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Using elastic neutron scattering measurements we have investigated the picosecond dynamics of dry and hydrated powders of DNA in the double-stranded (dsDNA) and single-stranded (ssDNA) state in the temperature range from 20 to 300 K. The extracted mean square displacements of DNA hydrogen atoms exhibit an onset of anharmonicity at around 100 K. The dynamics of the hydrated samples shows a further anharmonic contribution appearing at a temperature T-d=230-240K. Such dynamical behaviour is similar to the well-studied dynamical transition found in hydrated protein powders. The mean square displacements of dsDNA and ssDNA are practically superimposed in tile whole temperature range for both dry and hydrated samples. This suggests that the DNA local mobility in the picosecond timescale does not depend on the single- or double-stranded conformation.

We examine the rheological transition from an arrested to a fluid state for different mixtures of star polymers with varying functionality and size ratios. As a general trend, we find that addition of smaller star polymers in an arrested, concentrated solution of larger ones brings about melting of the large star glass. At the same time, the dependence of the amount of additives needed to melt the glass has a nontrivial dependence on the size ratio. Theoretical analysis, based on effective interactions and Mode Coupling Theory, reproduces the experimental results and helps identify two distinct types of glasses in the composite system.

We report on the self-assembly and molecular dynamics in a series of poly(gamma-methyl-L-glutamate-co-gamma-stearyl-L-glutamate) [poly(MLG-co-SLG)] using thermodynamic (differential scanning calorimetry), structural (wide-angle X-ray scattering and (13)C solid-state NMR), as well as dynamic (dielectric spectroscopy and site-specific solid-state NMR) techniques. Stearyl crystallization takes place in the copolymers with W(PSLG) >= 18% which reflects changes of the hydrocarbon side chains conformation. Extruded fibers of the copolymers are composed of both alpha-helical and beta-sheet secondary structures oriented along the fiber axis with a bottleneck structure with MLG/SLG sequences forming the neck and more distant SLG sequences forming the bottle. This supramolecular organization has direct consequences on the dynamics. Two dielectrically active processes with strong temperature dependences were found. Site-specific NMR allowed the identification of the geometry of motions. One is associated with small-amplitude motions of amorphous segments along the polypeptide backbone and at the chain ends that freeze at the liquid-to-glass transition. The slower process is associated with the relaxation of the alpha-helical peptide secondary structure. The analysis of the dielectric strength associated with the slower process revealed that helices are objects of low persistence. Macromolecules

The enthalpy relaxation mechanism of a low molecular weight synthesis of polymethylmethacrylate was investigated by means of calorimetric experiments. The data were analyzed in terms of a kinetic approach treating nonlinearity in a different manner with respect to the Tool-Narayanaswamy-Moynihan model. The relaxation isotherms recorded at four different temperatures were well reproduced by this approach that, however, failed in describing the relaxation asymmetry towards the equilibrium after opposite temperature jumps. A modification of the model was proposed with an additional free parameter accounting for the stretching of the relaxation function. In this way all the experimental data were reproduced fairly well.

We present the first detailed analysis of a transient grating (TG) experiment on a supercooled molecular liquid, m-toluidine, from 330 K (1.75 T-g) to 190 K (1.01 T-g) based on the theoretical model proposed in Pick et al (2004 Eur. Phys. J. B 39 169). This hydrodynamic model of the TG response function yields a complete description of the experimental data and a characterization, over a wide dynamical range, of the different physical phenomena giving rise to the signals, including the rotation-translation coupling process. Using a disentanglement procedure of the isotropic from the anisotropic part of the TG response and a careful fitting analysis, we obtain detailed information on the rotation-translation coupling function. We also extract the structural relaxation times related to the 'longitudinal' viscosity over almost 10 decades in time. The value of some other parameters and information on their thermal behaviour is also reported.

The steady state fluorescence (SSF) technique was used to study the sol-gel transition for the solution free radical crosslinking copolymerization of N-isopropyl acrylamide (NIPA), with N,N'-methylenebis (acrylamide) (BIS) as crosslinker in the presence of ammonium persulfate (APS) as an initiator. Pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt, HPTS) was used as a floroprobe for monitoring the polymerization. Pyranine molecules start to bind to NIPA polymer chains upon the initiation of the polymerization, thus the spectra of the bonded pyranines shift to the shorter wavelengths. The get fraction exponent beta and the weight average degree of polymerization exponent gamma' agree best with the mean-field (Flory-Stockmayer) results near the gel point for various crosslinker contents. Publisher's Version Q3 WOS:000243586600006

The compaction dynamics of a granular media subject to a sequence of vertical taps made of fluid pulses is investigated via Molecular Dynamics simulations. Our study focuses on three different levels: macroscopic (volume fraction), mesoscopic (Vorono\"{\i} volumes, force distributions) and microscopic (grain displacements). We show that the compaction process has many characteristics which are reminiscent of the slow dynamics of glass forming systems, as previously suggested. For instance the mean volume fraction slowly increases in time and approaches a stationary value following a stretched exponential law, and the associated compaction time diverges as the tapping intensity decreases. The study of microscopic quantities also put in evidence the existence of analogies with the dynamics of glass formers, as the existence of dynamical heterogeneities and spatially correlated motion of grains; however it also shows that there are important qualitative differences, as for instance in the role of the cage effect. Correlations between geometry and dynamics of the system at the grain level are put in evidence by comparing a particle Vorono\"{\i} volume and its displacement in a single tap.

We study lattice gas models with the imposition of a constraint on the maximum number of bonds (nearest-neighbour interactions) that particles can participate in. The critical parameters, as well as the coexistence region, are studied using the mean field approximation and the Bethe-Peierls approximation. We find that the reduction of the number of interactions suppresses the temperature-density region where the liquid and gas phases coexist. We confirm these results from simulations using the histogram reweighting method employing grand canonical Monte Carlo simulations.

We review some recent results on Statistical Mechanics approach to dense granular media. In particular, by analytical mean field investigation we derive the phase diagram of a monodisperse system. We show that '' jamming '' corresponds to a phase transition from a '' fluid '' to a '' glassy '' phase. The nature of such a '' glassy '' phase turns out to be the same found in mean field models for glass formers. This gives quantitative evidence to the idea of a unified description of the '' jamming '' transition in granular media and thermal systems, such as glasses.

We present a numerical study of the statistical properties of the potential energy landscape of a simple model for strong network-forming liquids. The model is a system of spherical particles interacting through a square well potential, with an additional constraint that limits the maximum number of bonds, $N_{\rm max}$, per particle. Extensive simulations have been carried out as a function of temperature, packing fraction, and $N_{\rm max}$. The dynamics of this model are characterized by Arrhenius temperature dependence of the transport coefficients and by nearly exponential relaxation of dynamic correlators, i.e. features defining strong glass-forming liquids. This model has two important features: (i) landscape basins can be associated with bonding patterns; (ii) the configurational volume of the basin can be evaluated in a formally exact way, and numerically with arbitrary precision. These features allow us to evaluate the number of different topologies the bonding pattern can adopt. We find that the number of fully bonded configurations, i.e. configurations in which all particles are bonded to $N_{\rm max}$ neighbors, is extensive, suggesting that the configurational entropy of the low temperature fluid is finite. We also evaluate the energy dependence of the configurational entropy close to the fully bonded state, and show that it follows a logarithmic functional form, differently from the quadratic dependence characterizing fragile liquids. We suggest that the presence of a discrete energy scale, provided by the particle bonds, and the intrinsic degeneracy of fully bonded disordered networks differentiates strong from fragile behavior., Final version. Journal of Chemical Physics 124, 204509 (2006)

The structure and the associated dynamics of a series of poly(y-benzyl-L-glutamate)-b-poly(dimethyl siloxane)b-poly(y-benzyl-L-glutamate) (PBLG-b-PDMS-b-PBLG) triblock copolymers were investigated using small- and wide-angle X-ray scattering, NMR, transmission electron microscopy, and dielectric spectroscopy, respectively. The structural analysis revealed phase separation in the case of the longer blocks with defected alpha-helical segments embedded within the block copolymer nanodomains. The alpha-helical persistence length was found to depend on the degree of segregation; thermodynamic confinement and chain stretching results in the partial annihilation of helical defects. Biomacromolecules

Electron spin resonance spectroscopy has been employed to analyze the probe rotational dynamics in a narrow distribution poly(methyl acrylate) melt. Dynamic regions have been found in the investigated temperature range and the coupling of the probe dynamics to the main and secondary polymer relaxations has been studied. The temperature dependence of the shear viscosity has been also measured, and, in the supercooled state of the host, it has been used to reveal the effects on the probe dynamics of both the local stiffness of the polymer and the occurrence of cooperativtity and dynamic heterogeneities in the matrix. Below T, the rotational dynamics follows activated regimes. (c) 2006 Elsevier B.V. All rights reserved.

The dynamics of lysozyme in the picosecond timescale has been studied when it is in dry and hydrated powder form and when it is embedded in glycerol, glycerol–water, glucose and glucose–water matrices. The investigation has been undertaken through elastic neutron scattering technique on the backscattering spectrometer IN13. The dynamics of dry powder and embedded-in-glucose lysozyme can be considered purely vibrational up to 100 K, where the onset of an anharmonic contribution takes place. This contribution can be attributed to the activation of methyl group reorientations and is described with an Arrhenius trend. An additional source of anharmonic dynamics appears at higher temperatures for lysozyme in hydrated powders and embedded in glycerol, glycerol–water and glucose–water matrices. This second process, also represented with an Arrhenius trend, corresponds to the so-called protein dynamical transition. Both the temperature where such a transition takes place and the magnitude of the protein mean square displacements depend on the environment. The dynamical response of the protein to temperature is put in relationship with its thermal stability.

We present a study of photoinduced transitions occurring in a low ${T}_{G}$ liquid crystalline polymer containing the azobenzene group in the side chain. The structural phase of this material can be optically switched between the glassy, nematic and isotropic states. We show the equivalence of light and temperature in causing these effects, promoting this material as a tool in the investigation of structural relaxation at transitions in confined geometry and fast temperature variation as in the case of the optically induced quenching that we report. The results allow us to propose also alternative writing methods for the application of these materials to high density optical memories.

We discuss how the anomalous increase of the viscosity in colloidal systems with short-range attraction can be related to the formation of long-living clusters. Based on molecular dynamics and Monte Carlo numerical simulations of different models, we propose a similar picture for colloidal gelation at low and intermediate volume fractions. On this basis, we analyze the distinct role played by the formation of long-living bonds and the crowding of the particles in the slow dynamics of attractive colloidal systems.

The rotational dynamics of the spin probe cholestane dissolved in a narrow distribution poly(n-butyl acrylate) sample has been investigated via electron spin resonance (ESR) spectroscopy. The measurements were carried out in a wide temperature range: different dynamic regions have been recognized, and the coupling of the probe dynamics to the α and secondary relaxations has been revealed. In particular, the coupling with the structural relaxation is ruled by two fractionary Vogel-Fulcher laws (VF). The crossover from one VF region to the other occurs at the temperature T(C) = 1.17T(g), signalling the onset of the cooperativity in the dynamics and confirming a behaviour previously observed in ESR studies carried out on polymeric glass-formers. Furthermore, in this work we discuss the activated regime at the highest temperatures and show that the activation energy does not depend on the length of the polymer main- and side-chains, while its onset temperature linearly depends on the chain length.

The phonon frequency spectrum g(omega) of a crystal, such as body centred cubic (bee) Rb, is known to be characterized by the Van Hove singularities at omega not equal 0. However, for a liquid metal like Rb, g(w) has a single, hydrodynamic-like singularity, namely it cusp proportional to omega((1/2)), at omega = 0. Here, we note first that computer simulation on liquid Rb near freezing has revealed a rather well-defined Debye frequency omega(D). Therefore, we propose here it zeroth-order model g(0)(omega) of g(omega) for Rb, which combines the Debye model with the 'hydrodynamic' omega((1/2)) cusp. The corresponding velocity autocorrelation function (v(t) . v(0)) has correctly a long-time tail proportional to t(-(3/2)). The terms from g(0)(omega) involving omega(D) are then damped by weak exponential factors exp(-alpha(i)t), and the resulting first-order approximation, g(t)(omega) say, to the frequency spectrum is found to have features in common with the molecular dynamics (MD) simulation form. Thus omega(D) is fixed, as well as transport coefficients for the known thermodynamic state. The article concludes with a more qualitative discussion on supercooled liquids, and on metallic glasses such as Fe, for which MD simulations exist.

We describe ail experimental Study Of the dynamic structure factor of liquid glycerol performed by complementary inelastic techniques Such as Brillouin visible, ultraviolet and x-ray scattering. The spectra have been collected as a function of both temperature and momentum transfer. The relevant hypersonic parameters are evaluated from the spectral lineshape analysis modelling the data with a simple hydrodynamic profile. The Study of their frequency dependence allows us to observe the Occurrence of ail active structural relaxation and to measure the related timescale. We also find signatures of further relaxation processes occuring below the accessible frequency window. As a result, the dynamic window traditionally probed in spectroscopic experiments is greatly extended and partially bridges the gap between MHz and THz techniques.

A study on high performance poly(ether-ether-ketone) (PEEK) composites prepared by incorporating aluminum oxide (Al2O3), 0 to 50 wt % by hot compaction at 15 MPa and 350°C was described. Density, thermogravimetric analysis/differential scanning calorimetry, and scanning electron microscopy (SEM) were employed to evaluate their density, thermal stability, crystallinity, and morphology. Experimental density was found higher than theoretical density, which indicates that composite samples are sound. It was found that the addition of micron sized (< 15 μm) Al2O3 increased the peak crystallization temperature by 12°C when compared with neat PEEK with insignificant increase in melting temperature. Half-time of crystallization is reduced from 2.05 min for the neat PEEK to 1.08 min for PEEK incorporated with 30 wt % Al2O3 because of the strong nucleation effect of Al2O3. The thermal stability of composites in air atmosphere was increased by 26°C. However, thermal stability in nitrogen atmosphere decreases at lower concentration of Al2O3 but increases above 20 wt % of Al2O3. Uniform dispersion of Al2O3 particles was observed in PEEK polymer matrix by SEM. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4623–4631, 2006

We have studied by inelastic x-ray scattering, at wave vectors $1\text{ }\text{ }{\mathrm{nm}}^{\ensuremath{-}1}\ensuremath{\le}q\ensuremath{\le}15\text{ }\text{ }{\mathrm{nm}}^{\ensuremath{-}1}$, the high-frequency dynamics of epoxy-amine mixtures as the monomers irreversibly polymerize. We find that chemical bonding, while inducing molecular ordering on a mesoscopic length scale, also efficiently realizes the mechanism of dynamical arrest described by the mode-coupling theory, as manifested by a cusp singularity in the behavior of the nonergodicity factor as a function of the number of chemical bonds. These results confront positively the mode-coupling theory with a new control parameter.

We will consider the use of the bond model in characterizing, by the direct calculation route, the full excitation profile. The present work sets the temperature behaviour of the excessive value of some thermodynamic quantities, such as configurational entropy and heat capacity of two homologous disaccharide aqueous solutions. The findings represent important data for understanding the better lyoprotectant effectiveness of trehalose in comparison with sucrose.

Glass transition during bulk polymerization was studied in free-radical cross-linking copolymerization (FCC) of methyl methacrylate (MMA) using the steady-state fluorescence (SSF) technique. Naphthalene (N) was used as a monomer and excimer forming probe. Changes in the viscosity of the pregel solutions due to gel formation dramatically enhance both monomer and excimer fluorescent yield of N molecules. The reaction time at which the monomer and excimer intensities exhibit a sudden increase corresponds to the reaction time at which the rate of polymerization becomes maximum resulting from the gel effect. This effect was used to study the gelation of MMA, as a function of time, in various N concentrations. The results were interpreted in the view of percolation theory. The gel fraction, beta, and weight average degree of polymerization, gamma, exponents beta=0.40 +/- 0.02 and gamma=1.70 +/- 0.07 were found in agreement with percolation results for both monomer and excimer measurements, respectively. Publisher's Version Q3 WOS:000230649700003

The static structure factor and associated dynamics have been investigated in a series of block copolymers of poly(methyl vinyl ether) (PMVE) and poly(isobutyl vinyl ether) (PiBVE) using x-ray scattering and dielectric spectroscopy (DS). The origin of the dynamic arrest at the glass temperature $({T}_{g})$ of PiBVE has been explored by temperature- and pressure-dependent DS and pressure-volume-temperature measurements. Both temperature and volume are responsible for the segmental dynamics but temperature has a stronger effect. The copolymers display a minimal dynamic asymmetry $(\ensuremath{\Delta}{T}_{g}\ensuremath{\sim}7\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, nevertheless, are spatially and dynamically heterogeneous. Increasing pressure, unlike temperature, enhances the dynamic heterogeneity. This effect originates from the distinctly different pressure sensitivities of the homopolymers and can be traced back to differences in local packing.

The dynamic miscibility of poly(4-vinylphenol)/poly(vinyl ethyl ether) (PVPh/PVEE) blends, with a T-g contrast of 186 K, as well as the PVEE segmental dynamics have been investigated by temperature- and pressure-dependent dielectric spectroscopy. In PVEE the pressure coefficient of T-g amounts to 0.215 KMPa, and its apparent activation volume displays the usual T dependence. Although both temperature and volume contribute to the segmental dynamics, the former has a stronger influence within the T and P investigated. In the blends, dynamic heterogeneity is suppressed because of hydrogen bonds that couple the components segmental dynamics. In the PVEE-rich blends, increasing temperature and pressure results in the broadening of the distribution of relaxation times through the weakening of hydrogen bonds and the associated decoupling of the segmental dynamics. A central result of the present study is the identification of a critical temperature above which the system becomes increasingly heterogeneous. Macromolecules

We study the dynamic behaviour of concentrated colloidal hard spheres using time resolved correlation, a light scattering technique that can detect the slow evolution of the dynamics in out-of-equilibrium systems. Surprisingly, equilibrium is reached a very long time after sample initialization, the non-stationary regime lasting up to three orders of magnitude more than the relaxation time of the system. Before reaching equilibrium, the system displays unusual ageing behaviour. The intermediate scattering function decays faster than exponentially and its relaxation time evolves non-monotonically with sample age.

An expression for the velocity u of migration of a diffuse simple crystal-melt interface has been derived on the basis of the theory of atomic mobility in supercooled liquids: u = K-0 (T / T-m) DeltaT, where DeltaT = T-m - T the undercooling below the melting point T-m; K-0 is the kinetic coefficient of atomic attachment, which is used in models of crystal growth. It has been calculated for a number of metals. u(max) = K0Tm / 4 is the theoretical limit of the velocity of crystal growth. For a number of FCC metals the theoretical limit of crystal growth has been found to be of order of 200 m/s. The crystal growth kinetics has been shown to be limited by the atomic self-diffusion in the interface, for which the strong dependence on the orientation of the crystal/melt interface has been explained. Publisher's Version WOS:000223641200085

The dynamics of shape-persistent macromolecules have not been investigated to the same extent as of glass-forming liquids or amorphous polymers. Herein we investigate the dynamics of systems possessing intrinsic orientational order. Do such systems exhibit a glass transition and if yes, what is its origin? Do they show a dual relaxation scenario (alpha and beta-processes) as glass-forming liquids and polymers? If such processes exists what is their pressure dependence? Is the a-process controlled by temperature or by the available (free) volume? To answer these fundamental questions we employ some of the most common polymers exhibiting intrinsic orientational order, namely: (a) side-chain liquid crystalline polymers, (b) hairy-rod polymers and (c) synthetic polypeptides with alpha-helical secondary structure. We show that these molecularly diverse but shape-persistent macromolecules share some common features in their dynamic response. (C) 2004 Elsevier Ltd. All rights reserved. Progress in Polymer Science

Literature on the crispy/crunchy behavior of cellular solid foods with a crust is discussed. The emphasis is on products with a dry crispy or crunchy crust as bread and various snacks and especially on mesoscopic and macroscopic aspects. Successively, the sensory sensations involved, the mechanical and fracture behavior of crispy/crunchy products, morphological aspects, and the relation between crispy and crunchy behavior and mobility of the macro-molecules and plasticizer (primarily water) involved, are discussed. Finally, some ideas for an integrated approach of crispy/crunchy behavior of cellular solid foods with a dry crust will be discussed.

The influence of the cross-linking degree on the dynamics of the segmental motions close to the glass transition of poly(methyl methacrylate), PMMA, networks was investigated by three different mechanical spectroscopy techniques: thermally stimulated recovery (TSR), dynamic mechanical analysis (DMA), and creep. The application of the time-temperature superposition principle to isothermal DMA and creep results permitted to successfully construct master curves for PMMA networks with distinct cross-linking degrees. The former results were fitted to the KWW equation. The obtained variation of beta(KWW) for the distinct networks indicated that the relaxation curves tend to broaden as the cross-linking degree increases. TSR results clearly revealed a significant shift of the alpha-relaxation to longer times and a broader relaxation as the cross-linking degree increases, what was also observed by DMA and creep. A change from a Vogel to an Arrhenius behavior was detected by the three techniques with the decrease of temperature below T-g. The temperature dependence of the apparent activation energies (E-a) was calculated from DMA, creep, and TSR experiments; above T-g a good agreement was seen between the E-a values for all the techniques. Furthermore, the effect of the cross-linking degree on the fragility of PMMA networks was evaluated. For these materials an increase of fragility with increasing cross-linking degree was observed.

We analyze structural and conformational properties in a simulated bead-spring model of a non-entangled, supercooled polymer melt. We explore the statics of the model via various structure factors, involving not only the monomers, but also the center of mass (CM). We find that the conformation of the chains and the CM-CM structure factor, which is well described by a recently proposed approximation [Krakoviack et al., Europhys. Lett. 58, 53 (2002)], remain essentially unchanged on cooling toward the critical glass transition temperature of mode-coupling theory. Spatial correlations between monomers on different chains, however, depend on temperature, albeit smoothly. This implies that the glassy behavior of our model cannot result from static intra-chain or CM-CM correlations. It must be related to inter-chain correlations at the monomer level. Additionally, we study the dependence of inter-chain correlation functions on the position of the monomer along the chain backbone. We find that this site-dependence can be well accounted for by a theory based on the polymer reference interaction site model (PRISM). We also analyze triple correlations by means of the three-monomer structure factors for the melt and for the chains. These structure factors are compared with the convolution approximation that factorizes them into a product of two-monomer structure factors. For the chains this factorization works very well, indicating that chain connectivity does not introduce special triple correlations in our model. For the melt deviations are more pronounced, particularly at wave vectors close to the maximum of the static structure factor., Comment: REVTeX4, 16 pages, 16 figures, accepted for publication in Physical Review E

Breakdown of the Einstein-Stokes relation in undercooled liquids is one of the unsolved problems in the theory of liquids. The self-diffusion coefficient follows the temperature dependence of the Einstein-Stokes equation D = kT / 6pietar at high temperatures but only down to approximately 1.2T(g) (T-g - glass-temperature). Below 1.2T(g) the temperature behavior of the diffusion coefficient is weaker than 1/eta. In the present study we show that this is a consequence of increasing correlations in the Brownian motion of the constituting particles of the liquid. We derive a relation, which includes the Einstein-Stokes equation as a limiting case for high temperatures. Publisher's Version WOS:000223641200084

Recent experiments have demonstrated that the deviation of the single-particle displacement distribution from Gaussian form in a dense quasi-two-dimensional colloid suspension is a result of heterogenous dynamics that involves cooperative motions of neighboring colloid particles [J. Chem. Phys. 47, 9142 (2001)]. In this paper, we report the results of molecular dynamics (MD) simulations of a quasi-two-dimensional assembly of nearly hard-sphere colloid particles. The colloid-colloid interaction we use is short ranged and everywhere repulsive; it is related to the Marcus-Rice (MR) and modified MR interactions used in a previous study [Phys. Rev. E 58, 7529 (1998)]. As is the case for those systems, the one we study supports liquid, hexatic, and solid phases. Our calculations show that the deviation of the single-particle displacement distribution from Gaussian form is present in the liquid phase, and that a sharp increase in its magnitude occurs at the liquidus density and extends into the crystalline phase. For densities greater than the liquidus density we find three dynamical relaxation processes that include, at intermediate times, a slowing down in the rate of growth of the diffusive displacement of a particle due to the cage effect. As the density increases toward the solidus density, the dependence of the mean squared displacement on time, at intermediate times, changes from sublinear to zero. The onset of the long-time relaxation mode corresponds to the time at which the deviation of the particle displacement distribution from Gaussian form is a maximum. At this time, which increases exponentially with the density, the self-part of the van Hove function exhibits multiple maxima with respect to r while the distinct part of the van Hove function is a maximum at the origin, thereby signaling jump dynamics. At long times the particle mean square displacement has diffusive character at all densities including solid phase densities. A remarkable feature of our findings is the continuity of character of the particle displacement from the liquid phase through the hexatic phase and into the solid phase. Cooperative jumps that lead to diffusive process in crystals can be explained by a mechanism that involves many such correlated hops in random locations and random directions (but along the crystallographic axes) thereby generating effective random walk behavior. We argue that the collective motion we have found is generated by superpositions of instantaneous normal mode vibrations along diffusive paths. The diffusive paths are along the directions with strong bond orientation correlation, and start to grow in amplitude rapidly on entry into the hexatic phase.

The interaction of crystalline amylose and of crystalline and amorphous amylopectin with the plasticisers glycerol or ethylene glycol in the absence of water was studied, by using differential scanning calorimetry (DSC) and solid state nuclear magnetic resonance (NMR) spectroscopy. Upon heating starch freshly mixed with plasticisers, a strong exothermal interaction enthalpy of ΔH∼−35 J/g was detected by DSC. At room temperature glycerol interacts mainly with the amorphous starch regions, the interaction taking 8 days to reach equilibrium. For ethylene glycol the interaction is faster, taking four days to reach equilibrium, and the rate is not affected by crystallinity. Ethylene glycol interacts in a more ordered manner with amorphous than with crystalline material, resulting in a narrower ethylene glycol cross-polarisation magic angle spinning (CP/MAS) signal when equilibrium is reached at room temperature. Upon heating, more glycerol or ethylene glycol is immobilised, but in a less ordered manner than upon storage at room temperature. This results in a more intense, but broader plasticiser CP/MAS signal upon heating. Interaction in a more ordered manner probably implies interaction with more of the hydroxy groups of the plasticiser. The polysaccharide mobility is increased more when the plasticiser interacts in a more ordered manner, as observed by small starch signals in HP/DEC spectra.

A method was developed to prepare films based on industrial wheat gluten, from aqueous dispersion at neutral pH. An essential step in this procedure is to prepare aqueous dispersions in such a way that coagulation of the vital wheat gluten is prevented. In contrast to current procedures, adjustment of the pH or addition of organic solvents can be avoided. It was shown that processing variables, e.g. pH and drying temperature, influenced the properties of the films (containing 20% (w/w) of glycerol). The moisture content of the films had an even larger influence on the properties of the film than the processing conditions. At a relative humidity of 11 and 79%, films showed a stress and strain of about 23 MPa and 1%, and 1.6 MPa and 350%, respectively. It was shown that these variations are a result of the influence of the moisture content on the glass transition temperature of wheat gluten.

The dynamics of poly(methyl-p-tolyl-siloxane) (PMpTS) have been studied as a function of temperature (in the range from 143 to 413 K), pressure (0.1-300 MPa), frequency (10(-2)-10(6) Hz), and molecular weight. Independent pressure-volume-temperature (PVT) measurements (for temperatures in the range from 293 to 393 K and for pressures in the range from 10 to 200 MPa) allowed calculation of the relevant thermodynamic parameters. Two dielectrically active channels of relaxation were found, one in the glassy state reflecting a localized motion of the substituted phenyl ring and one at higher temperatures reflecting the usual segmental (alpha) relaxation. In PMpTS, there are two dominant control variables; both density and temperature have a strong influence on the segmental dynamics. The PVT results allowed us to follow distinct thermodynamic (T,P) paths resulting in states bearing the same density. These isodensity states are characterized by an apparent activation energy (Q(V)) that is not very different from the corresponding activation energy under isobaric conditions (Q(V)/Q(P)similar to0.55) reflecting the importance of thermal effects. At temperatures above the glass temperature (T(g)), strong orientation correlations exist above some critical pressure that depends on temperature. This state extends from T(g) up to 1.08 T(g) and separates a normal liquid at higher temperatures from an oriented liquid at lower temperatures. Using the "phase diagram" we discuss separately the influence of the temperature and density on the PMpTS dynamics. Physical Review E

One of the central questions in glass physics is the dynamic nature of the glass transition. Essential issues are the type of atomic motions involved and their homogeneity or heterogeneity. Previous experimental studies of the dynamic heterogeneity performed by special NMR techniques for times >10 −6 s and by incoherent neutron scattering were restricted to the α-relaxation process. Here we review the results of neutron scattering studies focused on the picosecond time domain showing that fast β-process corresponds to large amplitude cluster like heterogeneous motion.

Ageing of gelatinised and partly gelatinised potato starch and wheat starch were investigated in the presence of plasticisers with increasing size and number of OH groups (ethylene glycol, glycerol, threitol, xylitol, glucose, and for potato starch also maltose). The influences of these plasticisers and of granular remnants (ghosts) on recrystallisation were determined by using X-ray diffraction. Recrystallisation of potato starch samples in the presence of plasticisers resulted in crystallinity indices of ∼0.5. The largest reduction in potato starch recrystallisation is found for threitol (4 OH) and xylitol (5 OH). In the plasticiser range examined, the crystallisation inducing effect of granular potato starch remnants is reduced better when the plasticiser contains more OH groups. Wheat starch recrystallises to a lesser extent than potato starch, resulting in crystallinity indices of ∼0.4. The results for wheat starch do not show clear trends for the influences of plasticiser size and of ghosts. The difference in behaviour of the two starches is probably caused by wheat starch having shorter amylopectin chains. Resulting from these shorter amylopectin chains, the remaining structure in wheat starch ghosts may resemble A-type crystallinity, making it more difficult to form B-type crystals. Alternatively, the trends as found for potato starch may occur, but are less manifest for wheat starch, due to the lower total extent of recrystallisation. Solid state CP/MAS NMR spectra of the wheat starch samples containing ethylene glycol were obtained, in order to compare completely and partly gelatinised systems. The spectra were identical, confirming that the ghost structures do not influence wheat starch recrystallisation. Apparently, wheat starch ghosts do not act as nuclei for crystallisation. Similarly, the influence of various malto-oligosaccharides in combination with granular remnants (ghosts) was investigated on wheat starch ageing. Gelatinised and partly gelatinised wheat starch were plasticised with maltose, maltotriose, maltotetraose, maltopentaose or maltohexaose. This resulted in crystallinity indices of ∼0.2, with the largest reduction in recrystallisation for maltotriose and maltotetraose. No trend was found for the influence of ghosts. The presence of ghosts did not influence the 13C solid state HP/DEC NMR spectra. Less recrystallisation took place than with the previously mentioned smaller plasticisers that resulted in crystallinity indices of ∼0.4. The finding that maltose was able to reduce retrogradation better than glucose could be of practical importance.

This contribution is in continuation of our preceding publication (Lasjaunias J C, Brison J P, Monceau P, Staresinic D, Biljakovic K, Carcel C and Fabre J M 2002 J. Phys.: Condens. Matter 14 837) in which we have considered general aspects of the low-temperature thermodynamical properties of the quasi-one-dimensional organic salts based on sulphur donors with different ground states: (TMTTF)(2)PF6 in the spin-Peierls state and (TMTTF)(2)Br (TMTTF = tetramethyltetrathiafulvalene) with commensurate spin modulation. In this part, part 11, we present our results on the dynamical aspects related to the non-equilibrium phenomena measured below I K. The metastable states excited to slightly higher temperature (by only a few per cent of the starting T-0) relax very slowly to the heat bath environment, depending on the duration of the heat pumping. We compare the features observed in the relaxation rates in these two sulphur compounds with those measured for the incommensurate spin-density-wave compound (TMTSF)(2)PF6. We discuss the possible nature of the long-living low-energy excitations lying at the origin of this complex dynamical behaviour, in relation to their ground states.

We study the relaxation process in a two-dimensional lattice gas model, where the interactions come from the excluded volume. In this model particles have three arms with an asymmetrical shape, which results in geometrical frustration that inhibits full packing. A dynamical crossover is found at the arm percolation of the particles, from a dynamical behavior characterized by a single step relaxation above the transition, to a two-step decay below it. Relaxation functions of the self-part of density fluctuations are well fitted by a stretched exponential form, with a $\beta$ exponent decreasing when the temperature is lowered until the percolation transition is reached, and constant below it. The structural arrest of the model seems to happen only at the maximum density of the model, where both the inverse diffusivity and the relaxation time of density fluctuations diverge with a power law. The dynamical non linear susceptibility, defined as the fluctuations of the self-overlap autocorrelation, exhibits a peak at some characteristic time, which seems to diverge at the maximum density as well., Comment: 7 pages and 9 figures

We review the scientific history of random close packing (RCP) of equal spheres, advocated by J D Bernal as a more plausible alternative to the non-ideal gas or imperfect crystal as a structural model of simple liquids. After decades of neglect, computer experiments are revealing a central role for RCP in the theory of liquids. These demonstrate that the RCP amorphous state of hard spheres can be well defined, is reproducible, and has the thermodynamic status of a metastable ground state. Further evidence from simulations of square-well model liquids indicates an extended role of RCP as an amorphous ground state that terminates a supercooled liquid coexistence line, suggesting likewise for real liquids. A phase diagram involving percolation boundaries has been proposed in which there is no merging of liquid and gas phases, and no critical singularity as assumed by van der Waals. Rather, the liquid phase continuously spans all temperatures, but above a critical dividing line on the Gibbs density surface, it is bounded by a percolation transition and separated from the gas phase by a colloidal supercritical mesophase. The colloidal-like inversion in the mesophase as it changes from gas-in-liquid to liquid-in-gas can be identified with the hypercritical line of Bernal. We therefore argue that the statistical theory of simple liquids should start from the RCP reference state rather than the ideal gas. Future experimental priorities are to (i) find evidence for an amorphous ground state in real supercooled liquids, (ii) explore the microscopic structures of the supercritical mesophase, and (iii) determine how these change from gas to liquid, especially across Bernal's hypercritical line. The theoretical priority is a statistical geometrical theory of RCP. Only then might we explain the coincident values of the RCP packing fraction with Buffon's constant, and the RCP residual entropy with Boltzmann's ideal gas constant. info:eu-repo/semantics/publishedVersion

A method for investigating the nature of thermally activated relaxations in terms of their cooperative character is tested in both polymer and low molecular weight crystal systems. This approach is based on analysis of the activation entropy in order to describe thermally activated relaxations. The betaine arsenate/phosphate mixed system of low molecular weight crystals was selected for investigation because pure compounds of this system show ferro-/antiferroelectric phase transitions and the mixed crystals undergo different kinds of relaxation processes involving both dipole-dipole and dipole-lattice interactions. The polymer chosen was a side chain liquid-crystalline polysiloxane, which shows the beta -relaxation characteristic of disordered systems and amorphous materials. The cooperative versus local character of the relaxations is described in terms of "complex'' and ''simple'' relaxations based on calculations of the activation entropies. The initial assumptions of the theory, as well as the resulting equations, were found to be applicable to the systems studied.

The reorientation process of a probe molecule in a supercooled polymeric liquid crystal has been studied by non linear Electron Spin Resonance. The temperature dependence of the correlation time of the spinning motion of the probe τ∥ and the collective viscoelastic relaxation, as given by dynamic-mechanical measurements, have been compared. In amorphous samples the Williams-Landel-Ferry law is observed, which in annealed samples is replaced by a dual Arrhenius law broken at the glass transition. A scaling law τ∥ ∝ ηξ, η being the polymer viscosity, is found in both amorphous (ξ = 4.7 10−2 ± 2.8 10−3) and partly crystalline samples (ξ = 5.9 10−2 ± 6.5 10−3).

Linear and non-linear Electron Spin Resonance spectroscopies have been designed to study the decay of the orientation correlations on different time scales. The reorientation of radicals («spin probes») dissolved in host phases (in the present case a polymeric liquid crystal (PLC)) may be investigated in the time window 10−11–10−5 s. Evidence of the non-exponential regression of fluctuations is given. The temperature dependence of the correlation time of the spin probe orientation is discussed in the case of semi-crystalline PLCs and contrasted with the case of amorphous PLCs.

A marked shortening of T, relaxations in solid aromatic polymers can be attributed to O2 adsorbed on the aromatic rings. This effect, observed as a function of temperature, is modulated by the amount of adsorbed O2, Which, in turn, varies according to the molecular packing and crystalline/amorphous ratio. Within the temperature range 77-400 K, semicrystalline syndiotactic polystyrenes (s-PS) in the alpha, beta, and gamma-polymorph modifications were studied by pulse H-1 NMR at 30 MHz. Due to the presence of many relaxation components, 1,2,2-trideuterio-s-PS was synthesized, crystallized, and characterized by X-ray powder diffraction. At low temperature, when both the O2 diffusion and the spin-diffusion processes are slow enough, the backbone-deuteriated polymers exhibit multiple T1 relaxations: three for alpha and beta s-PS and two for gamma s-PS. These T1 components are attributed to crystalline and amorphous phases. From the study of these components as a function of temperature and from direct comparison of T1 values in the different phases, the following conclusions were drawn: (i) Clear differentiation was possible between different s-PS polymorphs. (ii) Determination of the crystalline vs amorphous ratio fully consistent with X-ray diffraction data was achieved. (iii) The amorphous component of different semicrystalline s-PS polymorphs could be distinguished.

Ortho-positronium (o-Ps) is used as a probe in positron annihilation lifetime spectroscopy (PALS) experiments, to characterise the behaviour of free volumes in natural starch samples, as a function of temperature (T). Up to about 540 K, the o-Ps intensity, I(3), remains constant at 26.2% while its lifetime, tau(3), is found to increase linearly. Both parameters undergo a decrease above this T, due to the onset of decomposition, which results in a shrinking of the sample pellets. The results indicate that the glass transition temperature should be above 501 K. Data from thermal gravimetry analysis (TGA) measurements are well described by supposing a first order process for the survival probability (p) of the starch lattice, with an activation energy, E(act) = (1.52 +/- 0.05) eV, and a frequency factor, ln(k(0), s(-1)) = 25.3 +/- 0.4. In the decomposition region, the PALS data show the unexpected correlation (tau(3n))(3) = I(3n), linking the normalised values of tau(3), tau(3n), and of I(3), I(3n). This is explained by considering that the changes in I(3) with T arise from those in the surviving volume fraction of the lattice, p, whereas the changes in tau(3) reflect the shrinking of the radius of the free volumes, the latter decreasing in proportion to p(1/3). Quantitative approaches on these bases lead to satisfactory fitting of all PALS data, yielding an activation energy, E(act) = (1.53 +/- 0.03) eV, and frequency factor, ln(k(0), s(-1)) = 25.4 +/- 0.2, in excellent agreement with the values derived from TGA.

We study the glassy phase of a thin polydimethylsiloxane film by high sensitivity dielectric measurements. We can locate its glass and melting transition temperatures and discuss its aging properties and frequency dependencies. Our data are framed into the general picture of glassy phenomena and compared, in particular, to the so called universal dielectric response model of frequency dispersion. The effect of a dc bias is also analyzed. (c) 2007 American Institute of Physics.