Long-range depletion forces induced by associating small molecules

This is the first report of experimental observations of depletion interactions in solutions of a (hydrogen-bonded) reversible supramolecular polymer. Depletion forces were measured directly by colloidal probe atomic force microscopy. The range of the depletion force is consistent with existing independent experimental data. The interaction can be tuned by adding monofunctional chain stoppers to the solution, a possibility which is unique to supramolecular polymers. The depletion force is shown to be strong enough to induce phase separation in a colloidal suspension.     

Coexistence of single-mode and multi-longitudinal mode emission in the ring laser model

A homogeneously broadened unidirectional ring laser can emit in several longitudinal modes for large enough pump and cavity length because of Rabi splitting induced gain. This is the so called Risken-Nummedal-Graham-Haken (RNGH) instability. We investigate numerically the properties of the multi-mode solution. We show that this solution can coexist with the single-mode one, and its stability domain can extend to pump values smaller than the critical pump of the RNGH instability. Moreover, we show that the multi-mode solution for large pump values is affected by two different instabilities: a pitchfork bifurcation, which preserves phase-locking, and a Hopf bifurcation, which destroys it.     

高相对分子质量8-羟基喹啉锂聚合物的制备和性能

高分子电致发光显示器(PLED)是近几年来国际、国内的研究热点,取得了很大的进展,其中高分子化金属配合物是一类很有价值的功能材料。通过甲基丙烯酸甲酯(MMA)、苯乙烯(S)和含有8-羟基喹啉的单体共聚合成模板聚合物,再与氢氧化锂作用,实现了8-羟基喹啉锂配合物的高分子化,获得了一种能够溶解在普通的溶剂中的高相对分子质量的含喹啉锂配合物的发光聚合物,并利用元素分析、¹H-NMR、FTIR、UV、PL光谱、DSC、TGA、GPC等方法对其结构和性能作了表征。紫外吸收和光致发光(PL)光谱说明合成共聚物的发光来自于Liq基团,引入的可聚合的链段以及共聚物中的甲基丙烯酸甲酯或苯乙烯链段,并没有影响发光波长的改变。亲核溶剂改变8-羟基喹啉金属配合物分子结构,使共聚物光谱明显红移20nm左右。     

Scattering from solutions of star polymers

We study the scattering intensity of dilute and semi-dilute solutions of star polymers. The star conformation is described by a model introduced by Daoud and Cotton. In this model, a single star is regarded as a spherical region of a semi-dilute polymer solution with a local, position dependent screening length. For high enough concentrations, the outer sections of the arms overlap and build a semi-dilute solution (a sea of blobs) where the inner parts of the actual stars are embedded. The scattering function is evaluated following a method introduced by Auvray and de Gennes. In the dilute regime there are three regions in the scattering function: the Guinier region (low wave vectors, ) from where the radius of the star can be extracted; the intermediate region () that carries the signature of the form factor of a star with f arms: ; and a high wavevector zone () where the local swollen structure of the polymers gives rise to the usual q ^-5/3 decay. In the semi-dilute regime the different stars interact strongly, and the scattered intensity acquires two new features: a liquid peak that develops at a reciprocal position corresponding to the star-star distances; and a new large wavevector contribution of the form q ^-5/3 originating from the sea of blobs. Received: 3 September 1997 / Revised: 13 January 1988 / Accepted: 31 March 1998     

The stochastic heat equation: Feynman-Kac formula and intermittence

We study, in one space dimension, the heat equation with a random potential that is a white noise in space and time. This equation is a linearized model for the evolution of a scalar field in a space-time-dependent random medium. It has also been related to the distribution of two-dimensional directed polymers in a random environment, to the KPZ model of growing interfaces, and to the Burgers equation with conservative noise. We show how the solution can be expressed via a generalized Feynman-Kac formula. We then investigate the statistical properties: the two-point correlation function is explicitly computed and the intermittence of the solution is proven. This analysis is carried out showing how the statistical moments can be expressed through local times of independent Brownian motions.     

Nonstationary heating during VUV photochemical ablation of polymers

According to a previously developed pure photochemical model of VUV laser ablation of polymers, the velocity of ablation front is proportional to surface intensity, and a stationary value of the surface temperature does not depend on laser intensity. Previous estimations show, however, that this stationary surface temperature could be too high to be relevant to the photochemical mechanism. This raises a question of whether the stationary value of the surface temperature can be achieved for a given time shape of light intensity coming to the surface irradiated by a laser pulse of high enough fluence. The intensity time shape is connected not only with the time shape of a laser pulse but also with screening of laser radiation by the plume. This problem is discussed in the present communication. Specifically, it is shown that with a hyperbolic surface intensity time shape, heat diffusion can successfully compete with laser heating decreasing maximum surface temperature compared to its stationary value. The hyperbolic surface laser intensity corresponds to a rectangular laser pulse screened by plume during the photochemical ablation. This allows one to estimate that the photochemical model for a multiple-pulse VUV laser ablation with a high plume extinction coefficient is self-consistent even for a high value of stationary temperature and for high enough laser fluences. PACS 42.62.-b; 44.05.+e; 82.53.-k     

可溶性高聚物-多壁碳纳米管复合物的吸收光谱和光限幅性能研究

合成了三种可溶性高聚物嫁接的多壁碳纳米管复合物,包括聚乙烯基咔唑-多壁碳纳米管复合物(MWNTs-PVK),聚苯乙烯-多壁碳纳米管复合物(MWNTs-PSt)和聚甲基丙烯酸甲酯-多壁碳纳米管复合物(MWNTs-PMMA)。合成样品的TEM图显示出高聚物成功地包裹在碳纳米管外。样品的氯仿溶液UV-Vis 吸收谱显示,复合物的紫外区与纯的多壁碳纳米管不同,有一些复杂的特征吸收峰,反映了高聚物与碳纳米管共价连接的作用。利用Z扫描技术通过527 nm的纳秒激光脉冲激发研究了这三种复合物的非线性光学性质和光限幅性能。这三种复合物的氯仿溶液的光限幅性能很相似,并且优于纯多壁碳纳米管氯仿悬浮液和C60甲苯溶液。为解释观察到的结果,研究了样品的非线性折射,非线性吸收和非线性散射机制。结果显示,非线性吸收是引起样品光限幅的主要机制。     

非线性声波方程二次谐波高阶近似解及实验验证

非线性声波方程的传统二次谐波二阶近似解不够精确,对测量材料的非线性系数造成较大误差.利用摄动法展开非线性声波方程得到一系列非齐次偏微分方程,根据低阶解的性质拟定高阶特解的形式,通过符号计算求出高次谐波特解,对所有二次谐波成分求和最终获得二次谐波的高阶近似解.在水中开展非线性声学实验验证高阶近似解,结果表明:二次谐波相对...     

Review on the properties of the ferrorelaxor polymers and some new recent developments

It has been found that by introducing defects into the P(VDF-TrFE) copolymers, it is possible to convert the polymer from a normal ferroelectric to a relaxor ferroelectric. A new class of ferroelectric polymers, i.e., the terpolymers of P(VDF-TrFE-CFE) or of P(VDF-TrFE-CTFE), was developed from the normal ferroelectric PVDF-TrFE polymer by employing proper defect modifications which eliminate detrimental effects associated with a normal first order F?CP transition while maintaining high material responses. Relevant studies show that this class of electroactive polymers offers unique properties in comparison with other ferroelectric polymers. The syntheses of these relaxor ferroelectric polymers have been done by a combination of the suspension polymerization process and an oxygen-activated initiator at a temperature of 40°C. Films from cast solution can be made in different lengths and thicknesses. Stretching of these films increases the performance as well as the mechanical properties. These relaxor-ferroelectric terpolymers P(VDF-TrFE-CFE), P(VDF-TrFE-CTFE) are multifunctional, i.e., electrostrictive material, dielectric for electric energy storage. The terpolymer exhibits high electrostrictive strain (>7%) with relatively high modulus (>0.4?GPa). Examples of devices applications using unimorphe systems are presented. Micropump and Optical device concerning a liquid-filled varifocal lens on a chip are described.     

Self-Attractive Random Walks: The Case of Critical Drifts

Self-attractive random walks (polymers) undergo a phase transition in terms of the applied drift (force): If the drift is strong enough, then the walk is ballistic, whereas in the case of small drifts self-attraction wins and the walk is sub-ballistic. We show that, in any dimension d ≥ 2, this transition is of first order. In fact, we prove that the walk is already ballistic at critical drifts, and establish the corresponding LLN and CLT.     

Applicability of partial monomer volumes derived from the regular solution theory

We demonstrate that the regular solution theory is qualitatively successful in describing the composition dependence of partial monomer volumes in a compressible binary mixture of polymers only when the mixture is symmetric (see text for proper definition). For asymmetric mixtures, the partial monomer volumes can be non-monotonic with composition under ordinary conditions of temperature and pressure, which contradicts the prediction of the regular solution theory. The investigation enables us to conclude that the composition dependence of the partial monomer volumes obtained from the regular solution theory can be considered trustworthy only for a low degree of asymmetry.     

Confined space and effective interactions of multiple self-avoiding chains

We study the link between three seeming-disparate cases of self-avoiding polymers: strongly overlapping multiple chains in dilute solution, chains under spherical confinement, and the onset of semidilute solutions. Our main result is that the free energy for overlapping n chains is independent of chain length and scales as n9/4, slowly crossing over to n3, as n increases. For strongly confined polymers inside a spherical cavity, we show that rearranging the chains does not cost an additional free energy. Our results imply that, during cell cycle, global reorganization of eukaryotic chromosomes in a large cell nucleus could be readily achieved.     

A brief review on the wear mechanisms and interfaces of carbon based materials

Abstract

The demand for reducing wear and friction has become the chief aim in the automotive industry nowadays. The usage of lubricant is not considered enough as there is still room for improvements. As a solution, much research has arisen towards what we called self-lubricating ideas, in order to reduce friction better than lubricant. This paper presents an overview wear mechanism and the interface of carbon-based materials. This paper will also discuss the interfaces by carbon as substrate and coating layer. The findings show that for metals, the predominant wear mechanisms were abrasion and fatigue. Meanwhile, for polymers and coating (DLC), they were abrasive along with adhesive wear. The surface roughness of the substrate plays a crucial role in increasing the excellent performance of the DLC coating. The interfaces of carbon elements definitely give huge impact on both self-lubricant materials and coatings where the coefficient of friction and wear rate changes drastically even with 1 wt.% addition. Nevertheless, a clear understanding of the factors that affect the tribological performance is very essential in performance improvement for potential applications.     

Silane-modified polymers as interphases in glass-fibre-reinforced composites: 2. Grafting and crosslinking of interphase materials

_—This paper presents an interphase engineering technique suitable for grafting silane-modified polymers onto glass fibres to be used in composites with enhanced impact tolerance. The silane-modified polymers include ethylene polymers grafted with γ-methacryloxypropyltrimethoxysilane (MPS) and a copolymer of butyl acrylate (BuA) and MPS. The grafting of functionalized interphase materials onto glass fibres is performed in solution. By changing the concentrations of the solutions, different amounts of polymer can be deposited on the fibres. Water crosslinking of the polymer gives the possibility of producing stabilised interfacial polymer coatings over a range of thicknesses. It is concluded that acidic conditions (1) promote the grafting of silane-modified polymers on glass fibres and (2) for a given reaction time, increase the amount of crosslinked polymer in the interphase, i.e. yield more stable interphases. It is also likely that preserving acidic conditions at the fibre/polymer interface is important for maintaining bonding across the interface. It is shown that polystyrene/glass-fibre composites having SEBS at the interface are promising candidates for high-impact-tolerance composites.     

具有共轭结构的导电聚合物及其应用

具有共轭结构的导电聚合物在微电子学和光电子领域有着广泛的应用前景。文章简要介绍了这类聚合物材料的主要合成方法、导电机理及其应用概况，并对目前该领域的研究进展作了扼要评述。     

Preparation and Characterization of New Biodegradable Films Made from Poly(L-Lactic Acid) and Chitosan Blends Using a Common Solvent

Poly-(L-lactic acid) (PLLA) has been widely used for various biomedical applications due to its interesting properties such as its mechanical behavior, processability, biocompatibility, and biodegradability. Blending this polymer with chitosan that, besides being biodegradable and hydrophilic, can interact with anionic glycosaminoglycans, proteoglycans, and other negatively charged molecules of the extracellular matrix, could constitute an excellent way to improve the biological performance of PLLA in these kinds of applications. Such blends could also be used in environmental applications. In this work a new and simple method of preparing biodegradable blends of chitosan and PLLA at room temperature was developed. To the best of our knowledge, this is the first time that a common solvent for the two polymers has been used, hexafluor-2-propanol (HFIP), to produce a homogeneous solution containing both PLLA and chitosan. We also anticipate that this solvent can also be used to compatibilize other combinations of natural and synthetic polymers. Membranes were then obtained by solvent casting. Films with different fractions of each component were successfully prepared and didn't show visible phase separation. The prepared films were characterized by differential scanning calorimetry (DSC) in order to analyze the miscibility of the two components as a function of the composition of the film.     

Influence of pressure on yield and fracture in polymers

Some new data on the effects of pressure on a poly(p-oxybenzoyl) polymer are presented and existing data on various other polymers are reviewed and analyzed. It is demonstrated that the effect of pressure on the elastic response of a polymer depends on the location of T_g relative to room temperature, and that the modulus-pressure data can be used to estimate the pressure shift of T_g. Also, the pressure coefficient of the modulus increase can be deduced from considerations of finite strain elasticity theory. There is a marked increase of tensile and compressive yield strengths with pressure and this can be interpreted in terms of a Mohr-Coulomb type of yield criterion. In some polymers, hydrostatic pressure inhibits cold drawing and reduces the elongation to fracture. However, in other polymers which at atmospheric pressure fracture prior to yielding, increasing the pressure above some critical value can cause significant increases in ductility. This effect is utilized to show that even rigid high-temperature polymers, like polymide, can be successfully cold extruded at room temperature if a proper high-pressure environment is present. The nature of the changes occurring near the brittle-ductile transition pressure have been investigated by use of the scanning electron microscope and the possible influence of the pressure medium has been examined. SEM pictures of fracture surfaces and fracture modes of various polymers will be presented.     

Direct numerical simulation study of the interaction between the polymer effect and velocity gradient tensor in decaying homogeneous isotropic turbulence

Direct numerical simulation of decaying homogeneous isotropic turbulence (DHIT) of a polymer solution is performed. In order to understand the polymer effect on turbulence or additive-turbulence interaction, we directly investigate the influence of polymers on velocity gradient tensor including vorticity and strain. By visualizing vortex tubes and sheets, we observe a remarkable inhibition of vortex structures in an intermediate-scale field and a small-scale field but not for a large scale field in DHIT with polymers. The geometric study indicates a strong relevance among the vorticity vector, rate-of-strain tensor, and polymer conformation tensor. Joint probability density functions show that the polymer effect can increase "strain generation resistance" and "vorticity generation resistance", i.e., inhibit the generation of vortex sheets and tubes, ultimately leading to turbulence inhibition effects.     

含联二萘的PPV类共轭聚合物发蓝光材料的设计合成

利用Wittig和Wittig-Horner反应合成了四种含有联二萘和不同芳香基团的具有线性或超支化结构的发光共聚物,并对共聚物的光物理性质和电化学性质进行了研究。结果表明,这些线性共聚物都具有较高效率的蓝光发射。具有超支化结构的共聚物BN-TPPV在氯仿溶液中的荧光量子效率高达95%,在氯仿溶液和膜中的发射光谱峰值分别位于393,428nm,实现了高效率的蓝光发射。说明将联二萘生色团引入聚合物是一种实现发蓝光材料的重要途径。