preparation-liquid-crystal-networks-for-macroscopic-oscillatory

Preparation of Liquid Crystal Networks for Macroscopic Oscillatory Motion Induced by Light

A strategy based on doped liquid crystalline networks is described to create mechanical self-sustained oscillations of plastic films under continuous ...

Technical University of Eindhoven

Chemical networks and molecular switches dominate the area of research geared toward macroscopic motion of materials. A counter-intuitive approach to create self-sustained oscillation by light irradiation of ordinary photostabilizers in splay-aligned liquid-crystalline networks made from commercial mesogens is developed. Photostabilizers or any molecules that are able to quickly dissipate the absorbed light through heat, by vibrational and/or rotational modes, can reach self-oscillating macroscopic motion where self-shadowing plays a critical role. The mechanical self-oscillation is linked to temperature oscillations and the asymmetric response over the film thickness. Only a localized responsive zone, acting as hinge, activates the oscillation of a beam-shaped device. The outcome of this research is extended from UV to near-IR actuation, making bulk applications to convert sunlight into mechanical work within reach.

Mastering the Photothermal Effect in Liquid Crystal Networks: A General Approach for Self-Sustained Mechanical Oscillators.

Oscillating materials that adapt their shapes in response to external stimuli are of interest for emerging applications in medicine and robotics. For example, liquid-crystal networks can be programmed to undergo stimulus-induced deformations in various geometries, including in response to light. Azobenzene molecules are often incorporated into liquid-crystal polymer films to make them photoresponsive; however, in most cases only the bending responses of these films have been studied, and relaxation after photo-isomerization is rather slow. Modifying the core or adding substituents to the azobenzene moiety can lead to marked changes in photophysical and photochemical properties, providing an opportunity to circumvent the use of a complex set-up that involves multiple light sources, lenses or mirrors. Here, by incorporating azobenzene derivatives with fast cis-to-trans thermal relaxation into liquid-crystal networks, we generate photoactive polymer films that exhibit continuous, directional, macroscopic mechanical waves under constant light illumination, with a feedback loop that is driven by self-shadowing. We explain the mechanism of wave generation using a theoretical model and numerical simulations, which show good qualitative agreement with our experiments. We also demonstrate the potential application of our photoactive films in light-driven locomotion and self-cleaning surfaces, and anticipate further applications in fields such as photomechanical energy harvesting and miniaturized transport.

Making waves in a photoactive polymer film.

We report on the fabrication of a rewritable and reprogrammable dual photoresponsive liquid crystalline-based actuator containing an azomerocyanine dye that can be locally converted into the hydroxyazopyridium form by acid treatment. Each dye absorbs at a different wavelength giving access to programmable actuators of which the folding can be controlled by different colors of light. The acidic patterning is reversible and allows to erase and re-write new patterns in the polymer film, giving access to reusable, adjustable soft actuators.

Anne Helene Gelebart

Institute for Complex Molecular Systems (ICMS),
Department of Chemical Engineering and Chemistry,
Laboratory for Functional Organic Materials and Devices (SFD),
Department of Chemical Engineering and Chemistry, Laboratory for Functional Organic Materials and Devices (SFD)

Technical University of Eindhoven

Preparation of Liquid Crystal Networks for Macroscopic Oscillatory Motion Induced by Light