Making waves in a photoactive polymer film

Abstract

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.

Extended data Figure 1. Synthetic routes for constituent compounds.

Components of the LCN films include AzoPy, compound I and compound II.

Extended data Figure 2. Thermal characterization of the mixtures used in the study.

a, Differential Scanning Calorimetry (DSC) scans (second runs, exotherm downwards) showing the phase behavior of all mixtures investigated. The nematic to isotropic transition occurs at 90°C. b, DSC scan of a polymerized sample showing the change in specific heat at the glass transition temperature (Tg). The table summarizes the Tg data of the various polymerized compositions. c, Normalized absorption spectra of the various mixtures investigated.

Extended data Figure 3. Relaxation kinetics of the azo-derivatives embedded in the LCN.

Thermal relaxation from the photostationary cis state to the trans state of (a) A6MA, (b) compound I, (c) compound II, and (d) DR1A at various temperatures.

Extended data Figure 4. Pictures taken at different angles showing the created curvatures inducing the shadow effect.

Scale bar: 5 mm. At 90°, the bump is formed but since no shadow is created the wave cannot propagates and the film remains in that position.

Extended data Figure 5. Temperature measured at the front of the wave.

a, Influence of the intensity on the temperature increase at the front of the wave. The red layer helps to visualize the glass transition region. b, Temperature measured for the uniaxial oriented sample. despite the rubbery character of the films, no motion was observed.

Extended data Figure 6. Temperature measurements during wave propagation.

a,c, Thermal pictures of the wave taken at different time. b, Temperature profile over the length of the film during wave propagation at t = 0 s (black line), t= 0.67 s (dark gray line and t=1.40 s(light gray line). d, Temperature profiles for the planar up sample at t = 0 s (black line), t= 0.11 s (dark gray line) and t=0.22 s (light gray line).

Extended data Figure 7. ¹H-NMR spectra of constituent compounds.

a, ¹H-NMR of the compound AzoPy used to form the compound I. b, ¹H-NMR of the compound II.

Extended data Figure 8. Transmission spectra of LCN films.

compound I (green), II (black), A6MA (red), AzoPy (pink) and DR1A (blue). Thickness 20µm. The films containing A6MA, I and AzoPy are actuated with 405nm light. At that wavelength the transmission are 6.3 %, 8.9 % and 4.1 % for A6MA, AzoPy and I, respectively. The samples containing DR1A and II are illuminated with a 455 nm light. At this wavelength the initial transmission is 13 % and 26 % for II and DR1A respectively.

Figure 1. Azo dyes and their cis to trans relaxation

a, Chemical structures of azo-derivatives and liquid crystal mesogens. b, Exponential decrease of their half-life time as a function of temperature. Circle, square, triangle star and diamond for the A6MA, I, II, AzoPy and DR1A, respectively.

Figure 2. Mechanism of wave propagation and parameters that influence propagation speed

a, Experimental set-up, schematic. b, Frequency versus incident light angle. Planar side up, distance end-to-end L=22 mm (black) and 17 mm (red) and homeotropic side up, L=22 mm (blue). Error bar: s.d. n= 3. c, Influence of light intensity on wave velocity. Planar side up, L= 22 mm. d, e, Comparison of simulation (left) and experimental data (right) for both planar (d) and homeotropic (e) side up (video S4 and S5). Incident light: 10° from the left, L=22 mm. Scale bar: 5 mm. Films made of mixture I, size 23 mm x 4 mm x 20µm.

Figure 3. Temperature traces recorded by an IR-thermal camera during wave propagation

Mixture I based LCN films exposed to UV-light (500 mW.cm^-2). a, b, Temperature profiles during exposure at the homeotropic side and c, d, at the planar side. a, c, Local hot spots. b, d, Oscillating temperatures at positions 1 (black), 2 (red) and 3 (blue) as indicated in a and c. Distance end-to-end: 22 mm.

Figure 4. Two examples of applications demonstrating rejection of contaminants and oscillatory transport of a framed film.

a, Photoactuated wave motion ejects sand from the film’s surface via a snap-through release of energy, demonstrating the mechanism for a self-cleaning surface. See video S6. b, Schematic representation of the photoactuated locomotion along a flat substrate. The short ends of the active film, planar side up (c) or homeotropic side up (d), are fixed to a passive frame. The direction is dependent on the side exposed. See also video S7 and S8. Active film composition and dimension: see Figure 2. Plastic frame: 15 mm (length) x 5 mm (width). Scale bar: 5 mm. Dashed line represents the top surface of the glass plates.