Photoresponsive liquid crystalline and amorphous polymers

Isomerization processes of azobenzene dyes dissolved in a glassy polymeric matrix or attached in glassy amorphous or liquid crystalline polymers to the backbone as side groups are induced by light. The isomerization process, in turn, causes the dye to reorient provided that polarized light is used: the long axis of the dye is oriented perpendicular to the polarization direction in the stationary case. Such a reorientation gives rise to strong modifications of the optical properties. This contribution is concerned with the analysis of the correlation between the nature of the azobenzene dyes, the isomerization, reorientation and modulations discussed above and with possible applications in the optical holographic storage. Considered are, in particular, dye/matrix combinations giving rise to nonlinear holographic responses, two photon holography, transient holographic modes applicable for holographic displays and the optical switching of other than optical properties.     

Photo-reorientation of azobenzene side groups of thermotropic ‘hairy rod’ polyglutamate in LB multilayers

A polyglutamate with photochromic azobenzene side groups combines thermotropic properties with the capability to form Langmuir-Blodgett (LB) multilayers. On irradiation with linearly polarised light the azobenzene side groups may be reoriented in spin-coated films and LB multilayers perpendicular to the electric field vector of the incident light. But the photochemically induced reorientation process depends strongly on the supramolecular order and on the irradiation conditions. Thus, the stable nematic-like order of the ‘hairy rod’ backbones restricts the photo-reorientation process of the azobenzene side groups within the LB multilayer, in contrast to the spin-coated film. In this case the degree of the photo-induced anisotropy depends on the primary orientational order of the multilayer, the molecular aggregation of the azobenzene moieties and on the conditions of light exposure. The photo-reorientation process becomes more efficient after weakening the intermolecular interactions by a suitable preparation of the films or by an intermediate photochemical decoupling of the supramolecular order caused by an ultraviolet irradiation step.     

光致取向调控偶氮苯侧链液晶聚合物的阻隔特性研究

研究了液晶分子的排列方式对聚合物膜阻隔特性的影响,采用473 nm线偏振光照无定形偶氮液晶聚合物,使其介晶基元发生从无序到有序的取向排列.用膜透射率变化和锥光干涉图表征了分子的取向,其锥光干涉图为粗黑十字,说明在线偏振光下作用下液晶分子取向形成了单相畴沿面内排列的有序态.用金属表面氧化法进一步研究了取向态聚合物膜的阻隔...     

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.     

An infrared spectroscopic study of photo-induced reorientation in dye containing liquid-crystalline polymers

Infrared spectroscopy is used to study simultaneously the orientational behaviour of different segments of dye containing liquid-crystalline side group copolymers in sandwich type films of about 2 μm thickness. Under continuous irradiation with polarized light above and below T_g of the polymers both azobenzene and phenyl benzoate side groups reorient preferentially normal to the film plane leading to a strongly biaxial orientation distribution. The analysis of the kinetics reveals that the reorientation is essentially a mono-exponential process with an additional faster process only found for the azobenzene dye and assigned to the initial trans to cis isomerization step. Investigation of an isotropic copolymer system containing azobenzene in the side groups shows that an anisotropy can be induced through irradiation with polarized light that is strongly dependent on temperature.     

Photon control of liquid motion on reversibly photoresponsive surfaces

The movement of a liquid droplet on a flat surface functionalized with a photochromic azobenzene may be driven by the irradiation of spatially distinct areas of the drop with different UV and visible light fluxes to create a gradient in the surface tension. In order to better understand and control this phenomenon, we have measured the wetting characteristics of these surfaces for a variety of liquids after UV and visible light irradiation. The results are used to approximate the components of the azobenzene surface energy under UV and visible light using the van Oss-Chaudhury-Good equation. These components, in combination with liquid parameters, allow one to estimate the strength of the surface interaction as given by the advancing contact angle for various liquids. The azobenzene monolayers were formed on smooth air-oxidized Si surfaces through 3-aminopropylmethyldiethoxysilane linkages. The experimental advancing and receding contact angles were determined following azobenzene photoisomerization under visible and ultraviolet (UV) light. Reversible light-induced advancing contact-angle changes ranging from 8 to 16 degrees were observed. A large reversible change in contact angle by photoswitching of 12.4 degrees was achieved for water. The millimeter-scale transport of 5 microL droplets of certain liquids was achieved by creating a spatial gradient in visible/UV light across the droplets. A criterion for light-induced motion of droplets is shown to be consistent with the response of a variety of liquids. The type of light-driven fluid movement observed could have applications in microfluidic devices.     

Molecular addressing? Studies on light-induced reorientation in liquid-crystalline side chain polymers

Polarized light leads to an effective reorientation of the optic axis in the glassy state of liquid-crystalline side chain polymers containing azobenzene mesogenic groups, via a trans-cis and cis-trans isomerization. Using a combination of U V and IR dichroitic studies it is shown for copolymers consisting of chromophores (azobenzene) and non-chromophores (phenylbenzoate) that only the chromophores are reoriented by light as far as the glassy state is concerned. Individual chromophores are thus addressed by photoselection. Photoselection in the fluid nematic state, on the other hand, leads also to a reorientation of the non-chromophores.     

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.     

Photoswitchable orientational patterns of confined domains in monolayers

We report here a photoswitch process that involves collective molecular reorientation in a monolayer of an azobenzene derivative. Using polarized light we force the transition between two clearly distinguishable orientational mesoscopic configurations that can be monitored by reflection optical microscopy. A model that combines thermodynamic and kinetic arguments is proposed, and it is able to reproduce both the two states and the mechanism involved in the transition. We conclude that the phenomenon reported here is essentially different from the usual electric-field-induced molecular alignment often found in liquid crystalline materials and devices. Instead, it involves a photoexcitation concomitant with an H-aggregation process.     

Surface organization, light-driven surface changes, and stability of semifluorinated azobenzene polymers

A series of polymers with 4-perfluoroalkyl-modified azobenzene side groups was investigated for its light-induced changes in surface properties. The ultraviolet (UV) light activated trans to cis isomerization of the azobenzene group, and the influence of molecular order and orientation on this process were studied using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and water contact angle measurements. Light-induced molecular reorganization in the near-surface region was studied by NEXAFS using in situ UV irradiation of polymer thin films. Differential scanning calorimetry and wide-angle X-ray scattering studies showed that sufficiently long fluoroalkyl groups formed well-ordered smectic mesophases in the bulk, as well as on the surface, which was evidenced by NEXAFS. The disruption of mesogen packing by photoisomerization was found to be influenced by the fluoroalkyl segment length. Surfaces with perfluorohexyl and perfluorooctyl groups that showed high orientational order were also highly resistant to light-induced changes. In such cases, the trans-cis isomerization resulted in greater lowering of the azobenzene phenyl ring order parameters than the perfluoroalkyl order parameters. UV exposure caused reorientation of the phenyl rings of the azobenzene group, but the terminal perfluoroalkyl segments remained more or less ordered.     

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.     

Synthesis of a liquid crystalline azobenzene monomer for photoalignment of ferroelectric liquid crystals

A liquid crystalline dimethacrylate-based monomer containing an azobenzene group was synthesized. The miscibility of the azo monomer and its crosslinked polymer network with ferroelectric liquid crystals (FLCs) was improved by the liquid crystallinity of the monomer. By performing thermal polymerization of the azo monomer dissolved in a FLC host under linearly polarized visible light irradiation, following a pre-irradiation with unpolarized UV light at room temperature, bulk alignment of the FLC could be induced in the absence of surface orientation layers, as a result of the photo-orientation of azobenzene moieties related to the reversible trans-cis photoisomerization of the chromophore. The optical and rubbing-free alignment of a FLC could be achieved with as few as 0.5 wt % of the azo polymer network. This represents a step toward the preparation of uniform samples of photoaligned FLC by reducing the amount of the azo polymer network that may be phase-separated from the FLC host.     

光致变色液晶聚合物的光致再取向性能研究进展

光致变色分子经掺杂或键合作用嵌在液晶聚合物中可形成光致变色液晶聚合物,在线性偏振光或非偏振光的照射下,此物质中光致变色分子的构型变化会引起整个液晶分子的二维或三维光致再取向的形成,本文综述了光致变色液晶聚合物的光致再取向的近期研究进展.     

Synthesis and characterization of novel liquid‐crystalline copolymers containing thermally stable photochromic groups

To overcome the defects of the thermal instability of azobenzene, a series of novel photochromic, chiral, liquid‐crystalline monomers and polymers were synthesized from (+)‐camphor. The copolymerization of the photochromic monomers with comonomers was carried out. The synthesized monomers and polymers were identified with nuclear magnetic resonance, Fourier transform infrared, and elemental analysis. The composition of the copolymers was estimated with elemental analysis. The specific rotation of the chiral compounds and polymers was evaluated. The thermal stability and phases of the polymers during heating and cooling cycles were studied with differential scanning calorimetry and thermogravimetric analysis. The phases of the polymers were identified with polarized optical microscopy textures and X‐ray diffraction analysis. The distance between the layers of smectic liquid crystals was estimated from the diffraction angles. Photoisomerization of the configurational E/Z structures was investigated with an ultraviolet–visible spectrophotometer with 300‐nm ultraviolet irradiation. The thermal stability of the Z‐structural segment in the polymers was confirmed through the heating of the polymer at 70 °C for over 10 h. The photoisomerization and thermal stability of the C?C bond in the polymeric materials were demonstrated through a series of novel chiral polymers synthesized in this investigation. Both the polarity of the center part and the molecular length at the ends of the molecules were found to be necessary factors for the formation of liquid‐crystalline molecules. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2026–2037, 2007     

Orientational order of guest molecules in aligned liquid crystal as measured by EPR and UV–vis techniques

Orientational order of guest molecules in aligned liquid crystal 4-cyano-4′-pentylbiphenyl (5CB) is studied via optical dichroism and electron paramagnetic resonance (EPR) spectra measurements. The guest molecules used are bifunctional molecules bearing paramagnetic nitroxide group and photochromic azobenzene moiety. The bifunctional probe with rigidly bonded nitroxide and azobenzene moieties was found to align as a whole, while flexible long spacer between the moieties provides independent alignment for the nitroxide and azobenzene parts. Intermolecular interactions responsible for the alignment of azobenzene and nitroxide moieties of the probe molecules are discussed. The molecules with cis-configuration of azobenzene moiety are able to align in the liquid-crystalline medium, but to a lesser extent than the molecules with trans-configuration. Directions of orientational axes and characteristics of rotational mobility of spin probes are determined. Second, fourth and, in some cases, sixth rank order parameter values are found.     

基于原子转移自由基聚合技术的偶氮苯星形液晶聚合物的合成与表征

利用原子转移自由基聚合(ATRP)技术合成了含不同端基取代基的偶氮苯三臂星形侧链液晶聚合物. 均苯三酚与2-溴异丁酰溴通过酯化反应制备三官能团引发剂, 引发偶氮苯单体6-[4-(4-甲氧基苯基偶氮)酚氧基]己基甲基丙烯酸酯(MMAzo)或6-[4-(4-乙氧基苯基偶氮)酚氧基]己基甲基丙烯酸酯(EMAzo)的ATRP反应. 利用核磁共振氢谱(¹H NMR)、凝胶色谱(GPC)、差示扫描量热法(DSC)和偏光显微镜(POM)等手段对星形聚合物进行表征. 星形聚合物的液晶性与相应均聚物相似, 但偶氮苯端基取代基的不同导致星形聚合物的液晶性差别显著. 在紫外/可见光照射下星形聚合物呈现明显的异构化转变.     

Synthesis and photoresponsive behavior of azobenzene-functionalized polythiophene films

In this paper, two kinds of azobenzene-functionalized polythiophene liquid-crystalline (LC) polymers with different spacer lengths (n = 6 and 11) were synthesized. The photochromic behaviors and photoresponsive property of these polymer films were investigated by means of spectrofluorophotometer, polarized optical microscope and ARC UV lamp. The results have shown that these liquid-crystalline polythiophene films exhibit a quite fast photochemical phase transition speed and a better opticalswitching property. Furthermore, the existence of the azobenzene moiety in the side chain has also rendered the polythiophene some interesting optical properties that can be modulated by UV light irradiation, e.g., the intensity of photoluminescent emission associated with the conjugated polythiophene main chain was found to decrease upon UV irradiation and the effect becomes more prominent when shorter spacers are used in between the azobenzene group and the main chain.     

Orientational optical nonlinearity of nematic liquid crystals induced by high-molecular-mass azo-containing compounds

Processes of light-induced reorientation of nematic liquid-crystalline molecules induced by the addition of low concentrations (0.1–2.0 wt %) of comb-shaped polymers and carbosilane dendrimers containing azobenzene fragments are studied. When the molecular structure of the above compounds becomes more complicated, the induced orientational nonlinearity increases. The introduction of 2G and 3G dendrimers into a nematic has for the first time made it possible to visualize and study a purely optical first-order Freedericksz transition in the field of a linearly polarized wave.     

Fast and stable recording of birefringence and holographic gratings in an azo-polymethacrylate using a single nanosecond light pulse

Pulsed light-induced recording in azobenzene polymers has recently been studied due to its potential use in optical storage applications. In this paper we study the photoinduced birefringence (Deltan) and holographic grating recording in an azobenzene side chain liquid-crystalline polymethacrylate irradiating with a single 4 ns light pulse at 532 nm. For some irradiation conditions, Deltan grows in less than 50 ns reaching an essentially stable value of about 10(-2). Holographic gratings have been registered using intensity and polarization patterns. Fast response and stability, similar to those of Deltan, was observed in the holographic recording process. Both light-induced anisotropy and relief contributions have been found in the case of gratings recorded using intensity patterns, relief being the dominant contribution at high recording energies. Polarization gratings have been recorded using two orthogonally circularly polarized beams. The resultant gratings showed stable efficiencies up to 0.8% (measured at 633 nm in 1-mum-thick films) and no measurable relief was observed.     

Phase Transfer Catalysis in the Polycondensation Processes. XXV. The Relationship Between Structure and Supramolecular Ordering of Some Aromatic Polyethers Containing Flexible Spacer

Abstract

The paper presents a study concerning the influence of the chemical structure on the supramolecular ordering [crystalline, liquid crystalline (LC), or amorphous] of some aromatic polyethers. The polymers were synthesized using a phase transfer catalysis technique, in a liquid/liquid system, starting from 1,4‐dichlorobutane or 1,5‐dichloropentane and various bisphenols, as follows: 4,4′‐dihydroxyazobenzene (DHAB), 4,4′‐dihydroxydiphenyl (DHD), bisphenol A (BPA), and 2,7‐dihydroxynaphtyl (DN). All polymers were characterized by using ¹H‐NMR, DSC, and optical microscopy in polarized light. Some samples containing azobenzene moieties showed LC properties, but the majority of them resulted as semi‐crystalline compounds. If a bent bisphenol was used (BPA) to obtain homopolymers or copolymers, only amorphous products were obtained. The low thermal stability of the LC products generated some difficulties regarding the characterization of the mesophase. Generally, the copolymerization reaction influenced only the crystalline melting points, the isotropization temperature of the LC polymers being situated near their thermal degradation limit.