Particle-forming precipitation polymerization under unusual conditions
Precipitation polymerization of N-isopropylacrylamide (NIPAM) with a small amount of crosslinker in water gave monodisperse thermosensitive hydrogel particles at 70 °C but not at 50 °C. Monodisperse hydrogel particles could be obtained even at 50 °C when co-nonsolvent systems such as ethanol/water were used as the polymerization medium. Polymerization in ethanol/water mixture gave poly-NIPAM particles even at room temperature. In this type of polymerization, stirring was unnecessary for the formation of particles. Particles prepared in ethanol/water were highly swellable. This was due to frequent chain transfer reaction in which ethanol takes part, resulting in low-molecular-weight polymers and low crosslinking efficiency. DMSO/water mixture is also suitable for giving similar kinds of particles. However, it is unclear whether the mechanism of particle formation is the same as in ethanol/water mixture.
KeywordsPrecipitation polymerization N-isopropylacrylamide Micro-phase separation ·Co-nonsolvency effect Volume phase transition temperature
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