Local Relaxation Processes Research Articles

Structure and water dynamics of aqueous peptide solutions in the presence of co-solvents

We perform neutron diffraction and quasi-elastic neutron scattering (QENS) to probe hydration water structure, and dynamics down to supercooled temperatures, of a concentrated amphiphilic peptide system with the co-solvents glycerol and dimethyl sulfoxide. We find that the kosmotropic co-solvent glycerol preserves the hydration structure near the peptide that is observed in the water solvent alone, that in turn preserves the dynamical temperature trends of two water relaxation processes--one corresponding to a localized relaxation process of the peptide bound surface water and a second relaxation process of the outer hydration layers. By contrast the chaotropic co-solvent, by disrupting the hydration layer near the peptide surface, eliminates the inner hydration layer relaxation process induced by the peptide, to show a single time scale for translational water dynamics.

Ferroelectric Poly(vinylidene fluoride‐trifluoroethylene‐chlorotrifluoroethylene)s: Effect of Molecular Weight on Dielectric Property

AbstractA series of ferroelectric poly(vinylidene fluoride‐trifluoroethylene‐ chlorotrifluoroethylene), P(VDF‐TrFE‐CTFE), have been synthesized by a two‐step approach. The first step is copolymerization of VDF and CTFE via solution or suspension methods to produce P(VDF‐CTFE) copolymers with different molecular weights. The second step is partial de‐chlorination to convert copolymers into P(VDF‐TrFE‐CTFE) terpolymers with precisely controlled compositions. The effect of molecular weight, molecular weight distribution and uniaxially stretching on the dielectric properties has been investigated over a broad range of temperature and frequency. The X‐ray diffraction patterns and DSC curves demonstrate the coexistence of the multiple phases in the terpolymers. The dielectric spectra depict the local relaxation processes and relaxor ferroelectric behavior on the basis of the dielectric loss tangent as a function of temperature.

Structural Dependence of Phase Transition and Dielectric Relaxation in Ferroelectric Poly(vinylidene fluoride−chlorotrifluoroethylene−trifluoroethylene)s

A series of ferroelectric poly(vinylidene fluoride-chlorotrifluoroethylene-trifluoroethylene)s, P(VDF-CTFE-TrFE), with systematically varied chemical compositions have been synthesized via a two-step approach consisting of copolymerization and dechlorination. The effect of polymer structure on polarization responses and dielectric properties has been investigated over a broad frequency and temperature range. As shown in the X-ray diffraction patterns, multiple phases coexist within the terpolymers as a result of the gauche conformation induced by the CTFE unit. The polarization hysteresis loops reveal the variation of remanent polarization and coercive electric field with the CTFE content due to the changes of crystallinity and crystalline phase. The observed broad dielectric constant peak with Vogel-Fulcher dielectric dispersion behavior suggests a transformation from a normal ferroelectric to a ferroelectric relaxor of the polymers. The relationship between the local relaxation process and relaxor ferroelectric behavior has been examined on the basis of the dielectric and mechanical loss tangents as a function of temperature.

The M = 5.1 1980 Arudy earthquake sequence (western Pyrenees, France): a revisited multi-scale integrated seismologic, geomorphologic and tectonic investigation

This paper presents a combination of seismic imaging, geomorphologic, and tectonic data and an interpretation of the M = 5.1 1980 Arudy earthquake sequence putting in relation the seismicity, the inherited faults, and the geomorphologic (Wurm and postwurm) markers in this region of the Pyrenees. Since the anticlockwise rotation of the regional compression axis in Oligocene time, western Pyrenees are under a dextral regime and the resulting motion is accommodated along major inherited E–W dextral strike-slip faults. The Arudy aftershocks sequence is controlled by antecedent horsetail splay faults built at the boundary between two shallow Mesozoic crustal blocks most probably due to their differing rheology. This boundary has played the role of a seismic barrier stopping the E–W slip motion. The Arudy earthquake has reactivated the eastern segment of the main E–W strike-slip fault, while the post-seismic aftershocks correspond to local relaxation processes in normal tectonic behavior.

Oxygen Dynamics and Diffusion Mechanism in Intergrowth Bi2WO6-Bi3TiNbO9 Ferroelectrics

Dielectric and impedance spectroscopies were employed to study intergrowth Bi 5 TiNbWO 15 ferroelectrics. It was observed one ferroelectric phase transition at 1096 K and two dielectric relaxation processes involving bulk and grain boundary effects. It was also found that the bulk response within the material is of localized relaxation process, the temperature dependence of the relaxation strength is analogous to the Curie-Weiss law and the interactions between the oxygen vacancies in the bulk material follow the traditional point defects relaxation theory. Based on the analysis of the crystal structure, a short range diffusion mechanism of oxygen ions coordinating A sites was proposed.

The dynamics of copolymers in homopolymer matrices

The ability of a polymer chain to relax when it is deformed, and the ease to which this relaxation occurs, dramatically influences the macroscopic properties of the polymeric material. However, the local segmental relaxation processes in multi-component polymer systems are not well understood. The impact of the dynamics of one component on the dynamics of the other is not simply proportional to the relative amounts of each component, as one might expect. This paper discusses recent results using neutron techniques and Monte Carlo simulation that monitor the dynamic properties of a copolymer in a homopolymer matrix. In particular, the results indicate that altering either copolymer sequence distribution or copolymer composition will dramatically impact the dynamics of the copolymer that is surrounded by homopolymers. These results provide important fundamental information on the coupling of the dynamics of two components in a multi-component polymer system. This data also offer insight into the local segmental relaxation processes in multi-component polymer systems, which are not well understood and yet influence the ultimate properties of these mixtures.

Dynamics of liquid silica as explained by properties of the potential energy landscape

The dynamics of silica displays an Arrhenius temperature dependence, classifying silica as a strong glass-former. Using recently developed concepts to analyze the potential energy landscape, one can get a far-reaching understanding of the long-range transport of silica. It can be expressed in terms of properties of the thermodynamics as well as local relaxation processes, thereby extending the phenomenological standard picture of a strong glass-former. The local relaxation processes are characterized by complex correlated sequences of bond breaking and reformation processes.

Dielectric, Impedance, and Electric Modulus Spectroscopies of Mixed-Layer Aurivillius Phase Bi5Ti1.5W1.5O15

Dielectric, impedance, and electric modulus spectroscopies were employed to study the dielectric anomalies in ceramics. It was found that the ferroelectric transition temperature is at . Besides, two relaxation processes attributed to bulk and grain boundary effects were observed both in the dielectric and impedance spectra. However, only one relaxation process corresponding to the bulk response was observed in the electric modulus spectra. The relaxation parameters were deduced. It was found that the bulk response of the material was of localized relaxation process at low temperatures, and long-range conductivity only emerged at high temperatures. Finally, the relaxation species and dynamics of the bulk response were discussed.

Dielectric and Calorimetric Studies of Hydrated Purple Membrane

Purple membranes (PM) from halobacteria were hydrated to ∼0.4 and ∼0.2g H2O/g of PM and studied by dielectric spectroscopy and differential scanning calorimetry between 120 and 300K. The dielectric process, attributed to a local (β) relaxation of the confined supercooled water, shows an Arrhenius temperature behavior at low temperatures. In the case of the most hydrated PM a small deviation from the Arrhenius behavior occurs at 190–200K together with a pronounced endothermic process and an increased activation energy. The observed crossover is accompanied by a reduction of the interlayer spacing due to the partial loss of the intermembrane water. All these effects at ∼200K are consistent with a scenario where the local relaxation process merges with a nonobservable α-relaxation of the interlayer water, giving rise to a more liquid-like behavior of the interfacial water. For the less hydrated sample the effects are less pronounced and shift to a slightly higher temperature.

Dielectric relaxation behavior of poly(methyl methacrylate) under high‐pressure carbon dioxide

AbstractAn in situ dielectric measurement for atactic poly(methyl methacrylate) (at‐PMMA) was performed under high‐pressure CO2 under various pressures and temperatures. The at‐PMMA has the acetate side group with a large dipole moment. In the glassy state, a local relaxation process (β‐process) can be observed using dielectric measurement. In the rubbery state, the micro‐Brownian motion of main chain (α‐process) occurs, and the β‐process changes into αβ‐process coordinated with the α‐process. The dielectric loss (ε″) spectrum of at‐PMMA in the glassy state is asymmetric because of the density fluctuation for the amorphous structure. The loss peak frequency shifted to higher frequencies, and the relaxation strength increased with increasing CO2 pressure. In the glassy state, the shape of ε″ spectrum became more symmetric with increasing CO2 pressure. These show that the molecular mobility enhanced by the plasticization effect of CO2 allows the dipolar side groups in the high‐density region to contribute to the relaxation process. We also found that the apparent activation energy decreased under high‐pressure CO2. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2951–2962, 2005

Local relaxation mechanisms and collective stochastic dynamics

Damping and thermal fluctuations have been introduced to collective normal modes of a magnetic system in recent modeling of dynamic thermal magnetization processes. The connection between this collective stochastic dynamics and physical local relaxation processes is investigated here. A system of two coupled magnetic grains embedded in two separate oscillating thermal baths is analyzed with no a priori assumptions. It is shown explicitly that by eliminating the oscillating thermal bath variables and using a Markovian approximation, stochastic dynamics occur in the normal modes of the magnetic system in the form of independent damped harmonic oscillators driven by thermal fields. The grain interactions cause the local coupling to the thermal bath to be felt by the collective system and the resulting dynamic damping to reflect the system symmetry. This form of stochastic dynamics is in contrast to a common phenomenological approach where a thermal field is added independently to the dynamic equations of each discretized cell or interacting grain. The dependence of this collective stochastic dynamics on the coupling strength of the magnetic grains and the relative local coupling to thermal bath is discussed.

PBT chain mobility from the ESR saturation spectra studies

Temperature and microwave power dependence of ESR spectra of the spin probes in PBT has been studied. The observed changes of ESR signal allowed to determine the long correlation times (τ 10 - 4 -10 - 5 s) as well as other parameters characterizing the saturation processes and involved relaxation processes. It was established that at low temperature range ( 120-200 K) the rotational mobility of used nitroxide radicals was related to local relaxation processes of the methylene sequences (3-4 CH 2 groups) in an amorphous region of PBT. The obtained results show the possibility of that type of investigation also for other polymers. Determination of polymer properties via analysis of parameters obtained in saturation experiments is also raised.

A simple model for roughening in the instantaneous electrodeposition process

Abstract We introduce a very simple model for the instantaneous electrodeposition process that takes into account the kinetic roughening during the film growth. The main ingredients of the model are the characteristic time for the cation particles to attain their steady-state velocity and the choice of a velocity-dependent probability for the particles to relax on the substrate. The model is capable of reproducing the experimental results for the density current as a function of time for a fixed value of the external potential during the electrodeposition experiment, as well as typical voltammograms. The model was investigated through a suitable continuous Monte Carlo algorithm where the number of particles to be deposited in a Monte Carlo step (MCs) changes with time. For small values of the particle velocities the deposition is essentially described by a pure random deposition model, while for high values of velocity a dominant local relaxation process is assumed. At each MC we compute the surface width in order to determine the roughness of the surface. From the plots of the surface width versus time we found the growing ( β ), dynamic ( z ) and roughness ( α ) exponents for this model.

Locally heterogeneous dynamics in miscible blends of poly(methyl methacrylate) and poly(vinylidene fluoride)

Comparative measurements of specific heat capacities (temperature interval between 2 and 500 K), and of low frequency mechanical spectroscopy (temperature interval between 120 and 400 K) in poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) amorphous blends show the existence of single calorimetric and mechanical glass transition temperatures, as a clear indication of the existence of homogeneous single-state structures. Below Tg, the experimental data reveal distinct local relaxation processes within the backbone of the individual components, while the heat capacities below 15 K can be explained in terms of a two-phase model (i.e., a simple linear overlap of the contributions from wholly amorphous PMMA and PVDF, weighted by their proportions). These findings are associated with locally heterogeneous relaxation and vibrational motions, and are regarded as experimental evidence for the existence of a nanoscopic length scale where the dynamics of a blend exhibits a heterogeneous regime.

Non-linear spin transport in magnetic semiconductor multiple quantum wells

We present a theoretical model for vertical transport in DC-biased magnetically-doped II–VI semiconductor multiple quantum wells. Non-, partly- and fully-polarized spin distributions are taking into account within a self-consistent scheme. We investigate the formation of electric field domains under the combined influence of band electron interactions with polarized local moments and spin relaxation processes.

Dielectric study of the relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer system

The high-energy electron irradiated poly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE), copolymer exhibits many features resembling the relaxor ferroelectric behavior. In polymer systems, there are local dipolar motions at the monomer or unit cell scale, which manifest themselves as various relaxation processes. In this paper we investigate the relationship between the relaxor ferroelectric behavior, especially, Vogel-Fulcher (V-F) behavior and these local dipolar relaxation processes in irradiated P(VDF-TrFE) 65/35-mol % copolymer. In order to cover the change in polarization dynamics of the copolymer system, the dielectric behavior of copolymer is measured over a broad frequency (0.01 Hz--10 MHz) and temperature (-40 to 80 \ifmmode^\circ\else\textdegree\fi{}C) range. The results indicate that there is an increased coupling among the local dipolar motions with reduced temperature in the crystalline region. On the other hand, the randomness introduced in the irradiation prevents the formation of a polar phase, on both the macroscale and the microscale, in the polymer. The observed relaxor behavior is a consequence of the competition of these two effects. The results further show that the V-F process of the irradiated copolymer system is different from the glass transition, which occurs in the amorphous phase of the copolymer.

Lateral Force Microscopy Study of the Frictional Behavior of Self-Assembled Monolayers of Octadecyltrichlorosilane on Silicon/Silicon Dioxide Immersed in n-Alcohols

Lateral force microscopy (LFM) was used to evaluate the frictional behavior of surfaces modified with self-assembled monolayers (SAMs) and immersed in n-alcohols (CH3(CH2)xOH where x = 0−8,11) as a function of applied normal load, sliding velocity, and solvent chain length. SAMs were formed from octadecyltrichlorosilane (OTS) on silicon/silicon dioxide substrates. The objective was to investigate how the solvent environment affected the frictional behavior of OTS and to characterize the effectiveness of OTS as a boundary lubricant in liquid environments for applications such as fluidic self-assembly and microelectromechanical devices. Three characteristic frictional regimes were observed at low, intermediate, and high loads. Maxima as a function of the sliding velocity appeared in the frictional forces for intermediate applied normal loads of ∼20−40 nN for x = 1−8. These maxima shifted to lower sliding velocities with increases in the applied normal load and with increases in the chain length of the solvent. The frictional maxima were interpreted by adapting concepts of viscoelasticity for bulk polymer systems to the two-dimensional systems of SAMs. Maxima were interpreted to result from localized relaxation processes in the SAMs that depend on the extent of solvent partitioning in the compressed region under the tip. The characteristic relaxation times of the alkyl chains increased with increased applied normal load and with increased solvent chain length. The behavior as a function of x was consistent with both a mechanism of solvent partitioning controlled by the free volume distribution in the SAM and a mechanism of insertion into defects. The relaxation times of the alkyl chains were related to a molecular model of energy dissipation based on the adsorption and desorption of the chain tails from the surface of the atomic force microscopy tip. The total frictional forces was consistent with superposition of relaxation processes and viscous drag on the tip and plowing effects that become dominant at high applied normal loads.

Quasiparticle relaxation dynamics in cuprates and lifetimes of low-energy states: Femtosecond data from underdoped to overdoped YBCO and mercury compounds

We show that low-energy spectral features in the cuprates can be separated into different components by the measurement of the recombination dynamics of different low-energy excitations in real-time using femtosecond laser spectroscopy. Quasiparticle (QP) recombination across the gap and intra-gap localized state relaxation processes exhibit qualitatively different time- and temperature-dependences. The relaxation measurements also show the existence of two distinct coexisting energy gaps near optimum doping and in the overdoped region, one more or less temperature independent (which exists above and below T c ) and one which closes at T c in a meanfield like fashion. Thus systematic studies of QP recombination as a function of doping and temperature suggest that the ground state of the cuprates is a mixed Boson-Fermion system with localised states present over the entire region of the phase diagram.

Kinetic peculiarities of the simplest reactions in solids

Two extreme cases of the formal kinetic description are discussed. One of them is the rate constant distribution. In this case it is assumed that reagent in solids may be divided into subensembles. Each subensemble reacts according to simple kinetic law and the overall kinetics is a sum of the individual ones. The second case corresponds to a uniformly proceeding reaction. However, its rate constant k(t)Zk(t) takes place will be discussed in detail. It is the case of oxidation of alkyl and matrix radical by diffusing oxygen in methanol glass at 77–90 K. Time dependence of k(t) is compared with the diffusion coefficient time-dependence. It is concluded that both the slowing down of oxygen migration due to matrix relaxation and the local relaxation process in the vicinity of radical lead to a decrease in oxidation rate constants.

Polarized light-induced photoisomerization in glassy poly(methyl methacrylate) and local relaxation processes of the polymer matrix

Photoisomerization kinetics of trans 4-methoxystilbene induced by irradiation with linearly polarized light was investigated in the glassy state of poly(methyl methacrylate)(PMMA). The reaction was strongly selected by linearly polarized light as revealed by a large dichroic absorption generated upon irradiation. The optical anisotropy increases with increasing irradiation time and passes through a maximum before approaching an equilibrium value, which strongly depends on the experimental temperatures. From the decay of the total absorbance of the trans-isomer observed upon irradiation with 313 nm UV light, it was found that the rates of the photoisomerization trans → cis were not significantly changed within the temperature range of the experiments. On the other hand, the reorientational relaxation times of the stilbene, which were directly measured by the annealing-after-irradiation techniques, depend strongly on temperature. These relaxation data were compared to the previous results obtained with dopants of the size larger than the stilbene and were discussed in conjunction with the local relaxation processes of the PMMA matrix.