13C solid state NMR investigation of structural relaxation of the polymer backbone in poly (ethylmethacrylate)

Geometry and time scale of structural relaxation of poly(n-alkylmethacrylates) above the glass transition is studied by temperature dependent one- and two-dimensional 13C-NMR spectroscopy. The geometry of the isotropization of the polymer backbone as deduced from detailed analysis of spectral line shapes is identified as random angular jumps. Analysis of echo decays confirms that at a given temperature this isotropization can adequately be described with a single correlation time. The results are discussed in terms of conformational memory and local structure recently identified in these polymeric glasses.     

Non-markovian properties of dielectric relaxation in liquids

Statistical molecular memory is investigated in dielectric relaxation in a liquid medium. A new method of closing the infinite chain of kinetic equations for the time correlation function is proposed, and used to obtain an equation permitting the calculation of the smallest-order memory functions. The spectrum of the non-Markovian parameter obtained indicates that dielectric relaxation in liquid CH₃I is a significantly non-Markovian process.Kazan' State Pedagogical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 27–31, October, 1995.     

Screening and fundamental length scales in semidilute Na-DNA aqueous solutions

The fundamental length scales in semidilute Na-DNA aqueous solutions have been investigated by dielectric spectroscopy. The low- and the high-frequency relaxation modes are studied in detail. The length scale of the high-frequency relaxation mode at high DNA concentrations can be identified with the de Gennes-Pfeuty-Dobrynin correlation length of polyelectrolytes in semidilute solution, whereas at low DNA concentrations and in the low added salt limit the length scale shows an unusual exponent reminiscent of semidilute polyelectrolyte chains with hydrophobic backbone. The length scale of the low-frequency relaxation mode corresponds to a Gaussian chain composed of correlation blobs in the low added salt limit, and to the Odijk-Skolnick-Fixman value of the single chain persistence length in the high added salt limit.     

Low-temperature relaxations in amorphous polyolefins

The dynamic mechanical relaxation behavior (1cps) of two series of amorphous polyolefins, ─(CH₂)_mC(CH₃)₂─ and ─(CH₂)_mC(CH₃)(CH₂ CH₃)─ where m = 1, 2, 3 was investigated from 4.2°K to the glass transition. Most of the polymers show a damping maximum or pleateau in the 40 to 50°K region. Various mechanisms which have been suggested for cryogenic relaxations in amorphous polymers are considered as they might relate to the polyolefins. Two secondary relaxation processes above 80°K are distinguished. A relaxation at about 160°K (β) in the second and third member of each series is associated with restricted backbone motion. This process requires a certain degree of chain flexibility since it is not observed in the first member of each series. A lower Temperature process (γ) is observed in each member of the second series and is attributed to motion of the ethyl side group.     

Dielectric relaxation spectrum of water studied by the site—site generalized Langevin/modified mode-coupling theory

The dielectric relaxation spectrum of water is calculated from the site-site generalized Langevin/modified mode-coupling theory. The main part of the relaxation follows the Debye-type function, and a small deviation from the Debye relaxation is found on the high-frequency side. This tendency is consistent with recent experiments, although the absolute relaxation time does not agree with the experimental value quantitatively. The time development of the longitudinal polarization function resembles the dielectric part of the memory function, and we consider that this is because the dielectric friction dominates the collective reorientation of the dipole moment of water. We performed calculations with different dielectric constants using the reference interaction-site model integral equation, and found that the large gap between the time scales of the dielectric relaxation and the longitudinal polarization relaxation causes the Debye-type dielectric relaxation in our theory when the dielectric friction is dominant in the friction on the collective reorientation of the dipole moment. Namely, the longitudinal polarization relaxation is fast enough to be considered as a white noise to the dielectric relaxation process, so that the relaxation becomes a Markov process. The large gap between the two relaxation times originates from a large local field correction owing to the large dielectric constant of water. It is also suggested that the deviation from the Debye relaxation at the high-frequency side is the manifestation of the slow memory caused by the long-time part of the longitudinal polarization relaxation in the low-wavenumber region.     

13C direct detected COCO-TOCSY: a tool for sequence specific assignment and structure determination in protonless NMR experiments

A novel experiment is proposed to provide inter-residue sequential correlations among carbonyl spins in (13)C detected, protonless NMR experiments. The COCO-TOCSY experiment connects, in proteins, two carbonyls separated from each other by three, four or even five bonds. The quantitative analysis provides structural information on backbone dihedral angles phi as well as on the side chain dihedral angles of Asx and Glx residues. This is the first dihedral angle constraint that can be obtained via a protonless approach. About 75% of backbone carbonyls in Calbindin D(9K), a 75 amino acid dicalcium protein, could be sequentially connected via a COCO-TOCSY spectrum. 49 [Formula: see text] values were measured and related to backbone phi angles. Structural information can be extended to the side chain orientation of aminoacids containing carbonyl groups. Additionally, long range homonuclear coupling constants, (4)J(CC) and (5)J(CC), could be measured. This constitutes an unprecedented case for proteins of medium and small size.     

Propagation and relaxation of tension in stiff polymers

We present a unified theory for the longitudinal dynamic response of a stiff polymer in solution to various external perturbations (mechanical excitations, hydrodynamic flows, electrical fields, temperature quenches, etc.) that can be represented as sudden changes of ambient/boundary conditions. The theory relies on a comprehensive analysis of the nonequilibrium propagation and relaxation of backbone stresses in a wormlike chain. We recover and substantially extend previous results based on heuristic arguments. New experimental implications are pointed out.     

Generalized hydrodynamic equations and correlation functions for thermoviscoelastic fluids

For very viscous liquids a phenomenological theory of thermoviscoelasticity is formulated, in which the retarded reaction of thermal variables, which arises from structural relaxation, is taken into account. The theory describes the effect of the slowing down of the structural relaxation near a glass transition on the fluctuation spectra of density and entropy; in particular, the intensity of the slow relaxational component of the fluctuation spectra, which is frozen in the glass below the glass transition, is derived. Conditions for positive energy dissipation and symmetry relations are obtained in the framework of thermodynamic relaxation theory, and the memory functions occurring in the Mori-Zwanzig projection operator formalism are calculated.Dedicated to K. Dransfeld on the occasion of his 60th birthday     

Detection of correlated dynamics on multiple timescales by measurement of the differential relaxation of zero- and double-quantum coherences involving sidechain methyl groups in proteins

Multiple effects may lead to significant differences between the relaxation rates of zero-quantum coherences (ZQC) and double-quantum coherences (DQC) generated between a pair of nuclei in solution. These include the interference between the anisotropic chemical shifts of the two nuclei participating in formation of the ZQC or DQC, the individual dipolar interactions of each of the two nuclei with the same proton, and the slow modulation of the isotropic chemical shifts of the two nuclei due to conformational exchange. Motional events that occur on a timescale much faster than the rotational correlation time (ps-ns) influence the first two effects, while the third results from processes that occur on a far slower timescale (mus-ms). An analysis of the differential relaxation of ZQC and DQC is thus informative about dynamics on the fast as well as the slow timescales. We present here an experiment that probes the differential relaxation of ZQC and DQC involving methyl groups in protein sidechains as an extension to our recently proposed experiments for the protein backbone. We have applied the methodology to (15)N, (13)C-labeled ubiquitin and used a detailed analysis of the measured relaxation rates using a simple single-axis diffusion model to probe the motional restriction of the C(next)H(next) bond vector where C(next) is the carbon that is directly bonded to a sidechain methyl carbon (C(methyl)). Comparison of the present results with the motional restriction of the C(next)C(methyl) bond (S(axis)(2)) reveals that the single-axis diffusion model, while valid in the fringes of the protein and for shorter chain amino acids, proves inadequate in the central protein core for long chain, asymmetrically branched amino acids where more complex motional models are necessary, as is the inclusion of the possibility of correlation between multiple motional modes. In addition, the present measurements report on the modulation of isotropic chemical shifts due to motion on the mus-ms timescale. Three Leu residues (8, 50, and 56) are found to display these effects. These residues lie in regions where chemical shift modulation had been detected previously both in the backbone and sidechain regions of ubiquitin.     

Slow dynamics of self-diffusion in metastable colloidal fluids

A simple theory for nonequilibrium density fluctuations in concentrated hard-sphere suspensions of interacting Brownian particles with both hydrodynamic and direct interactions is proposed. The correlation effects due to the many-body hydrodynamic interactions among particles are shown to cause a structural arrest in the relaxation of nonequilibrium density fluctuations. A volume fraction dependence of slow relaxation process in concentrated colloidal suspensions is thus explored from a new unifying point of view.     

Glassy vortex dynamics induced by a random array of magnetic particles above a superconductor

The magnetic relaxation of a Nb film covered with a random array of permalloy particles has been studied using various procedures. When the sample undergoes a field-cooled process, the magnetic relaxation becomes logarithmic in time. The relaxation rate is nearly temperature independent at low temperature and characteristic glassy dynamics-aging and memory effects-are observed. These results are interpreted as the consequence of pinning by the statistical variation of the number of nanoparticles within the area of a vortex core.     

Response of lysozyme internal dynamics to hydration probed by13C and1H solid-state NMR relaxation

We have studied the hydration dependence of the internal protein dynamics of hen egg white lysozyme by naturally abundant¹³C and¹H nuclear magnetic resonance (NMR) relaxation. NMR relaxation timesT ₁, off-resonanceT _1p and proton-decoupled on-resonanceT _1p (only for carbon expriments) were measured in the temperature range from 0 to 50°C. The spectral resolution in carbon cross-polarization magic angle spinning spectrum allows to treat methine, methylene and methyl carbons separately, while proton experiments provide only one integral signal from all protons at a time. The relaxation times were quantitatively analyzed by the well-established correlation function formalism and model-free approach. The whole set of the data could be adequately described by a model assuming three types of motion having correlation times around 10^?4, 10^?9 and 10^?12 s. The slowest process originated from correlated conformational transitions between different energy minima, the intermediate process could be identified as librations within one energy minimum, and the fastest one is a fast rotation of methyl protons the symmetry axis of methyl groups. A comparison of the dynamic behavior of lysozyme and polylysine obtained from a previous study (A. Krushelnitsky, D. Faizullin, D. Reichert, Biopolymers 73, 1–15, 2004) reveals that in the dry state both biopolymers are rigid on both fast and slow time scales. Upon hydration, lysozyme and polylysine reveal a considerable enhancement of the internal mobility, however, in different ways. The side chains of polylysine are more mobile than those of lysozyme, whereas for the backbone a reversed picture is observed. This difference correlates with structural features of lysozyme and polylysine discussed in detail. Due to the presence of a fast spin diffusion, the analysis of proton relaxation data is a more difficult task. However, our data demonstrate that the correlation functions of motion obtained from carbon and proton experiments are substantially different. We explained this by the fact that these two types of NMR relaxation experiments probe the motion of different internuclear vectors. The comparison of the proton data with our previous results on proton relaxation timesT ₁ measured over a wide temperature range indicates that at low temperatures lysozyme undergoes structural rearrangements affecting the amplitudes and/or activation energies of motions.     

Structure and Shape Memory Properties of Polyurethane Copolymers Having Urethane Chains as Soft Segments

Shape memory polyurethanes are usually fabricated with low-molecular weight polyols through a two-step copolymerization, which often results in difficulty attaining both desired shape memory switch temperature and optimal thermomechanical properties. Here we present a series of shape memory polyurethane copolymers having urethane chains as soft segments. The structure and shape memory properties of copolymers were investigated with differential scanning calorimetry, dynamic mechanical analysis, small angle x-ray scattering, and thermomechanical tests. Increasing the length of the urethane soft segments enhanced phase separation, while it brought little change to the glass transition temperature (T _g). Based on the urethane soft segments, some rigid chain extenders could be readily introduced into the backbone of copolymers, resulting in better phase separation. All polyurethane copolymers exhibited more than 90% of shape recovery. The shape recovery of the materials was proved to be inversely proportional to the fraction of hard phase and directly proportional to the stability of hard domains. The copolymers containing longer soft and hard segments and rigid chain extenders exhibited higher deformation stress and thus larger recovery stress. The copolymerization employing urethane chains as soft segments can greatly expand flexibility for molecular design and favor the optimization of shape memory properties.     

The Effect of Orientation Relaxation on Polymer Melt Crystallization Studied by Monte Carlo Simulations

We use dynamic Monte Carlo simulations to study the athermal relaxation of bulk extended chains and the isothermal crystallization in intermediately relaxed melts. It is found that the memory of chain orientations in the melt can significantly enhance the crystallization rates. The crystal orientation and lamellar thickness essentially depend on the orientational relaxation. Moreover, there is a transition of the nucleation mechanism during the isothermal crystallization from the intermediately relaxed melts. These results explain the mechanism of the self-nucleation by orientation and suggest that in flow-induced polymer crystallization, the orientational relaxation of chains decides the crystal orientation.     

Dynamics of conformations,hydrogen bonds and translational diffusion of poly(methacrylic acid) in aqueous solution and the concentration transition in MD simulations

The dynamic behaviour of chain conformations, hydrogen bonds and translational diffusion of aqueous poly(methacrylic acid) (PMA) solution as a function of polymer volume fraction Φ_p across dilute to concentrated regimes inclusive of the pure polymer amorphous state was studied by molecular dynamics simulations. The behaviour of the relaxation time (τ) of the backbone dihedral angle auto-correlation function (ACF) reveals slower relaxation at higher level of polymer concentration and the existence of a concentration-driven relaxation transition for the aqueous polymer solution which occurs in the polymer volume fraction range, specifically 54% < Φ_p < 82% for this system. The relaxation constant τ for backbone dihedral angle exhibits a linear variation with Φ_p, indicating a first-order kinetic transition. The intermittent ACF for decay of the H-bond correlation shows that H-bonds among water molecules relax faster than those of the PMA–PMA and PMA–water type. The relaxation rate of PMA–water H-bonds shows a decrease up to Φ_p = 72% and becomes faster at Φ_p = 82% due to the confining influence of neighbouring PMA chains. PMA–water and water–water H-bond dynamics show transitions around Φ_p = 72% PMA. With increase in Φ_p PMA diffusion coefficient decreases exponentially and water diffusion coefficient decreases linearly, in agreement with experimental observations using fluorescence and nuclear magnetic resonance (NMR) spectroscopic studies.     

Study on O-Terphenyl/Polystyrene Blends: How Plasticizer Affects the Glass Transition of Polystyrene?

The effect of plasticizer o-terphenyl on the glass transition of polystyrene was investigated by Fourier transform infrared spectroscopy from a new aspect. The peak areas of four conformation insensitive bands as a function of temperature were studied, these being assigned to the vibrational modes of main chain groups and side groups of polystyrene. It was shown that the reorientation relaxation temperature of the main chain around glass transition was lower than that of the side groups when polystyrene was plasticized by o-terphenyl. It was explained on the basis of cohesional entanglements of polystyrene chains. The reorientation relaxation region of the side groups was nearly the same as the macroscopically observable glass transition region of polystyrene, implying that the glass transition process of polystyrene was dominated by the reorientation of side groups.     

Structural relaxation in metastable alloys

A mechanism of stress relaxation in metastable alloys is considered. It is demonstrated that structural transformations during a relaxation test lead to anomalously high stress relaxation. Two stages of structural relaxation are revealed: formation of defect-free channels and decomposition of a solid solution. A new method of program hardening of metastable alloys based on the structural relaxation processes is suggested. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 56–61, November, 2007.     

一种改进的WOx忆阻器模型及其突触特性分析

忆阻器被定义为第四种基本电子元器件, 其模型的研究呈现多样性. 目前, 忆阻器模型与忆阻器实际特性的切合程度引起了研究者的广泛关注. 通过改变离子扩散项, 提出了一种新的WO_x忆阻器模型, 更好地匹配了忆阻器的实际行为特性. 首先, 新的模型不仅能够描述忆阻器的一般特性, 而且能够俘获记忆丢失行为. 另外, 将新的忆阻器作为神经突触, 分析了脉冲速率依赖可塑性、短期可塑性、长期可塑性, 并发现了与生物系统中极为相似的“经验学习”现象. 最后, 考虑到温度与离子扩散系数的关系, 探讨了温度对突触权值弛豫过程的影响. 实验表明, 新忆阻器模型比原来的模型更切合实际, 且更适合作为突触而应用到神经形态系统之中.     

Information processes in optical echo holography

A set-theoretical method for studying information processes in resonant media with phase memory is developed. The conversion of the classical information carried by the object laser pulse into the potential (structural) quantum information of the resonant medium that takes place during the recording of a phase echo hologram is studied. It is shown that a resonant medium with phase memory stores quantum information in the form of an information-phase grating (the distribution of qubits within the inhomogeneously broadened line of a resonant transition). The temporal evolution of this grating is studied as a function of the times of reversible and irreversible relaxation of the system.