A Photoresponsive Polymeric Actuator Topologically Cross-Linked by Movable Units Based on a [2]Rotaxane

Abstract

A photoresponsive polymeric material, in which [2]rotaxane units with α-cyclodextrin threading onto an azobenzene derivative is used as a topological cross-link for the main chains of the polymeric material in aqueous media, was achieved.[2]Rotaxane structures were found to act as movable links in the polymer network, and the mechanical properties of the material were enhanced to show a rupture strain of 2800%. The materials were reversibly deformed by irradiation with UV or visible light in aqueous media, which caused photoisomerization of the azobenzene moiety and changed the structure of the [2]rotaxane linker, leading to deformation of the polymer network. Surprisingly, the dry materials, which had been uniaxially extended in air, showed a faster response than the hydrogel. The orientation of the polymeric network in the materials enables the efficient response. This dry material (5.6 mg) performed 5.6 μJ of mechanical work within 10 s, which is approximately 50 times higher than that achieved in our previous work.

From Molecular Machines to Stimuli‐Responsive Materials

“…d) (i) Covalent polymer network built from a pseudo[2]rotaxane and linear PEG units, (ii) evolution of the Young modulus of the corresponding hydrogel (1) and xerogel (2) upon UV and Visible light irradiation over 5 cycles, (iii) (1) schematic representation of the lift weight experiment upon UV irradiation and (2) corresponding images. d) Reproduced with permission . Copyright 2018, American Chemical Society.…”

“…In this case, while the mobility of the mechanical bond is important to reach the desired physical property, it cannot be controlled by its precise actuation. Reaching materials with switching capabilities associated to a controlled actuation of the mechanical bond has been particularly challenging with [ n ]rotaxanes and, to the best of our knowledge, examples remained very limited . The first one was reported in 2011 by the groups of Stoddart and Heath, who synthesized bistable redox‐sensitive poly[ n ]rotaxanes with 1,5‐dioxynaphtalene and TTF stations for bis‐pyridinium macrocycles, in order to construct a solid‐state switch .…”

Light‐Responsive Shape‐Changing Polymers

“…Irradiating the gel with ultraviolet light (λ = 365 nm) from the left side bent the gel to the right, whereas irradiating the bent gel with visible light (λ = 430 nm) from the same side restored the initial condition . This approach was later modified and expanded to prepare dry actuators with the opposite behavior that contract under UV light and expand when illuminated with visible light . Here, a more complex rotaxane molecular structure was employed to achieve the feat, where the azobenzene moieties were part of the main chain of the polymer hydrogel (Figure c).…”

“…c) Schematic illustration of the preparation of the αCD‐Azo hydrogel as a topological material. Reproduced with permission . Copyright 2018, American Chemical Society.…”

Emerging Material Technologies for Haptics

“…Many of those have been used to realize actuators . For example, solvent‐responsive gels, photoresponsive gels, magnetoresponsive gels, and electroresponsive gels have been used for actuator design. There are also several examples of tactile displays using gels .…”