static dielectric constants (εs up to ∼140 at room temperature) than the monomers from which they were synthesized, suggesting that polymerization encourages the observed synergistic dipole alignment (g > 1).