Confinement Promotes Hydrogen Bond Network Formation and Grotthuss Proton Hopping in Ion-Conducting Block Copolymers

Proton-conducting polymer membranes frequently contain nanoscale structural and ion-conducting phases. In addition to enhancing the mechanical properties, this mesoscale structure often leads to a significant increase in ion dynamics; however, the molecular underpinnings behind this phenomenon are not well understood. Here, a model proton-conducting polymeric ionic liquid (PIL) block copolymer is shown to have conductivity up to an order of magnitude higher than an analogous homopolymer. Variable temperature H-1 solid-state magic angle spinning (MAS) NMR spectroscopy reveals that confinement in the block copolymer PIL decreases the segmental motion of the PIL and increases the structural rigidity of the ionic functional groups. This increased structural rigidity is found to dramatically increase the connectivity of the hydrogen bonding network in the block copolymer PIL according to double quantum-single quantum H-1 MAS NMR. This nanoscale restructuring leads to a significant increase in Grotthuss proton hopping dynamics in the block copolymer PIL compared to the homopolymer.