Light-induced mechanical response in crosslinked liquid-crystalline polymers with photoswitchable glass transition temperatures

Abstract

Energy conversion of light into mechanical work is of fundamental interest in applications. In particular, diligent molecular design on nanoscale, in order to achieve efficient photomechanical effects on macroscopic scale, has become one of the most interesting study topics. Here, by incorporating a "photomelting" azobenzene monomer crosslinked into liquid crystalline (LC) networks, we generate photoresponsive polymer films that exhibit reversible photoswitchable glass transition temperatures (T_g) at room temperature (~20 °C) and photomechanical actuations under the stimulus of UV/visible light. The trans-to-cis isomerization of azo chromophores results in a change in T_g of the crosslinked LC polymers. The T_g of the polymer network is higher than room temperature in the trans-form and lower than room temperature in the cis-form. We demonstrate the photoswitchable T_g contribute to the photomechanical bending and a new mechanism for photomechanical bending that attributes the process to an inhomogeneous change in T_g of the film is proposed.

Fig. 1

Schematic representations of molecular structures and their packing modes in the polymer film. a The crosslinked polymer network structure in the film. b Schematic illustration of the fabrication procedure of the polymer film in a molecule alignment cell to realize macroscopic ordering in the film. c An as-prepared free-standing film that shows high transparency

Fig. 2

Photographic images of the M-azo compounds that show photomelting under UV light. a, b M-azo compounds before (a) and after (b) UV irradiation (λ = 365 nm, 125 mW cm⁻²) at room temperature. c A solid-to-liquid phase transition can be observed in the UV-irradiated areas, whereas the regions not irradiated (covered from UV exposure by using a metal mask) retained the solid state. Scale bar, 5 mm. d Electron microscope image clearly revealing the boundary (dashed-dotted line) between the solid and liquid states in M-azo. Scale bar, 100 μm

Fig. 3

Structural characterization of DGI monomers and the polymer films. a DGI monomer revealing anisotropic aggregations with a liquid crystal texture under POM. b The patterns in the DGI liquid crystals look similar to those of a muscle-fiber structure under POM. Scale bar, 100 μm. c X-ray diffraction pattern of the free-standing polymer film at room temperature that shows a peak at about 2θ = 2.3°, corresponding to bilayer distances (d) of ~38 Å. d The molecular length of the DGI bilayer shown in the fully extended conformation is about 47 Å. The cross-section of the film was tilted at e 0° and f 45° angles relative to the transmission axis of the analyser under POM. The white bidirectional arrows on the top right represent the directions of the analyser (A) and polarizer (P). Scale bars, 30 μm

Fig. 4

Photoswitchable T_g, UV/Vis absorption spectra and photomechanical response of the film. a DSC measurements of T_g during the first heating process of the polymer in trans-, and cis-form. b Absorption spectra of a DGI/M-azo film before (red line) and after (blue line) irradiating with a light of λ = 365 nm for 4 s (light intensity = 100 mW cm⁻²). c Absorption spectra of a DGI/M-azo film before (red line) and after (blue line) irradiation using light of λ = 465 nm for 2 s (light intensity = 100 mW cm⁻²). d The relationship between the photoresponse speed of the fresh DGI/M-azo fiber and UV light intensity. e The continuous photomechanical response of the polymer film under light irradiation. The UV irradiation (λ = 365 nm, 11 mW cm⁻²) caused bending of the film; the visible light (λ = 465 nm, 30 mW cm⁻²) subsequently straightened the bent film. T_g-front, T_g value of the film on the front side; T_g-back, T_g value of the film on the backside