The photothermally responsive materials were able to be reprogrammed by adjusting the PCE as demonstrated in Fig. 1a. The materials could be encoded into various states by different types of masks and then showed different motions under illumination on account of the different PCE of each part. The high PCE part efficiently converted light into large amount of heat, but the unprogrammable parts were poor at generating heat. The material could also be erased back to its original state for the next program. Our reprogrammable photothermal responsive material also consisted of the actuation base material and the photothermal conversion material. The representative LCE materials prepared by acrylate mesogenic monomers 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene, commonly referred to as RM82, was selected as the actuation base material because of its huge thermal uniaxial contraction and stable preparation.30 Bismuth (III) compounds were chosen for use in the photothermal conversion material because of their fast switchable PCE and high photothermal property. And we found that the PCE of the bismuth compounds in LCE was capable of changing under UV illumination. As depicted in Fig. 1d, the traditional liquid crystal cell technique was used to prepare the LCE film. Moderate bismuth (III) neodecanoate was added into the precursor as the bismuth source during the preparation. The precursor was composed of the materials shown in Fig. 1c. After heat treatment, oligomer films were formed, which were then uniaxially stretched and UV crosslinked to form bismuth (III)-doped LCE films (Bi-LCE films). As observed in the cross section of Bi-LCE films (Fig. 1b), nano- to micrometer-sized particles were uniformly distributed in the films. To confirm the combination between LCE networks and bismuth compounds, Bi-LCE films were repeatedly soaked into methylbenzene. The resulting methylbenzene solution mixed with thiols was exposed to UV light. The solution remained transparent and colorless (Supplementary Fig. 1), indicating that no bismuth (III) ions or bismuth compounds leaked into the solution. It could be speculated that bismuth compounds were well encircled by LCE networks.
As the pictures photographed by orthogonal polarized optical microscopy (POM) (Fig. 1e), the Bi-LCE films were in a well uniaxial orientation at room temperature. The Bi-LCE film became softer than the pure LCE film due to the addition of bismuth compound dopants, potentially impeding the crosslinking of the networks. The larger molar ratio of functional groups of thiols between the di-functional flexible spacer (EDDET) and the tetra-functional crosslinking monomer (PETMP) resulted a looser network structure and a softer Bi-LCE film.31 The softer film presented larger shortage strain during thermal actuation. However, the film could not fully recover to the original shape when the functional group ratio exceeded 75:25. In order to balance the best mechanical and actuation property of the film, the functional group ratio of 75:25 of thiols between EDDET and PETMP was chosen to prepare the Bi-LCE film. The Young’s modulus of Bi-LCE films was 3.35 MPa, with the fracture strain of approximately 50% (Fig. 1f). According to the differential scanning calorimetry (DSC) diagram (Supplementary Fig. 2), the clearing point of the Bi-LCE film was 88℃, similar to the pure LCE films. Moreover, the Bi-LCE films showed approximately 45% shrinkage strain at 120℃ (Supplementary Fig. 3), which fed the requirement for actuation.
First, the photoactuation property of the Bi-LCE films in two different states was demonstrated. We designated the Bi-LCE films with high PCE as Bi-LCE-H films and the Bi-LCE films with low PCE as Bi-LCE-L films. In order to maximize the light power utilization, a device was set up as depicted in Fig. 2a. The infrared laser diode (LD) of 808 nm wavelength was used as the excited light source with a spot diameter of nearly 1 cm to drive the Bi-LCE films. Both ends of the Bi-LCE films were glued with tapes to facilitate weight hanging, so that planar motions could be executed rather than the three-dimensional motions caused by the asymmetric orientation produced during preparation.
As displayed in Fig. 2b and Supplementary movie 1–2, the Bi-LCE-L film contracted at a laser power of 1 W, and the shrinkage strain of the film increased along with increasing the light power. And the film reached the maximum shrinkage strain of less than 30% at the light power of 2.5 W. The response time, defined as the time when the film reached 85% of the maximum shrinkage strain, was about 15 seconds. Both sides of the Bi-LCE-L film were then exposed to UV to obtain the Bi-LCE-H film. The Bi-LCE-H film performed shrinkage of over 15% at the light power of 0.5 W and achieved the maximum shrinkage of more than 35% at the light power of 2 W, applying work of approximately 0.05 J by lifting the clamps 200 times its own weight. The smaller photothermal shrinkage compared to the thermal shrinkage might be caused by the light spot of incomplete coverage on the film and the inhomogeneous distribution of the light power. The response time was about 5 seconds, and the shrinkage strain and the response time promoted little with increasing the light power. All films recovered to their original shape within 5 to 8 seconds. The Bi-LCE-H films contracted much more sharply than the Bi-LCE-L films, clearly separating two different motion states of the films.
To quantify the photothermal conversion effect, an infrared sensor was set up to detect the surface temperature of the films. The films were irradiated continuously until they reached a steady temperature, followed by cooling to the ambient temperature after the laser was turned off. It could be seen in Fig. 2c that when the samples were irradiated with the infrared laser of 808 nm at the power of 0.757 W, the temperature of the Bi-LCE-H film increased from the ambient temperature of 22.6℃ to 77℃ at the linear rate of about 3℃ per second. The temperature reached a steady state and then slowly decreased. In contrast, the Bi-LCE-L film was heated warm (32.3℃) at the same light power. In addition, there was little temperature variation in the pure LCE film, indicating that the networks of LCE contributed little to the thermal conversion. Obviously, there was an enormous difference in the PCE between the Bi-LCE-H film and the Bi-LCE-L film. This was the direct reason for the different photothermal actuation performance of the Bi-LCE films.
We attempted to explore the mechanism underlying the switchable PCE in Bi-LCE films. It was well-known that bismuth (III) ions and thiols could readily react to form precipitates. Typically, bismuth-sulfur compounds were direct bandgap semiconductors.32 The bandgap of Bi-LCE-L film and the Bi-LCE-H film calculated by diffuse reflectance ultraviolet-visible near-infrared spectrum (Fig. 3a) was 3.08 eV and 2.23 eV, respectively. The narrower bandgap of the Bi-LCE-H film enabled it to absorb more photon energy to produce hot electrons and holes, facilitating heat generation through electron-phonon scattering and subsequent phonon emission processes. Figure 3b exhibited the XPS spectra of sulfur 2p and bismuth 4f for the Bi-LCE film before and after UV treatment. The peaks at around 164.0 eV and 158.9 eV before UV treatment were consistent with the binding energy of bismuth 4f5/2 and 4f7/2, similar to the value for Bi2S3.33 After UV irradiation, the two bismuth peaks shifted to 165.3 eV and 159.7 eV, close to the value for Bi2O3.34 This meant that the valence state of bismuth (III) stayed unchanged before and after UV treatment. In addition, two peaks at around 169.4 eV and 168.2 eV were detected in the Bi-LCE film, which were similar to the value for BaSO4 and K2SO4 contributing to the binding energy of sulfur 2p, respectively.35,36 The intensity of the two peaks became much stronger after UV treatment. Besides, one peak was observed weaker after UV treatment at about 164.7 eV, which was the binding energy of sulfur 2p equal to the value for Cucl2·2DMSO37 or C2H4S/Mo38. This indicated the presence of sulfoxide (-S = O-) or sulfur bridge (-S-) in the film. As seen in the Fourier transform infrared spectra shown in Fig. 3c, a sharp peak at 1071 cm− 1 on behalf of the ν(S = O) band indicated the presence of sulfoxide (-S = O-) groups in the films,39 whereas only peaks related to the sulfur atom were found at 1255 cm− 1and 694 cm− 1, corresponding to Bi-S40 or C-S41, respectively. That is, there were both sulfoxide (-S = O-) as well as a small amount of sulphone (-SO2-) bound to bismuth (III) ions in the original Bi-LCE film. When the film was exposed to UV light, most of the sulfoxide (-S = O-) was oxidized into sulphone (-SO2-) with bismuth (III) ion in the air.42 Therefore, the reversible PCE changes that happened in the Bi-LCE film could be speculated to the change in the sulfur functional groups, whose chemical structure could be expressed by the following reaction:
$$\left\{\begin{array}{c}{ (\text{R}-\text{S}\text{O}-\text{R}{\prime })}_{3}{\text{B}\text{i}}^{3+} \underrightarrow{UV, { O}_{2}}{ (\text{R}-\text{S}{\text{O}}_{2}- {\text{R}}^{{\prime }})}_{3}{\text{B}\text{i}}^{3+}\\ { (\text{R}-\text{S}{\text{O}}_{2}-\text{R}{\prime })}_{3}{\text{B}\text{i}}^{3+} \underrightarrow{heat, thoil}{ (\text{R}-\text{S}\text{O}- {\text{R}}^{{\prime }})}_{3}{\text{B}\text{i}}^{3+}\end{array}\right.$$
The sulfoxide (-S = O-) in bismuth (III) compounds could be oxidized to sulphone (-SO2-) under the UV treatment, which caused the film to have a narrower bandgap and rapidly generate heat. And sulphone (-SO2-) could be reduced back to the sulfoxide (-S = O-) by heating in the thiol solution, thus restoring the wider bandgap and reducing the heat generation. These reversible chemical reactions offered the convenience for the reprogrammable nature of the Bi-LCE film.
Next, we investigated the performance of the Bi-LCE-H films under repetitive cycling. The infrared light was turned on for 10 seconds for actuation and turned off for 10 seconds for recovery, with the light power of 2 W for each cycle. As shown in Fig. 4a and Supplementary movie 3, the photoresponsive shrinkage actuation was repeated ten times. But the shrinkage strain at the tenth time declined to 25% from the maximum strain of over 35%. The actuation decrease could be attributed to the heat generated by the infrared light source, which not only actuated the film but also caused it to recover to the original state. This phenomenon was due to the thermal instability of the sulphone (-SO2-) in bismuth (III) compounds, which provided opportunities for a rapid reprogrammable process. From the overall effect, the Bi-LCE films demonstrated a sufficient level of repeatability, achieving over 25% shrinkage strain, which was suitable for most applications.
Then, we evaluated the reprogrammable performance of the Bi-LCE films. The Bi-LCE-L film was taken as the original state, which was programmed to become the Bi-LCE-H film using both-side UV illumination with a 365-nm wavelength for 15 minutes. The programmed Bi-LCE-H film could be erased and recovered to the Bi-LCE-L film by heating it in 0.1 wt% EDDET/ethanol solution at 80 ℃ for 5 minutes. The entire reprogrammable procedure, including programming and erasing, took only 20 minutes to complete. The photoactuation performance of the Bi-LCE film was tested after the programming and erasing treatment. Figure 4b and Supplementary movie 4 exhibited that the Bi-LCE film could be reprogrammed for 10 times. The Bi-LCE-H film, which was programmed for the first time, shrank by about 30% strain. However, the shrinkage strain declined along with the increasing number of programmable times, which descended to over 15% after the tenth programmable procedure. This decline could be attributed to other byproducts irreversibly oxidized from the sulfoxide (-S = O-) in the film, which was unable to produce heat efficiently. Based on preliminary analysis, it could be speculated that the sulphone (-SO2-) products were produced with an approximate yield of 90%.42 In contrast, the Bi-LCE-H film could recover well to the Bi-LCE-L film. The Bi-LCE-L film maintained less than 5% shrinkage strain after ten times of erasure processes, which was similar to the original state. Although there was a slight decline in the photoactuation property after reprogramming processes, the film could reach the same performance by prolonging the exposure time or increasing the irradiation power.
The patterning method could be used to create complex deformation patterns with responsive light illumination. Therefore, the same Bi-LCE film could be reprogrammed to present diverse deformations under irradiation. The 1.1 mm * 0.6 mm Bi-LCE film was locally covered by three different photomasks to prevent the UV light from encoding the film. As shown in Fig. 5c, Fig. 5e, Fig. 5g, Fig. 5i and Supplementary movie 5–8, the encoded part of the film turned brown while the covered part remained light. The film was first encoded from the original state (Fig. 5a-5b) with a 1-mm strip (Fig. 5c), followed by a 5 mm * 6 mm area (Fig. 5e). Then, two squares with 2 mm sides were programmed in the center of the film (Fig. 5g) and finally three squares were encoded (Fig. 5i). A bandpass filter in the wavelength range of 510 ± 40 nm was used during photography to filter out the red light and to clarify the appearance of the film. Only the brown regions contracted along with the orientation under right above infrared illumination at the light power of 2.5 W. The encoded film under uniform infrared illumination showed different three-dimensional deformations in about ten seconds, involving bending, contracting and wrinkling in sequence, as shown in Fig. 5d, Fig. 5f, Fig. 5h and Supplementary movie 5–7. The film immediately regained its flatness after the light was turned off. The film was erased after the demonstration. Besides, the light power also influenced the shrinkage strain of the film, which also affected the final shape of the film. As demonstrated by Fig. 5j-5l and Supplementary movie 8, the film turned plicated under 1.5W-power infrared illumination whereas it transformed into saddle shape at the light power of 2.5 W. But when the light power was further increased etchas illustrated in Fig. 5l and Supplementary movie 8, the final actuation motion varied little because the coded parts reached their maximum shrinkage strain, corresponding to the former performance.
The reprogrammable process is not limited to the plane film. It can also be extended to other structures. For example, a hollow tube (Fig. 6a-6b) made of the Bi-LCE film could be patterned with simple UV treatment. The tube had a diameter of 6 mm and a length of 8 mm, and its orientation was vertical to the long side. The infrared illumination was also directly above the tube. Under irradiation, the top surface closer to the light source contracted more than the bottom surface, and the side part of the tube was transformed into a 5-mm diameter oval (Fig. 6c-6d and Supplementary movie 9) at the light power of 2.0 W. Then, the tube was reprogrammed to the configuration shown in Fig. 6e, and the cross-section of the tube was deformed into a flower shape (Fig. 6f and Supplementary movie 10) at the light power of 3.0 W. While it could not be observed sunk on the left side of the tube, possibly because the encoded part was deflected form the light source and absorbed less light power, resulting in a less pronounced shape change. Next, the tube was encoded again to the configuration shown in Fig. 6g, and it presented the rough surface of the tube (Fig. 6h and Supplementary movie 11) at the light power of 2.5 W. Therefore, the single tube was able to be reprogrammed to achieve different complicated shapes at once under infrared irradiation with appropriate light power, including the diameter adjustment, cross-sectional configuration, and surface morphology. The encoding and the erasing methods were the same as for the film mentioned above. It was difficult for other molding approaches to realize the complex deformation change. It would be easy to control the speed, quantity and state of the fluid by transforming the shapes of the tube in appropriate programs to the tube, which was highly desirable in microfluidics field.