Phase Transition Behavior Of Temperature-induced Poly(N-isopropylacrylamide)/polyethyleneimine Microgel Suspension

As a matrix material,the intelligent microgel can also form a macroscopic hydrogel with certain mechanical strength by in-situ gelation.At the same time,it also gives the injectability of this type of hydrogel material,which provides a possibility to further expand its application in the biomedical field.In this paper,poly（N-isopropylacrylamide）/polyethyleneimine（PNIPAM/PEI）copolymer microgel was prepared by soap-free emulsion polymerization.The obtained microgels were characterized in detail by transmission electron microscopy（TEM）,scanning electron microscopy（SEM）,Fourier transform infrared spectroscopy（TF-IR）,dynamic light scattering（DLS）,and nuclear magnetic resonance spectroscopy（1H NMR）.The results show that the copolymer microgel is a relatively uniform sphere of submicron size,and has a typical core-shell structure with a PNIPAM component as a core and a PEI component as a shell.The size of the microgel continued to decrease with increasing temperature,and its volume phase transition temperature（VPTT）was approximately 40.0℃.Under temperature induction,the microgel dispersion which added the electrolyte（Na₂HPO₄）forms a physical cross-linked gel;on the basis of physical cross-linking,genipin is added,which spontaneously reacts with the amino group on the PEI to form a chemical cross-linked gel.The PNIPAM/PEI microgel dispersion,physical cross-linked gel and chemical cross-linked gel were scanned by dynamic rheometer and dynamic frequency.The results showed that when the mass concentration of the microgel dispersion is low,it appears as a viscous liquid in the temperature window under study;when the mass concentration is high,the microgel dispersion system is a viscoelastic solid at a low temperature,then becomes a viscous liquid at a high temperature.That is,a solid-liquid state transition occurs during the heating process;it can be seen from the dependence of the dynamic modulus and the angular frequency that the viscous solid is a typical colloidal glass.The microgel system which the electrolyte is added exhibits a physical cross-linked gel-liquid transition during the temperature rise.And the gelation temperature decreases as the electrolyte content and the microgel mass concentration increase.The addition of electrolytes and genipin microgel system showed a chemical cross-linked gel-liquid transition during the heating process.The gelation temperature of the chemical cross-linked gel decreases with the increasing of electrolyte content,the genipin content,and the microgel mass concentration.At the same time,through dynamic time-scan curves we found that both physical cross-linked gels and chemical cross-linked gels need to be induced by temperature,and the addition of electrolytes is a necessary condition for the formation of chemical cross-linked gels.