Phase Behavior and Conductivity of Sulfonated Block Copolymers Containing Heterocyclic Diazole-Based Ionic Liquids

Abstract

We have investigated morphologies and conductivities of ionic liquids (ILs) incorporated poly(styrenesulfonate- b -methylbutylene) (PSS- b -PMB) block copolymers by varying kinds of heterocyclic diazoles in ILs. A low molecular weight PSS- b -PMB copolymer (3.5–3.1 kg/mol) with sulfonation level of 17 mol % was employed as a matrix polymer, which indicates disordered morphology at entire temperature examined. The addition of different ILs results in the emergence of various ordered morphologies such as lamellar, hexagonal cylinder, and gyroid structures. Interestingly, it has been revealed that ring structures and alkyl substituents in diazoles play an important role in determining the morphologies of ILs impregnated PSS- b -PMB copolymers, attributed to the dissimilar strength of ionic interaction. Heating the ILs doped PSS- b -PMB copolymers causes intriguing thermoreversible order–order and order–disorder phase transitions, which can be rationalized by classical block copolymer thermodynamics. From conductivity measurements, it has been found that the enhanced conductivity could be achieved by increasing number of protic sites in heterocyclic diazoles. Upon exploring morphology effects on conductivities of ILs-containing PSS- b -PMB copolymers, with decoupled segmental motion of polymer chains and ion transport, similar morphology factor of 0.4 has determined if the morphologies are appeared to be lamellar and/or hexagonal cylinder structures. In contrast, the gyroid-forming sample revealed apparently high morphology factor in the range of 0.6 to 0.7, which is intimately related to better connectivity of ionic channels along cocontinuous PSS phases.