Morphology and Ionic Conductivity of Humidity-Responsive Polymerized Ionic Liquid Block Copolymers

We present the ionic conductivity and morphology of humidity-responsive polymerized ionic liquid block copolymers (PIL BCPs), poly(methyl methacrylate-$b$-1-[2-(methacryloyloxy)ethyl]-3-butylimidazolium-X), where X is a bromide (Br) or hydroxide (OH) anion, as a function of relative humidity (RH), temperature, and PIL composition ($\phi_{PIL})$. PIL BCPs were characterized by in situ small-angle X-ray scattering and electrochemical impedance spectroscopy. These PIL BCPs have microphase separated morphologies and long-range order increases as $\phi_{PIL}$ increases. Notably, ionic conductivity increases 3 to 4 orders of magnitude when RH increases from 30 to 90 percent. When $\phi_{PIL}$ is greater than 0.37, BCP ionic conductivity approaches or exceeds that of the homopolymer, suggesting that the dynamics in PIL microdomains mimic the homopolymer and long-range order aids ion transport. Moreover, over 60 percent of the BCP is nonconductive without a penalty in ion transport. When $\phi_{PIL}$ is less than 0.37, BCP conductivity is 1 to 2 orders of magnitude less than the homopolymer and non-conductive PMMA segments dominate ion transport, as expected. Ionic conductivities at 80 $^{^{\circ}}$C, 90 percent RH, are 7.6 mS/cm for the Br-containing BCP with $\phi_{PIL} \quad =$ 0.53 and 25.0 mS/cm for the OH-containing BCP with $\phi_{PIL} \quad =$ 0.50.