Proton Hopping and Diffusion Behavior of Sulfonated Block Copolymers Containing Ionic Liquids

Abstract

We have investigated the dependence of proton hopping and diffusion behavior of nanostructured poly(styrenesulfonate-b-methylbutylene) (PSS-b-PMB) block copolymers containing imidazolium-based ionic liquids (ILs) on the type and the composition of ILs. Pulsed-field gradient spin-echo nuclear magnetic resonance experiments suggest that fast proton hopping is facilitated by higher imidazolium contents in the ILs. On the contrary, the alkyl substituent at the protic position of imidazole results in a 2-fold reduction in the self-diffusion coefficient for proton hopping, becoming comparable to vehicle diffusion in the temperature window examined. The anion of ILs makes a significant impact on the activation barrier for proton conduction, leading to conductivities differing by a few orders of magnitude, depending on the type of anion in the ILs. Notably, a high proton transference number of 0.81 was achieved for the IL-containing PSS-b-PMB copolymers (compared to the low value of 0.52 for neat ILs) by optimizing the composition and the type of ILs. This increase is attributed to specific interactions between the ILs and the polymer matrix, affecting the nature of hydrogen bonds and ionic aggregate structures. This study provides valuable insights into the factors affecting the proton transport efficiency of IL-containing polymers, enabling the design of new polymer electrolytes characterized by both fast proton conduction and high proton transference numbers.