非均匀剪切流场中液晶聚合物微观结构的无网格模拟

基于宏观流场控制方程与微观分子取向扩散方程耦合的微-宏观双尺度模型,率先采用无网格方法对液晶聚合物在非均匀剪切流场中的微观结构进行了模拟研究.无网格方法精度高、稳定性好的特性保证了模拟结果的可靠性.研究了Deborah数对平板Poiseuille流中液晶聚合物微观结构的影响,预测出非均匀剪切流场中液晶聚合物的一种单一结构和五种复合结构.指出在复合结构的过渡区,分子运动具有不稳定性,可能产生瑕疵. 关键词： 液晶 微观结构 双尺度 无网格     

Study of polyethylene structure by means of low-frequency Raman spectroscopy: Longitudinal acoustic mode and boson peak

The opportunities given by the low-frequency Raman spectroscopy for studying the ordered structures in polymers are presented and illustrated by examples of application of two techniques. The longitudinal acoustic modes (LAM) excited in straight chain segments of macromolecules are characteristic in linear semicrystalline polymers, while the Boson peak (BP) manifests itself in amorphous (glassy) polymers. Both these features are due to the laser light scattering from the acoustic vibrations localized either in the ordered one-dimension structures (LAM) or in the mid-range order correlation sphere (BP).     

掺铒聚合物狭缝波导放大器的增益特性研究

设计了一种高浓度稀土铒掺杂聚合物填充硅狭缝结构的平面光波导放大器（工作波长1 550 nm,泵浦波长1 480 nm）,能够在低泵浦下获得高增益,可以应用于硅基光互联的损耗补偿。通过扫描电镜照片观察发现,合成的铒掺杂聚合物材料具有良好的纳米狭缝填充能力。考虑铒离子的合作上转换和激发态吸收,利用铒离子四能级跃迁模型,建立原子速率方程和光功率传输方程,数值仿真分析了聚合物光学性质、狭缝波导结构参数及信号光泵浦光功率等放大器增益特性的影响因素。这种具有纳米截面尺寸的光波导放大器,获得4.5 dB的信号光相对增益仅需要1.5 mW的泵浦光,展现了良好的集成光学应用前景。为了进一步提高增益,引入了多层狭缝结构,四层狭缝波导的重叠积分因子比一层狭缝的高42%。     

Polymer Characterization by Raman Spectroscopy

Abstract

Although the Raman spectrum of polystyrene was measured as long ago as 1932 [1]. and the value of vibrational spectroscopy for the characterization of polymer structures became evident a few years later. Raman spectroscopy failed to make a significant impact in the field for many years. Indeed. in 1964 [2]. Nielsen commented that while infrared spectroscopy had yielded a great deal of information about the molecular and crystalline structure of high polymers. Raman spectroscopy had not been widely used or been very helpful in the investigation of polymers. He went on to note that the main reason for this state of affairs was that it had generally been quite difficult to obtain their Raman spectra but. nevertheless. efforts to overcome the difficulties facing the Raman spectroscopy of polymers would be very worthwhile.     

Definition and properties of ideal amorphous solids

It is proposed that two ideal amorphous structures, type I and type II, based on maximally random jammed packing of spheres of equal size, form a distinct class of ideal amorphous solids. The ideal amorphous structures contain wide variations in local density, limited by the condition of solidity. Four distinct characteristics, based on statistical geometry and topology, are shown to define this class. Voronoi tessellations carried out on simulated cells of random packed spheres and amorphous polymers give a broad distribution of individual volumes, skewed, with a tail at the high volume end.     

Silicon photonic structures with embedded polymers for novel sensing methods

At present time research and development of a new generation of optical sensors using conjugated polymers, in particular sensors of explosives are actively underway. Nevertheless, the problems of the sensitivity, selectivity, and stability of such sensors are still of great interest. One of the ways to solve the problem is the creation of luminescence sensors based on photonic crystals with a high specific surface area, which have significant sorption ability and allow to effective modulate emission properties of luminophores. In this paper, porous silicon microcavities with embeded organic polyphenylenevinylene- (PPV) and polyfluorene- (PF) type polymers were created. It was shown that polymer infiltration in porous silicon microcavities leads to modification of their luminescence properties, which is expressed in narrowing of the emission spectrum and changing of its directional pattern. It was demonstrated that such structures exhibit sensitivity to saturated vapors of trinitrotoluene. The structures proposed can be treated as a basis for development of new type of sensors used for detection of vapors of nitroaromatic compounds.     

Ab initio study of the electronic structures and conduction properties of some novel low band-gap donor-acceptor polymers

The results of the electronic structures and conduction properties of four novel donor-acceptor polymers based on polysilole, obtained on the basis of ab initio Hartree-Fock crystal orbital method using their optimized geometries, are reported. The repeat unit of these polymers consists of bicyclopentadisilole unit bridged by an electron-accepting group Y(Y=CCH₂ in PSICH, CO in PSICF, CCF₂ and CC(CN)₂ in PSICN). All the polymers on the basis of their geometries and π-bond order values are found to have benzenoid-type electronic structures. Comparison of the important electronic properties such as ionization potential, electron affinity and band-gap of these polymers indicates PSICN to be the best candidate for intrinsic conductivity and reductive (n-) doping while PSICH is predicted to be the best candidate for oxidative (p-) doping. All these polymers are estimated to have band-gap values ranging between 1 and 2 eV. The low band-gap values of these polymers are rationalised on the basis of the patterns of their frontier orbitals.     

Electronic structure of conjugated polymers: consequences of electron–lattice coupling

Conjugated organic polymers can be doped, via oxidation or reduction chemistry or via acid–base chemistry, to induce very high electrical conductivity. Conjugated polymers are beginning to find uses, in both the neutral and the doped states, in prototype molecular-based electronics applications and in electronic and opto-electronic devices. The physical basis for the many of the unusual properties of these new materials is discussed, at a sufficient level of approximation to enable an understanding of the important issues by the general condensed matter physicist. In particular, emphasis is placed on the interconnections of the electronic, geometric and chemical structures, in the ground state and especially in the excited states. The important role of electron–electron and electron–lattice interactions are pointed out, and justified through a combined experimental–theoretical approach.     

Substrate tuned reconstructed polymerization of naphthalocyanine on Ag(110)

The linkage structures between monomers make great influence on the properties of polymers.The synthesis of some special linkage structures can be challenging,which is often overcome by employing special reaction conditions.Here,we build dihydropentalene linkage in poly-naphthalocyanine on Ag(110)surface.Scanning tunneling microscopy(STM)and non-contact atomic force microscopy(nc-AFM)measurements confirm the dihydropentalene linkage structure and a possible formation path with reconstruction steps is proposed.The controlled experiment on Ag(100)surface shows no dihydropentalene structures formed,which indicates the grooved substrate is necessary for the reconstruction.This work provides insights into the surface restricted reactions that can yield special structures in organic polymers.     

Simultaneous process of embossing and poling at elevated temperatures: a simple technique for nonlinear grating formation in polymer films

A simple technique for fabrication of nonlinear gratings in polymer films, based on simultaneous embossing and poling, is proposed and demonstrated. A master grating consisting of a metal electrode with a dielectric die was fabricated and used for repeated embossing of the grating structures into nonlinear optical polymers at elevated temperatures. At the same time, we applied high voltage to the polymer films to induce second-order nonlinearity. The grating profile and the nonlinearity were estimated, as well as the mass productivity of nonlinear gratings.     

Pyrolysis of cellulose and lignin

X-ray and UV-induced photoelectron spectroscopy (XPS and UPS) and scanning electron microscopy (SEM) have been performed to characterise the pyrolysis of cellulose and lignin and their interaction with methanol. Clean highly oriented pyrolitic graphite (HOPG) was also analysed as a reference material. Asymmetric C1s core level fits and valence band XPS of the samples indicate a graphitic-like structure after the pyrolysis at 1200 °C. Due to the low polar contents in pyrolysed cellulose and lignin, an interaction with methanol under high vacuum conditions could not be identified. From a technical viewpoint a temperature of 1200 °C is attainable without high costs. Therefore, the pyrolysis of wood-based polymers containing high amounts of cellulose and lignin are potential low-cost materials for various applications. If it is possible to generate graphite in complex structures made of wood-based polymers, a cheap and energy-efficient method will become available for producing bipolar plates for fuel cells. Technical problems like form instability and foaming are discussed as well as further development and possible modifications of the ground material to achieve optimal compositions.     

Synthesis of conjugated polymers for application in light-emitting diodes (PLEDs)

In the field of electroluminescent organic materials, conjugated polymers have attracted much attention over recent years owing to their versatile synthesis, their relative ease of processing and the possibility of establishing predictive structure–function relationships between chemical structures and optical properties. This review article highlights the advances made in the synthesis of conjugated polymers for use in light-emitting devices (LEDs) covering the last two years. Research efforts were largely directed towards the improvement of the synthesis of monomers and polymers involving classical polymer structures such as poly( p -phenylene vinylene)s (PPVs), poly( p -phenylene)s (PPPs), poly(2,7-fluorene)s (PFs), or poly(2,5-thienylene)s (PTs). Control of the color of emission by modulation of the effective conjugation length, improving balanced charge injection and transport properties by introduction of electron-donating or -withdrawing moieties directly into the polymer backbone or enhancement of the emission efficiency by attempts to influence film morphology are some examples of recent research directions. All these investigations contributed to a significantly better understanding of the chemical and physical processes spanning topics from the manufacturing process to the operation of LEDs and leading to the announcement of the first commercial products. In addition to the classical π -conjugated polymers a few examples of rather unusual structures have also emerged.     

Optical Properties of Carbazole-Based Photorefractive Polymers

The optical properties of several carbazole-based photorefractive polymers which we prepared were determined. Through analysis of the UV-vis spectrum, the ratio of the azo nitrobenzene was adjusted to control the content of azo chromophore functional components. The solid transient fluorescence emission spectra of the photorefractive polymers were tested with a 325 nm emission wavelength. The results showed that the J-aggregates and several large closed-loop structures formed in the photorefractive polymers resulted in a long lifetime of their transient fluorescence. Spin-orbit coupling among the molecules increased the intersystem crossing rate, and induced delayed fluorescence.     

Polymer ablation: From fundamentals of polymer design to laser plasma thruster

UV-Laser ablation of polymers is a well-established method to structure and deposit polymers, but the mechanisms of ablation are still controversial, i.e. photothermal or photochemical processes. An approach to probe the ablation mechanisms and to improve ablation is to incorporate photoactive groups into the polymer structure.The investigation of the ablation behavior of designed triazene polymers showed that the ablation mechanism is always a combination of both photothermal and photochemical processes, but the ratio can be changed by using different polymers and irradiation wavelengths. Also the quality of structures in the triazene polymers is superior at an irradiation wavelength of 308 nm compared to commercially available polymers.Polymers can be designed not only for UV irradiation, but also for applications in the IR range, but with different requirements. One application for designed polymers in the near-IR range is as fuel for the laser plasma thruster, which is used as propulsion system for small satellites. With commercially available polymers the necessary thrust could not be achieved. A specially designed polymer-absorber system for this application produce more energy in the form of thrust, than the laser delivered.     

光在大分子聚合物中的解偏和散射

文章报道了光在大分子聚合物中的前向散射和解偏现象,并用Mie散射和Fraunhofe衍射理论对其进行了解释,提出了小角度散射的光解偏理论。     

Comparative Study of Tribological Properties of Polyphenylene Sulfide (PPS), Polyethersulfone (PES), and Polysulfone (PSU)

The tribological properties of polyphenylene sulfide (PPS), polyethersulfone (PES) and polysulfone (PSU), which have similar molecular structures, were investigated using an end-face contact tribometer and a reciprocating tribometer. The thermomechanical behavior of the polymers was analyzed using dynamic mechanical analysis (DMA). PPS exhibited a maximum friction coefficient with increasing load and sliding speed, while the friction coefficients of PES and PSU decreased only slightly. The wear rate of PPS was much lower than that of PES and PSU under high loads and speeds. It is suggested that the main factors influencing the friction and wear properties of the neat polymers are their condensed state and heat resistance. Amorphous PES and PSU showed liquid-like behavior and very low friction when the frictional surface was in the molten-flow state. The macromolecular crystals of crystallizable PPS give it some solid-like behavior and load-carrying capacity; hence PPS exhibited lower wear than PES and PSU.     

Pressure effect on phase transition in ferroelectic polymers

Pressure effects of phase transition behaviour in two kinds of ferroelectric polymers of poly(vinylidene fluoride), PVDF, and copolymers of vinylidene fluoride and trifluoroethylene, (VDF/TrFE), are discussed. In the case of PVDF, several high-pressure treatments including a high-pressure annealing and a uniaxial compression were shown to induce a crystal transformation from a non-polar Form II crystal to a polar Form I crystal, which has ferroelectric characteristics and high piezoelectric activity. In addition, substantial pressure effects on ferroelectric phase transition points as well as crystal structures were observed for (VDF/TrFE) copolymers with different VDF contents. The most significant pressure effects were observed for copolymer samples with unstable ferroelectric structures at atmospheric pressure. From high-pressure X-ray and Raman scattering studies, these pressure effects were suggested to originate from the pressure-induced conformational transition from gauche to trans in the molecular chains.     

Studies on the Electronic Transport Properties of some New Chelate Compounds Containing Siloxane

The temperature dependence of the electrical conductivity and the Seebeck coefficient of some new complex polymeric structures containing metal chelate sequences alternating with silane units were studied. The measurements were performed using thin films deposited from solution. The investigated polymers have interesting semiconducting characteristics. The correlations between these characteristics and the molecular structure of the respective polymers are discussed.     

Long-delay engraving and recovery of chirped femtosecond pulses by spectro-temporal holography in spectral hole-burning molecular solids

The persistent spectral hole-burning (PSHB) phenomenon observed in molecular doped polymers cooled down to liquid helium temperatures allows the engraving of spectral structures in the inhomogeneous absorption profile of the material. Therefore, a PSHB molecular-doped solid can be programmed in the spectral domain and then converted in an optical processor capable to achieve user-defined optical functions. We demonstrate the high storage capacity of naphthalocyanine-doped polymer materials by engraving and retrieving the phase information stored in femtosecond-chirped pulses, even with nanosecond time delay, which correspond to information registered with sub-GHz spectral resolution. Perspectives for the coherent control of light fields or photochemical processes are also evoked.     

On the role of pressure in elasto-inertial turbulence

The dynamics of elasto-inertial turbulence is investigated numerically from the perspective of the coupling between polymer dynamics and flow structures. In particular, direct numerical simulations of channel flow with Reynolds numbers ranging from 1000 to 6000 are used to study the formation and dynamics of elastic instabilities and their effects on the flow. Based on the splitting of the pressure into inertial and polymeric contributions, it is shown that the polymeric pressure is a non-negligible component of the total pressure fluctuations, although the rapid inertial part dominates. Unlike Newtonian flows, the slow inertial part is almost negligible in elasto-inertial turbulence. Statistics on the different terms of the Reynolds stress transport equation also illustrate the energy transfers between polymers and turbulence and the redistributive role of pressure. Finally, the trains of cylindrical structures around sheets of high polymer extension that are characteristics of elasto-inertial turbulence are shown to be correlated with the polymeric pressure fluctuations.