Thermal analysis of fully zwitterionic copolymers for safer electrochemical energy storage

Current ionic liquid electrolytes used in lithium ion batteries are both volatile and flammable. A safer alternative to liquid electrolytes is to use solid conductive polymer gel electrolytes. In this study, the thermal properties of a group of zwitterionic copolymers designed for electrochemical energy storage are investigated using temperature modulated differential scanning calorimetry (TMDSC) and thermogravimetry (TG). Investigating the thermal properties will reveal how the zwitterionic moieties can affect transition phenomena. Sulfobetaine vinylimidazole (SBVI) and 2-methacryloyloxyethyl phosphorylcholine (MPC) were dissolved in an ionic liquid solution in varying molar ratios and subsequently polymerized to produce fully zwitterionic random copolymers. TG reveals two degradation steps at 290 ^oC and 390 ^oC due to MPC degradation and one step at 350 ^oC due to SBVI degradation. TMDSC at 5 ^oC/min was used to identify the glass transition of these materials, showing a decrease in T_g from 60 ^oC to -35 ^oC with increasing addition of SBVI. A melting endotherm at -18 ^oC was observed on heating, attributed to the presence of residual ionic liquid.