Interplay Between Hydroxyl Density and Relaxations in Poly(vinylbenzyltrimethylammonium)-<i>b</i>-poly(methylbutylene) Membranes for Electrochemical Applications

Abstract

Anion-exchange membranes (AEMs) consisting of poly(vinyl benzyl trimethylammonium)-b-poly(methylbutylene) of three different ion exchange capacities (IECs), 1.14, 1.64, and 2.03 mequiv g^-1, are studied by High-Resolution Thermogravimetry, Modulated Differential Scanning Calorimetry, Dynamic Mechanical Analysis, and Broadband Electrical Spectroscopy in their OH^- form. The thermal stability and transitions are elucidated, showing a low temperature T_g and a higher temperature transition assigned to a disorder-order transition, T_δ, which depends on the IEC of the material. The electric response is analyzed in detail, allowing the identification of three polarizations (only two of which contribute significantly to the overall conductivity, σ_EP and σ_IP,1) and two dielectric relaxation events (β₁ and β₂), one associated with the tolyl side groups (β₁) and one with the cationic side chains (β₂). The obtained results are integrated in a coherent picture and a conductivity mechanism is proposed, involving two distinct conduction pathways, σ_EP and σ_IP,1. Importantly, we observed a reordering of the ion pair dipoles which is responsible for the T_δ at temperatures higher than 20 °C, which results in a dramatic decrease of the ionic conductivity. Clustering is highly implicated in the higher IEC membrane in the hydroxyl form, which reduces the efficiency of the anionic transport.