While the block/graft/branched structures are widely studied to favor the reversible physical gelation, there are no reports regarding the steric hindrance-induced
sol-gel transition of
PNIPAm-based
nanogels above their phase transition temperature (Tp). Generally, the introduction of hydrophobic components into
poly (N-isopropylacrylamide) (
PNIPAm)-based
nanogels only led to collapse and lower viscosity instead of the
sol-gel transition upon heating above the Tp. Herein, the results of temperature-variable 1HNMR and FTIR confirm that the introduction of hydrophobic N-
tert-butylacrylamide (TBA) with the large steric hindrance of side groups of N-tert-butyl to form NIPAm/TBA copolymer
nanogels can dramatically slow down the
dehydration of all the hydrophobic alkyl groups, thus resulting in the formation of thermally induced
sol-gel transition above the Tp. Furthermore, the N-acrylamido-
L-phenylalanine (APhe) monomer composed of a strongly water absorbing carboxyl group and a phenyl group with larger steric hindrance is studied to form P(NIPAm/TBA/APhe) terpolymer
nanogels which can self-assemble into colorful colloidal crystals. Surprisingly, owing to the synergistic effect between the water absorbing carboxyl group and the steric hindrance group on the same side group, these colloidal crystals can achieve
sol-gel transition above Tp, forming a physically crosslinked colorful
hydrogel. This work is expected to greatly advance the design, synthesis, and application of the
sol-gel transition of
PNIPAm-based
nanogel systems.