Ionic strength/temperature-induced gelation of aqueous poly(N- isopropylacrylamide-co-vinylimidazole) solution

A high molecular weight copolymer of N-isopropylacrylamide (NiPAAm) and vinyl imidazole (VI) was synthesized and its phase transition behavior in aqueous solutions (5 wt%) by simultaneous changes of ionic strength and temperature was investigated. At low ionic strengths, the copolymer solution showed two phases (clear and opaque solutions), which were freely mobile, as increasing temperatures up to 65 °C due to repulsion interaction of positive charges developed by basic imidazole group on the polymer aggregates. However, at the physiological condition (I=0.15, T=37 °C), four distinctive phases (clear solution, opaque solution, gel and shrunken gel) were observed because of charge shielding effect by added salts. The gel state was stable and maintained from 32 °C to 55°C. In particular, the phase transition from opaque solution to gel rapidly occurred by the change in ionic strength (from ∼ 0 to 0.15) at 37 °C. This characteristic can be utilized as a liquid embolic agent.