Interactions between ionic liquid and fully zwitterionic copolymers probed using thermal analysis

Understanding the interactions between ionic liquids and polymers is crucial for the design of higher conductivity solid polymer electrolytes. In this work, thermal analysis is used to study the interactions between a hydrophobic ionic liquid, 1-ethyl-3-methylimadazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI), and random zwitterionic copolymers of 2-methacryloyloxyethyl phosphorylcholine (MPC) and sulfobetaine vinylimidazole (SBVI). The zwitterionic copolymers are highly hygroscopic, readily absorbing water from the ambient atmosphere. After absorption, the water displaces EMI TFSI from the zwitterionic groups resulting in crystallization of most of the ionic liquid when cooled to sub ambient temperatures. Upon reheating, absorbed water is removed and the ionic liquid remains in close association with the copolymers. Removal of the water leads to strong interactions between the zwitterions and the ionic liquid, causing plasticization that lowers the glass transition temperature (T-g) and inhibits EMI TFSI crystallization. Analysis of the glass transition reveals that the ionic liquid decreases T-g significantly below the predicted copolymer T-g, and that the copolymer backbone is stabilized by the formation of dipolar crosslinks between zwitterionic side groups of the same species.