Two types of novel reactive linear liquid crystal polymers (LLCPs) with different azotolene concentrations have been synthesized and processed into films and fibers by solution and melting processing methods. Then, the LLCPs in the obtained monodomain fiber and polydomain film were easily cross-linked with difunctional primary amines. The resulted cross-linked liquid crystal polymers (CLCPs) underwent reversible photoinduced bending and unbending behaviors in response to 445 and 530 nm visible light at room temperature, respectively. The post-cross-linking method provides a facile way to prepare the CLCP films and fibers with different shapes from LLCPs, which can be processed by traditional melting and solution methods.Keywords: azotolene; good processability; linear polymer; liquid crystal; photoinduced deformation;

Crosslinked liquid crystal polymers (CLCPs) containing azobenzene mesogens have been developed as stimuli-responsive materials, which can undergo photodeformation and thus convert light energy into mechanical force. The deformation behavior of CLCPs is strongly influenced by the alignment of the mesogens; however, a precise control of the alignment domain at micro-scale is still a challenge. Here we report complex molecular alignment in the CLCP film by using photoalignment technology. First, azo dye SD1 is aligned in-plane by UV light with a discrete alternating striped director profile. The SD1 molecules in adjacent strips are aligned orthogonal, and the widths of the strips are controlled in several hundred micrometers by a photomask with grating patterns. Then the liquid crystal molecules in the CLCP film are aligned by SD1 through the anchoring effect on one side (SD1 side), and aligned perpendicular by the polyimide (PI) alignment layer on the other side (PI side). With these alignments, two kinds of splayed structures are formed through the depth of the film. When irradiated by UV light, the film bends toward the SD1 side with the bending direction along the diagonal of the film, determined by the resultant direction of molecular alignment on the SD1 side. When irradiated by blue light and heat, the bending direction is along the edge of the film. This dual-responsive deformable film with complex alignment is anticipated to be used in shape-changing biomedical devices, multiple controllable switches, and microactuators.

Cross-linked liquid-crystalline polymers containing azobenzene have been drawing great research interest of scientists due to their significant values in the design of light-driven soft actuators. However, poor processabilities due to the chemical cross-linking severely prevent their practical applications. Here, we designed and synthesized a novel reactive block copolymer PEO-b-PAZO bearing N-hydroxysuccinimide carboxylate substituted azobenzene groups. This copolymer is compatible with the traditional polymer processing methodology including melt and solution processing methods, providing a facile way to prepare the photoresponsive films and fibers, and enabling post-crosslinking after processing through a mild reaction between N-hydroxysuccinimide carboxylate substituents and a difunctional amine. Most importantly, the prepared fibers and films showed reversible photodeformation regulated by alternating irradiation of UV and visible light (365 nm/470 nm). We envision that this novel liquid crystalline polymer will open up new possibilities for low-cost fabrication of high performance photo-driven actuators on a large scale.

Photoresponsive cross-linked liquid crystal polymer (CLCP) films with different surface topographies, submicropillar arrays, and submicrocone arrays were fabricated through colloidal lithography technique by modulating different types of etching masks. The prepared submicropillar arrays were uniform with an average pillar diameter of 250 nm and the cone bottom diameter of the submicrocone arrays was about 400 nm, which are much smaller than previously reported CLCP micropillars. More interestingly, these two species of films with the same chemical structure represented completely different wetting behavior of water adhesion and mimicked rose petal and lotus leaf, respectively. Both the submicropillar arrayed film and the submicrocone arrayed film exhibited superhyrophobicity with a water contact angle (CA) value of 144.0 ± 1.7° and 156.4 ± 1.2°, respectively. Meanwhile, the former demonstrated a very high sliding angle (SA) greater than 90°, and thus, the water droplet was pinned on the surface as rose petal. On the contrary, the SA of the submicrocone arrayed CLCP film consisting of micro- and nanostructure was only 3.1 ± 2.0°, which is as low as that of lotus leaf. Furthermore, the change on the wettability of the films was also investigated under alternating irradiation of visible light with two different wavelengths, blue light and green light.Keywords: colloidal lithography; liquid crystal polymer; submicrocone; submicropillar; wettability

Novel photo and thermal dual-responsive inverse opal films were fabricated based on a crosslinked liquid crystal polymer (CLCP) containing azobenzene. Accompanying the deformation of the CLCP, switchable behavior on the reflection spectra of the inverse opal film was induced by light or temperature. We found that the optical properties were drastically decreased by thermal or photoinduced phase transitions of the CLCP. It was also found that the extent of decrease in the heat-induced inverse opal film is much more than in the one induced by light.

A red-light-controllable soft actuator has been achieved, driven by low-power excited triplet–triplet annihilation-based upconversion luminescence (TTA-UCL). First, a red-to-blue TTA-based upconversion system with a high absolute quantum yield of 9.3 ± 0.5% was prepared by utilizing platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP) as the sensitizer and 9,10-bis(diphenylphosphoryl)anthracene (BDPPA) as the annihilator. In order to be employed as a highly effective phototrigger of photodeformable cross-linked liquid-crystal polymers (CLCPs), the PtTPBP&BDPPA system was incorporated into a rubbery polyurethane film and then assembled with an azotolane-containing CLCP film. The generating assembly film bent toward the light source when irradiated with a 635 nm laser at low power density of 200 mW cm–2 because the TTA-UCL was effectively utilized by the azotolane moieties in the CLCP film, inducing their trans–cis photoisomerization and an alignment change of the mesogens via an emission–reabsorption process. It is the first example of a soft actuator in which the TTA-UCL is trapped and utilized to create photomechanical effect. Such advantages of using this novel red-light-controllable soft actuator in potential biological applications have also been demonstrated as negligible thermal effect and its excellent penetration ability into tissues. This work not only provides a novel photomanipulated soft actuation material system based on the TTA-UCL technology but also introduces a new technological application of the TTA-based upconversion system in photonic devices.

A micro-arrayed liquid crystal polymer film, cross-linked in the nematic phase, was successfully prepared from an optimized azobenzene precursor and polydimethylsiloxane-soft-template-based secondary replication. Regulated by alternating irradiation of UV-visible light (365 nm/530 nm), the micro-arrayed film showed an ideal quick (<1 min) and reversible switch of superhydrophobic adhesion.

Both polymer gels and crosslinked liquid-crystalline polymers (CLCPs) have unique properties to be developed as stimuli-responsive soft materials. While the former are suitable to be used in a wet circumstance, the latter are able to be applied as dry actuation materials. Among various stimulating sources, light has attracted more interest because it is a kind of clean energy which can be manipulated conveniently and controlled in situ. Photodeformable polymer gels and CLCPs undergo macroscopic shape changes such as contraction and bending induced by light, thus providing a photomechanical effect. This review will describe the advances in the field of photomanipulation regarding both the polymer gels and the CLCPs including the design of these smart materials, controlled actuation in response to light, and ability to tailor the materials properties for different applications.

When upconversion nanophosphors were incorporated into an azotolane-containing cross-linked liquid-crystal polymer film, the resulting composite film generated fast bending upon exposure to continuous-wave near-IR light at 980 nm. This occurs because the upconversion luminescence of the nanophosphors leads to trans–cis photoisomerization of the azotolane units and an alignment change of the mesogens. The bent film completely reverted to the initial flat state after the light source was removed.

An acrylate monomer and diacrylate crosslinker containing a spacer of undecylene were synthesized and crosslinked liquid-crystalline polymer films with different crosslinking densities were prepared by photopolymerization of mixtures of the monomer and the crosslinker. Due to the relatively long spacer, distinct bending behavior was observed in the film with a low crosslinking density: when the temperature was lower than 90 °C, the film first bent away from the light source and then towards it upon the irradiation of UV light. As the temperature was raised above 90 °C, the film bent directly towards the light source. X-ray diffraction was used to probe the phase structures of the film at different temperatures and a plausible mechanism was presented to explain the distinct phenomenon. In addition, when the crosslinking density was increased, all the films bent directly towards the light source with faster speed, which is related to their modulus and order parameters.

Monodomain crosslinked liquid-crystalline polymer (CLCP) films with three crosslinking densities were prepared by thermal polymerization of the mixtures of mono and diacrylates, both of which contain azotolane chromophores. Thermodynamic properties of the monomer, the crosslinker, their mixtures and the CLCP films were determined by differential scanning calorimetry. Mesomorphic properties were studied using a polarizing optical microscope and a polarized UV-vis absorption spectrometer. Due to a longer conjugated structure of the azotolane moieties in side chains, free-standing CLCP films underwent photoinduced bending upon exposure to short-wavelength visible light at 436 nm and the bent films returned to their initial states by alternating the wavelength of the actinic visible light to 577 nm. The bending could also be induced by 530 nm visible light though the bending speed decreased with the increasing of the wavelength of the actinic light. It was also observed that the bending speed of the films was related to the order parameter, the light intensity and the temperature. The mechanical force of the films generated upon exposure to visible light at 436 nm increased with the increasing of the crosslinking density and the light intensity but decreased with the increment of the temperature.

Visible-light-driven fully plastic microrobots were successfully prepared from crosslinked liquid-crystalline polymer and polyethylene bilayer films.

Photoinduced bending and unbending behavior of crosslinked liquid-crystalline polymers containing azotolane moieties in side chains occurred upon irradiation with sunlight, according to the trans–cisphotoisomerization of the azotolane moieties.

Photoinduced bending and unbending behavior of crosslinked liquid-crystalline polymers containing azotolane moieties in side chains occurred upon irradiation with sunlight, according to the trans–cisphotoisomerization of the azotolane moieties.

Novel photo and thermal dual-responsive inverse opal films were fabricated based on a crosslinked liquid crystal polymer (CLCP) containing azobenzene. Accompanying the deformation of the CLCP, switchable behavior on the reflection spectra of the inverse opal film was induced by light or temperature. We found that the optical properties were drastically decreased by thermal or photoinduced phase transitions of the CLCP. It was also found that the extent of decrease in the heat-induced inverse opal film is much more than in the one induced by light.

Cross-linked liquid-crystalline polymers containing azobenzene have been drawing great research interest of scientists due to their significant values in the design of light-driven soft actuators. However, poor processabilities due to the chemical cross-linking severely prevent their practical applications. Here, we designed and synthesized a novel reactive block copolymer PEO-b-PAZO bearing N-hydroxysuccinimide carboxylate substituted azobenzene groups. This copolymer is compatible with the traditional polymer processing methodology including melt and solution processing methods, providing a facile way to prepare the photoresponsive films and fibers, and enabling post-crosslinking after processing through a mild reaction between N-hydroxysuccinimide carboxylate substituents and a difunctional amine. Most importantly, the prepared fibers and films showed reversible photodeformation regulated by alternating irradiation of UV and visible light (365 nm/470 nm). We envision that this novel liquid crystalline polymer will open up new possibilities for low-cost fabrication of high performance photo-driven actuators on a large scale.

An acrylate monomer and diacrylate crosslinker containing a spacer of undecylene were synthesized and crosslinked liquid-crystalline polymer films with different crosslinking densities were prepared by photopolymerization of mixtures of the monomer and the crosslinker. Due to the relatively long spacer, distinct bending behavior was observed in the film with a low crosslinking density: when the temperature was lower than 90 °C, the film first bent away from the light source and then towards it upon the irradiation of UV light. As the temperature was raised above 90 °C, the film bent directly towards the light source. X-ray diffraction was used to probe the phase structures of the film at different temperatures and a plausible mechanism was presented to explain the distinct phenomenon. In addition, when the crosslinking density was increased, all the films bent directly towards the light source with faster speed, which is related to their modulus and order parameters.

Three kinds of novel planar chiral azobenzenophane derivatives for controlling the helical structures of cholesteric liquid crystals were synthesized and found to exhibit excellent chiroptical properties in both chloroform and liquid crystal media. The results of doping experiments showed that all the chiral switches possess high helical twisting power (HTP) values in nematic liquid crystal hosts (>80 μm−1) so that they can effectively transfer the chirality to the host LC medium resulting in a helical supramolecular structure even at very low doping concentrations (<1.0 wt%). In addition, the dependency of function-structure on various cyclic azobenzenophane scaffolds and flexible chains was also discussed in detail. The results show that the introduction of ordered segments and flexible long alkyloxy chains in the dopants can increase the cholesteric induction ability of dopants effectively. And the screw direction of the formed chiral nematic liquid crystal was dependent upon the planar chiral configuration of the molecular switch. Moreover, in this mixed system, reversible tuning between orange and green reflection colors were achieved upon irradiation with alternating UV/Vis light.

Monodomain crosslinked liquid-crystalline polymer (CLCP) films with three crosslinking densities were prepared by thermal polymerization of the mixtures of mono and diacrylates, both of which contain azotolane chromophores. Thermodynamic properties of the monomer, the crosslinker, their mixtures and the CLCP films were determined by differential scanning calorimetry. Mesomorphic properties were studied using a polarizing optical microscope and a polarized UV-vis absorption spectrometer. Due to a longer conjugated structure of the azotolane moieties in side chains, free-standing CLCP films underwent photoinduced bending upon exposure to short-wavelength visible light at 436 nm and the bent films returned to their initial states by alternating the wavelength of the actinic visible light to 577 nm. The bending could also be induced by 530 nm visible light though the bending speed decreased with the increasing of the wavelength of the actinic light. It was also observed that the bending speed of the films was related to the order parameter, the light intensity and the temperature. The mechanical force of the films generated upon exposure to visible light at 436 nm increased with the increasing of the crosslinking density and the light intensity but decreased with the increment of the temperature.