On the dynamics of polymers in dense systems – Results of neutron spin echo spectroscopy

One of the basic problems in the dynamics of polymers concerns the importance of geometrical or topological interactions which are directly related to the large scale molecular structures. In the famous reptation model these constraints are pictured in terms of a tube of localization following the average chain profile and confining the chain motion to the curve‐linear tube. Recently studying the dynamic structure factor of a single labeled chain in a polymer melt by means of neutron spin echo spectroscopy (NSE) led to a direct observation of these tube constraints. Here I shall summarize these neutron spin echo experiments. I shall address the NSE technique, present results on the entropy driven segmental chain dynamics, discuss the dynamics of single chains in the melt where the chain length is increased through the transition to “reptation” dynamics and display NSE measurements on long chain systems which revealed the molecular existence of the entanglement distance. Their magnitudes agree very well with tube diameters derived from dynamical mechanical measurements on the basis of the reptation model proving thereby the basic assumption of this Nobel Price winning concept. This revised version was published online in August 2006 with corrections to the Cover Date.     

Oxygen-enhanced atomic chain formation

We report experimental evidence for atomic chain formation during stretching of atomic-sized contacts for gold and silver, that is strongly enhanced due to oxygen incorporation. While gold has been known for its tendency to form atomic chains, for silver this is only observed in the presence of oxygen. With oxygen the silver chains are as long as those for gold, but the conductance drops with chain length to about 0.1 conductance quantum. A relation is suggested with previous work on surface reconstructions for silver (110) surfaces after chemisorption of oxygen.     

CoTiSb基体中Ni元素诱导的单自旋通道研究

沿半Heusler结构CoTiSb合金的[001]晶体学方向, 利用Ni元素连续替换一条原子链上的Ti, Sb原子, 在半导体性CoTiSb基体中设计了一系列均匀分布的Ni基单原子链阵列. 采用第一性原理方法, 研究了Ni基单原子链的电子结构和磁性质, 发现Ni-Sb单原子链具有高度自旋极化率和空穴导电特性, Ni-Ti及Ni-Ni单原子链具有100%的自旋极化率, 并且在CoTiSb基体中形成了以这种Ni基单原子链为中心的、尺寸非常小的单自旋纳米柱通道.     

Effect of hydrogen on the formation of the atomic structure of linear carbon chains: An ab initio approach

An ab initio study of the features of the formation of the atomic structure of carbon chains, which are structural parts of films of linear carbon chains, has been performed using the electron density functional theory. It has been shown that the formation and stabilization of experimentally observed kinks of carbon chains require the presence of hydrogen impurities in the chamber during the synthesis of linear carbon chains. The kinks of a carbon chain are formed in the process of the adsorption of hydrogen atoms on a chain. The optimal kink angle of carbon chains has been determined. The stability of kinks of carbon chains has been estimated as a function of the length of linear fragments. An additional mechanism has been proposed for the formation of kinks in carbon chains owing to the joining of short carbon chains with hydrogen ends.     

Dielectric behaviour of emeraldine base polymer–ZnO nanocomposite film in the low to medium frequency

Emeraldine base (EB) polymer–ZnO nanoparticles composite films has been synthesized by solution casting technique on ITO-coated glass substrate and characterized by XRD, FTIR and TEM for their structure and morphology. Dielectric behaviour of these composite films has been investigated in the very low frequency region to medium frequency region (1 kHz–1 MHz). The dielectric constant of the composite with 30% nanoparticles is almost one-tenth of the pure EB. The dielectric value becomes constant in the frequency region greater than 400 kHz. The change in dielectric behaviour of the composite is explained on the basis of multilayered interface formed between the ZnO nanoparticles and emeraldine chains. Nanoparticles have high energy surface which is responsible for the decrease of free volume for the orientation of polymer chains consequently decrease in dielectric constant of the composite. TEM images shows about 10 nm ZnO particles embedded in the emeraldine matrix. From the XRD data it has been observed that the lattice parameters of ZnO have been modified due to the alignment of polymer chains along the basal planes of the nanoparticles. The shift of N=Q=N and N–B–N vibration bands to higher wave number in IR indicates that interaction between emeraldine chain and nanoparticles which provides stability to emeraldine matrix.     

Anomalous polymer sedimentation far from equilibrium

A single flexible polymer in strong sedimentation fields is investigated using hydrodynamic simulations and scaling arguments. For short chains and small fields compaction is observed. For elevated fields or long chains the chain stretches and the sedimentation coefficient decreases, in agreement with ultracentrifuge experiments on linear as well as circular DNA. For very large fields a tadpole forms consisting of a compact leading head and a trailing stretched tail.     

Quality Control Kit for Determination of Insulin by Radioimmunoassay Method

Insulin is one of early discovered hormones, in 1889. Its structural formula has been discovered by Sanger and it consists of two long polypeptide chains binded with two disulphide bonds. It is considered that in the labelled insulin the 75 per cents out of the total radioactivity was binded in a chain A, having acidic properties and 21 amino-acid, while the chain B has alkali properties and contains 30 amino acids.     

Electronic structure of long chain polyenes in the Peierls-PPP model

On the basis of the semi-empirical Peierls-PPP Hamiltonian (i.e. the PPP Hamiltonian with electron-phonon coupling included) a theory of the electronic structure of long atomic chains with half filled bands has been worked out within the framework of the mean field approximation. The general theory is then applied to long chain polyenes. The results of the numerical analysis are in good agreement with those of other authors and show that the ground state of a long chain polyene is very likely not antiferromagnetic, but a charge density wave state with an associated Peierls distortion.     

Dynamics of strongly entangled polymer systems: activated reptation

The effect of excluded-volume interactions on the reptation dynamics of long polymer chains is considered theoretically. It is shown that interactions give rise to an exponential increase of the reptation time, , if polymer chains are long enough: , where is the number of monomers per entanglement. We propose a novel dynamical mechanism of activated reptation implying that neighboring chains exchange conformations of their terminal fragments. It is shown that the exchange mechanism is compatible with the equilibrium polymer chain statistics and that it provides a bridge between the previous theories. Received: 25 July 1997 / Accepted: 8 October 1997     

Mixed PbSi dimer chains on Si(1 0 0): a first-principles study

It has been a common belief that the one-dimensional structures observed by STM at low coverage of Pb on Si(1 0 0) are buckled Pb-Pb dimer chains. However, using first-principles density functional calculations, we found that it is energetically favorable for Pb adatoms to intermix with Si atoms to form mixed dimer chains on Si(1 0 0), instead of Pb-Pb dimer chains as assumed in previous studies. Up to a Pb coverage of 0.125 ML, mixed PbSi dimer chain is 0.19 eV per Pb atom lower in energy than Pb dimer chain.     

Effects of N-Phosphoproline on the Phospholipid of Human Erythrocytes Membrane

The effects of phosphorylated proline on the packing and conformation of the membrane phospholipid have been studied. Generally it causes the polar parts of the phospholipid to become more mobile, the packing of the long chain of phospholipid to become more loose, the long chain to be in a lower trans/gauche ratio, and the end of the long chain to become more orderly. Any change in the level of phosphorylated proline affects the fluidity of the membrane phospholipid. Free proline and phosphorylated proline have different effects on the aliphatic chains of phospholipid.     

Initial stage of adsorption of octithiophene molecules on Cu(111)

The initial stage of the adsorption of octithiophene (8T) molecules on a Cu(111) surface is investigated using a scanning tunneling microscope at room temperature. We find a characteristic molecular chain structure of 8T molecules on a terrace of the Cu(111) surface, which has not been reported so far for adsorption of oligothiophene molecules on metal surfaces. Up to the adsorption of 0.26 monolayer (ML), 8T molecules in the molecular chain align with their long axis parallel to the Cu<11-2> direction. With increasing coverage, there appear 8T molecules that align with their long axis parallel to the Cu<110> direction. The appearance of different molecular orientations is understood by the decrease of the number of the adsorption sites for extending the molecular chains. Fragments of 8T molecules, such as single thiophene molecules, are also observed in this work. They are trapped only at the step edges of the Cu(111) surface at the beginning and later trapped in a small Cu(111) region surrounded by 8T molecules.     

基于分布链修正的磁流变弹性体的物理模型

在考虑磁流变弹性体中链的方向分布的基础上,对磁流变弹性体的偶极子模型作出了修正.用局部场的方法计算了链的势能,引入了分布函数来描述链的分布,并分析了与磁场方向不一致的斜链的磁流变效应,进而通过积分叠加求得含有分布链的磁流变弹性体的磁流变效应.在磁流变弹性体的理论模型中,引入了制备磁场和基体性质等影响因素.     

Effects of Intrinsic Decoherence on Information Transport in a Spin Chain

Considering Milburn＇s intrinsic decoherence effect on quantum communication through a spin chain, we show that the transfer quality for quantum state and entanglement will obviously decrease with the increasing intrinsic decoherence rate. Some odd chains are much higher than even ones for the state transfer efficiency. The state transfer of a long chain is very sensitive to the intrinsic decoherence, which turns out to be an obstacle for information transport.     

Dynamics of chain molecules in disordered materials

The dynamic behavior of hard chains in disordered materials composed of fixed hard spheres is studied using discontinuous molecular dynamics simulations. The matrix induces entanglements in the chain fluid, i.e., for high matrix densities the diffusion coefficient D scales with the chain length N as D is approximately N(-2). At high matrix densities the rotational relaxation time becomes very large but the translational diffusion is not affected significantly; i.e., the chains display a dynamic heterogeneity reminiscent of probe diffusion in supercooled liquids and glasses. We show that this is because some chains are trapped, and move via a hopping mechanism. There are no signatures of this dynamic heterogeneity in the matrix static structure, however, which is identical to that of a hard-sphere liquid.     

A review of arc-discharge method towards large-scale preparation of long linear carbon chains

Linear carbon chains as new one-dimensional (1D) nanomaterials attract attention for the predicted outstanding properties. However, the high reactivity of linear carbon chains hampers further experimental research. To date, different methods have been developed to synthesize new materials containing linear carbon chains. Among them, the arc-discharge method is a practical way to prepare both finite and infinite linear carbon chains. This review provides a brief discussion of the recent progress in the techniques to prepare carbon chain-based materials and then focuses on the arc-discharge method. The configuration of apparatus, optimal conditions, and the corresponding mechanism of arc-discharge method to prepare long linear carbon chain inside multi-walled carbon nanotubes are summarized in detail. The characterization techniques are introduced to evaluate the quality of products. Moreover, remaining challenges and perspectives are presented for further investigation of long linear carbon chains.     

Operator product expansion on a fractal: The short chain expansion for polymer networks

We prove to all orders of renormalized perturbative polymer field theory the existence of a short chain expansion applying to polymer solutions of long and short chains. For a general polymer network with long and short chains we show factorization of its partition sum by a short chain factor and a long chain factor in the short chain limit. This corresponds to an expansion for short distance along the fractal perimeter of the polymer chains connecting the network vertices and is related to a large mass expansion of field theory.

The scaling of the second virial coefficient for bimodal solutions is explained. Our method also applies to the correlations of the multifractal measure of harmonic diffusion onto an absorbing polymer. We give a result for expanding these correlations for short distance along the fractal carrier of the measure.     

Spectral self-similarity,one-dimensional Ising chains and random matrices

Partition functions of one-dimensional Ising chains with specific long distance exchange between N spins are connected to the N-soliton τ-functions of the Korteweg-de Vries (KdV) and B-type Kadomtsev-Petviashvili (BKP) integrable equations. The condition of translational invariance of the spin lattice selects infinite-soliton solutions with soliton amplitudes forming a number of geometric progressions. The KdV equation generates a spin chain with exponentially decaying antiferromagnetic exchange. The BKP case is richer. It comprises both ferromagnets and anti ferromagnets and, as a special case, includes an exchange decaying as 1/(i − j)² for large |i − j|. The corresponding partition functions are calculated exactly for a homogeneous magnetic field and some fixed values of the temperature. The connection between these Ising chains and random matrix models is considered as well. A short account of the basic ideas underlying the present work has been published in JETP Lett. 66 (1997) 789.     

Exact solutions in the dynamics of alternating open chains of spins s = 1/2 with the XY Hamiltonian and their application to problems of multiple-quantum dynamics and quantum information theory

A method for exactly diagonalizing the XY Hamiltonian of an alternating open chain of spins s = 1/2 has been proposed on the basis of the Jordan-Wigner transformation and analysis of the dynamics of spinless fermions. The multiple-quantum spin dynamics of alternating open chains at high temperatures has been analyzed and the intensities of multiple-quantum coherences have been calculated. The problem of the transfer of a quantum state from one end of the alternating chain to the other is studied. It has been shown that the ideal transfer of qubits is possible in alternating chains with a larger number of spins than that in homogeneous chains.     

Energy structure of a finite Haldane chain in Y2BaNi0.96Mg0.04O5 studied by high field electron spin resonance

This Letter presents the fine structure of energy levels for the edge states of a Haldane chain. In order to investigate the edge states, we have performed high field and multifrequency electron spin resonance (ESR) measurements of finite length S=1 antiferromagnetic chains in Y2BaNi0.96Mg0.04O5. Owing to the high spectral resolution by high fields and high frequencies, observed ESR signals can be separated into the contributions of the finite chains with various chain lengths. Our results clearly show that the edge spins actually interact with each other through the quantum spin chain and the interaction depends on the chain length N. This N dependence has been obtained experimentally for the first time, and shows that the correlation length xi in the real system is somewhat larger than that calculated by a simple Heisenberg model.