Photoswitching of holographic polymer-dispersed liquid crystals doped with chiral dopant

Chiral dopants were added to the formulation of holographic polymer-dispersed liquid crystals and the effects studied in terms of grating formation dynamics, morphology, diffraction efficiency, contrast ratio and electro-optical properties of the films. A gradual increase of real-time diffraction efficiency, decrease of droplet size and increase of diffraction efficiency of the composite film were obtained with the addition and increasing content of chiral dopant, due to the increased viscosity of the liquid crystal (LC) doped with the chiral dopant leading to decreased droplet coalescence. The contrast ratio decreased with increasing content of chiral dopant due to the difficult orientation of LC molecules caused by the formation of a helical structure. Addition of a small amount of the chiral dopant increased the driving voltage slightly, whereas the decay time is decreased significantly as a result of the high twisting of the helical structure.     

Dual effects of fullerene doped to holographic polymer dispersed liquid crystals

Doping of conductive fullerene particles to the formulation of conventional holographic polymer dispersed liquid crystal‐induced dual effects of reducing both droplet coalescence and operating voltage. Fullerene induced an induction period which otherwise does not exist, followed by a gradual increase of diffraction efficiency to a saturation value being increased with increasing fullerene content. The increased diffraction efficiency was caused by the decreased droplet coalescence which was due to the hindered migration of LC by the fullerene particles. On the other hand, doped fullerene particles augmented the conductivity of polymer phase and hence the local electrical field imposed on LC droplet, which overcome the threshold for driving and reduced operating voltage and response times. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5590–5596, 2007     

Transmission Holographic Polymer Dispersed Liquid Crystals Based on a Siloxane Polymer

The low surface energy and the great immiscibility of poly (dimethylsiloxane) (PDMS) with liquid crystals (LCs) are used in the fabrication of holographic polymer dispersed liquid crystals (HPDLCs). By adding increasing amounts of PDMS, the extent of the phase separation between the polymer and the LC, the LC channel width, and—eventually—also the diffraction efficiency of the film can be increased, while keeping the droplet size essentially the same. In addition, the presence of PDMS causes a decrease in the switching voltage and an increase in the response time. At an optimum content of PDMS (PUA40), a minimum switching voltage of 4 V μm^?1, a rise time of 0.20 ms, and a decay of 14.75 ms were obtained. Regarding the effect of the LC content, an overshoot of the diffraction efficiency was observed when the amount of LC exceeded 35 %, which can be attributed to droplet coalescence.     

Effects of multiwalled carbon nanotube on holographic polymer dispersed liquid crystal

The morphology and electro‐optical performance of holographic polymer dispersed liquid crystal (HPDLC) were investigated with the addition of multiwalled carbon nanotube (CNT), both pristine and chemically modified one (CNT‐C?C). With the addition of CNT, the diffraction efficiency increased and showed a maximum at 0.6% while nucleation was delayed due to the increased mixture viscosity. Film was driven only with CNT due to the induced local electric field of polymer to overcome the threshold resistance. Among the two types of CNT, chemically modified one gave finer CNT dispersion, lower mixture viscosity, larger liquid crystal (LC) droplet, higher diffraction efficiency, and shorter response time while the pristine CNT decreased the driving voltage. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of silicone monomer on the grating formation of holographic PDLC

To augment polymer-LC phase separation and driving characteristics of holographic polymer dispersed liquid crystals (HPDLCs), various types and contents of silicone monomer viz. vinyloxytrimethylsilane (VOTMS), 1,3-divinyltetramethyldisiloxane (DVTMDS) and 3-dicyclohexyl-1,1,3,3-tetrakis (dimethylvinylsilyloxy) disiloxane (DCHTDS) were added to the conventional formulations for HPDLC in a broad range of content. It was found that grating formation becomes fast in proportion to its content regardless of the type of silicon monomer due to the enhanced polymer-LC phase separation driven by the increased immiscibility. SEM and AFM showed that DCHTDS gave by far the greatest extent of phase separation. VOTMS and DVTMDS preferentially migrated toward the surface or polymer-LC interface, which effectively lowered the surface anchoring and reduced driving voltage. Due to the droplet coalescence and poor migration of DCHTDS toward the interfaces, DCHTDS gave low diffraction efficiency and high driving voltage in spite of its great and fast phase separation with LC molecules.     

Synthesis and properties of azo dye aligning layers for liquid crystal cells

The synthesis and properties of azo dyes that can be used for photoaligning liquid crystals (LCs) have been investigated. The structures and the synthetic procedure for the azo dyes are presented. The photoaligning of azo dyes takes place purely due to the reorientation of the molecular absorption oscillators perpendicular to the UV light polarization. The qualitative model for the phenomenon in terms of the rotational diffusion of the azo dye molecules in the field of the polarized light is discussed. The order parameters S = -0.4 (80% of the maximum absolute value S_m = -0.5) were measured from the polarized absorption spectra at the wavelength 372 nm. A temperature stable pretilt angle of 5.3° was obtained by a two-step exposure of the azo dye film using normally incident polarized light followed by oblique non-polarized light. The azimuthal anchoring energy of the photoaligned substrate was A_ϕ ≈10^-4 J m^-2, which is the same as the anchoring of the rubbed polyimide (PI) layer. The voltage holding ratio value of a photoaligned LC cell was found to be even higher than for a rubbed PI layer, which enables the applications of azo dyes as aligning layers in active matrix liquid crystal displays. The thermal stability of the photoaligned azo dye layers is sufficiently high, but UV stability has to be improved, e.g. by polymerization. A new LCD aligning technology based on polymerized azo dye layers is envisaged.     

Surfactant effects on morphology and switching of holographic PDLCs based on polyurethane acrylates.

Effects of octanoic acid (OA) on the morphology, diffraction efficiency, and electro-optic properties of the transmission mode of holographic polymer-dispersed liquid crystals (HPDLC) are studied. Droplet size decreases with increasing OA content (0-9 %), and this leads to a monotonic increase in off-state diffraction with increasing OA content. However, on-state diffraction decreases with increasing applied voltage and shows a minimum at 6 % OA, for which minimum switching voltage (5 V microm(-1)) and maximum contrast ratio (10) are obtained. Rise time and decay time decrease with increasing OA content. Interposition of OA between polymer and LC droplet is theoretically predicted by the spreading coefficient (lambda>0) calculated on the basis of the solubility parameter, while the coalescence behavior of droplets is described by a dimensionless group (gamma d rho / mu(2)) called coalescence number.     

Dye-dependent studies on droplet pattern and electro-optic behaviour of polymer dispersed liquid crystal

In order to study the droplet pattern and electro-optic (EO) behaviour of polymer dispersed liquid crystal (PDLC) with the addition of dye, dichroic polymer dispersed liquid crystal (DPDLC) films were prepared using a nematic liquid crystal (NLC), photo-curable polymer (NOA 65) and anthraquinone blue dichroic dye (B2), in equal ratio (1:1) of polymer and liquid crystal (LC) by polymerisation induced phase separation (PIPS) technique. Dichroic dye was taken in different concentration (wt./wt. ratio) as 0.0625%, 0.125%, 0.25%, 0.5% and 1% of the LC mixture in DPDLC films. Initially, in an open circuit when there is no proviso for external electric field (0 V), LC droplets in polymer matrix exhibited bipolar pattern, though on closing the circuit with the increase of electric field pattern of droplets starts changing, LC molecules align along the direction of applied electric field and aligned completely relatively at higher field (30 V), which illustrate vertical radial pattern. Further, results show that the DPDLC film containing 0.0625% dye concentration with consistent average droplet size ~4.30 μm, exhibits the best transmission at lower operating voltage.     

Cholesteric liquid crystalline siloxanes with azo dye. Generation of additional reflection bands with linearly polarized light

Photosensitive cholesteric polysiloxanes, which contain an azo dye, were irradiated with linearly polarized light. The cholesteric samples were oriented in the Grandjean texture. Before irradiation they reflected circularly polarized light in the near infrared region. For perpendicular incidence, only one order of reflection was observed. Upon irradiation with linearly polarized light, which is absorbed by the azo dye, additional reflection bands appeared in the visible part of the spectrum. It turned out that the additional reflection is caused by a new Bragg type grating which shows higher reflection orders. The formation of the grating is based on the periodic deformation of the helical ordering of the molecules. The deformation is periodic, as due to photoselection, only dye molecules in equidistant layers with a suitable orientation absorb radiation. For low exposure, the grating reflects linearly polarized light. After continued irradiation, the reflection bands disappear almost completely. High birefringence, strongly dichroic dye absorption and the loss of the reflecting properties prove that a planar nematic texture has developed. The formation of this texture from the Grandjean texture is a new example for photoinduced rotational diffusion.     

Ag/MgO串联协同催化降解偶氮染料

以纳米氧化镁为载体,采用浸渍法制备一系列过渡金属负载型催化剂。测试其对染料的降解性能后筛选出了效果最优的催化剂Ag/MgO,并采用X射线衍射(XRD)、X射线光电子能谱(XPS)、透射电子显微镜(TEM)、扩展X射线吸收精细结构谱(EXAFS)以及X射线吸收近边结构(XANES)等表征方法对该催化材料的结构特征、微观形貌进行分析。通过表征分析发现Ag是以纳米簇的形式均匀地分散在MgO表面,Ag和Mg之间形成了双金属位点,且催化剂中Ag的电子密度较高,从而具有较高的催化活性。在甲醛溶液中,在室温、无需光热等条件下即可高效降解偶氮染料AR1。该反应体系中影响降解效果的因素主要是温度和甲醛浓度,温度升高,降解效率增大,但甲醛浓度有最优值(1 mol·L^-1)。通过自由基捕获实验测得反应中有2种自由基在发挥作用,即具有还原性的氢自由基和具有氧化性的超氧自由基,它们的协同作用可以将染料分子中的显色基团(—N=N—)轻易地破坏,这种"还原-氧化"的协同作用机制提高了反应效率。此外,醛类也是常见污染物之一,将其作为助剂的同时达到了"以污治污"的效果。     

Orientational control of liquid crystal molecules via carbon nanotubes and dichroic dye in polymer dispersed liquid crystal

Polymer dispersed liquid crystals (PDLCs) using nematic liquid crystal and photo-curable polymer (NOA 65) were prepared by polymerisation-induced phase separation technique, in equal ratio (1:1) of polymer and liquid crystal (LC). We demonstrate that doping of small amount (0.125%, wt./wt.) of multiwall carbon nanotubes (CNTs) and orange azo dichroic dye in PDLC generously controlled the molecular orientation, dynamics of LC in droplet and size of droplets. The effects of multiwall CNTs and dye on PDLCs were studied in terms of transition temperature, droplet morphology, transmittance characteristic, contrast ratio and response time. The results exhibited that the values of the threshold electric fields were reduced from 8 V/µm (pure PDLC) to 1.18 and 1.72 V/µm, doped with multiwall CNTs and dye, respectively. The CNTs-doped PDLC shows faster switching response as compared with pure PDLC and dye-doped PDLC. However, dye-doped PDLC shows much higher contrast among all PDLC samples. Further, the results also illustrate that the birefringence value of LC in PDLCs was changed with doping of CNTs and dye.     

Control of liquid crystal droplet configuration in polymer dispersed liquid crystal with macro-iniferter polystyrene

A polystyrene macro-iniferter was applied to control the alignment of liquid crystal molecules at the droplet wall of polymer dispersed liquid crystal (PDLC) films. The aspects of the alignment were monitored by observing the droplet in the PDLC film. With increasing the macro-iniferter polystyrene in the composition, the configuration of LC droplets changes from bipolar to radial. This is because the high concentration of the macro-iniferter polystyrene results in a small surface interaction between the LC and the polymer matrix, which favours the formation of radial configuration. The radial configuration was stable under our conditions. However, increasing the LC and the initiator concentrations resulted in the change from radial to bipolar.     

Selective response of cyclodextrin-dye hydrogel to metal ions

The design and preparation of novel hydrogels have gained much interest recently, and in spite of the promising results to date, increasing pressure remains to develop novel gels based on stimuli-responsive systems. Here we present an anisotropic supramolecular gel of gamma-cyclodextrin and a simple azo dye in which physical gelation is completed through specific host?Cguest interaction. The obtained hydrogel exhibits respective morphological transitions based on supramolecular assembly and dissociation, leading to either precipitation or a sol-to-gel transition. It can identify different classes of metal ions, and, among them, naked-eye differentiation of lead ion is possible due to the coordination-induced unthreading of dye molecules. Gel-to-sol transitions can also be realized using chromium ions as a denaturing reagent. Accompanying structural changes were identified by various characterization techniques, including 2D-ROESY, HR-MAS, UV?CVisible absorption, small-angle X-ray scattering, and induced circular dichroism measurements. The hydrogel properties discussed here will be useful in analytical applications such as metal ion sensing and removal applications.     

Polymerization-induced phase separation III. Morphologies and contrast ratios of polymer dispersed liquid crystals

Polymerization induced phase separation in mixtures of liquid crystals (LCs) and acrylates (Merck TL205/PN393) proceeds by liquid-gel demixing, in most cases of practical interest. At high LC content or low temperature of polymerization liquid-liquid separation cannot be excluded. Depending on the elasticity and homogeneity of the polymer network at the onset of phase separation, spherical or non-spherical LC domains are observed; non-spherical domains reflect an inhomogeneous gel structure. The change from spherical to non-spherical occurs in a very narrow range of LC concentrations and curing temperatures. The transition between these two morphologies can be explained using conversion phase diagrams obtained from the Flory-Huggins-Dusek theory. The contrast ratio of PDLCs made from the Merck mixture passes through a maximum when the droplet shape at the onset of phase separation changes from spherical to non-spherical. Lowering the LC content or increasing the temperature leads to smaller LC domains which scatter less efficiently. The reverse changes lead to early phase separation and large LC domains which also scatter inefficiently. It is speculated that the maximum of the contrast ratio is related to secondary phase separation, leading to subdomains of an appropriate size.     

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 %.     

Anti-corrosion and anti-microbial evaluation of novel water-soluble bis azo pyrazole derivative for carbon steel pipelines in petroleum industries by experimental and theoretical studies

In this research, we investigated the synthesis of a novel water-soluble bis azo pyrazolin-5-one (ABP) which was synthesized efficiently via the regioselective reaction of hydrazine with coumarin hydrazone (CMH). Also, we evaluate their anti-corrosion and anti-bacterial behavior. The inhibition efficiency of ABP in an acidic medium (1.0 M HCl) was evaluated using various electrochemical and surface morphology measurements. The novel bis pyrazole-based azo dye ABP (16 × 10^?6 M) demonstrated a higher protection capacity (93.3 %). Tafel curves revealed that ABP was a mixed-type inhibitor. The adsorption of ABP on the C-steel (CS) surface is proven by the alteration in (R_ct and C_dl) impedance characteristics and obeyed the Langmuir isotherm model. SEM/EDX, AFM, and XPS surface examinations confirmed the enhancement of an adsorbed film protects the CS surface from acid corrosion at the appropriate dose. Furthermore, theoretical calculations using DFT and MC simulations were performed to identify the active sites on ABP molecules in charge of the adsorption and surface protection of the CS. The adsorption of bis pyrazole-based azo dye on the metal surface explained the protection mechanism. Moreover, the ABP screened for its antimicrobial activity against sulfate-reducing bacteria (SRB), and the calculated inhibition efficiency was 100 %. The current work presents significant results in manufacturing and producing novel water-soluble bis pyrazole-based azo dye derivative with high anti-corrosion and anti-microbial efficiency.     

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 %.     

新型复合电极与偶氮染料分子的氧化降解反应的研究

介绍了具有合成H_2O_2和光催化性能的双功能型复合电极、双功能复合电极是 将TiO_2/C光催化剂负载在具有合成H_2O_2性能的新型载体上形成的。在光反应器 中,复合电极作阴极,钌-钛不溶性电极作阳极,低压汞灯作光源,实现了光化学 氧化与光催化氧化在同一电极/溶液界面上的联合作用。当反应器工作时,复合电 极中的TiO_2/C光催化层表面进行着光催化反应,在载体的三相界面上进行着O_2的 2电子H_2O_2的电化学反应,电流效率达82%（J = 15 mA/cm~2）,为·OH自由基的 生成提供了物质源,而且氧的2电子还原反应电位使其表面的TiO_2获得相对于平带 电位约+0.5 V的阳极偏压,改善了TiO_2的光催化活性。实验结果表明,复合电极 对提高偶氮染料分子活性艳红（K-2BP）的氧化降解速度起了重要作用。实验发现 偶氮染料分子在复合电极表面的吸附量与反应速度密切相关。文中讨论了复合电极 的作用原理及偶氮染料分子的氧化降解过程。     

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     

Influence of oil polarity on droplet growth in oil-in-water emulsions stabilized by a weakly adsorbing biopolymer or a nonionic surfactant

The influence of oil type (n-hexadecane, 1-decanol, n-decane), droplet composition (hexadecane:decanol), and emulsifier type (Tween 20, gum arabic) on droplet growth in oil-in-water emulsions was studied. Droplet size distributions of emulsions were measured over time (0-120 h) by laser diffraction and ultrasonic spectroscopy. Emulsions containing oil molecules of low polarity and low water solubility (hexadecane) were stable to droplet growth, irrespective of the emulsifier used to stabilize the droplets. Emulsions containing oil molecules of low polarity and relatively high water solubility (decane) were stable to coalescence, but unstable to Ostwald ripening, irrespective of emulsifier. Droplet growth in emulsions containing oil molecules of relatively high polarity and high water solubility (decanol) depended on emulsifier type. Decanol droplets stabilized by Tween 20 were stable to droplet growth in concentrated emulsions but unstable when the emulsions were diluted. Decanol droplets stabilized by gum arabic exhibited rapid and extensive droplet growth, probably due to a combination of Ostwald ripening and coalescence. We proposed that coalescence was caused by the relatively low interfacial tension at the decanol-water boundary, which meant that the gum arabic did not absorb strongly to the droplet surfaces and therefore did not prevent the droplets from coming into close proximity.