Synthesis and optical properties of poly(2,11-triphenyleneethynylene-alt-m-phenyleneethynylene)s

Triphenylene units are introduced into the backbone of a poly(m-phenyleneethynylene) (mPE). The new polymer, mPPET, exhibits strong intra-chain folding in good solvents in which the polymer is soluble, such as chloroform, benzene, and THF. In poor solvents, where the polymer is insoluble, such as acetonitrile, hexane, or DMSO, interchain aggregation dominates. In a solvent mixture of a good solvent and a poor solvent with appropriate ratios, mPPET appears to adopt a random coil conformation. Among the three conformations (intra-chain folding, interchain aggregation, and random coil chain), the intra-chain polymeric foldamer exhibits the highest fluorescence quantum yield. These results indicate that introducing a planar polynuclear aromatic compound into mPE not only increases its intra-chain folding propensity, but also may enhance its fluorescent properties. This approach may be applied to synthesize a variety of conjugated foldamers containing planar polynuclear aromatic hydrocarbons with interesting photophysical properties.     

Enhanced performance of P3HT/PCBM bulk heterojunction photovoltaic devices by adding spin ½ radicals

We have studied bulk heterojunction organic photovoltaic devices (solar cells) based on polymer/fullerene blends using both electrical and magneto-optical methods. We show that adding spin ½ galvinoxyl radicals to the device active layer, consisting of a regio-regular polythiophene/fullerene derivative [P3HT/PCBM] blend, significantly improves the device performance. Compared to pristine photovoltaic devices, the radical-rich devices show improved short-circuit current density, fill factor, and power conversion efficiency. The enhanced device performance is attributed to a reduced geminate recombination rate and improved carrier transport, both of which result from spin–spin interactions between the radical impurities and the photogenerated carriers. Optically detected magnetic resonance, a technique that is sensitive to spin-lattice relaxation rates, is used to verify the proposed mechanism.     

Polaron formation dynamics in conducting polymers

In the framework of the extended Su–Schrieffer–Heeger model, the formation mechanism of a polaron is studied by calculating the interaction force between a hot electron and the lattice in a conducting polymer. It is found that the formation of a polaron depends upon the energy of the hot electron. Regardless of whether an electron comes from an injection, doping or a dissociated state of a polaron, it will be trapped by the lattice to form a polaron only when its energy is lower than a critical value. The force field between the lattice and the hot electron with a lower energy has a localized characteristic, which causes polaron formation in a conducting polymer. For a hot electron with much higher energy, the force field is extended over the whole polymer chain, which cannot trap the electron and therefore a polaron does not form in this case.     

A study of the different structural phases of the polymer poly(9,9′-dioctyl fluorene) using Raman spectroscopy

The polymer poly(9,9′-dioctyl fluorene) (F8) can be easily driven between a number of different structural phases by different thermal or solvent treatments. The phases which we have identified include (i) as-spin-coated, (ii) semi-crystalline, (iii) glassy, (iv) with an extended intrachain conformation. Atomic force microscopy has been used to investigate the surface structures associated with the different phases of the polymer and different solvents used to cast the film. Raman spectroscopy has been performed on these samples to understand how the polymer conformation changes with the phases. Variations in the relative intensity of the peaks and shifts in energy are observed. We have compared the F8 spectrum with a trimer of fluorene with butyl side chains and acrylate/hydroxy endgroups attached to the fluorene main chain via hexyl spacers. We identify a very small frequency dispersion in the vibrational modes connected to an increase of the conjugation length. Therefore, the assignment of the vibrational modes of the trimer is very important for understanding the F8 spectrum.     

线圈外置式磁流变减摆器设计与实验研究

针对传统的线圈内置式磁流变减摆器线圈散热性差、线圈密封性不好等问题,设计了一种新型的线圈外置式磁流变减摆器。通过磁流变减摆器的磁路设计、结构设计,建立了磁路结构参数之间的约束关系方程组,为优化设计提供了理论依据;并且利用参数化建模和有限元优化分析方法,优化了外置式磁流变减摆器的磁路结构参数。优化和仿真结果验证了最终结构参数取值的合理性。并通过静态阻尼特性实验验证了分析结果的正确性。     

Mesoscopic study of bipolar charge transport in polymer-based devices

We present generalised Monte-Carlo calculations of bipolar charge carrier transport in polydiacetylene (PDA) to assess effects of some key factors on the properties of polymer-based light-emitting diodes (PLEDs). Our mesoscopic model includes specific realisations of the polymer network and examines the effect of polymer structural order on current flow and charge recombination within the polymer layer. Specifically addressed are the issues concerning the fractions of polymer strands contributing to the charge injection processes at electrode interfaces and to electroluminescence (EL). Our results suggest that radiative recombination increases as short chain concentration increases, whereas current efficiency shows an opposite behaviour. Radiative recombination decreases as charge injection efficiency decreases and its internal efficiency seems to saturate for a luminescent chain concentration >30%. Our results also suggest that both current efficiency and space-charge increases as charge injection efficiency decreases. Polymer disorder due to inclusions in the network does seem to contribute to reduce both current and recombination efficiencies.     

Dynamics of charge transfer in molecular switches

With impurity molecules working as switches, the charge transfer on a single conducting polymer chain is studied. The chain is modeled by a modified tight-binding Hamiltonian extended to include the effects of an external field and the parameters of the switching molecules. The charge transfer through the sites that work like a switch is analyzed by the numerical integration of the equations of motion. Two basic types of molecular switches are studied: single and pairs of donor—acceptor molecules bonded to the chain. The main differences between these two models of switches are determined. We have found that the single radical switch has an anisotropic character and only works for solitons with the same parity of bonding site. For the donor—acceptor pair we have encountered that the chain offers a wider range of devices, from simple switches to perfect molecular rectifiers. The influence of the parameters of the molecules on the charge transfer and the changes they must undergo to characterize the molecular switch are obtained. The role of the length of separation between the sites where the donor and acceptor molecules bond is clarified. The optimum switch configuration is determined.     

The thermochromism of poly(3-alkylthiophene)s: the role of the side chains

A combined atomistic simulation involving static lattice and molecular dynamics simulation methods investigates the structure of poly(3-alkylthiophene)s in order to study their thermochromism. Taking the shortest alkyl chain (butyl) that makes the polymer thermochromic, conformational changes are monitored at different temperatures. The simulations demonstrate that there is a tendency for the side chain to twist between 40 and 70° about the C–C bond linking it to the thiophene ring, although the carbon atoms in the side chain largely retain a trans conformation to above room temperature. These results, and the prediction of a non-zero setting angle, are qualitatively in accord with lattice powder diffraction structures. There is a step-like increase in the mean angle between the thiophene rings as a function of temperature at T=340 K, suggesting the occurence of an abrupt reduction in the planarity of the conjugated chain at this temperature that should lead to an increased band width and thermochromism. This is shown to be concerted with similar changes in side chain torsions at the same temperature, on the basis of which a tentative mechanism for the thermochromism is proposed.     

Thermochromic and solvatochromic effects in poly(3-hexylthiophene)

We report thermochromic and solvatochromic effects in the soluble conductive polymer poly(3-hexylthiophene). Thin solid films of the polymer change colour at higher temperatures. The colour of solutions of the polymer can be changed by varying the composition of the solvent. Optical absorption in good solvents is similar to that of thin solid films at higher temperatures, while in poor solvents, absorption features similar to those found in solid films at low temperatures are observed. We discuss these effects with reference to the existence of a hitherto disregarded type of conformational defect, conformons. These are rotational defects on the polymer chain, leading to loss of planarity. High vacuum studies of the thermochromic effect in thin films using X-ray photoemission spectroscopy support this interpretation. The standard series of defects in conductive polymers, solitons, polarons and bipolarons, may have to be extended to include conformons.     

Stabilizing ceramic suspensions using anionic polyelectrolytes: adsorption kinetics and interparticle forces

The interactions between adsorbed polyelectrolyte layers have been studied using direct force measurements with an atomic force microscope. The results were correlated to adsorption studies using ellipsometry and we found that interactions can be described as electrosteric where the relative importance of the electrostatic and steric contribution varies with the adsorption and solution conditions. For adsorption at high pH (same charge on polyelectrolyte and surface), the adsorbed amount is relatively low but the adsorbed chains attain a relatively extended conformation. Reducing the pH results in a collapse of the adsorbed layer with a larger change in layer thickness at low ionic strength. For polyelectrolyte adsorbed at low pH (opposite charge on a weakly dissociated polyelectrolyte and surface), the adsorbed amount is higher and little chain expansion was observed when increasing pH.     

特种合金软接触电磁近终成形技术的基础研究

以低熔点金属锡作为特种合金液模拟工质 ,对高频磁场作用下模壳内合金液的弯月面形态进行热模拟研究 ,测定了弯月面高度及软接触高度 ,掌握了电源功率、感应线圈参数和磁化套参数对弯月面形态的影响效果。研究表明 :电源功率、线圈高度和工作位置及磁化套高度和工作位置将影响弯月面形态及稳定性 ,影响合金液与模壳的软接触 ,是影响软接触成形过程稳定性的重要参数。     

Effect of alkyl side-chain length and solvent on the luminescent characteristics of poly(3-n-alkylthiophene)

In poly(3-n-alkylthiophene) (P3AT), the length of alkyl side chain as well as the solvent used for film casting affected the conformation and eventually the luminescent characteristics of the polymers. When the length of alkyl side chain was shorter than 12 carbon atoms, the electronic absorption and PL spectra of P3AT film showed red shift as the length of the alkyl side chain increased. This is because, when the length of the alkyl side chain is not too long, solvation is promoted with increase in the length of the alkyl side chain, leading to expanded chain structures with longer π-conjugation lengths. Conversely, when the length of the alkyl side chain was longer than 12 carbon atoms, the electronic absorption and PL spectra of P3AT film showed blue shift as the length of the alkyl side chain increased. This is because, when the length of the alkyl side chain is too long, the steric hindrance becomes predominant leading to highly distorted chain conformation with shorter π-conjugation lengths. The PL intensity of the film cast from THF was higher by an order of magnitude than that of the film cast from chloroform, toluene, or p-xylene. In the P3AT films cast from a poor solvent such as THF, the polymer chains are highly distorted leading to high PL intensity due to exciton localization. In the EL spectra of P3AT films cast from various solvents, λ_max shifted as much as 140 nm depending upon the solvent characteristics. This result demonstrates that the color of EL can be varied from blue to orange simply by casting the same polymer from a different solvent.     

基于LabVIEW的远场涡流管道检测系统

为了提高管道无损检测的精度,实现管道内外壁缺陷的检测,采用远场涡流的检测方法,依据检测线圈接收到的信号是激励线圈产生磁场两次穿过管壁后的信号,携带了管壁内外缺陷信息的原理,设计了远场涡流传感器、检测信号处理电路。对检测线圈接收信号进行放大、滤波处理,信号采集;采用LabVIEW编程计算检测信号幅值,互相关算法计算检测信号相位,确定缺陷深度。对内径36mm的有伤管道进行了试验。结果表明,所设计的仪器能检测出管道上深度为0．5mm及以上缺陷。利用LabVIEW对相位差计算提高了检测精度,为实际应用提供参考。     

Generating and detecting torsional guided waves using magnetostrictive sensors of crossed coils

In this work, we propose a crossed-coil magnetostrictive sensor consisting of a solenoid coil, a toroidal coil, and a ferromagnetic patch for generating and detecting torsional waves for pipe inspection. We show that the direction of the magnetic field of the ferromagnetic patch can be controlled by adjusting the input current to the toroidal coil. In addition, it is found that the amplitudes of the signals reflected from the pipe end using a crossed-coil sensor are much larger than those using pre-magnetization techniques, and excitation energies are used to generate the torsional waves only. From results on pipes with multiple circumferential notches, any defects larger in diameter than three times the pipe thickness were detected using the crossed-coil magnetostrictive sensor.     

Determination of the density of recombination centers in a photorefractive guest—host polymer

The density of deep traps for charge carriers significantly determines both the amplitude and the phase of the internal space-charge modulation in photorefractive polymers. We have measured the photoconductivity of a photorefractive guest—host polymer as a function of the external electric field and the sample temperature. Using theoretical models for charge carrier generation, transport and recombination, the density of recombination centers can be extracted. It is closely related to the density of deep traps. The results are compared with the density of pertinent traps, as obtained from phase shift measurements by holographic two-beam coupling techniques.     

On the photovoltaic spectral response of polyacetylene films

Photocurrent spectral responses of trans-polyacetylene films in both Al Schottky junctions and liquid junctions are measured and analysed in the range 400–1000 nm. The role of charge photogeneration is discussed and some evidence is presented on the importance of processes involving geminate recombination and dissociation of electron-hole pairs.     

The search for east transits using the Auston switch technique in single crystal polydiacetylenes

Studies of pulsed photoconduction in polydiacetylenes with a 35ps temporal resolution, employing the Auston switch electrode geometry, are interpreted as showing that at short times, prior to formation of a Solitary Wave Acoustic Polaron, a small number of hot carriers are responsible for the observed signal. At longer times, carriers which had earlier thermalised within the potential well of their twin and subsequently avoided geminate recombination, give rise to a slowly decaying signal (ρ∼50ns) as they drift with a saturated velocity close to the speed of sound. Because of the surface electrode geometry and the high electrical anisotropy of polydiacetylene crystals, decays in narrow gap samples are unable to reveal a well defined transit time and a featureless decay is observed irrespective of the gap width.     

Single-photon emission from a single nanoparticle consisting of a single conjugated polymer chain

Single-photon behavior in the emission from a single nanoparticle consisting of a single poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV, molecular weight: 2,600,000 amu) chain has been investigated. Photon correlation measurements of a number of single nanoparticles revealed that the probability of single-photon emission from the single nanoparticles clearly increases, compared with that of single chains embedded in host polymer matrices, poly(methyl methacrylate) (PMMA) and polystyrene (PS). The result suggested that even a single MEH-PPV chain with high molecular weight, which does not exhibit single-photon emission in PMMA and PS, is forced to exhibit single-photon emission by adopting “compact” chain conformation like nanoparticle. Efficient exciton migration and exciton annihilation processes in the compact chain conformation most probably result in the efficient single-photon emission from the single nanoparticle. The present results indicate that multi-quantum systems consisting of a large number of chromophores, such as the MEH-PPV, can be made to behave as single-photon sources by appropriately controlling their size.     

Optically detected magnetic resonance (ODMR) of polarons and charge-conjugation symmetry in conducting polymers

The main narrow photoluminescence(PL)-enhancing g = 2.0025 ± 0.0010 X-band ODMR of poly(3-alkylthiophene) (P3AT) and poly(2,5-dialkoxyparaphenylenevinylene) (PDOPV) films, solutions, and blends is described and discussed. The resonances all closely match a sum of narrow and broad Gaussians. In P3AT the narrow Gaussian saturates at a lower power than the broad Gaussian. While this ODMR is believed to result from intrachain ‘distant pair’ polaron recombination, the assignment of each Gaussian to the proper polaron is undecided. The g-values of the two Gaussians are nearly identical in PDOPV, apparently reflecting a very high degree of charge-conjugation symmetry (CCS). They are, however, quite distinct (g = 2.0032, 2.0019, respectively) in P3AT, possibly due to the sulphur heteroatom on the thiophene rings. The ramifications of this CCS violation for the PL quantum yield are discussed. In all cases, the spectral dependence of the narrow resonance is similar to the PL spectrum, indicating that the polaron levels are close to the band edges and to the levels of the (presumably) short-lived geminate singlet excitons. The sensitivity of the ODMR intensity to sample processing suggests that it may possibly be affected by an unidentified native, possibly conformational, defect. The decay modes competing with polaron recombination are also discussed.     

应重视发展钢材深度加工领域

论述了我国钢材整体产能过剩形势下,发展钢材深度加工的重要性;对钢材深度加工的概念、分类和范围作了阐述;并指出钢材生产产业链的延伸是提高钢材使用效率、发展循环经济的有效途径.