Synergistic Thermoresponsive Optical Properties of a Composite Self-Healing Hydrogel

A fast (30 s) thermoresponse and rapid (30 min) self-healing were achieved by noncovalently incorporating a small fraction of poly(N-isopropylacrylamide) (PNIPAM) nanogels (≤0.18 wt %) into a benzyl methacrylate-co-octadecyl methacrylate-co-methacrylic acid (BOMA-16) polymer matrix. The PNIPAM serves as the thermoresponsive component while the BOMA-16 provides self-healing via hydrophobic associations. The PNIPAM/BOMA-16 composite showed tunable lower critical solution temperature and up to 95% more visible light modulation (ΔT) than the same concentration of PNIPAM by itself. Conversely, the PNIPAM nanogels (≤0.18 wt %) served as reinforcements and increased the storage modulus of the PNIPAM/BOMA-16 hydrogel by up to 2.7 times relative to a BOMA-16 hydrogel. Using different analytical techniques, we show that the synergistic thermoresponsive light modulation is due to a hydrophilic–hydrophobic phase transition and clustering of the PNIPAM nanogels facilitated by the BOMA-16. The synergistic optical effect is also observed for mixtures of PNIPAM and other polyelectrolytes.