Enhanced End Group Effect on the Thermoresponsive Properties of Nanoscale Poly(N-isopropylacrylamide) Assemblies

Herein, we report how the thermoresponsive behavior of nanoscale poly(N-isopropylacrylamide) (N-PNIPAM) assemblies differs from that of molecularly dissolved PNIPAM (M-PNIPAM). PNIPAM-modified gold nanoparticles (Au/PNIPAM) and block copolymer micelles composed of polystyrene core and PNIPAM shell (PS/PNIPAM) were used as two representative N-PNIPAM, which showed thermoresponsive behaviors distinct from those of M-PNIPAM. Systematic investigation of a series of N-PNIPAM with varying end groups revealed that a small end group is mainly responsible for the distinct thermoresponsive behavior of N-PNIPAM. While the end group influences the lower critical solution temperature (LCST) of both M-PNIPAM and N-PNIPAM, its effect is dramatically amplified in N-PNIPAM, leading to widely varying thermoresponsive behaviors ranging from no macroscopic phase transition to irreversible phase segregation. Furthermore, the end group effect results in greater hysteresis and enhanced bistability in N-PNIPAM, which was utilized to fabricate a heat-driven color display. These findings highlight the importance of the small end group of thermoresponsive polymers, especially when they are assembled into nanostructures.