Improvement of LPS-based command surfaces: effect of inserting a flexible disiloxane segment into the azo side chain on photo-driven response

A new synthetic route for azo grafted ladder-like polysilsesquioxanes (LPS) used for fabricating command surfaces has been developed. In comparison with previous work, the grafting ratio of azobenzene on the LPS backbone could be controlled more accurately and the low molar mass impurity, which might influence the concerted action of the azobenzene units as command moieties on the surrounding LC molecules, could be readily removed by repeated precipitation. The results of our measurements indicate that inserting a flexible disiloxane segment into the spacer of the side group can improve the photo-driven response. The response steepness of a LC cell is 20 times that of our previous result. However, the response speed also depends on the spacer length and the content of azo side groups. Using azo grafted LPS with a shorter spacer as the material for the command surface, the response speed is faster than with a longer spacer. Optimum response can be achieved at an azo content of 10 mol %.     

Synthesis,characterization and photoinduction of optical anisotropy in liquid crystalline diblock azo‐copolymers

Diblock copolymers with polymethyl methacrylate and side chain liquid crystalline (LC) azopolymethacrylate blocks were synthesized by atom transfer radical polymerization (ATRP). The azobenzene content in these copolymers ranges from 52 to 7 wt %. For an azo content down to 20% they exhibit a LC behavior similar to that of the azo homopolymers. Thin films of these copolymers were characterized by transmission electron microscopy (TEM). A lamellar nanostructure was observed for azo content down to 20 wt %, while no structure is observed for the copolymer with a 7% azo content. The optical anisotropy induced in these films by illumination with linearly polarized 488 nm light was studied and the results compared with those of the azo homopolymer and of a random copolymer with a similar composition. The formation of azo aggregates inside the azo blocks is strongly reduced in going from the homopolymer to the copolymers. Photoinduced azo orientation perpendicular to the 488 nm light polarization was found in all the polymers. The orientational order parameter is very similar in the homopolymer and in the block copolymers with an azo content down to 20 wt %, while it is much lower in the random copolymer and in the 7 wt %. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1899–1910, 2007     

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.     

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     

Twin liquid crystals and segmented thermotropic polyesters containing azobenzene—effect of spacer length on LC properties

We report systematic studies on a homologous series of twin liquid crystalline (LC) molecules based on phenyl and naphthyl azobenzene ( PnP and NpnNp ) as well as segmented copolyesters based on them. The twin series had the structure azobenzene–oligooxyethylene–azobenzene, where the ethyleneoxy length was varied from 2 to 6 units. The LC properties of the twin series depended on the chemical structure of the azochromophore and also the length of the central oligooxyethylene segment. The PnP series exhibited smectic LC properties for n > three oligooxyethylene units. Conversely, NpnNp series exhibited spherulitic phases only for the shortest member – Np2Np . One non‐LC short spacer twin ( P2P ) and one LC long spacer twin ( P6P ) were incorporated as part of a main chain polyester composed of fully aliphatic segments of sebacate and di or tetraethylene glycol (DEG/TEG) units by melt polycondensation. Non‐LC P2P formed LC polymers even at low (5 mol %) incorporation in DEG‐based copolymers, whereas the LC‐ P6P could do so only at 30 mol % incorporation. The LC properties of the twin molecules as well as copolymers were studied using differential scanning calorimetry, polarized light microscopy (PLM) along with variable temperature wide angle X‐ray diffraction. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Unique molecular orientation in a smectic liquid crystalline polymer film attained by surface-initiated graft polymerization

Liquid crystalline (LC) polymer brushes containing a mesogenic azobenzene (Az) moiety are synthesized on a quartz or silicon substrate by surface-initiated atom transfer radical polymerization. The molecular orientation of the Az units and the LC properties in the grafted chains are evaluated by UV-vis spectroscopy, polarized optical microscopy, and grazing incidence X-ray diffraction measurements. The Az side chains of the grafted chains exhibited a smectic LC phase in which the smectic layers are oriented perpendicular to the substrate with a parallel orientation of the mesogens. In contrast, a spincast film of the identical LC polymer without grafting to the surface shows layer structures parallel to the substrate. A drastic effect of tethering one end to the substrate on the LC orientation is demonstrated.     

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     

两种侧链偶氮聚电解质自组装膜中生色团取向研究

用偏振紫外光谱研究了两种侧链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 .讨论了不同的组装条件对自组装膜中偶氮生色团取向的影响 .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 ,侧链偶氮聚电解质自组装膜中的偶氮生色团存在一定程度的面内取向 ,自组装的各种影响条件和聚合物结构等 (pH值、侧链柔性间隔基团长度、以及偶氮生色团官能度等 )与自组装膜中偶氮生色团的面内取向程度存在一定的相关性 .通过研究偶氮生色团的取向和影响因素 ,可以深入认识侧链偶氮聚电解质的自组装行为     

Photoluminescent behavior of poly(3‐hexylthiophene) derivatives with a high azobenzene content in the side chains

A series of polythiophene derivatives with substantially higher azobenzene contents in the side chains were prepared via copolymerization of 3‐hexylthiophene with four different types of 4‐((4‐(phenyl)azo)phenoxy)alkyl‐3‐thienylacetate. The alkyl spacers with different lengths, i.e. butyl, hexyl, octyl and undecyl groups were used between the azobenzene group and the thiophene ring. The compositions, structures and thermal properties of these polythiophene derivatives were characterized. The structural dependence of photoluminescent emission, photochromic behavior of these copolymers were systematically studied and compared with poly(3‐hexylthiophene). The results show that the azobenzene substitution renders the polythiophene some interesting optical properties that can be modulated by UV light irradiation. In the azobenzene modified polythiophene, the intensity of photoluminescent emission associated with the conjugated polythiophene main chain was found to decrease significantly upon UV irradiation. The finding suggests that the photo‐induced trans‐cis isomerization of the azobenzene pendant groups has a significant effect on photoluminescent emission, particularly when short spacers are used between azobenzene groups and the main chain. However, the effect becomes less prominent when longer spacers are used between the azobenzene group and the main chain. Furthermore, UV irradiation of the copolymers also resulted in an increase in intensity and broadening of bandwidth for the absorption peak associated with the polythiophene backbones. Again the magnitude of intensity changes upon UV irradiation were found to be dependent on the spacer length between the azobenzene group and polythiophene main chain. Copyright © 2005 John Wiley & Sons, Ltd.     

胆甾侧链聚硅氧烷高分子液晶的合成与相行为研究

本工作利用双键与Si-H键的加成反应合成了以丙烯酸胆甾醇酯为侧链的硅氧烷梳状聚合物液晶ECS.聚合物的结构得到了核磁与红外分析的证明.偏光显微镜和DSC对ECS的相态转变研究表明,ECS在一定的温度范围内能够形成液晶相,液晶相的存在范围与聚合物的结构密切相关.     

Stable photoalignment layer for nematic liquid crystals based on ladder-like polysilsesquioxanes

A new kind of aligning material for liquid crystal cells, ladder-like polysilsesquioxanes (LPS) grafted with cinnamoyl side groups, has been developed to improve the thermal stability of the photoalignment layer. The LC aligning ability of the LPS-based alignment layers, fabricated by linearly polarized UV-induced polymerization (LPP), was characterized by polarizing optical microscopy, conoscopic observations and electro-optic response measurements. In particular, a practical and severe annealing test was adopted to examine the thermal stability of the alignment layer; this showed that even when LC cells were annealed at 100 C (much higher than the clearing point of the LC) for several hours, good LC orientation could remain when the cell was cooled to a constant measurement temperature. The results confirmed that the photoalignment layers exhibited not only good LC aligning ability, but also excellent thermal stability, so heralding their potential application in LCDs.     

Synthesis and characterisation of reactive liquid crystals containing an azo group

A new series of liquid crystal (LC) monomers – not only contain a double bond but also contain an azo group – were designed and synthesised. The length of side groups in the LC monomers containing azobenzene ester varied from 1 to 2 methylene units, and the length of the substituted groups in the main chain varied from 1 to 3 methylene units. The molecular structures of the intermediates and target compounds were confirmed by Fourier transform infrared, ultraviolet and visible spectrum and nuclear magnetic resonance (NMR) spectroscopy. The thermal phase behaviour of the LC monomers was investigated by polar optical microscopy coupled with hot stage and differential scanning calorimetry (DSC). In this paper, the effect on the LC zone with the substituents were investigated, and with the increased methylene of main chain, the melting point and the phase transition temperature of the substance will be lowered and LC regions will be narrower.     

Azobenzene‐Containing Block Copolymers as Light‐Induced Reworkable Adhesives: Effects of Molecular Weight,Composition, and Block Copolymer Architectures on the Adhesive Properties

We previously reported that ABA‐type triblock copolymers with azobenzene‐containing terminal blocks can be utilized as a light‐induced reworkable adhesive that enables repeatable bonding and debonding on demand. The reworkability was based on the photoisomerization of the azobenzene moiety and concomitant softening and hardening of the azo blocks. Our aim in this study is to investigate the effect of the composition, molecular weight, and block copolymer architectures on the reworkable adhesive properties. For this purpose, we prepared AB diblock, ABA triblock, and 4‐arm (AB)₄ star‐block copolymers consisting of polymethacrylates bearing an azobenzene moiety (A block) and 2‐ethylhexyl (B block) side chains and performed adhesion tests by using these block copolymers. As a result, among the ABA block copolymers with varied compositions and molecular weights, the ABA triblock copolymers with an azo block content of about 50 wt % and relatively low molecular weight could achieve an appropriate balance between high adhesion strength and low residual adhesion strength upon UV irradiation. Furthermore, the 4‐arm star‐block structure not only enhances the adhesion strength, but also maintains low residual adhesion strength when exposed to UV irradiation. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 806–813     

Silicon‐containing azo polymers with paired mesogens and their applications to optical memory media

We synthesized a novel photoresponsive monomer, silicon‐containing azo monomer with paired mesogens in the side chain, by reacting 3‐methacryloxypropylmethyldichlorosilane with 2‐[2‐(4‐cyano‐azobenzene‐4′‐oxy)ethylene‐oxy]ethyl alcohol, a mesogenic group. Corresponding homopolymer and copolymers with methyl methacrylate were generated via radical polymerization with AIBN as a radical initiator. Investigations of their thermal properties and optical textures confirmed the monomer and the homopolymer have smectic structures. Homo‐ and copolymer films showed high potential as reversible data recording media via photoinduced alignment of azobenzene groups with irradiation of a linearly or circularly polarized light. Out of all the samples, the copolymer films with high azo dye contents showed the best resolution in the recorded data as well as the fastest response to a pump beam due to large optical birefringence induced in a write‐in process. Strong dependence of the stability of the data stored in the films on the glass transition temperature of the polymers was also observed. In addition, high‐quality holographic grating patterns were inscribed even on the copolymer film with azo molar content of only 7.0% using a modified two‐wave mixing technique. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6734–6745, 2008     

Morphology and switching of holographic gratings containing an azo dye

The effects of an azo dye on the diffraction efficiency, morphology and electro-optic properties of the transmission mode of a holographic polymer dispersed liquid crystal (LC) have been studied. The azo dye induced an induction period which otherwise does not exist, followed by a gradual increase of the diffraction efficiency to a saturation value which increased with increasing azo dye content, as a result of the azo dye reorientating LC molecules within the droplet. The increased diffraction efficiency was caused by the decreased droplet coalescence which was due to the hindered migration of the LC by the dye molecules, and to LC orientation induced by azo dye molecules giving a high refractive index contrast. The droplet size decreased with increasing dye content. The dye also lowered the threshold voltage due to the high dielectric anisotropy caused by the presence of a strong on-axis dipole moment and decreased the response time.     

Orientation of the Azobenzene Spacer of Carboxylic Methyl Ester Gemini Surfactants in Langmuir Monolayer

The Langmuir monolayer of carboxylic methyl ester Gemini surfactants with the azobenzene spacer, referred to as MC_m(azo)MC_m, was prepared and the π‐A isotherms were measured. The result revealed an orientational picture of the azobenzene spacer at the air/water interface. Before irradiation, the planar trans‐azobenzene group adopted an approximate configuration parallel to the interface and lay on the air/water interface. After UV‐light irradiation, the spacer became the twisted cis‐one. Because of the location of the polar headgroups at the air/water interface, the two phenyl rings were also forced to take a near‐parallel orientation with the interface to reduce the free energy. As a result, the spacer thin‐layer was "thickened", which caused an elevation in collapse pressure.     

Phototunable Liquid‐Crystalline Phases Made of Nanoparticles

The properties of liquid‐crystalline (LC) hybrid systems made of inorganic nanoparticles grafted with photosensitive azo compounds are presented. For materials with a large density of azo ligands at the surface, the LC structure can be reversibly melted by UV light, and the return to the LC state does not require the absorption of visible light. For systems with a lower density of azo ligands, UV light causes shortening of the distance between metal sublayers in the lamellar phase. Interestingly, the azo derivatives attached to the nanoparticle surface show very different kinetics of cis/trans conformational change as compared to the free molecules. The cis form of free ligands in solution is stable for days, whereas the isomerization of molecules attached to the nanoparticle surface to the trans form takes only a few minutes. Apparently, owing to the crowded environment, azo ligands immobilized at a metal surface behave as they would in the condensed state.     

偶氮共轭聚合物光电特性研究进展

芳香环或杂环通过NN双键连接形成的化合物如偶氮苯、偶氮吡咯等具有π共轭结构,此类分子有顺反两种构型,他们可以在光照条件下相互转换。分子构型转变会影响电子的共轭程度及其离域特性,因此含环结构的偶氮共轭分子具有光调制特性。反式偶氮苯分子为平面结构,顺式构型分子两个苯环有一定角度的扭转分子不在同一平面,实验和理论计算结果表明偶氮苯分子的键长、键角等受溶剂和取代基影响;光照可以实现偶氮苯分子的导电性改变,目前认为其导电性改变的原因主要是光致顺反异构而改变分子尺寸而引起。通过氮氮双键连接的杂环共轭分子能显著地降低分子的能隙,并使共轭化合物在更宽的波长范围内有强吸收,能提高太阳能光伏电池的转换效率,是理想的有机光伏材料。文章还对偶氮共轭聚合物的合成方法做了介绍,分析了含偶氮结构的共轭聚合物的光相应研究现状及其未来发展趋势。     

Supramolecular side chain liquid crystalline polymers assembled via hydrogen bonding between carboxylic acid-containing polysiloxane and azobenzene derivatives

Supramolecular side chain liquid crystalline polymers were prepared from poly(3-carboxypropylmethylsiloxane) (PSI100) and azobenzene derivatives through intermolecular hydrogen bonding (H-bonding) between the carboxylic acid groups in the PSI100 and the imidazole rings in the azobenzene derivatives. The existence of H-bonding has been confirmed using FTIR spectroscopy. The polymeric complexes behave as liquid crystalline (LC) polymers and exhibit stable mesophases. The LC behaviour of these H-bonded polymeric complexes was investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. The complexes exhibit nematic LC phases identified on the basis of Schlieren optical textures. On increasing spacer length or the concentration of the H-bonded mesogenic unit in the complex, the clearing temperature and the temperature range of the LC phase of the polymeric complex increase. The terminal group plays a critical role in determining the LC properties of the polymeric complexes. A terminal methoxy group is more efficient than a nitro group in increasing the clearing temperature. The electron donor-acceptor interactions between the H-bonded mesogenic units containing methoxy and nitro terminal groups in supramolecular 'copolymeric' complexes lead to an increase in the clearing temperature and a wider temperature range for the LC phase.     

Supramolecular side chain liquid crystalline polymers assembled via hydrogen bonding between carboxylic acid-containing polysiloxane and azobenzene derivatives

Supramolecular side chain liquid crystalline polymers were prepared from poly(3-carboxypropylmethylsiloxane) (PSI100) and azobenzene derivatives through intermolecular hydrogen bonding (H-bonding) between the carboxylic acid groups in the PSI100 and the imidazole rings in the azobenzene derivatives. The existence of H-bonding has been confirmed using FTIR spectroscopy. The polymeric complexes behave as liquid crystalline (LC) polymers and exhibit stable mesophases. The LC behaviour of these H-bonded polymeric complexes was investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. The complexes exhibit nematic LC phases identified on the basis of Schlieren optical textures. On increasing spacer length or the concentration of the H-bonded mesogenic unit in the complex, the clearing temperature and the temperature range of the LC phase of the polymeric complex increase. The terminal group plays a critical role in determining the LC properties of the polymeric complexes. A terminal methoxy group is more efficient than a nitro group in increasing the clearing temperature. The electron donor-acceptor interactions between the H-bonded mesogenic units containing methoxy and nitro terminal groups in supramolecular 'copolymeric' complexes lead to an increase in the clearing temperature and a wider temperature range for the LC phase.