Controlling the LCST-Phase Transition in Azobenzene-Functionalized Poly (<i>N</i>-Isopropylacrlyamide) Hydrogels by Light

Poly(<i>N</i>-isopropylacrylamide) PNIPAAm hydrogels were modified with a new azobenzene-containing co-monomer. In this work, light responsiveness as an additional functionality, is conceptualized to induce two phase transitions in the same material, which can be controlled by light. For a hydrogel with merely 2.5 mol% of this co-monomer, the lower critical solution transition temperature (LCST) was lowered by 12 °C (to 20 °C) compared to PNIPAAm (LCST at 32 °C), as analyzed by differential scanning calorimetry (DSC). The untreated unimodal endotherm split into a bimodal peak upon irradiation with UV-light, giving a second onset due to the switched (<i>Z</i>) isomer-rich regions, LCST*<b>_H2.5%</b>_{-<b>(<i>Z</i>)</b>} = 26 °C. On irradiation with 450 nm, leading to the reverse (<i>Z</i>) to (<i>E</i>) isomerization, the endotherm was also reversible. Thus, a photo-switchable hydrogel whose LCST and structure are tunable with the hydrophobicity-hydrophilicity of the (<i>E</i>) and (<i>Z</i>) isomeric state of azobenzene was obtained. The influence of the increase in the mol% of azoacrylate on the LCST was evaluated via DSC, in combination with NMR studies, UV-vis spectroscopy and control experiments with linear polymers. The large light-driven modulation of the LCST adds bistability in thermoresponsive hydrogels, which may open diverse applications in the field of soft robotics actuators.