Dynamics of Polymerized Ionic Liquids and their Monomers

Dielectric spectroscopy determines the static dielectric constants ($\varepsilon_{s} )$ of polymers with imidazolium pendant structures containing a combination of alkylene and ethyleneoxy units as spacers between the backbone and the imidazolium cation. All monomers and their polymers exhibited two dipolar relaxations, assigned to the usual segmental motion ($\alpha )$ associated with the glass transition and a lower frequency stronger relaxation ($\alpha_{2} )$, attributed to ions rearranging. While ion pairs in conventional (smaller) ionic liquids prefer antiparallel alignment (Kirkwood $g\approx 0.1$ with $\varepsilon_{s} $ $\approx $ 15), because their polarizability volumes strongly overlap, ion pair dipoles in the larger ionic liquid monomers display $g$ of order unity and $50\le \varepsilon_{s} \le 110$. Longer spacers lead to higher static dielectric constant, owing to a significant increase of the relaxation strength of the $\alpha_{2} $ process, which is directly reflected through an unanticipated increase of the static dielectric constant with ionic liquid molecular volume. The ionomers consistently exhibit 1.5 - 2.3 times higher static dielectric constants than the monomers from which they were synthesized, suggesting that polymerization encourages the observed synergistic dipole alignment ($g>1)$. Comparison of dielectric and linear viscoelastic responses reveals a strong connection between the time scales of polymer segmental motion ($\alpha )$, ion rearrangements ($\alpha_{2} )$ and the viscoelastic softening associated with the glass transition. For all polymers with imidazolium side chains and a wide variety of counter-anions, there is a strong correlation between glass transition temperature and repeat unit molecular volume. Large side chains have low T$_{\mathrm{g}}$ $\approx $ -50 $^{\mathrm{o}}$C and their ionic conductivity increases as ethylene oxide repeats are incorporated into the side chains.