New insights into photoactivated volume generation boost surface morphing in liquid crystal coatings

Abstract

Photoactivated generation of disorder in a liquid crystal network produces free volume that leads to the controlled formation of dynamic corrugations at its surface. The liquid crystal order amplifies the deformation of copolymerized azobenzene, which takes place on molecular length scales, to a micrometre-sized macroscopic phenomenon based on changes in density. We postulate a new mechanism in which continuous oscillating dynamics of the trans-to-cis isomerization of the azobenzene overrules the net conversion, which is currently considered as the origin. This is supported by a significant local density decrease when both the trans and cis isomers are triggered simultaneously, either by dual-wavelength excitation or by the addition of a fluorescent agent converting part of the light to the cis-actuating wavelengths. This new insight provides a general guideline to boost free volume generation leading not only to larger macroscopic deformations but also to controllable and faster non-equilibrium dynamics.

Figure 1. Kinetic mismatch between photochemistry and mechanical response.

(a) Change in absorbance during thermal relaxation in dark of the cis to the trans state of azobenzene in an LCN* measured at room temperature. The insert shows the change in absorption with time fitted by a single exponential decay of the cis state. (b) Mechanical response measured as a height change of the thin film versus time upon actuation by 365 nm light of intensity 78 mW cm⁻² and a subsequent relaxation in dark. The film thickness is 4 μm, the corresponding density decrease is 1.3%.

Figure 2. Photostationary state of azobenzene in the LCN.

The equilibrium end conversion of trans azobenzene to its cis state under various light illumination conditions using a blend of 365 and 455 nm light, both generated simultaneously by an LED light source. The horizontal axis gives the intensity ratio of both wavelength.

Figure 3. Density change and volume increase under various illumination conditions.

(a–c) Decrease in density of LCN* under the different illumination conditions when 365 nm LED light is blended with 455 nm LED light. The error bars give the deviation within five separate measurements. (d,e) Interference microscopy measurements of the surface profile during (d) exposure to single 365 nm light, and (e) exposure to simultaneous 365 and 455 nm. (f) Corresponding surface profiles measured at a 10-μm film found during one-wavelength 365 nm exposure (red curve) and two-wavelength 365 nm +455 nm exposure (black curve).

Figure 4. Schematic view of oscillatory LCN deformation.

Volume increase induced by copolymerized di-acrylate azobenzene (a) and mono-acrylate azobenzene (b).

Figure 5. Comparison of LCN* actuation with and without fluorescent dye.

(a) Excitation/emission of the fluorescent dye. (b) Chemical structure of the fluorescent dye and comparison of the density decrease at various dye concentrations. (c) Interference microscopy measurement of the surface deformation of a 10-μm LCN* coating with dye (black line) and without dye (red line), and (d,e) the corresponding three-dimensional surface profiles.

Figure 6. Applied materials.

Liquid crystal monomers (1-3), polymerizable chiral dopant (4), polymerizable azobenzene (5), photoinitiator (6), fluorescent dye (7), inert ultraviolet absorber (8).