Multiple-stimulus-responsive hydrogels of cationic surfactants and azoic salt mixtures

New hydrogels having high water content, ～96 wt%, composed of cationic surfactants, alkyltrimethylammonium bromides (C _n TAB, n?=?12, 14, 16, and 18), and a small dye molecule, sodium azobzenzene 4,4′-dicarboxylic acid (AzoNa₂), was firstly obtained. The three-dimensional network structures of hydrogels were determined by transmission electron microscopy images, scanning electron microscopy images, ¹H nuclear magnetic resonance, and small-angle X-ray scattering measurements. The mechanism of hydrogel formation was also illustrated. The rheological data were obtained to investigate the mechanical strength of hydrogels, which were turned out to be strong mechanical strength (～10⁴ Pa) materials. We found that the strength of the hydrogel depends on the fiber density, which can be controlled by changing the proportion of the two compounds, concentration of surfactants, temperature, and the chain length of the surfactant. Interestingly, the hydrogels were found to have a multiple-stimulus response property. A reversible thermal, UV–vis, or a chemical response was investigated in the mixtures of cationic surfactants and azoic salt for the first time. These findings may find potential applications such as sensors, actuators, shape memories, and drug delivery systems, etc.