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Light-responsive and self-healing behavior of azobenzene-based supramolecular hydrogels
Martina Salzano de Luna1, Valentina Marturano2, Marina Manganelli3
1Department of Chemical, Materials and Production Engineering (INSTM Consortium - UdR Naples), University of Naples Federico II, P.le Tecchio 80, 80125 Naples, Italy; Institute for Polymers, Composites and Biomaterials, National Research Council of Italy, P.le E. Fermi 1, 80055 Portici, Italy.
Hypothesis:
Multi-component supramolecular hydrogels are gaining increasing interest as stimuli-responsive materials. To fully understand and possibly exploit the potential of such complex systems, the hierarchical structure of the gel network needs in-depth investigations across multiple length scales. We show that a thorough structural and rheological study represents a crucial pillar for the exploitation of this class of functional materials.
Experiments:
Supramolecular hydrogels are prepared by self-assembly of hexadecyltrimethylammonium bromide (CTAB) and azobenzene-4,4'-dicarboxylic acid (AZO) in alkaline aqueous solution. The CTAB/AZO concentration was varied from ϕ = 0.25 to 4 wt% keeping the CTAB:AZO molar ratio fixed at 2:1. The systems were thoroughly studied through a combination of X-ray scattering, microscopy, rheological and spectroscopic analyses.
Findings:
The CTAB/AZO solutions form a self-supporting gel with nanofibrillar structure below ~30 °C. The critical gelation concentration is ϕc = 0.45 wt%. Above this threshold, the gel elasticity and strength increase with CTAB/AZO content as ~(ϕ-ϕc)1. The hydrogels exhibit self-healing ability when left at rest after a stress-induced damage. Moreover, the light-induced isomerization of the AZO moieties provides the gel with light-responsiveness. Overall, the multi-stimuli responsiveness of the studied CTAB/AZO hydrogels makes them a solid starting point for the development of sensors for mechanical vibrations and UV/visible light exposure.
