Remarkable sol-gel transition of PNIPAm-based nanogels via large steric hindrance of side-chains

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 (T-p). 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 T-p. Herein, the results of temperature-variable (HNMR)-H-1 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 T-p. 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 T-p, 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.

Automated summary

It is discovered that introducing hydrophobic N-tert-butylacrylamide (TBA) with large sterically hindering side groups can slow down the dehydration of hydrophobic alkyl groups in PNIPAm-based nanogels, resulting in thermally induced sol-gel transition above the phase transition temperature (T-p). Furthermore, the introduction of N-acrylamido-L-phenylalanine (APhe) monomer with a strongly water-absorbing carboxyl group and larger steric hindrance can self-assemble PNIPAm/TBA/APhe terpolymer nanogels into colorful colloidal crystals that undergo sol-gel transition above T-p.