Polarization and Ion Transport in Poly(ionic liquid)-Grafted Nanoparticles

Poly(ionic liquid)-grafted nanoparticles (PILgNPs) are building units of PIL-based hybrid electrolytes, where the NP structures and the connectivity of polyelectrolyte chains determine ion transport properties. This connectivity is enhanced by increasing the molecular weight of grafted PIL chains, which further enhances their molar conductivity, that is attributed to the chain conformations helping in the development of ion-conducting pathways. In this work, molar ionic conductivities of four PILgNP samples with distinct graft lengths, two of which differ in graft density (low vs high), are presented. We design dielectric measurements, where the conductivity of the samples is measured under a large voltage gradient, to test whether PIL-grafted chains can be polarized by electric fields and vary in conformations. Samples at low graft density exhibit negative and positive deviations in conductivity in reference to their unperturbed (no-field) state. Their response is observed to be irreversible as the conductivity continues to increase even in the absence of oscillating external fields during zero-field rest intervals. In contrast, the steady conductivity measured for the high graft density sample is attributed to the constraints where the rearrangements of chains are entropically hindered. These results are explained by the polarization-induced chain reorientation under electric fields, corroborated through molecular dynamics simulations. The findings help us to understand the behavior of hybrid electrolytes under electrified conditions, with direct relevance to sensors and actuators.