Influence of Sulfonic Acid-Containing Ionic Liquid Groups on the Thermal, Mechanical, and Conductive Properties of Covalently Cross-Linked Ionene Networks

A series of covalently cross-linked networks and ionogels were prepared, whereby a protic sulfonic acid group was introduced either through a polymerizable or free ionic liquid (IL) group. The resulting ionene networks and ionogels were synthesized using thiol–ene photopolymerization. As the amount of protic IL (polymerizable or free) was increased, a reduction in the DSC glass transition temperature (Tg) and DMA storage modulus in the rubbery plateau region (E′) was observed as the IL groups generally acted as plasticizers. Introduction of a mixed anion [NTf2]/[OTf] system into the ionogel series led to a synergistic reduction in DSC Tg and an increase in ionic conduction (anhydrous conductivity of ∼10–5 at 30 °C). The conductivity was further enhanced when the film was exposed to varying degrees of relative humidity, leading to conductivities approaching 10–2 S/cm at 90 °C and 70% RH. This study underscores the potential of protic ionic liquid-containing ionene networks, in particular those that contain a combination of anions, as membranes in fuel cell applications.