Controllable transformation of UCST and LCST behaviors in polyampholyte hydrogels enabled by an association-disassociation theory-based switch mechanism

The potential temperature-sensitive characteristics of polyampholyte hydrogels have not been explored yet, despite their excellent mechanical properties and universality as supramolecular materials. Here, polyampholyte hydrogels were prepared with anionic and cationic monomers at high concentrations and their thermosensitive behaviors were investigated systematically. The results of this study break through the traditional understanding that hydrogels prepared from zwitterionic copolymers could only exhibit UCST characteristics. Moreover, the "association-disassociation" theory was presented to explain the abnormal phenomenon, which could endow a controllable switch for transforming UCST and LCST in polyampholyte hydrogels; the thermosensitive properties of the polyampholyte hydrogels arise from the competition of "association force" and "disassociation force", based on which the polyampholyte hydrogels could be endowed opposite thermosensitive properties by regulating the monomer concentration and monomer ratio. Accordingly, essential conditions required to form physically crosslinked UCST hydrogels could be concluded: satisfactory solubility of monomers; high-enough monomer concentration; appropriate hydrophilicity of ion pairs and suitable monomer ratio.