Polymerized Ionic Liquids with Enhanced Static Dielectric Constants

Abstract

Dielectric spectroscopy was used to determine the static dielectric constants (ε s ) of imidazolium acrylates and methacrylates and their ionomers, with different imidazolium pendant structures containing a combination of alkylene [(CH 2 ) n , n = 5 or 10] and ethyleneoxy [(CH 2 CH 2 O) 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 (α 2 ), 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 ε s values (≤30), because their polarizability volumes V p strongly overlap, whereas the dipoles in the larger ionic liquid monomers display g of order unity and 50 ≤ ε s ≤ 110. A longer spacer leads to higher static dielectric constant, owing to a significant increase of the relaxation strength of the α 2 process, which is directly reflected through an unanticipated increase of the static dielectric constant with ionic liquid molecular volume V m . The glass transition temperature of polymerized imidazolium ionic liquids with various counterions is also shown to simply be a monotonically decreasing function of V m . Furthermore, the ionomers consistently exhibit 1.5−2.3 times higher static dielectric constants (ε s 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).

Molecular Volume Effects on the Dynamics of Polymerized Ionic Liquids and their Monomers

“…In spite of the high dipole density in the ILs, a high dielectric constant may not be achieved if the dipoles are aligned primarily antiparallel to each other. Lopes and Padua [77] and Correlation between glass transition temperature T g and repeat unit molecular volume V m (including the counterion) for imidazolium-based PILs [37,38] with various side chains and counterions (filled symbols), monomers [37,56] (open symbols), alkali inorganic salts [55] (+ symbols), and ionic liquids [55,57] (X symbols). The solid curves are exponential fits to guide the eye, with the large V m limit having T g =-52 C (for the PILs), -70 C (for the monomers), and -110 C (for the simple ionic liquids).…”

“…This distinction can be understood through overlap of the polarizability volumes of ion pairs, which forces the ion pair dipoles to interact strongly [37]. Since the static dielectric constants of polar liquids, ionic liquids, IL monomers and PILs predominantly result from orientational polarization, the polarizability volume V p (defined by Debye [80]) can be determined by [81] …”

“…(1) using the dipole moment m listed in Table 1 [37,38]. Then, the overlap of polarizability volumes can be quantified as…”

“…3b, conventional (smaller) ILs, having a high density of ions, exhibit V p /V m ! 3.5, indicating that the ion pairs all interact strongly with multiple neighbours [37]. The IL monomers, PILs and polar liquids have 1 < V p /V m < 3.5, wherein the dipoles overlap less; as a result, the static dielectric constant increases with V m because the ion pair dipoles are better able to independently respond to the applied field.…”

“…In principle, these details might matter but herein all properties are correlated with repeat unit molecular volume V m (including the anion). Syntheses of these materials can be found in the literature [29,36,37,39,46].…”