Rheology and phase behavior of thermo‐reversible pentablock terpolymer hydrogel

Thermo‐reversible phase behaviors and rheological properties of a pentablock terpolymer solution, poly( N ‐isopropylacrylamide)‐ b ‐poly(ethylene oxide)‐ b ‐poly(propylene oxide)‐ b ‐poly(ethylene oxide)‐ b ‐poly( N ‐isopropylacrylamide) (PNIPAM 150 ‐PEO 136 ‐PPO 45 ‐PEO 136 ‐PNIPAM 150 ), are investigated in comparison with its precursor, PEO 136 ‐PPO 45 ‐PEO 136 (F108). It is found that the critical gelation concentration of the terpolymer solution is only about 11 wt %, which is significantly lower than that of F108 solution (∼22 wt %). The 11 wt % terpolymer solution displays higher viscosity, stronger gel strength, and fast thermo‐responsive behavior compared with the 22 wt % F108 solution. The 11 wt % terpolymer solution shows a typical Newtonian fluid behavior at 30 °C due to the presence of individual spherical micelles, and presents an elastic gel property at 41 °C because of the formation of the close‐packed micelle aggregates. Cryogenic transmission electron microscopy (cryo‐TEM) and variable‐temperature 1 H NMR results demonstrate that the sol–gel phase transition mechanism is mainly related to the hydrophilic/hydrophobic transition of PPO and PNIPAM groups by external temperature stimulus.