Synthesis and self-assembly behavior of light and thermally responsive rod-coil amphiphile in aqueous solutions

In this work, a novel single-tailed nonionic rod-coil amphiphile TriBAzoEO was synthesized, in which the introduced azobenzene moiety and polyoxyethylene ether headgroup endowed it with light- and thermo-responses, respectively. The surface activity of TriBAzoEO was rather weak, whereas it formed wormlike micelles in the aqueous solutions. The increased concentration of TriBAzoEO resulted in the formation of wormlike micelles with longer length, and thereby endowing the sample with higher viscosity. The trans- to cis-isomerization of azobenzene moiety in TriBAzoEO was occurred under ultraviolet (UV) light irradiation, inducing TriBAzoEO wormlike micelles to spherical micelles transitions. However, the reverse transitions cannot be realized by visible light irradiation, which might be caused by the strong π-π interactions and steric hindrance between cis-isomers of TriBAzoEO molecules in aggregates. Thermo-induced reversible transitions of TriBAzoEO aqueous samples were mainly caused by the temperature-dependent hydrogen bonding interactions between polyoxyethylene ether headgroups and water. The hydrophilicity of TriBAzoEO would be decreased upon heating because of the weakened hydrogen bonding interactions, facilitating the formation of wormlike micelles with larger size and showing corresponding thermo-induced thickening behavior. Once the temperature was decreased, both the hydrophilicity and micellar structure of TriBAzoEO would be recovered, thereby resulting in the decreased viscosity.