Azobenzene-containing Liquid Crystalline Polymer Research Articles

Photo-Programmed Deformations in Rigid Liquid Crystalline Polymers Triggered by Body Temperature.

Deformations triggered by body heat are desirable in the context of shape-morphing applications because, under the majority of circumstances, the human body maintains a higher temperature than that of its surroundings. However, at present, this bioenergy-triggered action is primarily limited to soft polymeric networks. Thus, herein, the programming of body temperature-triggered deformations into rigid azobenzene-containing liquid crystalline polymers (azo-LCPs) with a glass-transition temperature of 100 °C is demonstrated. To achieve this, a mechano-assisted photo-programming strategy is used to create a metastable state with room-temperature stable residual stress, which is induced by the isomerization of azobenzene. The programmed rigid azo-LCP can undergo large-amplitude body temperature-triggered shape changes within minutes and can be regenerated without any performance degradation. By changing the programming photomasks and irradiation conditions employed, various 2D to 3D shape-morphing architectures, including folded clips, inch-worm structures, spiral structures, and snap-through motions are achieved. When programmed with polarized light, the proposed strategy results in domain-selective activation, generating designed characteristics in multi-domain azo-LCPs. The reported strategy is therefore expected to broaden the applications of azo-LCPs in the fields of biomedical and flexible microelectronic devices.

Supramolecular hydrogen bond enables Kapton nanofibers to reinforce liquid-crystalline polymers for light-fueled flight

We report fabrication of photoresponsive liquid-crystalline polymers reinforced with highly-oriented Kapton nanofibers with a supramolecular hydrogen bonding interface. To enhance the interfacial strength, hydroxyl groups are introduced into the side chain of azobenzene-containing liquid-crystalline polymers, forming hydrogen bonds with the Kapton nanofibers, directly imaged by nano-FTIR. Interestingly, the composite film exhibits the hierarchical structure of dragonfly wings, while demonstrating relatively high elastic modulus (1.64 GPa), reduced modulus (72.8 GPa), and nanohardness (4.5 GPa); 20–30 times higher than natural dragonfly wings. The enhanced mechanical performance and bilayer structure enables the composite film to exhibit rapid photoresponsive behaviors independent of the direction of illumination, due to an unconventional deformation mechanism arising from the interactions at the fiber-polymer interface. In addition, the flapping frequency and bending angle of the composite films can be continuously tuned for a single device (0.1–5 Hz, and 1.5–15.8°) by modifying the pulsed photoirradiation. The composite films are assembled into an artificial dragonfly device, and the light-driven flight aerodynamics are demonstrated in windy conditions. These not only provide a solution of micro-aircraft wings, but also offer a good bionic model for emulating dragonfly wings.

Hydrogen Bond Enhances Photomechanical Swing of Liquid-Crystalline Polymer Bilayer Films.

Mechanical swing is common in nature, such as sound waves, wingbeat of birds, and heartbeat, which is important to convert input energy into continuous motion. Here, we report a photodriven swing actuator composed of commercially available polyimide (Kapton) and azobenzene-containing liquid-crystalline polymers. The liquid-crystalline polymers act as the photoactive layer, which were synthesized by copolymerization of one benzenecarboxylic acid-containing monomer (M6BCOOH) and one azobenzene-containing monomer (M6ABOC2) with different molar ratios. The Kapton layer with a high elastic modulus is photoinert and functions as the substrate layer. After thermal annealing, the film displays chaotic swing under continuous irradiation of actinic light. Interestingly, the swing amplitude is greatly enhanced by the existence of supramolecular hydrogen bonding in liquid-crystalline polymer films. It is the introduction of M6BCOOH to the copolymer that accelerates the trans-cis photoisomerization rate of azobenzenes. Also, it forms a hydrogen bond as physical crosslinking sites, enabling the polymer film to work as a whole. Thus, it enhances the driving force for photomechanical deformation. Moreover, it improves the elastic modulus of the photoactive layer and modulates the swing behavior of the bilayer strip. More importantly, the formation of a hydrogen bond in the form of acidic dimers has a spatial confinement effect, extending the timescale of photodriven swing. The photomechanical self-vibration of the bilayer film can be ascribed to the combination of the photoisomerization process of azobenzenes with the local photosoftening effect of liquid-crystalline polymers.

Bending Behaviors of Azobenzene-containing Liquid Crystalline Polymers (AZ-LCP): Factors Influencing Bending Direction, Frequency and Degree

Liquid crystal polymers (LCP) containing azobenzene and its derivatives are fascinating and topics of intense scientific curiosities. As the addition of photo-responsive azobenzene, the LCP is exerted with the photochemical response, which shows fascinating and useful photo-induced motions and consequently enables a variety of applications in a lot of fields. In this review, we detail the macroscale mechanical motion of azobenzene containing liquid crystalline polymer (AZ-LCP) materials upon irradiation, with emphasis on the external and internal factors of bending behaviors of AZ-LCP, including bending direction, frequency and degree. We end with an outlook of challenging and competitive application as soft actuators.

Optimization of the photo-orientation rate of an azobenzene-containing polymer based on a kinetic model of photoinduced ordering.

The explicit dependence of the rate of photoinduced ordering (photo-orientation) of an azobenzene-containing liquid-crystalline polymer on the photostationary concentration of cis-azobenzene has been predicted theoretically and found experimentally. The employed kinetic model based on the photoinduced rearrangement of the domain structure of a liquid-crystalline material predicts the maximum rate of photo-orientation at ca. 50% content of the cis-isomer in the photostationary state of irradiation. For experimental fine tuning of the photostationary trans-cis ratio, the simultaneous irradiation of material with two beams of light with different wavelengths was employed. The excellent agreement of theory and experiment indicates that the difference of photostationary fractions of cis-azobenzene fragments in adjacent domains of different orientations is the driving force of photoinduced ordering.

A Bright Future for Liquid Functional Materials?

Liquid metals were merely sci-fi fantasy a few decades ago, playing a prominent role in the Terminator film franchise. Current research is leading to an array of emerging functional liquids. Here, we discuss the exciting possibilities of such fluid materials.

Multiscale modeling of photomechanical behavior of photo-responsive nanocomposite with carbon nanotubes

We propose a scale-bridging methodology to link the microscopic photoreaction of an azobenzene-containing liquid crystalline polymer (LCP) and the macroscopic interfacial and elastic properties of carbon nanotube (CNT)-reinforced photo-responsive nanocomposites. The photo-isomerization of the azobenzene moieties is described by implementing a photo-switching potential that represents the light-excited energy transition path. The relevant time evolution of the molecular shape and the concurrent changes in the interfacial morphology are observed using molecular dynamics (MD) simulations. Finally, the effective elastic properties of the photo-responsive polymer (PRP) nanocomposite with respect to the isomerization ratio are numerically derived using the micromechanics-based homogenization method. It is verified that the size of the CNT and the photo-deformation of the azobenzene molecules influence the intermolecular interactions and the nematic phase of the LCP at the interfacial region. The continuum-scale finite element (FE) model, which reflects the microscopic information, clearly predicts the reinforcing effect of the CNT filler on the elastic properties of the composite and their variation under photo-actuation. We expect our results to shed light on designing the photomechanical energy conversion efficiency of nano-sized soft actuators composed of CNT-reinforced composites.

Photo-modulations of surface wrinkles formed on elastomer sheets

ABSTRACTSurface wrinkle formation is ubiquitous in nature, and has become of great importance from the technological viewpoints of surface patterning and fabrications. To date, most of studies on the surface wrinkle formation have been achieved with surface thin films of fixed physical properties formed by deposition or lamination on elastomeric sheets such as poly(dimethyl siloxane) films. This article introduces our recent results on tunable surface wrinkle formation in response to light using azobenzene-containing liquid crystalline polymers. Two strategies are proposed: i) anisotropic wrinkle formation using a photoalignable radical initiator in UV-curable liquid crystal films, and ii) modulations of wrinkle formation using a series of azobenzene liquid crystalline polymers.

Photo-responsive polysiloxane-based azobenzene liquid crystalline polymers prepared by thiol-ene click chemistry

ABSTRACTIn this work, a novel polysiloxane-based azobenzene-containing liquid crystalline polymer (LCP) PMMS-A44V6 has been designed and synthesized via a facile thiol-ene click chemistry method by grafting a side-on azobenzene mesogenic group A44V6 onto poly-[3-mercaptopropylmethylsiloxane] (PMMS) backbone. 1H NMR, 13C NMR, GPC, TGA, DSC, POM and WAXS were used to investigate the mesogenic properties and photo-responsive behaviour of PMMS-A44V6. Taking advantage of the azobenzene’s trans–cis isomerization effect, PMMS-A44V6 can perform an isotropization process under UV irradiation in its nematic phase. Initially, the isotropization process starts in a linearly decreasing manner with a rate of ca −6.5 × 105 intensity/s, and eventually finishes in an exponential decrease regime to form cis-azobenzene moieties. The reversible UV-response behaviour of PMMS-A44V6 can be performed in a relatively low temperature range of 30 ~ 75°C, which might help this azobenzene-containing LCP material to find potential application in control devices.

Photoresponsive Surface Wrinkle Morphologies in Liquid Crystalline Polymer Films

Laminated bilayer films comprised of photoresponsive azobenzene-containing liquid crystalline polymer skin layers adhered on a thick elastomeric substrate were prepared, and the wrinkle formation upon uniaxial compression was investigated. Irradiation with UV light at 365 nm led to the disappearance of the wrinkle or reduction of wrinkle wavelength, depending on the content of azobenzene unit in the polymer. Such photoresponsive modulations of wrinkle formation could be well correlated with the photoinduced changes in the Young’s modulus determined by indentation–retraction force curve measurements using an AFM cantilever. Additionally, the thickness modulation of the skin layer caused by the photoinduced mass migration can also be applied to modify the wrinkle wavelength. These photoresponsive surface wrinkle modulations are anticipated to offer new possibilities for the surface microfabrication technology.

Photoalignment of Liquid Crystalline Polymer Films Containing Mesostructures and a Free Surface Command Layer

Investigations on the photoaglinment processes of liquid crystalline materials started around 25 years ago, and currently great developments have been made in both academic and industrial areas. However, there remain many issues to be explored in this field. This short review introduces our recent advances regarding the photoalignment and realignment process using azobenzene-containing liquid crystalline polymer materials. Two topics are involved here: the photoalignment of microphase separation structures of block copolymer films, and photoalignment controls from the free surface of a liquid crystalline polymer film.

Recent advances in photoresponsive liquid-crystalline polymers containing azobenzene chromophores

Famous for their photoisomerization, azobenzene and its derivatives have been intensively studied as among the most fascinating advanced materials. In particular, azobenzene-containing liquid-crystalline polymer (LCP) materials show unique properties by combining the self-assembly of liquid crystals and photoresponsive performance of chromophores. Here, we highlight their intriguing properties and potential applications from photonics to photodriven motion as well as the novel nanotechnology. The photoresponsive features such as photochemical phase transition, photoinduced alignment and photo-triggered cooperative motion often result in a large modulation of the refractive index, which can be easily fixed in LCP films. This is very advantageous for their photonic applications. Upon forming connections by three-dimensional crosslinking, a large deformation can be photoinduced from the micro to the macro scale, enabling applications as photomechanical and photomobile materials to be found. Upon integrating with the microphase separation of well-defined block copolymers, they exhibit photocontrollable regular nanostructures on the macroscopic scale with excellent reproducibility and mass production, meaning they can be used as nanotemplates for nanoengineering and nanofabrication.

ESR and Optical Study of Photo-Orientation in Azobenzene-Containing Liquid-Crystalline Polymer

We report a comprehensive experimental study of photoinduced processes in azobenzene-containing liquid-crystalline polymer. The kinetic and optical characteristics of the photoisomerization reaction and photo-orientation process have been determined. The spin probe technique has been used to elucidate the details of orientation order and rotational mobility of molecular fragments within the liquid-crystalline polymer. The evolution of orientation order parameters <P2>, <P4>, and <P6> in the course of photoinduced ordering of the material as well as the change of liquid-crystalline domain sizes have been studied quantitatively. The obtained experimental results are compared with the predictions of the existing models of photo-orientation.

The Alignment of Carbon Nanotubes: An Effective Route To Extend Their Excellent Properties to Macroscopic Scale

To improve the practical application of carbon nanotubes, it is critically important to extend their physical properties from the nanoscale to the macroscopic scale. Recently, chemists aligned continuous multiwalled carbon nanotube (MWCNT) sheets and fibers to produce materials with high mechanical strength and electrical conductivity. This provided an important clue to the use of MWCNTs at macroscopic scale. Researchers have made multiple efforts to optimize this aligned structure and improve the properties of MWCNT sheets and fibers. In this Account, we briefly highlight the new synthetic methods and promising applications of aligned MWCNTs for organic optoelectronic materials and devices. We describe several general methods to prepare both horizontally and perpendicularly aligned MWCNT/polymer composite films, through an easy solution or melting process. The composite films exhibit the combined properties of being flexible, transparent, and electrically conductive. These advances may pave the way to new flexible substrates for organic solar cells, sensing devices, and other related applications. Similarly, we discuss the synthesis of aligned MWCNT/polymer composite fibers with interesting mechanical and electrical properties. Through these methods, we can incorporate a wide variety of soluble or fusible polymers for such composite films and fibers. In addition, we can later introduce functional polymers with conjugated backbones or side chains to improve the properties of these composite materials. In particular, cooperative interactions between aligned MWCNTs and polymers can produce novel properties that do not occur individually. Common examples of this are two types of responsive polymers, photodeformable azobenzene-containing liquid crystalline polymer and chromatic polydiacetylene. Aligning the structure of MWCNTs induces the orientation of azobenzene-containing mesogens, and produces photodeformable polymer elastomers. This strategy also solves the long-standing problems from the traditional mechanical rubbing method, which include production of broken debris and structure damage during fabrication and building up electrostatic charge during use. Aligning MWCNTs induces a conformational change in polydiacetylene, which causes the composite fibers to be electrochromatic, a previously unknown reaction in chromatic polymers. Due to their large surface area, flexibility, electrical conductivity, and remarkable electrocatalytic activity, aligned MWCNT films can be used as counter electrodes to produce highly efficient dye-sensitized solar cells. In addition, chemists have developed new electrodes from the aligned MWCNT fibers to make a family of high-performing, wire-shaped dye-sensitized solar cells.

Novel Generation of Liquid Crystalline Photo‐Actuators Based on Stretched Porous Polyethylene Films

The preparation of photo-actuators based on stretched porous polyethylene and an azobenzene-containing liquid crystalline polymer network is reported for the first time. It is revealed that this kind of photo-actuator possesses the following advantages: the lack of a need for using aligning coatings and cells preparation, high deformation of the actuator and its complete reversibility, good mechanical properties, and relatively low cost of fabrication. In addition some kinetic and thermodynamic features of the bending and unbending processes have been studied.

Simultaneous Analysis of Optical and Mechanical Properties of Cross-Linked Azobenzene-Containing Liquid-Crystalline Polymer Films

The photomechanical behavior of cross-linked azobenzene-containing liquid-crystalline polymer films was investigated by means of simultaneous measurement of their optical and mechanical properties. The connection between photoisomerization of the azobenzene moieties, photoinduced change in molecular alignment, photoinduced stress generation, and macroscopic bending was analyzed. Upon UV irradiation, the films exhibited bending due to gradient in cis-azobenzene content, and subsequent unbending when cis-azobenzene content became uniform throughout the film. The maximum photoinduced stress was generated in the same time scale as the time required to reach photostationary state in the cis-azobenzene concentration. The maximum values of photogenerated stress strongly depended on the crosslinker concentration, even if the azobenzene concentration and the cis-azobenzene content in the photostationary state were similar for all the polymer films. The stress is connected to the initial Young's modulus and also to the photoinduced change in birefringence of the polymer films. In addition, a significant photoinduced decrease in Young's modulus was for the first time observed in cross-linked azobenzene-containing liquid-crystalline polymers, which is likely to be an important factor in dictating their photomechanical behavior.

Crown-ether and azobenzene-containing liquid crystalline polymers: An influence of macromolecular architecture on optical properties and photo-orientation processes

A series of novel crown ether-containing photochromic comb-shaped liquid crystalline polyacrylates with different macromolecular structure of side groups were synthesized and investigated. Phase behavior, optical and photo-optical properties of thin spin-coated films of these polymers were studied. A special attention was paid to a comparative study of the photo-orientation phenomena occurring in the polymer films under a polarized light action. It was shown that complex formation with the potassium ions results in the decrease in degree of the photoinduced order that can be used for the creation of new materials for sensor devices. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Macromol. Rapid Commun. 20/2009

Front Cover: Upon irradiation of a linearly polarized laser beam, precise control of photoinduced birefringence is obtained in azobenzene-containing liquid-crystalline polymers by adjusting the azotolane content in the post-functionalization reaction. Further details can be found in the article by H. Yu,* T. Kobayashi, and Z. Ge on page 1725.

Photoresponsive Liquid Crystals for Tuning and Aligning Nanostructures

The photoalignment of liquid crystalline azobenzene-containing polymers has been an active area of research, which intends to apply for optical elements such as rewritable optical memory, photorefractive and photomechanical devices. However, the photoalignment behavior is hardly studied in nano-confined environments. We introduce herein our current activities on the new types of photoalignment controls of azobenzene-containing liquid crystalline polymer materials; (i) polymer chains tethered (grafted) onto a substrate and (ii) nanostructures formed by a block copolymer. New platforms for the photoalignment technologies are anticipated in these photoactive thin film systems.

Photoinduced Alignment and Multi-Processes of Refractive-Index Gratings in Pre-Irradiated Films of an Azobenzene-Containing Liquid-Crystalline Polymer

After being pre-treated upon irradiation of unpolarized light at 366 nm, an azobenzene-containing liquid-crystalline polymer film was photoinduced alignment by one linearly polarized laser beam at 488 nm, leading to a large birefringence of 0.21. Then holographic gratings were recorded in the pre-irradiated polymer films by interference of two coherent laser beams. Little surface relief was obtained by the holographic recording process, indicating that a refractive-index grating paid most of contributions to the diffraction efficiency. Because the cooperative motion of the azobenzene mesogens were eliminated in the cis-azobenzene-rich isotropic film by the pre-irradiation, multi-processes of the refractive-index gratings were successfully achieved. The obtained grating structures were clearly shown in the polarizing optical microscopic pictures, which were also verified by their diffraction patterns. The azobenzene-containing liquid-crystalline polymer used in this study has potential application in high-density information storage.