Photomechanical Bending Research Articles

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.

Visible Light Mediated Photopolymerization in Single Crystals: Photomechanical Bending and Thermomechanical Unbending

Recently several molecular crystals have been reported to show photomechanical bending mediated by chemical dimerization or rearrangement reactions or supramolecular changes in single crystals. Here we report a photomechanical bending involving polymerization reaction in single crystals for the first time. Bending is observed under visible light in a single-crystal-to-single-crystal (SCSC) fashion and a rare thermomechanical unbending (via decomposition reaction) upon heating in needle shaped crystals of 1,1'-dioxo-1H-2,2'-biindene-3,3'-diyl didodecanoate. The single crystalline polymeric material formed here via topochemical polymerization reaction may provide an opportunity to combine the attractive qualities of both the crystalline small molecules and durable polymers for realizing certain unique advantages, for instance for construction of durable high performance mechanical actuators.

Polymerization-induced photomechanical bending in single crystals

Polymerization-induced photomechanical bending in single crystals

Reversible Bending Behaviors of Photomechanical Soft Actuators Based on Graphene Nanocomposites.

Photomechanical nanocomposites embedded with light-absorbing nanoparticles show promising applications in photoresponsive actuations. Near infrared (nIR)-responsive nanocomposites based photomechanical soft actuators can offer lightweight functional and underexploited entry into soft robotics, active optics, drug delivery, etc. A novel graphene-based photomechanical soft actuators, constituted by Polydimethylsiloxane (PDMS)/graphene-nanoplatelets (GNPs) layer (PDMS/GNPs) and pristine PDMS layer, have been constructed. Due to the mismatch of coefficient of thermal expansion of two layers induced by dispersion of GNPs, controllable and reversible bendings response to nIR light irradiation are observed. Interestingly, two different bending behaviors are observed when the nIR light comes from different sides, i.e., a gradual single-step photomechanical bending towards PDMS/GNPs layer when irradiation from PDMS side, while a dual-step bending (finally bending to the PDMS/GNPs side but with an strong and fast backlash at the time of light is on/off) when irradiation from PDMS/GNPs side. The two distinctive photomechanical bending behaviors are investigated in terms of heat transfer and thermal expansion, which reveals that the distinctive bending behaviors can be attributed to the differences in temperature gradients along the thickness when irradiation from different sides. In addition, the versatile photomechanical bending properties will provide alternative way for drug-delivery, soft robotics and microswitches, etc.

Illumination-oriented and thickness-dependent photomechanical bilayer actuators realized by graphene-nanoplatelets

Photomechanical bilayer actuators, constituted by PDMS/GNPs thin layer and PDMS thin layer, are presented. Due to the photothermal effect of GNPs and differences in CTE of the two layers caused by the GNPs existed in PDMS/GNPs thin layer; the bilayer actuator can display photomechanical bending under nIR light irradiation. And it is interesting to notice that the bilayer actuators show a two-step photomechanical bending process when the nIR light is incident from the PDMS/GNPs thin layer while one-step photomechanical bending process when from the PDMS thin layer, which is different from the process commonly observed in thermal bilayer and bimorph actuators. According to preliminary analysis, it is revealed that the photomechanical two-step bending is the results of temperature gradient effect and thermal bilayer effect, while the one-step photomechanical bending process only caused by thermal bilayer effect. Meanwhile, the photomechanical bendings are both positively correlated with light intensities. By adjusting the rotation speed in spinning coating, various bilayer actuators with different thickness of PDMS thin layer were fabricated and tested. The photomechanical deflections decreased with the augment of rotation speed. In addition, the realization of micromachines with our proposed bilayer actuators was demonstrated.

Mechanical motion of molecular crystals induced by [4 + 4] photodimerisation

The photomechanical bending of crystals of a stilbene-type compound substituted with anthracene and indanone groups, (E)-2-(9-anthrylmethylene)-1-indanone (trans-1), was investigated. When a narrow plate-like microcrystal was irradiated with ultraviolet (UV) light at 365 nm, the crystal gradually bent away from the light source and finally reached a semicircular shape after more than 10 min. A larger rod-like crystal, approximately 10 mm in length, also exhibited a slight bending motion. The cessation of UV irradiation caused the bent crystals to return very slowly to their straight form. In the crystals, the anthracene planes of two neighbouring trans-1 molecules are arranged in a head-to-tail parallel manner, with a short plane-to-plane distance of only 3.72 A. The 1H nuclear magnetic resonance spectra of trans-1 crystals before and after UV irradiation revealed the intermolecular [4 + 4] photodimerisation of the two anthracene planes, while the trans-to-cis photoisomerisation was not significant. The UV-vis absorption spectra of the trans-1 powder crystals, obtained using a diffuse reflectance spectrophotometer, showed a gradual increase in absorbance between 200 and 500 nm with increasing UV irradiation time, reaching a maximum after 1 h. Thermal back-monomerisation was very slow in the dark, not recovering the initial spectrum even after 25 days. The fluorescence spectra at 570 nm, derived from the anthracene excimer, decreased in intensity with increasing UV irradiation time due to a decrease in the amount of anthracene chromophore via photodimerisation. In situ X-ray measurements revealed that the bending of the crystals was caused by slight elongation of the b axis of the unit cell, corresponding to the long axis of the rod-like crystals. Calculations revealed that the observed crystal elongation could be explained by an optimised head-to-tail [4 + 4] orientation of the anthracene dimer.

Light-driven Bending of Polymer Films in Which Salicylidenephenylethylamine Crystals are Aligned Magnetically

Abstract Silicone polymer films in which plate-like chiral and achiral crystals of salicylidenephenylethylamines were aligned by a magnetic field were prepared. The polymer films exhibited reversible photomechanical bending upon alternate irradiation with UV and visible light.

Light-Driven Bending Crystals of Salicylidenephenylethylamines in Enantiomeric and Racemate Forms

The photomechanical bending behavior of chiral crystals composed of S- and R-enantiomers of photochromic N-3,5-di-tert-butylsalicylidene-1-phenylethylamine in enol form [enol-(S)-1 and enol-(R)-1] has been compared with that of achiral crystals of the racemic compound [enol-(rac)-1] of S- and R-enantiomers. Both platelike chiral and achiral crystals, a few millimeters in length, exhibited similar reversible bending upon alternate irradiation with ultraviolet (UV) and visible light. The reversible bending of the achiral enol-(rac)-1 crystal could be repeated over 100 times, while the tip displacement angles of the chiral enol-(S)-1 and enol-(R)-1 crystals gradually decreased with repeated cycles. In situ X-ray measurements revealed that the bending motion of the chiral and achiral crystals was caused by a slight shrinkage of the a and b axes of the unit cell, respectively, corresponding to the long axis of the platelike crystals; shrinkage was induced by the shape change of component salicylidenephenylethy...

Photomechanical bending mechanics of polydomain azobenzene liquid crystal polymer network films

Glassy, polydomain azobenzene liquid crystal polymer networks (azo-LCNs) have been synthesized, characterized, and modeled to understand composition dependence on large amplitude, bidirectional bending, and twisting deformation upon irradiation with linearly polarized blue-green (440–514 nm) light. These materials exhibit interesting properties for adaptive structure applications in which the shape of the photoresponsive material can be rapidly reconfigured with light. The basis for the photomechanical output observed in these materials is absorption of actinic light by azobenzene, which upon photoisomerization dictates an internal stress within the local polymer network. The photoinduced evolution of the underlying liquid crystal microstructure is manifested as macroscopic deformation of the glassy polymer film. Accordingly, this work examines the polarization-controlled bidirectional bending of highly concentrated azo-LCN materials and correlates the macroscopic output (observed as bending) to measured blocked stresses upon irradiation with blue-green light of varying polarization. The resulting photomechanical output is highly dependent on the concentration of crosslinked azobenzene mesogens employed in the formulation. Experiments that quantify photomechanical bending and photogenerated stress are compared to a large deformation photomechanical shell model to quantify the effect of polarized light interactions with the material during static and dynamic polarized light induced deformation. The model comparisons illustrate differences in internal photostrain and deformation rates as a function of composition and external mechanical constraints.

Siloxane polymers containing azo moieties synthesized by click chemistry for photo responsive and liquid crystalline applications

AbstractThree new types of siloxane‐based photoactive liquid crystalline polymers containing azo side groups were synthesized through the click chemistry route. The polymers having molecular weight range of 14,000–34,000 g mol−1 were soluble in most of the polar solvents like chloroform, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, and dichloromethane. The photoresponsive trans–cis photoisomerization under UV radiation and cis–trans relaxation process in dark for the polymers were studied. The isomerization rate constants were found to be 0.01–0.04 sec−1 and 1.16*10−4–4.67*10−4 sec−1, respectively. It has been noted that the polymers showed high intensity absorption for n‐π* in chloroform. Both trans and cis forms of azide monomers having azo moiety exhibited molar extinction coefficient (ϵmax) in the range of 22,000–33,000 L mol−1 cm−1. The thermotropic behavior of the polymers was studied by polarizing optical microscope (POM) and differential scanning calorimetry (DSC) experiments. Polymer P1 showed liquid crystalline textures of nematic droplets, whereas P2 showed smectic focal conic texture and nematic droplets. Polymer P1 was also studied for photomechanical bending on exposure to UV radiation. The polymers showed initial degradation temperature in the range of 210–275°C. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Direction control of photomechanical bending of a photochromic molecular fiber

A photochromic molecular fiber was developed and the photomechanical bending motions of the fiber were demonstrated. The bending direction of the fiber was found to be controllable by altering the polarization direction of the irradiated light without changing the position of the light source and the wavelength of the light.

アゾベンゼン系フォトメカニカル結晶

Molecular machines based on the conversion of changes in molecular geometry to mechanical motion in macroscale materials are interesting from the perspective of basic research and offer a wide range of potential applications. Such machines are amenable to remote operation with external stimuli such a photon impingement and do not require direct contact. Azobenzenes are typical chromophores that undergo trans-cis photoisomerization. We have recently found photomechanical bending of trans-4-(dimethylamino)azobenzene microcrystals. Upon UV irradiation, the platelike microcrystal bent quickly in the direction opposite to the light source, reaching the maximum deflection after 0.5 s. The microcrystal returned to its initial flat shape 30 s after the illumination was stopped. This mechanical motion was reversible over repeated 100 cycles of UV irradiation. The bending effect was attributed to a gradient in the extent of UV-induced trans-cis photoisomerization as a function of light penetration, causing the expansion of the irradiated crystal surface to result in a bent macrostructure.

Mechanical Motion of Azobenzene Crystals upon Photoirradiation

Photomechanical bending of trans-4-(dimethylamino)azobenzene microcrystals was observed. Upon UV irradiation, the (001) face of the platelike microcrystals bent quickly in the direction opposite the light source, reaching the maximum deflection after 0.5 s. The microcrystal returned to its initial flat shape 30 s after the illumination was stopped. This mechanical motion was reversible over repeated cycles of UV irradiation. The bending effect was attributed to a gradient in the extent of UV-induced trans-cis photoisomerization as a function of light penetration, causing the expansion of the irradiated crystal surface along the b axis to result in a bent macrostructure.

Actuators Based on Photomechanical Polymer

New light-driven actuators based on films of polymer polyvinylidene fluoride are described. The actuators employ the photomechanical bending of the polymer film caused by low power (10mW and less) laser radiation. The photomechanical effect combines various physical mechanisms, such as anisotropic thermal expansion, converse piezoelectric mechanism along with photovoltaic and pyroelectric ones, while the mechanism of thermal expansion is dominant for slow motion. Mechanical vibrations of the strips of the photomechanical polymer were observed with periodic pulsed laser excitation. The resonance frequency is inversely proportional to the square of the length of the strip, in full agreement with the theory. Resonance frequency measurements were used to determine the modulus of elasticity of the films, which was close to 3.0x109Pa. Two possible applications were discussed: photonic switch and adaptive mirror. The proposed actuators have a potential of being used as the components of future light-driven micro/nano systems.