Theory of Light-Induced Deformation of Azobenzene Elastomers

Summary: A microscopic theory is proposed to describe light-induced deformation of photo-sensitive elastomers bearing azobenzene chromophores in their strands. We use an orientation approach in which it is assumed that the light-induced deformation is caused by reorientation of azobenzene chromophores with respect to the electric vector of the linearly polarized light, E , due to the trans-cis-trans photoisomerizaion process whose efficiency depends on the orientation of the chromophores with respect to the vector E . In the framework of the Gaussian approximation for elasticity of network strands it is shown that the value of the light-induced deformation depends on the chemical structure of network strands, namely, on the orientation distribution of chromophores around the main chains which is related to the length and elasticity of spacers. Depending on the chemical structure, azobenzene elastomers can demonstrate expansion or uniaxial contraction along the vector E , as well as non-monotonic deformation with increasing light intensity (expansion at small light intensities and contraction at high ones).     

Liquid crystalline polymers: From self-organization to functions and applications

Liquid crystals derive their unusual properties and their broad range of applications from their unique spatial and orientational order giving rise to specific symmetries, to strong anisotropies with respect to macroscopical properties and to a strong coupling to external fields. We have studied for liquid crystalline polymers modes of inducing strong modifications of the anisotropic optical properties locally in solid films by light. A storage process is described in this contribution which is based on light-induced trans-cis-trans-isomerization reactions of azobenzene chromophores attached to a polymer backbone as side groups in liquid crystalline polymers. The chromophores are able to rotate in the glassy state if subjected to linearly polarized light: the azobenzene units approach a saturation orientation which is perpendicular to the polarization direction of the light. The contribution discusses the molecular mechanism of this process as well as possible applications.     

Dynamic evolution of light-induced orientation of dye-doped liquid crystals in liquid phase studied by time-resolved optically heterodyned optical Kerr effect technique

Transient evolution of light-induced molecular reorientation both in 1-amino-anthraquinone (1AAQ) dye and azobenzene doped isotropic liquid crystals (LCs) were studied by time-resolved optically heterodyned optical Kerr effect method. The results give clear direct experimental proof that under short pulse (30 ps) excitation, LC molecules orientate toward the excitation light polarization direction in the 1AAQ/LC system. However, LC molecular orientation becomes orthogonal to the light polarization in azobenzene/LC system. Time-resolved excited-state absorption of 1AAQ and wavelength dependent excited-state absorption of azobenzene were also observed and their contributions to the early dynamics of the third order optical responses of the two systems were confirmed. A simplified two-level mean-field theory was derived to reveal the intensity dependence of orientation enhancement factor in azobenzene/LC system considering the photoisomerization process.     

Opposite photo-induced deformations in azobenzene-containing polymers with different molecular architecture: molecular dynamics study

Photo-induced deformations in azobenzene-containing polymers (azo-polymers) are central to a number of applications, such as optical storage and fabrication of diffractive elements. The microscopic nature of the underlying opto-mechanical coupling is yet not clear. In this study, we address the experimental finding that the scenario of the effects depends on molecular architecture of the used azo-polymer. Typically, opposite deformations in respect to the direction of light polarization are observed for liquid crystalline and amorphous azo-polymers. In this study, we undertake molecular dynamics simulations of two different models that mimic these two types of azo-polymers. We employ hybrid force field modeling and consider only trans-isomers of azobenzene, represented as Gay-Berne sites. The effect of illumination on the orientation of the chromophores is considered on the level of orientational hole burning and emphasis is given to the resulting deformation of the polymer matrix. We reproduce deformations of opposite sign for the two models being considered here and discuss the relevant microscopic mechanisms in both cases.     

光致形变液晶高分子

交联液晶高分子兼具液晶的各向异性和高分子网络的弹性,并且具有优异的分子协同作用.在交联液晶高分子中引入光响应基团,例如偶氮苯后,即可赋予其光致形变性能,利用分子协同作用可以将光化学反应引起的分子结构变化放大为宏观形变,从而将光能直接转化成机械能.通过合理的分子结构和取向设计可以使液晶高分子产生诸如伸缩、弯曲、扭曲、振动等多种形式的光致形变,并用于各类光控柔性执行器件的构筑,在人工肌肉、微型机器人、微量液体操控等领域呈现出独特的优势和广阔的应用前景.本文总结和评述了光致形变液晶高分子的研究,包括材料结构对光致形变性能的影响、新型可加工光致形变材料的研究、利用可见光和近红外光触发形变的策略,以及光致形变液晶高分子微执行器在微量液体操控中的应用,最后展望了该领域的发展方向.     

PET表面接枝偶氮聚合物和光致取向研究

合成了一种丙烯酸酯类含芳香族偶氮生色团的单体ANB .以二苯甲酮为引发剂 ,采用固相紫外光接枝方法将上述单体接枝到聚酯 (PET)膜的表面 ,得到了一种具有光响应性的接枝膜 .通过SEM研究了接枝膜的表面和断面的形貌 ,观察到偶氮接枝层均匀地覆盖了PET表面 ,接枝层厚度约 0 4μm .研究发现 ,当使用488nm的线偏振激光照射接枝膜时 ,偶氮生色团通过快速的顺反异构化反应在垂直于偏振光极化方向上发生取向 ,得到了具有光学各向异性表面的PET接枝膜 .接枝膜的取向是一个快速过程 ,取向有序度参数在 2min时即达到最大值 ,为 0 0 6左右     

Mechanotropic Elastomers

Liquid crystal elastomers (LCEs) are anisotropic polymeric materials. When subjected to an applied stress, liquid crystalline (LC) mesogens within the elastomeric polymer network (re)orient to the loading direction. The (re)orientation during deformation results in nonlinear stress‐strain dependence (referred to as soft elasticity). Here, we uniquely explore mechanotropic phase transitions in elastomers with appreciable mesogenic content and compare these responses to LCEs in the polydomain orientation. The isotropic (amorphous) elastomers undergo significant directional orientation upon loading, evident in strong birefringence and x‐ray diffraction. Functionally, the mechanotropic displacement of the elastomers to load is also nonlinear. However, unlike the analogous polydomain LCE compositions examined here, the isotropic elastomers rapidly recover after deformation. The mechanotropic orientation of the mesogens in these materials increase the toughness of these thiol‐ene photopolymers by nearly 1300 % relative to a chemically similar elastomer prepared from wholly isotropic precursors.     

Mechanotropic Elastomers

Liquid crystal elastomers (LCEs) are anisotropic polymeric materials. When subjected to an applied stress, liquid crystalline (LC) mesogens within the elastomeric polymer network (re)orient to the loading direction. The (re)orientation during deformation results in nonlinear stress‐strain dependence (referred to as soft elasticity). Here, we uniquely explore mechanotropic phase transitions in elastomers with appreciable mesogenic content and compare these responses to LCEs in the polydomain orientation. The isotropic (amorphous) elastomers undergo significant directional orientation upon loading, evident in strong birefringence and x‐ray diffraction. Functionally, the mechanotropic displacement of the elastomers to load is also nonlinear. However, unlike the analogous polydomain LCE compositions examined here, the isotropic elastomers rapidly recover after deformation. The mechanotropic orientation of the mesogens in these materials increase the toughness of these thiol‐ene photopolymers by nearly 1300 % relative to a chemically similar elastomer prepared from wholly isotropic precursors.     

Photoinduced supramolecular chirality in amorphous azobenzene polymer films

Supramolecular chirality was optically induced in amorphous and achiral azobenzene polymer films by irradiation of a laser beam with elliptical polarization. The chirality resulted from helical orientation of azobenzene chromophores by a combined process of circular and linear polarization. The helix-handedness could be controlled by incident light-handedness.     

Photoinduced deformation of amphiphilic azo polymer colloidal spheres

Photoinduced shape deformation of colloidal spheres made of an amphiphilic azo polymer has been demonstrated in this work. The polymer contains the donor-and-acceptor-type azobenzene chromophores and can form uniform colloidal spheres by dropwise adding water into its THF solution. When the colloidal spheres obtained were exposed to the interfering p-polarized Ar+ laser beams (150 mW/cm2), the colloidal spheres changed to prolates (i.e., "rugby-balls"), "spindles", and finally "rods", depending on the irradiation times. The elongated direction of the spheres was observed to be the same as the polarization direction of the laser beam. The average major-to-minor ratio of the ellipsoids could be easily adjusted by controlling the irradiation time. The deformation effect observed in this work can offer a new way to prepare nonspherical colloids from colloidal spheres and will shed new light on the correlation between the photodriven shape deformation and photoinduced surface relief gratings for the same type of polymers.     

DESIGN AND STUDY OF NEW AZOBENZENE LIQUID CRYSTAL/POLYMER MATERIALS

Discussion is presented on the use of the photoisomerization of azobenzene chromophore in the design andpreparation of novel functional materials. The two systems reviewed are azobenzene polymer-stabilized liquid crystals andazobenzene elastomers. In the first case, a polymer network containing azobenzene moieties is used to optically induce andstabilize a long-range liquid crystal orientation without the need of treating the surfaces of the substrates. This optical andrubbing-free approach was applied to nematic and ferroelectric liquid crystals. In the second case, an azobenzene side-chainliquid crystalline polymer is grafted onto a styrene-butadiene-styrene triblock copolymer to yield a photoactive thermoplasticelastomer. Coupled mechanical and optical effects make possible the formation of dimaction gratings that may be useful formechanically tunable optical devices.     

Azobenzene‐containing LC polymethacrylates highly photosensitive in broad spectral range

Two photosensitive chiral liquid crystalline azobenzene‐containing polymethacrylates having different length of flexible spacer connecting chromophores with backbone were synthesized and their phase behavior and photo‐optical properties were studied. Both polymers consist of lateral methyl substituents in ortho‐position of azobenzene chromophores providing high photosensitivity even in red spectral region as well as high thermal stability of photoinduced Z‐form of azobenzene chromophores. It is shown, that smectic phase (SmA*) formation in films of polymer with longer spacer predetermines its quite unusual spectral response to UV and subsequent visible light actions. The SmA* phase promotes spontaneous homeotropic alignment of azobenzene chromophores in polymer films. UV‐irradiation induces not only E‐Z isomerization but also results in disruption of homeotropic alignment, whereas subsequent visible light action enables to obtain films with the low degree of chromophores orientation. The photo‐orientation phenomena under the action of polarized light of different wavelength on polymer films were studied. The possibility of using red polarized light of moderate intensity for optical photorecording on polymer films is demonstrated. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2962–2970     

端基对侧链型偶氮聚电解质自组装膜内生色团取向的影响

用偏振紫外光谱研究了 4种带有不同端基的侧链型偶氮聚电解质静电逐层自组装膜中偶氮生色团的初始取向 .讨论了不同的端基对偶氮生色团在自组装膜中初始取向的影响 .进一步探讨了偶氮聚电解质自组装膜的结构特点 .研究表明 ,侧链型偶氮聚电解质自组装膜中偶氮生色团普遍存在一定程度的沿面取向 .偶氮生色团所带端基的类型对其在自组装膜中的取向程度有较大的影响 ,这主要取决于偶氮生色团与聚阳离子基底的电荷相互作用和极性相互作用等 .对偶氮生色团在水溶液中能形成H 聚集体的自组装膜来说 ,H 聚集体对生色团取向也有一定的影响 .结果表明 ,在制备需控制生色团取向性的自组装膜时 ,要考虑生色团上的不同端基对取向的影响     

Layer-by-layer deposited organic/inorganic hybrid multilayer films containing noncentrosymmetrically orientated azobenzene chromophores

Organic/inorganic hybrid multilayer films with noncentrosymmetrically orientated azobenzene chromophores were fabricated by the sequential deposition of ZrO2 layers by a surface sol-gel process and subsequent layer-by-layer (LbL) adsorption of the nonlinear optical (NLO)-active azobenzene-containing polyanion PAC-azoBNS and poly(diallyldimethylammonium chloride) (PDDA). Noncentrosymmetric orientation of the NLO-active azobenzene chromophores was achieved because of the strong repulsion between the negatively charged ZrO(2) and the sulfonate groups of the azobenzene chromophore in PAC-azoBNS. Regular deposition of ZrO(2)/PAC-azoBNS/PDDA multilayer films was verified by UV-vis absorption spectroscopy and quartz crystal microbalance measurements. Both UV-vis absorption spectroscopy and transmission second harmonic generation (SHG) measurements confirmed the noncentrosymmetric orientation of the azobenzene chromophores in the as-prepared ZrO2/PAC-azoBNS/PDDA multilayer films. The square root of the SHG signal (I(2omega)(1/2)) increases with the increase of the azobenzene graft ratio in PAC-azoBNS as the number of deposition cycles of the ZrO(2)/PAC-azoBNS/PDDA films remains the same, while the second-order susceptibility chi(zzz)(2) of the film decreases with the increase of the azobenzene graft ratio. Furthermore, the present method was successfully extended to realize the noncentrosymmetric orientation of azobenzene chromophores in multilayer films when small organic azobenzene compounds with carboxylic acid and/or hydroxyl groups at one end and sulfonate groups at the other end were used. The present method was characterized by its simplicity and flexibility in film preparation, and it is anticipated to be a facile way to fabricate second-order nonlinear optical film materials.     

Light-induced charge redistribution in the retinal chromophore is required for initiating the bacteriorhodopsin photocycle

Bacteriorhodopsin's photocycle is initiated by the retinal chromophore light absorption. It has usually been assumed that light primarily isomerizes a retinal double bond which in turn induces protein conformational alterations and biological activity. We have studied several artificial pigments derived from retinal analogues tailored to substantially reduce the light-induced chromophore polarization. The lack of chromophore polarization was reflected in an undetectable second harmonic generation (SHG) signal. It was revealed that these artificial pigments did not exhibit any detectable light-induced photocycle nor light acceleration of the hydroxylamine-bleaching reaction. We suggest that light-induced retinal polarization triggers protein polarization which controls the course of the isomerization reaction by determining the relative efficiency of forward versus back-branching processes.     

Photoinduced optical anisotropy in films of photochromic liquid crystalline polymers

The light-induced modification of the optical properties of photochromic liquid crystalline (side-group) polymers (LCPs) containing azobenzene moieties was studied. Films of such polymers were irradiated with unpolarized and linearly polarized light. Unpolarized irradiation results in a modification of the order parameter, whereas the director orientation remains constant. The light-induced disturbance of the supramolecular order is strongly dependent on the structure of the polymer. A correlation with the enthalpic stability of the liquid crystalline phases is given. Linearly polarized irradiation causes a modification of the order parameter and a reorientation of the side-groups towards a direction perpendicular to the electric vector of the actinic light. This reorientation process is caused by an angular-dependent photoselection within the steady state of the photoisomerization of the azobenzene units. The amount and kinetic of the reorientation differ considerably as a function of certain structural features of the polymers, such as the content of photochromic moieties and the lengths of the spacer chains between the polymeric backbone and the rod-like moieties. It is shown that under certain circumstances the angular-dependent photoselection process may cause a reorientation of the non-photochromic moieties by a cooperative process. The result is a light-induced rotation of the optical axis of the LCP and thus an efficient modification of the birefringent and dichroic properties. This effect can be used in optical data storage.     

Enhanced photochemical‐shift of reflection band from an inverse opal film based on larger birefringent polymer liquid crystals: Effect of tolane group on the photochemical shift behavior

Photo‐chemically tunable photonic band gap materials are prepared by infiltration of liquid crystal polymers having azobenzene groups into voids of SiO₂ inverse opal films. Linearly polarized (LP) light irradiation results in transformation from a random to an anisotropic molecular orientation of azobenzene side chains in the voids of the SiO₂ inverse opal film, leading to the reversible and stable shift of the reflection peak to longer wavelength more than 15 nm. To improve switching properties, we use copolymers of azobenzene monomer and tolane monomer, which have higher birefringence, as infiltration materials into the voids. The azobenzene‐tolane copolymers are found to show higher birefringence than azobenzene homopolymers by the LP light irradiation at higher temperature. Consequently, the reflection band of the SiO₂ inverse opal film infiltrated with the azobenzene‐tolane copolymer can be shifted to longer wavelength region more than 55 nm by the irradiation of LP light. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1981–1990, 2009     

How can azobenzene block copolymer vesicles be dissociated and reformed by light?

The underlying mechanism of UV light-induced dissociation and visible light-induced reformation of vesicles formed by an azobenzene diblock copolymer was investigated. These processes were studied in situ by monitoring changes in optical transmittance of the vesicular solution while being exposed to UV or visible light irradiation. The results indicate that the UV-induced dissociation of the vesicles results from their thermodynamic instability due to a shift of the hydrophilic/hydrophobic balance arising from the trans-cis isomerization, while their reaggregation takes place upon visible light irradiation that shifts the hydrophilic/hydrophobic balance in the opposite direction after the reverse cis-trans isomerization. The study suggests a specific design principle for obtaining UV light-dissociable and visible light-recoverable vesicles based on azobenzene block copolymers. On one hand, the structure of azobenzene moiety used in the hydrophobic block should have a small (near zero) dipole moment in the trans form and a significantly higher dipole moment in the cis form, which ensures a significant increase in polarity of the hydrophobic block under UV light irradiation. On the other hand, the hydrophilic block should be weakly hydrophilic. The conjunction of the two conditions can make the light-induced shift of the hydrophilic/hydrophobic balance important enough to lead to the reversible change in vesicular aggregation.     

液晶弹性体刺激形变研究

开发可以通过外部刺激产生机械形变的人工致动材料是一个近年来的研究热点。其中,液晶弹性体因结合了聚合物网络的橡胶弹性和液晶的有序性而具有独特的性质,在热、光、电等的外界刺激下可以产生可逆的形状记忆效应。本文综述了液晶弹性体响应多种外界刺激产生各种形变的行为,主要介绍了有关热致形变液晶弹性体、电致形变液晶弹性体、化学刺激导致形变的液晶弹性体及光致形变液晶弹性体的研究进展,阐述了各类液晶弹性体产生形变的机理包括热致、电致和光致相转变,讨论了影响其响应性能的主要因素,并展望了这一领域的发展前景。