Search

The mechanical characterization of rubber materials at the frequencies relevant for the tire function during braking (about 1 MHz) is extremely challenging. On the other hand, dielectric relaxation experiments provide a suitable tool for the analysis of relaxation phenomena over a broad frequency range with high sensitivity. Here we show that by analyzing dielectric relaxation experiments in the framework of a simple model the mechanical relaxation of simplified industrial systems based on mixtures of a styrene butadiene rubber (SBR) with a polystyrene oligomer can be anticipated, at least for the SBR rich mixtures that are more relevant for development of tire polymers. The model needs the description of the dielectric and mechanical relaxation processes of the single components of the mixture and uses concepts and ideas introduced in the past for describing the segmental dynamics of athermal polymer blends. The model description of dielectric relaxation experiments on the mixtures provides a quantification of the thermal fluctuations of concentration in the blends. With this input, the mechanical relaxation of the mixtures are calculated using well established equations for the mechanical properties of composite materials. The present approach should be helpful in a rational design of the rubber material depending on the specific requirements.

More than 100 years after Debye proposed his model for the dielectric relaxation of monoalcohols (MA), some fundamental questions about their dynamics and its relation with the supramolecular structures created by hydrogen bonding remain not fully understood. The recent detection of the dynamics of hydrogen bonded aggregates by techniques other than dielectric spectroscopy is leading to novel insights and opening new questions. In particular it was recently reported that the shear response of some MA present three relaxation components, at variance with their apparent bimodal dielectric response. We show by mixing an archetype MA with LiCl that both shear and dielectric measurements are consistent with the presence of three separated contributions. Results are discussed in the more general context of MAs with non-exponential slow dielectric relaxations. In particular, we propose the recently reported third process as the origin of the unusual broadening of the Debye-like dielectric relaxation of some MAs.

Single-chain polymer nanoparticles (SCNPs) are individual intrachain cross-linked polymer chains -typically with a size < 20 nm-which can be considered as emerging building blocks for bionanotechnology. Very recently, surfaces displaying molecular recognition motifs have been decorated with brushes of responsive SCNPs and transformed into a cross-linked polymeric monolayer, thus paving the way to controlled nanofilm formation around bioactive surfaces (e.g., virus capsids). In this work, the structural properties of brushes formed by topologically complex SCNPs on flat surfaces are investigated as a function of SCNP size and intrachain cross-linking degree. A detailed comparison is performed to the structural features displayed by brushes of other macromolecular architectures, thus revealing the tunable, unique properties of brushes comprising elastic SCNPs. Remarkably, this work paves the way to the potential design of brushes of SCNPs with drug delivery characteristics based on stimulus-mediated rupture of intrachain cross-links.

Understanding the translocation of soft, conformable single-chain polymer nanoparticles (SCNPs) with intricate topology through nanopores and nanoslits is of great interest for several potential applications, including modern single-molecule analytical characterization methods. This work focus on the ultrafiltration of an elastic SCNP of size R through a cylindrical pore of diameter D « R or a rectangular slit of width H « R under an elongational flow field. Concerning SCNP ultrafiltration through nanopores of different diameter, we find a scaling law in qualitative agreement with recent results concerning the ability of soft conformable nanoparticles to translocate through pores at least tenfold smaller in size. Moreover, SCNP ultrafiltration through rectangular nanoslits provides a simple way to determine the elasticity parameter of a real SCNP based on its experimentally determined minimum slit width for effective ultrafiltration.

We review the recent activity aiming to clarify glassy dynamics in nanostructured polymer glasses, in particular thin films, nanocomposites and nanospheres. Special emphasis is devoted to recent results on the out-of-equilibrium dynamics, that is, the way the system leaves equilibrium when cooling, marking the thermal glass transition, or recovers it once in the glassy state – the so-called physical aging. Apart from those systems exhibiting strong interactions at the interface, we show that a huge number of studies probing glassy dynamics in nanostructured glasses finds negative deviations from bulk Tg and accelerated physical aging. Analysis of the dependence of the rate of spontaneous fluctuations – namely the linear dynamics – on nanostructuring indicates that there exists a significant component exhibiting bulk-like dynamics. This is the case even in the most extreme case of nanostructuring, that is, semi-isolated polymer chains and freestanding thin film with thickness ∼10 nm. In the latter case this is found at temperatures around the bulk glass transition temperature (Tg), that is, in the range where deviations of the out-of-equilibrium dynamics are normally observed. All together these results indicate that the linear dynamics alone cannot provide an exhaustive description of the out-of-equilibrium dynamics in nanostructured systems. In this case, purely geometric factors must be included. We discuss recent approaches aiming to capture the phenomenology of glassy dynamics in nanostructured glasses. Special attention is dedicated to the free volume hole diffusion (FVHD) model.

In this paper we present a dielectric relaxation analysis of three acrylic–polyurethane gel samples in connection with their synthesis conditions. The dielectric analysis was carried out using broadband dielectric spectroscopy (BDS). Additionally, differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) measurements were performed. The thermodynamic characterization outcomes evidence very similar characteristics of the glass transition phenomena among the investigated samples without any signature of melting or crystallization. X-ray diffraction patterns evidence distinctive features for the three gel samples in two different regions, i.e., at high and low values of the scattering vector q. BDS measurements, identify three distinct dielectric relaxation processes above Tg. Particularly, differences in the dielectric parameters of the slowest relaxation process have been rationalized in connection with the synthesis method used. Our results show that the analysis of these acrylic–polyurethane gels by BDS – in combination with SAXS – provides access to characteristics of the network structure that would ultimately influence the properties of these materials.

In this work we present a dielectric relaxation study of Acrylic/Polyurethane polymers synthesized via miniemulsion photo-polymerization. Three different samples, both parts of the synthesized hybrid latex (SOL and GEL fractions) and a full acrylic sample as reference have been considered. Besides dielectric experiments using Broadband Dielectric Spectroscopy (BDS) technique, complementary Differential Scanning Calorimetry measurements have been performed. The thermodynamic characterization outcomes evidence very similar characteristics of the glass transition phenomena among the investigated samples without any signature of melting/crystallization. However BDS experiments show three well resolved dielectric relaxation processes above Tg, two of them evidence marked differences among the samples. A detail study allows us to provide a molecular origin for these three dielectric relaxation processes in connection with the sample characteristics. In particular, our results show that the characterization of these hybrid Acrylic/PU by BDS can provide access to some of the structural features that would ultimately influence the adhesives properties.

In the last years, a new AFM based dielectric spectroscopy approach has been developed for measuring the dielectric relaxation of materials at the nanoscale, the so called nanoDielectric Spectroscopy (nDS). In spite of the effort done so far, some experimental aspects of this technique remain still unclear. In particular, one of these aspects is the possibility of extending the experimental frequency range, to date limited at high frequencies by the resonance frequency of the AFM cantilever as a main factor. In order to overcome this limitation, the electrical excitation of cantilever higher eigenmodes for measuring the dielectric relaxation is here explored. Thus, in this work we present a detailed experimental analysis of the electrical excitation of the cantilever second eigenmode. Based on this analysis we show that the experimental frequency range of the AFM based dielectric spectroscopy can be extended by nearly two decades with a good signal-to-noise ratio. By using the combination of first and second cantilever eigenmodes we study dissipation processes on well known PVAc based polymeric samples. Both, relaxation spectra and images with molecular dynamics contrast were thus obtained over this broader frequency range.

The glass transition temperature (Tg) of polymer thin films has been a subject of controversy in the last two decades. Determinations of the thermal Tg, that is the temperature at which a discontinuity in thermodynamic coefficients is observed, generally suggest a significant depression, whereas the molecular mobility is found to be unchanged. These experimental evidences have been recently rationalized within the volume holes diffusion model. Such model was previously applied to the description of the out-of-equilibrium dynamics in different nanostructured systems, included polymer freestanding films. According to the model, Tg depression emerges from the ability of thinner films to maintain equilibrium, due to the shorter distance free volume holes have to diffuse to the polymer interface, the molecular motion determining the diffusion coefficient being thickness independent. The present study aims to extend the application of the model to supported films that is systems where adsorption at the polymer/substrate interface limits the amount of >free> interface available to free volume holes elimination. Our analysis demonstrates that the model is able to quantitatively capture the thickness as well as the cooling rate dependence of the Tg depression in a variety of sample configurations. © 2013 Elsevier B.V. All rights reserved.

Filler-polymer interactions in reinforced rubbers are mainly governed by a thin layer of polymer around the filler particles. The dynamics of this layer has long been a matter of intense investigation. Some researchers suggest that this >interacting layer> is immobilized and possesses slower dynamics compared to the bulk, whereas some others consider it equally mobile. In this work, we address this problem by studying the macroscopic characteristics of reinforced rubber using Broadband Dielectric Spectroscopy (BDS) and by measuring its local dielectric behavior using a new AFM based approach: nano-dielectric spectroscopy. This new approach enables us to study the dielectric response of the interacting polymer layer and the bulk dynamics locally, with a spatial resolution of about 20-30 nm. The total amount of interacting polymer within the compounds was also obtained by analyzing the local mechanical properties. Both macro- and nano-measurements show no differences in the polymer local dynamics for the compounds here analyzed. We propose that, for our particular system, the interacting polymer layer shares the same local relaxation dynamics with the bulk rubber due to the presence of a flexible link between the polymer chains and the nanoparticles. © 2013 Elsevier Ltd. All rights reserved.

We investigate the role of structure and chemical composition on the uptake of poly(ethylene oxide) by a series of graphite oxides (GOs) and thermally reduced GOs, leading to the formation of polymer-intercalated GO and polymer-adsorbed graphene nanostructures. To this end, a series of poly(ethylene oxide) (PEO) - GO hybrid materials exhibiting a variable degree of GO oxidation and exfoliation has been investigated in detail using a combination of techniques including X-ray photoelectron spectroscopy, X-ray diffraction, thermogravimetry, scanning-electron microscopy, and nitrogen adsorption. Intercalation of the polymer phase into well-defined GO galleries is found to correlate well with both the degree of GO oxidation and with the presence of hydroxyl groups. The latter feature is an essential prerequisite to optimize polymer uptake owing to the predominance of hydrogen-bonding interactions between intercalant and host. Unlike the bulk polymer, these intercalation compounds show neither crystallisation nor glass-transition associated with the polymer phase. Exfoliation and reduction of GO result in high-surface-area graphene layers exhibiting the highest polymer uptake in these GO-based materials. In this case, PEO undergoes surface adsorption, where we observe the recovery of glass and melting transitions associated with the polymer phase albeit at significantly lower temperatures than the bulk. © 2012 Elsevier Ltd. All rights reserved.

The physical aging process, namely the spontaneous evolution of the thermodynamic state occurring in glasses, has been monitored in poly(methyl methacrylate) (PMMA)/silica and polystyrene (PS)/silica nanocomposites following the time evolution of the enthalpy by means of differential scanning calorimetry (DSC). The effect on physical aging of the content of silica particles has been investigated in detail varying it over a wide range. Our results indicate accelerated physical aging in the nanocomposites in comparison to the corresponding pure polymer. Furthermore the acceleration is generally more pronounced in nanocomposites presenting a high silica content. This result cannot be attributed to a difference in molecular mobility of the polymers in the nanocomposites in comparison to pure PMMA and PS, since broadband dielectric spectroscopy (BDS) indicates no effect of silica nanoparticles on the polymer segmental dynamics. Moreover, calorimetric measurements reveal a reduction of the heat capacity jump at T g for the nanocomposites, as well as lower experimentally recoverable enthalpy values. This may result from the faster non-isothermal evolution of the glass state when cooling down the samples from their liquid state. To account for these experimental results the acceleration of physical aging has been rationalized in the framework of the diffusion of free volume holes model as previously proposed. This is able to adequately catch the dependence of the physical aging rate on silica content, determining the area of nanoparticles to the volume of polymer, that is the relevant variable within the diffusion model. © 2012 Published by Elsevier Ltd.

Treatment of graphite oxide (GO) with organic solvents via sorption from either liquid or gas phase, and subsequent desorption, induces profound changes in the layered GO structure: loss of stacking order, retention of trace amounts of solvents and decreasing decomposition temperature. This study presents new evidences of the effect of organic solvents on the thermal reduction of GO by means of thermogravimetric analysis, X-ray diffraction and X-ray photoelectron spectroscopy. The results reveal a relative higher decrease of the oxygen amounts in solvent-treated GO as compared to untreated GO and the restacking inhibition of the thermally reduced GO sheets upon slow heating. The kinetic experiments evidence changes occurring in the reduction mechanisms of the solvent-treated GO, which support the close link between GO structure and thermal properties. © 2011 Elsevier B.V. All rights reserved.

In this work we present a new AFM based approach to measure the local dielectric response of polymer films at the nanoscale by means of Amplitude Modulation Electrostatic Force Microscopy (AM-EFM). The proposed experimental method is based on the measurement of the tip–sample force via the detection of the second harmonic component of the photosensor signal by means of a lock-in amplifier. This approach allows reaching unprecedented broad frequency range (2–3×104 Hz) without restrictions on the sample environment. The method was tested on different poly(vinyl acetate) (PVAc) films at several temperatures. Simple analytical models for describing the electric tip–sample interaction semi-quantitatively account for the dependence of the measured local dielectric response on samples with different thicknesses and at several tip–sample distances.

Dielectric relaxation (DR) has shown to be a very useful technique to study dielectric materials like polymers and other glass formers, giving valuable information about the molecular dynamics of the system at different length and time scales. However, the standard DR techniques have a fundamental limitation: they have no spatial resolution. This is of course not a problem when homogeneous and non-structured systems are analyzed but it becomes an important limitation for studying the local properties of heterogeneous and/or nano-structured materials. To overcome this constrain we have developed a novel approach that allows quantitatively measuring the local dielectric permittivity of thin films at the nanoscale by means of Electrostatic Force Microscopy. The proposed experimental method is based on the detection of the local electric force gradient at different values of the tip–sample distance. The value of the dielectric permittivity is then calculated by fitting the experimental points using the Equivalent Charge Method. Even more interesting, we show how this approach can be extended in order to obtain quantitative dielectric images of insulating thin films with an excellent lateral resolution.

The sorption–desorption of water and different organic solvents (methanol, 1-propanol, benzene, chloroform and carbon tetrachloride) from the vapor phase in graphite oxide (GO) was studied. Changes observed in the GO interlayer distance, initially due to solvent uptake, were subsequently monitored during desorption by means of time-resolved X-ray diffraction so providing desorption kinetics. A limiting geometric stability in the GO interlayer distance was envisaged, corresponding to an expansion of about 60%, with an intercalation of about 45 mL of solvent/100 g of GO. Specific desorption kinetics behavior was found for each solvent to be related to their chemical nature and geometry. Finally, from this study it was also found that the layer distortions of GO, when exposed to water and organic solvent vapors, occurred in a lesser extent than that found when the intercalation occurred from the liquid solvents.

In this work we have studied the low-temperature dynamics of water molecules after isothermal crystallization at different temperatures (Tc) and times (tc) of an aqueous solution of poly(vinyl pyrrolidone) (PVP) by broadband dielectric spectroscopy (BDS) and optical microscopy. Three differentwater concentrations (cw=45, 50 and 55 wt.%) were explored. Two major different phases were observed by optical microscopy during crystallization: one related with the core ice formed during crystallization and the other one related with the amorphous phase. Between them a rich interface was also seen. By BDS, two dielectric processes (I and II) are observed in the amorphous samples whereas, in the partially crystallized samples a third relaxation (process III) emerges in the dielectric spectra. Process II is related with the rotational dynamics of water molecules in the amorphous phase with similar characteristics to that found in the complete amorphous solutions whereas the process III is attributed to the rotation of water molecules at the interface between the ice cores and the amorphous water–PVP phase. Typical characteristics of these processes on the relaxation map are discussed.

In this communication, we present first results on the dielectric response of various end-functionalized polystyrenes (PS). The functional polymers have been prepared by using a combination of anionic polymerization techniques and hydrosilation chemistry that gives well-defined polymers where the functional group can be placed with flexibility and high precision. The data show that for rather large functional groups, the overall polymer matrix properties are altered giving rise to a decrease in the glass transition temperature. The trend can be rationalized in terms of free volume effects caused by the bulky functional groups. However, for cyano-functionalized PS, i.e. PS with a CN group at the chain end, the inclusion of the group does not affect the matrix properties. We show that by taking advantage of the strong dipole moment of the CN group, we can obtain a strong dielectric signal that can be used to selectively study the specific dynamics where the group is located. In other words, by appropriately attaching cyano groups at different part of the chains, these can be used as dielectric probes that allow specific contributions to the alpha relaxation.

The results of phenomenological analysis of the ortho-positronium (o-Ps) lifetime, s3, from positron annihilation lifetime spectroscopy (PALS) and of the dielectric relaxation spectra from broadband dielectric spectroscopy (BDS) on poly(propylene glycol) (PPG) are presented. A number of coincidences between the characteristic PALS temperatures TPALS g , TL b1 and TL b2 obtained from a phenomenological analysis of a quasi-sigmoidal PALS response and the various characteristic BDS temperatures for the spectral and relaxation parameters have been revealed. Thus, an onset of the quasi-plateau level at the PALS temperature TL b2 ¼ 1:44TPALS g is characterized by the equality of the o-Ps lifetime, and the primary relaxation time: s3 ffi sa. Moreover, a deviation toward the quasi-plateau level at TL b2 is related to a saturation in both spectral width and relaxation strength parameters of the a-process and a merging of the relaxation times of the primary (a) and secondary (b)-processes. In addition, TL b2 lies in the vicinity of a high-temperature crossover of the main relaxation process from the non-Arrhenius to the Arrhenius behavior. The another characteristic PALS temperature, TL b1 ¼ 1:18TPALS g , is also connected with the change in temperature behavior of time and strength parameters of the secondary effective b-relaxation. All these findings indicate a close connection between the PALS response and the dielectric relaxation in PPG.

The dispersion of graphite oxide (GO) in organic solvents followed by their evaporation at relative high temperature resulted in a strong adsorption of the solvent molecules in the graphitic interlayers as confirmed by 13C magic-angle-spinning NMR. Three series of solvents, alcohols (1-methanol, 1-propanol, 1-pentanol, 1-heptanol), aromatics (benzene, toluene, p-xylene, chlorobenzene) and chloride compounds (dichloromethane, chloroform, carbon tetrachloride) were studied to understand the interaction of graphite oxide with solvents. The distribution of basal interlayer spacing changed due to solvent intercalation and this distribution was particularly different for each solvent series. Even though there was on average 1 solvent molecule per 100 carbon atoms of GO, they had a profound effect on the thermal properties of the resulting GO. The exfoliation temperature was drastically reduced by the presence of the solvents due to the increase of the interlayer spacing and the reducing power of the solvent.

We report on the dielectric and calorimetric results of water solutions of poly(vinyl methyl ether) (PVME), poly(vinyl pyrrolidone) (PVP), pentaethylene glycol (5EG) and tri-poly(propylene glycol) (3PG) for a temperature range of 130 K up to crystallization. For 3PG and 5EG two relaxation processes (Process I and II) were observed for all the hydration levels whereas for PVME and PVP a single peak was detected (Process II). We will show that the temperature dependence of the relaxation time of Process II, related to water molecules dynamics, presents a crossover at a temperature about the calorimetric glass transition of the whole system. We show that this crossover could be interpreted in terms of confinement effects due to the frozen-in of the whole solution. Finally, in spite of the fact that water molecules are surrounded by different environments, the results are very similar for the all the investigated systems, which suggest a general behavior for confined water. From this finding, we have obtained an intrinsic value of the activation energy for water dynamics in confining environments.

We have investigated the motion of different molecular groups in glassy polycarbonate below its glass transition temperature. The characterization of methyl group and phenylene ring motions on the one hand, and carbonate group on the other, were carried out by means of neutron scattering and dielectric spectroscopy respectively. The comparison of the results from both the techniques indicate that different motions within polycarbonate may be strongly coupled.

In a previous quasielastic neutron scattering study [I. Quintana, A. Arbe, J. Colmenero, B. Frick, Macromolecules 38 (2005) 3999] we characterized the hydrogen dynamics in glassy polyethersulfone as due to ‘fast processes’ plus flips and oscillations of the phenylene rings. Here we have extended this investigation by using thermal neutrons, that allowed exploring also much higher values of the momentum transfer Q. In the dynamic window accessed, a characterization as best accurate as possible of the fast dynamics is imperative in order to describe the experimental results, especially at very local length scales. While for Q ≲ 2.2 Å−1 (i.e., in the range previously studied) the proposed model perfectly describes the experimental data with the same parameters deduced from the earlier work, in the high Q-range (Q ≈ 3.5–4 Å−1) deviations are found. These could be overcome by assuming some kind of ‘effective softening’ of the fast process and slight deviations from harmonic behavior in the Q-dependence of the associated Debye–Waller factor.

Among the different dynamical processes that take place in polymers, methyl group rotation is perhaps the simplest one, since all the relevant interactions on the methyl group can be condensed in an effective mean-field one-dimensional potential. Recent experimental neutron scattering results have stimulated a new revival of the interest on methyl group dynamics in glasses and polymer systems. The existence of quantum rotational tunnelling of methyl groups in polymers was expected for a long time but only very recently (1998), these processes have been directly observed by high-resolution neutron scattering techniques. This paper revises and summarizes the work done on this topic over last 10 years by means of neutron scattering. It is shown that the results obtained in many chemically and structurally different polymers can be consistently described in the whole temperature range—from the quantum tunnelling limit to the classical hopping regime—as well as in the librational spectrum, in terms of the Rotation Rate Distribution Model (RRDM), which was first proposed in 1994. This model introduces a distribution of potential barriers for methyl group rotation, which is associated to the disorder present in any structural glass. The molecular and structural origin of the barrier distribution in polymers is discussed on the basis of a huge collection of investigations reported in the literature, including recent fully atomistic molecular dynamics simulations.

Time-of-flight measurements on three engineering thermoplastics and a polymer membrane have been carried out below their glass-transition temperatures. The vibrational properties as well as the non-harmonic contributions to the scattering functions of these systems are discussed in a comparative way.

Fully atomistic molecular dynamic simulations were carried out by using the Insight (Insight II 4.0.0 P version) and the Discover-3 programs from MSI with the polymer consortium force field. The model system used in these simulations was built using the Amorphous Cell module. The polymer system simulated was glassy polyisoprene (PI) as used in previous neutron scattering (NS) measurements. A first molecular dynamics at 363 K was run for 1 ns using the Discover-3 program collecting data every 0.01 ps and a subsequent one (taking the previous output sample as an input for the following dynamics) was run for 2 ns collecting data every 0.5 ps. The results of the second run agreed to those of the first run, indicating that the sample was well equilibrated at this high temperature. Starting from the obtained atomic trajectories we have calculated the partial static structure factors for NS corresponding to different PI samples with different levels of deuteration (PId3, i.e., methyl group deuterated and main chain protonated; PId5, i.e., methyl group protonated and main chain deuterated; PId8, i.e., fully deuterated and PIh8, i.e., fully protonated). The results obtained are compared to the coherent NS cross-sections measured on real samples by means of D7 spectrometer with polarization analysis (ILL, Grenoble). A good agreement is obtained between experimental and simulated data validating the simulated sample. Moreover, the dynamic evolution of these correlations has also been calculated from the simulations. With these time dependent functions, the magnitude measured in a neutron spin echo (NSE) experiment can be constructed. Here we present two examples dealing with the fully deuterated sample PId8 and a partially deuterated sample, PId5, that show how computer simulation constitutes an invaluable tool for interpreting NSE results.

We have carried out molecular dynamics (MD) simulations of polyisoprene at 363 K, about 150 K above the experimental glass transition temperature, using the insight and discover programs from MSI Inc. with the Polymer Consortium Force Field. The model system was built using the MSI amorphous cell construction protocol with periodic boundary conditions. Starting from the self part of the van Hove correlation function, the incoherent intermediate scattering function was calculated for the protons in the main chain and in the methyl groups (MGs). The dynamics of the latter ones can be well described assuming decoupled segmental dynamics and rotations in a threefold potential. The limits of such an approximation are also discussed. We find the existence of a distribution of potential barriers for MG rotation which is very similar to that deduced from low temperature (150 K) MD-simulation results and inelastic neutron scattering measurements. The glass transition would thus hardly modify this distribution.

Recent experimental neutron scattering results have stimulated a new revival of the interest on methyl group dynamics in glasses and polymer systems. The existence of quantum rotational tunnelling of methyl groups in polymers was expected for a long time. Only very recently, these processes have been directly observed by high-resolution neutron scattering techniques (IN16, ILL Grenoble). The goal of this paper is to revise and summarise the work done in these topics over the last few years by means of neutron scattering. It will be shown that the results obtained, for both quantum and classical regimes, in different polymers can be consistently described in terms of a model which assumes a pure single-particle threefold potential for methyl group dynamics together with a distribution of potential barriers. The microscopic origin of such distributions, which should be associated to the disorder present in any structural glass, can be investigated by molecular dynamics simulations in realistic polymers. It is concluded that the width of the distribution is mainly controlled by the non-bonded interactions.

In a previous work the incoherent scattering function of poly(vinyl chloride) (PVC) in the microscopic time range was successfully described in the frame of a model which assumes harmonic vibrations coexisting with relaxational contributions controlled by C–C torsional barriers. In the same frame the behaviour of the coherent scattering function well above the glass transition temperature Tg could also be interpreted. In this work, we investigate the applicability of such a model to coherent scattering of PVC below and around Tg. Within the experimental accuracy, the data can be well described starting from the results independently obtained from the study of the incoherent function. This gives additional support to the ideas involved in the proposed model.

We present a neutron scattering study of the methyl group dynamics at low temperature in sodium acetate trihydrate, Na(CH3COO) · 3H2O, in crystalline and glassy state. As previously found in glassy polymers, the spectra of this low molecular weight system show an apparent quasielastic feature in the glassy state instead of the characteristic rotational tunnelling lines observed in crystals. This can be understood in terms of an asymmetric distribution of rotational tunnelling lines resulting from a Gaussian distribution of potential barriers for methyl rotation. The average barrier of this distribution is found to be notably higher than the unique barrier present in the crystal.