Effects of chemical structure variation and salt addition on the glass transition behavior of polyzwitterion/LiCl salt complexes

Using temperature modulated differential scanning calorimetry (TMDSC), the glass transition process and the solid-state heat capacity of polyzwitterion (PZI) / LiCl salt complexes were investigated. Our goal is to characterize the influence of chemical structure variation and salt ratio on the formation of dipole-dipole sidechain electrostatic crosslinks. Starting with poly(sulfobetaine methacrylate), PSBMA, as the parent polymer, changes to the backbone and sidechain chemistries resulted in the following PZIs: poly(sulfobetaine acrylate), PSBA; poly(ethyl sulfobetaine methacrylate), PESBMA; poly(sulfobetaine-4vinylpyridine), PSB4VP; and poly(sulfobetaine-2vinylpyridine), PSB2VP. These changes result in variation of the onset of thermal degradation, the level of water uptake, and the solid and liquid state specific heat capacities. The PZI/LiCl complexes are both hydroscopic and hydrophilic, and bound water results in plasticization of the glass transition. Thermogravimetric analysis (TGA) quantified the change in mass with temperature during water removal. To accurately determine the effects of LiCl on the system bound water must be eliminated to achieve a dry solid state. For the dry PZI, as the molar ratio of LiCl increases, the glass transition temperature is lowered, and the heat capacity increment at the glass transition temperature is increased. These results are consistent with a model where the number of dipole-dipole crosslinks is decreased as LiCl content is increased.