The stimuli-responsive multiphase behavior of core–shell nanogels with opposite charges and their potential application in in situ gelling system

Concentrated p(N-isopropylacrylamide) (PNIPAM) nanogel dispersions exhibited rich temperature-sensitive sol-gel phase transition behavior. In the present work, the influence of electrostatic forces between nanogel particles, including attraction and repulsion, on the sol-gel phase transition behavior of PNIPAM nanogel dispersions has been studied. Both oppositely charged nanogels with core-shell structures (NIA and PND nanogels) were synthesized, and their shell charges were calculated to -0.33 and 0.082 mmol/g by potentiometric titration method. When mixed with various ratio of negative and positive charge (NC value), the resultant mixture dispersions of NIA and PND nanogel (OCNs) exhibited different aggregating behavior from NIA and PND nanogels. OCN-e aggregates (NC value=1/4), which exhibited temperature-independence of electric neutrality, had the maximum size, about 1.9-2.2 times larger than NIA or PND nanogels. Concentrated OCN-e dispersions exhibited stronger ability to form shrunken gel. Its CGC was about 2.0 wt%, 4-times lower than that of NIA and PND nanogels (about 8.0 wt%). In vitro and in vivo gelling results indicated that OCN-e aggregates could form free-standing gel with good mechanical strength, and were promising to be developed as new in situ gelling system.