Crosslinked Liquid-crystalline Polymers Research Articles

Circularly polarized organic room temperature phosphorescence activated by liquid crystalline polymer networks

Circularly polarized organic room temperature phosphorescence (CPRTP) withgmax= +0.098 was activated by the rigid environment of anin situcross-linked liquid crystalline polymer network.

Temperature-Dependent Gas Transport Behavior in Cross-Linked Liquid Crystalline Polyacrylate Membranes

Stable, cross-linked, liquid crystalline polymer (LCP) films for membrane separation applications have been fabricated from the mesogenic monomer 11-(4-cyanobiphenyl-4′-yloxy) undecyl methacrylate (CNBPh), non-mesogenic monomer 2-ethylhexyl acrylate (2-EHA), and cross-linker ethylene glycol dimethacrylate (EGDMA) using an in-situ free radical polymerization technique with UV initiation. The phase behavior of the LCP membranes was characterized using differential scanning calorimetry (DSC) and X-ray scattering, and indicated the formation of a nematic liquid crystalline (LC) phase above the glass transition temperature. The single gas transport behavior of CO2, CH4, propane, and propylene in the cross-linked LCP membranes was investigated for a range of temperatures in the LC mesophase and the isotropic phase. Solubility of the gases was dependent not only on the condensability in the LC mesophase, but also on favorable molecular interactions of penetrant gas molecules exhibiting a charge separation, such as CO2 and propylene, with the ordered polar mesogenic side chains of the LCP. Selectivities for various gas pairs generally decreased with increasing temperature and were discontinuous across the nematic–sotropic transition. Sorption behavior of CO2 and propylene exhibited a significant change due to a decrease in favorable intermolecular interactions in the disordered isotropic phase. Higher cross-link densities in the membrane generally led to decreased selectivity at low temperatures when the main chain motion was limited by the lack of mesogen mobility in the ordered nematic phase. However, at higher temperatures, increasing the cross-link density increased selectivity as the cross-links acted to limit chain mobility. Mixed gas permeation measurements for propylene and propane showed close agreement with the results of the single gas permeation experiments.

Development of novel network structures in crosslinked liquid-crystalline polymers

Crosslinked liquid-crystalline polymers (CLCPs) show macroscopic deformation along with a change in molecular order, which is triggered by various stimuli such as heat, electricity, and light. CLCPs containing photochromic moieties are realizable as photoresponsive soft actuators. This review focuses on recent developments in novel network structures of CLCPs: rearrangeable networks and interpenetrating polymer networks (IPNs). CLCPs with dynamic covalent bonds could be reshaped into 3D architectures through the rearrangement of the network topology, in contrast with conventional crosslinked polymers memorizing permanent shapes. The reshaped samples showed various photoinduced motions depending on their initial shapes. Furthermore, amorphous polymers were incorporated into conventional CLCP networks to control mechanical and photoresponsive properties. The sequential formation of CLCP and amorphous polymer networks resulted in IPN films with a homogeneous alignment of mesogens. The elastic moduli of IPN films were controlled through the selection of amorphous components. The incorporation of soft components such as poly(dodecyl methacrylate) and poly(dimethylsiloxane) (PDMS) significantly enhanced the rate of photoinduced bending. These strategies of controlling the network structures of CLCPs could enable the versatile design of photomobile polymer materials as soft actuators with desired architectures and functions. Novel network structures were applied to crosslinked liquid-crystalline polymers (CLCPs) with photomechanical effects to enhance their performance and function. CLCPs with rearrangeable networks could be reshaped into 3D architectures, in contrast with conventional crosslinked polymers memorizing permanent shapes. The reshaped samples showed various photoinduced motions depending on their initial shapes. Furthermore, interpenetrating polymer networks (IPNs) were developed to improve mechanical and photoresponsive properties of CLCPs. These strategies of controlling the network structures of CLCPs could enable the versatile design of photomobile polymer materials as soft actuators.

Photo Actuation Performance of Nanotube Sheet Incorporated Azobenzene Crosslinked Liquid Crystalline Polymer Nanocomposite.

Crosslinked liquid crystalline polymers (CLCPs) containing azobenzene (AZO-CLCPs) are a type of promising material due to their significance in the design of light-driven smart actuators. Developing AZO-CLCP composites by incorporating AZO-CLCPs with other materials is an effective way of enhancing their practicability. Herein, we report an AZO-CLCP/CNT nanocomposite prepared by the in situ polymerization of diacrylates containing azobenzene chromophores on carbon nanotube (CNT) sheets. The liquid crystal phase structure of CLCP matrix was evidenced by the two-dimensional X-ray scattering. The prepared pure AZO-CLCP films and AZO-CLCP/CNT nanocomposite films demonstrated strong reversible photo-triggered deformation under the irradiation of UV light at 366 nm of wavelength, as a result of photo-induced isomerization of azobenzene moieties in the polymer network. But compared to pure AZO-CLCP films, the AZO-CLCP/CNT nanocomposite films could much more rapidly return to their initial shapes after the UV light irradiation was removed due to the elasticity effect of CNT sheets. The deformation behavior of AZO-CLCP/CNT nanocomposite films under the light irradiation was also different from that of the pure AZO-CLCP films due to the interfacial interaction between a polymer network and CNT sheet. Furthermore, incorporation of a CNT sheet remarkably increased the mechanical strength and robustness of the material. We also used this AZO-CLCP/CNT nanocomposite as a microvalve membrane actuator, which can be controlled by light, for a conceptual device of a microfluidic system. The results showed that this AZO-CLCP/CNT nanocomposite may have great potential in smart actuator applications for biological engineering, medical treatment, environment detection and microelectromechanical systems (MEMS), etc.

A Cut-and-Weld Process to 3D Architectures from Multiresponsive Crosslinked Liquid Crystalline Polymers.

Crosslinked liquid crystalline polymers (CLCPs) have garnered extensive attention in recent years for their significant values in the design of light-driven soft actuators. However, poor processabilities due to the insoluble and infusible crosslinked networks prevent their practical applications severely. In this study, a weldable azobenzene-containing CLCP is designed with photo- and humidity-responsive actuations, which enables a cut-and-weld process to 3D CLCP architectures. The tensile properties and stability are almost unchanged after welding, much better than those of the films pasted by common adhesive tapes. Meanwhile, the mechanisms of the welding process are clarified on the base of surface hydrogen bonding and further crosslinking. By taking advantage of the cut-and-weld process, a 3D "claw" integrated into a robotic arm is realized for grabbing millimeter-scale objects by remote control. This work enhances significantly not only the processability of CLCP films but also the utilization of leftover pieces, which provides an efficient approach to create functional 3D structures from film precursors for the potential application in the smart materials.

Multiscale Study of the Relationship between Photoisomerization and Mechanical Behavior of Azo-Polymer Based on the Coarse-Grained Molecular Dynamics Simulation

Cross-linked liquid crystalline polymers (CLCPs) incorporated with photochromic azobenzene moieties can exhibit large and reversible deformations under the exposure of actinic lights. To practicall...

Photoinduced deformation and isomerization of azobenzene liquid-crystalline polymer films at cryogenic temperature

We prepared 2 types of crosslinked liquid-crystalline polymers (CLCPs) (polyacrylates) with the same side-chain length but a different position of an azobenzene moiety in the side chain. We evaluated photoinduced deformation behavior of the CLCP films and photoisomerization behavior of azobenzene in the films at cryogenic temperature. Azobenzene moieties in both CLCP films showed no trans-cis isomerization and neither film exhibited bending behavior upon irradiation with UV light in liquid nitrogen [77 K]. On the other hand, when the film included azobenzene with a long spacer and a short tail, cis-trans isomerization of azobenzene and photoinduced deformation were observed in liquid nitrogen.

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

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 (Tg) 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 Tg of the crosslinked LC polymers. The Tg 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 Tg contribute to the photomechanical bending and a new mechanism for photomechanical bending that attributes the process to an inhomogeneous change in Tg of the film is proposed.

Opto-mechanical behavior and interfacial characteristics of crosslinked liquid crystalline polymer composites with carbon nanotube fillers

Azobenzene-containing crosslinked liquid crystalline polymer (CLCP) exhibits macroscopic deformation through irradiation with light, and recently, carbon nanotubes (CNTs) were inserted to improve the efficiency of the photo-mechanical energy conversion. We conducted molecular dynamics simulations to study the effects of the arrangement of the single-walled CNT (SWCNT) and photo-isomerization of the photochromic moieties on the opto-mechanical properties of the composite. We characterized the composites in terms of the interfacial shear strength (IFSS), photostrain, and elastic modulus according to the isomerization ratio. The SWCNT-CLCP composite enables 19–29% larger photostrain and 20–41% higher modulus compared to those of neat azo-PRP. As the angle between the CNT axis and the nematic director increases, mechanical reinforcing effect decreases. Nevertheless, the applicable photo-shrinkage increases because more space for polymer diffusion is made by the nematic defects in the interphase. The isomerization of azobenzene weakens the interfacial adhesion due to the decrease in the π-π interaction. However, changes in the elastic stiffness of the composite can be modulated by adjusting the arrangement between the SWCNT and matrix. These results provide insight into the mechanism of changes in the interfacial and opto-mechanical properties of the CNT-PRP composite caused by photo-isomerization and the mechanical design of a photo-responsive actuator.

Light-Induced Bending and Buckling of Large-Deflected Liquid Crystalline Polymer Plates

Cross-linked liquid crystalline polymers (LCPs) are smart materials for large light-activated variation or bend to transfer luminous energy into mechanical energy. We study the light-induced behavior of homeotropic nematic network polymer plates. The perturbation method is applied to find approximate solutions under uniform illumination and compared with finite element simulations. Moreover, situations of nonuniform laser illumination are investigated. Unlike single solution obtained from small displacement assumption, multiple solutions within Föppl–von Kármán nonlinear geometry framework are found in the post-buckling regime and the effect of various boundary conditions is discussed. The nonuniform bending moment generated by the inhomogeneous light-induced strain, membrane force and boundary effects lead to the unconventional nonsymmetric buckling behavior that is rarely observed under traditional mechanical or thermal loading.

Photoinduced deformation of crosslinked liquid crystalline polymers and soft actuators

Photoresponsive polymeric materials are a type of functional materials that can absorb photo energy and undergo intra- or inter-molecular physical or chemical reactions. As a result, the materials exhibit macroscopic property changes when exposed to light, such as shape, color or refractive index. They attract more and more attention for the reason that light is an environmental-friendly and remotely controllable stimulus source. Liquid crystals exhibit cooperative motion of molecules in changing the molecular alignment by external stimuli. That means that only a small amount of energy is needed to bring about the alignment change in the whole system. Crosslinked liquid crystalline polymers (CLCPs) can be prepared by crossliking conventional liquid crystal polymers into a network, which possess both the order of liquid crystals and the elasticity of elastomers. The cooperative effect of liquid crystals coupled with polymer network structure gives rise to the characteristic properties of CLCPs. The initial ordering of mesogens can be fixed by the crosslinkers in CLCPs, which might cause a quick change in shape due to a fast order-disorder transition. CLCPs have been a hot topic because large deformation can be induced by changing the alignment of mesogens in CLCPs by external stimuli such as electric fields, changes in temperature, humidity and light. Incorporation of photochromic moieties into CLCPs provides the materials with photo controllable properties. Photoresponsive CLCPs can generate sophisticated movements including contraction/expansion, bending and twisting which convert photo energy into mechanical energy directly. Therefore, the CLCPs can be fabricated into various soft smart actuators by rational design, which brings about broad applications in artificial muscles, microrobots, microvalves, sensors and so on. In this review, the photoresponsive mechanism is firstly elucidated involving photochemical phase transition and photo realignment. Then, we detail the historical development of photoresponsive CLCPs, with emphasis on light- driven mechanical responses concerning two dimensional and three dimensional movements and surface property changes caused by deformation. We also summarize the recent progress of light-driven soft actuators made of photoresponsive CLCPs. We end with an outlook of existing challenges and opportunities.

Remoldable Crosslinked Liquid-Crystalline Polysiloxane with Side Chain Mesogens Based on Exchangeable Crosslinks

Crosslinked liquid-crystalline polymers (CLCPs) show unique behavior of deforming macroscopically when stimulated with heat, electricity and light. However, they cannot be remolded and reshaped once prepared. Here, we designed crosslinked polysiloxane with side chain mesogens that contain exchangeable crosslinking points based on transesterification between phenyl benzoates and hydroxy groups. After the formation of a polymer network, a base catalyst was introduced into the CLCP. When heated at 80°C, a piece of the CLCP was reshaped into a film and separate pieces of the CLCP were fused together, indicating that the CLCP is remoldable.

Photomobile polymer materials: photoresponsive behavior of cross-linked liquid-crystalline polymers with mesomorphic diarylethenes.

Cross-linked liquid-crystalline (LC) polymers with a mesomorphic diarylethene were prepared to demonstrate a versatile strategy for cross-linked photochromic LC polymers as photomobile materials. Upon exposure to UV light to cause photocyclization of the diarylethene chromophore, the cross-linked polymer films bend toward an actinic light source. By irradiation with visible light to cause a closed-ring to open-ring isomerization, the bent films revert to the initial flat state. Without visible-light irradiation, the bent films remain bent even at 120 °C, indicating high thermal stability of the cross-linked diarylethene LC polymers.

Photoinduced Bending Behaviour of Cross-linked Liquid Crystalline Polymers with High Cross-linking Densities

Highly cross-linked liquid crystalline polymer (CLCP) films with a long spacer and azobenzene chromophores (of varying concentrations and positioned differently) were prepared. The effects of various factors on the photoinduced bending behaviour of the highly CLCPs were explored. Long spacer groups and high cross-linking density resulted in considerably faster bending of the CLCP films relative to those reported in the literature that feature shorter spacer groups and smaller cross-linking densities. The azobenzene chromophores at the cross-linking points significantly influenced the photoinduced bending behaviour of the CLCP films relative to those at side chains. Furthermore, reduced concentrations of the azobenzene chromophores produced faster bending and larger photoinduced stress of the CLCP films that contained chromophores at the side chains as opposed to those incorporating chromophores at the cross-linking points. Moreover, the CLCP film with relatively low azobenzene concentration and small thickness exhibited bending in the reverse direction following maximum bending under continuous UV irradiation, thereby showing that photomobile behaviour of the material system.

Photomechanical energy conversion based on cross-linked liquid-crystalline polymers

Cross-linked liquid-crystalline (LC) polymers with a photochromic moiety show photoinduced deformation with change in molecular shape and alignment of photochromic compounds. Molecular-level photoisomerization of the photochromic moieties can give rise to macroscopic deformation of the materials, allowing one to convert light energy directly into mechanical work. The photomechanical effects extend the applicability of azobenzene-containing polymers towards light-driven actuators and artificial muscles. Recently, the effect of structure–property relationships and crosslinking density on the photomechanical property of photochromic polymers was investigated. Various motions based on the photoinduced deformation of the LC polymers were achieved by forming the polymer materials. This chapter summarizes the recent progress in photoinduced movements and light-driven actuation property of the LC polymers, in particular cross-linked LC polymers with a photochromic property.

Photoinduced Deformation of Crosslinked Liquid‐Crystalline Polymer Film Oriented by a Highly Aligned Carbon Nanotube Sheet

Biegsame Filme: Eine ausgerichtete Schicht von Kohlenstoffnanoröhren orientiert vernetzte flüssigkristalline Polymere durch einen einfachen Schmelzprozess (siehe Bild). Der entstehende Kompositfilm kann durch abwechselnde Bestrahlung mit UV- und sichtbarem Licht gebogen und begradigt werden. Der Film zeigt außerdem exzellente mechanische und elektrische Eigenschaften.

Photodeformable polymer gels and crosslinked liquid-crystalline polymers

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.

Location of the Azobenzene Moieties within the Cross-Linked Liquid-Crystalline Polymers Can Dictate the Direction of Photoinduced Bending.

We present a simple way to control the photoinduced bending direction of azobenzene-containing cross-linked liquid-crystalline polymers. By changing the location of the photoactive azobenzene moieties from cross-links to side-chains, the bending direction of the sample is reversed under identical irradiation conditions. In addition to providing a versatile route toward directionality control of the photoinduced macroscopic motions, this observation highlights the complicated nature of the photomechanical response of azobenzene-containing cross-linked liquid-crystalline polymers, showing that the photomobile behavior can be determined by seemingly small details on the materials design.

Photodeformable CLCP material: study on photo-activated microvalve applications

A kind of photodeformable polymer material, crosslinked liquid-crystalline polymer (CLCP) incorporated with azobenzene moieties, is studied as microvalve membrane actuator for microfluidic system applications. In the photomechanical analysis, the photo-induced bending effect of such polymer film is equivalent to a moment Tm and an axial force Nm applied on it, which are related to irradiation time and intensity of the driving UV light. A linear elastic beam model is established to calculate the maximal force that the valve actuator can resist and to analyze the deformation at any irradiation time in FEA. An experimental setup is built to record the flow rate in one operate cycle under various irradiation or preload conditions. The open time can reach 8 s and the close time 6 s. The results show that CLCP films have great potential in microvalve applications in biological engineering, drug delivering, etc. It has advantages of wireless, remote control and green energy.

Preparation of Crosslinked Liquid-Crystalline Polymers by Electron-Beam Irradiation and Their Photoinduced Bending Behavior

We proposed a new approach to fabricate bilayer films with a crosslinked azobenzene liquid-crystalline (LC) polymer. The polyethylene (PE) substrates were coated with linear azobenzene LC polymers, and then the azobenzene layers were crosslinked by irradiation with electron beams (EBs). In the EB-irradiated films with ordered azobenzene moieties, exposure to UV light caused trans-cis isomerization and the change in alignment of azobenzene moieties. Bending of the bilayer films could be induced toward an actinic light source when they were irradiated from the side of the azobenzene layer, while they bent away from the actinic light source upon UV irradiation from the side of the PE layer. In both cases, the bent films reverted to the initial flat state by turning off the light, which could be repeated at room temperature.