In situ thermal gelation of water-soluble poly(N-isopropylacrylamide-co-vinylphosphonic acid)

A copolymer baaed on N-isopropylacrylamide (NIPAAm) and vinylphosphonic acid (VP) was synthesized to investigate its thermal gelation behaviors in the presence of calcium ion. The copolymer showed a variety of temperature-sensitive phase transition properties as a function of temperature. In an aqueous solution, it exhibited 3 distinctive phase transitions with gradually increasing the temperature: a transparent solution state, a cloudy and white solution state (sol), and a white and semisolid (gel) state. Particularly, in situ reversible sol-gel transition behaviors could be observed over a wide range of temperatures in the presence of varying concentrations of calcium ion. This physical gel that initially was formed homogeneously without changing its dimension in the solution container tended to slowly phase-separate out into a polymer-rich phase and a water-rich phase over the following several days, indicating that a synerisis occurred. The in situ thermal gelation behavior was utilized in the controlled release formulation of a model compound.