Effects of LiCl and water on thermal properties of polyzwitterions

The thermal and structural properties of five polyzwitterion (PZI) and salt (LiCl) complexes were measured using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and temperature modulated DSC, and Fourier transform inferred (FTIR) spectroscopy. Poly(sulfobetaine methacrylate), PSBMA, poly(sulfobetaine acrylate), PSBA, poly(ethyl sulfobetaine methacrylate), PESBMA, poly(ethyl sulfobetaine acrylate), PESBA, and poly(sulfobetaine methacylamide), PSBMAm, were synthesized with variations in the sidechain and backbone. Complexes are cast from solution containing LiCl in amounts ranging from 0.06 to 1.65 mol%. The total and reversing heat capacity (c_p) and glass transition temperatures (T_g) are measured using DSC and TMDSC. A standard drying treatment in DSC is applied to ensure no surface or molecularly bound water. We use TGA to quantify the initial bound water content and develop an expression for the change in mass as a function of temperature. Addition of LiCl interrupts the intra- and inter-molecular crosslinking formed by the zwitterionic moieties resulting in a decrease in T_g and an increase in the heat capacity increment at T_g.  FTIR confirms the interruption of the crosslinks through an increase in the asymmetric stretching of the S=O side chain group.