Effects of lithium chloride on the solid state properties of polyzwitterions*

Polyzwitterions (PZIs) have ideal properties such as ionic conduction, thermal stability, and mechanical stability, making them prime candidates to be used as solid polymer electrolytes. In this study we investigate thermal properties of the solid state for a series of specially synthesized PZIs including: poly(sulfobetaine acrylate), PSBA; poly(sulfobetaine methacrylate), PSBMA; and poly(ethyl sulfobetaine acrylate), PESBA. These PZIs can dissolve molar ratios of salt which increases their electrical conductivity. We added LiCl to PZIs at 1-20 wt.% and studied the impact of added salt on water uptake, degradation temperature, solid state heat capacity, and the glass transition temperature, T_g. Thermal gravimetry shows the percentage of bound water ranges from 5-18 wt.%, and increases as LiCl content increases. Using differential scanning calorimetry, we perform cyclic heating and cooling studies where each successive heat cycle reaches a higher maximum temperature. This procedure allows systematic removal of bound water and observation of the plasticization effect on T_g. Results show that addition of LiCl reduces the glass transition of dry PSBA (i.e., containing no bound water) from about 200°C to 150°C for the PSBA/LiCl blend at a 1:1 molar ratio.