Heterogeneous dynamics at the glass transition in van der Waals liquids, in the bulk and in thin films

It has been shown over the last few years that the dynamics close to the glass transition is strongly heterogeneous, both by measuring the diffusion coefficient of tagged particles or by NMR studies. Recent experiments have also demonstrated that the glass transition temperature of thin polymer films can be shifted as compared to the same polymer in the bulk. We propose here first a thermodynamical model for van der Waals liquids, which accounts for experimental results regarding the bulk modulus of polymer melts and the evolution of the density with temperature. This model allows us to describe the density fluctuations in such van der Waals liquids. Then, by considering the thermally induced density fluctuations in the bulk, we propose that the 3D glass transition is controlled by the percolation of small domains of slow dynamics, which allows to explain the heterogeneous dynamics close to T _g. We show then that these domains percolate at a lower temperature in the quasi-2D case of thin suspended polymer films and we calculate the corresponding glass transition temperature reduction, in quantitative agreement with experimental results of Jones and co-workers. In the case of strongly adsorbed films, we show that the strong adsorption amounts to enhance the slow domains percolation. This effect leads to 1) a broadening of the glass transition and 2) an increase of T _g in quantitative agreement with experimental results. For both strongly and weakly adsorbed films, the shift in T _g is given by a power law, the exponent being the inverse of that of the correlation length of 3D percolation. Received 21 March 2000 and Received in final form 4 December 2000     

Viscoelastic phase separation in polymer blends

In this paper, the dynamics and morphology of viscoelastic phase separation in polymer blends is investigated based on the two-fluid model in two dimensions. At critical composition, we have carefully checked the role of shear modulus, without taking account of bulk modulus. The results show that the higher shear modulus component tends to form a dispersed phase in the intermediate stage of phase separation, if the difference between the shear moduli of the components is large enough. This is opposite to the role of bulk modulus, that the higher bulk modulus component forms a networklike pattern without taking account of the shear modulus even if it is the minority phase. The morphological formation is determined by the competition of opposite effects of shear modulus and bulk modulus. For polymer blends at critical composition, the bulk modulus difference leads to a networklike pattern formed by the higher modulus component in the intermediate stage of phase separation. But if the difference between the shear moduli of the components is large enough, a co-continuous structure is observed, resulting from the competition between shear and bulk moduli. For off-critical composition, difference in bulk modulus also leads to a networklike pattern of the component with higher bulk modulus in the intermediate stage of phase separation, but phase inversion is observed rapidly. A small difference between the shear moduli of the components can support the networklike pattern to continue for longer time. But the networklike pattern does not occur for large difference between shear moduli.Received: 9 September 2004, Published online: 10 November 2004PACS: 64.75. + g Solubility, segregation, and mixing; phase separation - 83.80.Tc Polymer blends     

The single-fibre pull-out method: its advantages,interpretation and experimental realization

Past results from the single-fibre pull-out tests are reviewed and new results obtained with carbon in thermoplastics are presented. The force-distance curve during pull-out indicates some pseudo-ductility, in some cases, as the applied force builds up to the failure load. However, the failure itself is not ductile; rather, it is sudden, suggesting brittle fracture. The debonding force vs. embedded length (L) plots range from straight lines intersecting the origin, through smooth curves, to scatter diagrams, depending on the fibre and polymer. Interfacial shear strengths estimated from the shorter embedded lengths are often high and, quite frequently, higher than that of the polymer matrix. In addition, there is normally no correlation between interface properties (strength or work of fracture) and the corresponding polymer properties. This could well be due to the growth of an oriented interphasial layer, with a modulus and strength up to six times greater than that of the bulk polymer, as has been observed for modulus at least, with particulate reinforced materials.     

Proton conducting interpenetrating polymer networks

In order to improve stability and performance of the polymer electrolyte-based hydrogen sensor developed for on-line analysis, modifications to the PVA/H₃PO₄ proton conducting polymer blend were made. Water insoluble, increased bulk modulus, and higher conductivity polymer membranes have been fabricated by development of interpenetrating polymer networks that incorporate the PVA/H₃PO₄ (host) blend. The guest polymer is a three dimensional polymer network composed of methacrylic acid and methylenebisacrylamide. High protonic conductivity results from the phosphoric acid and water, the poly (methaacrylic acid-methylenebisacrylamide) contributes to the increased modulus water insolubility. Hydrogen sensors have been demonstrated using these membranes.     

Structural relaxation and dynamic heterogeneity in a polymer melt at attractive surfaces

Molecular dynamics simulations of polymer melts at flat and structured surfaces reveal that, for the former, slow dynamics and increased dynamic heterogeneity for an adsorbed polymer is due to densification of the polymer in a surface layer, while, for the latter, the energy topography of the surface plays the dominant role in determining dynamics of interfacial polymer. The dramatic increase in structural relaxation time for polymer melts at the attractive structured surface is largely the result of dynamic heterogeneity induced by the surface and does not resemble dynamics of a bulk melt approaching T(g).     

Entropic wetting and many-body induced layering in a model colloid-polymer mixture

We develop an efficient simulation scheme to study a model suspension of equally sized colloidal hard spheres and nonadsorbing ideal polymer coils, both in bulk and adsorbed against a planar hard wall. The many-body character of the polymer-mediated effective interactions between the colloids yields a bulk phase diagram and adsorption phenomena that differ substantially from those found for pairwise simple fluids; e.g., we find an anomalously large bulk liquid regime and, far from the bulk triple point, three layering transitions in the partial wetting regime prior to a transition to complete wetting by colloidal liquid.     

Mean-field theory for polymer adsorption on curved surfaces

We discuss the influence of polymer adsorption on the curvature energy of an interface. Following an article by Clement and Joanny (J. Phys. II 7, 973 (1997)), a mean-field theory is used to calculate the surface tension, rigidity constants and spontaneous curvature associated with both reversible and irreversible polymer adsorption. In the case of irreversible polymer adsorption it is assumed that the amount of adsorbed polymer remains constant upon curving the interface. Unfortunately, constraining the amount of polymer by adding a Lagrange multiplier affects the thermodynamic state of the (free) polymer far away from the interface. Clement and Joanny solve this problem by removing the polymers in the bulk. We allow for the presence of free polymers, but to achieve this we have to apply a local external field to keep the adsorbed amount fixed. The results of the two approaches are compared and a physical interpretation is given. Received 25 July 2001 and Received in final form 5 December 2001     

First-principles study of structural,elastic, lattice dynamical and thermodynamical properties of GdX (X = Bi,Sb)

The results are presented of first-principles calculations of the structural, elastic and lattice dynamical properties of GdX (X = Bi, Sb). In particular, the lattice parameters, bulk modulus, phonon dispersion curves, elastic constants and their related quantities, such as Young's modulus, Shear modulus, Zener anisotropy factor, Poisson's ratio, Kleinman parameter, and longitudinal, transverse and average sound velocities, were calculated and compared with available experimental and other theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacities, Grüneisen parameter and Debye temperatures were predicted in wide pressure (0?50 GPa) and temperature ranges (0–500 K). The plane-wave pseudopotential approach to the density-functional theory within the GGA approximation implemented in VASP (Vienna ab initio simulation package) was used in all computations.     

Collapsed and Adsorbed States of a Directed Polymer Chain in Two Dimensions

A phase diagram for a surface-interacting long flexible partially-directed polymer chain in a two-dimensional poor solvent, where the possibility of collapse in the bulk exists, is determined using exact enumeration methods. We used a model of self-attracting self-avoiding walks and evaluated 30 steps in series. An intermediate phase between the desorbed collapsed and adsorbed expanded phases, having the conformation of a surface-attached globule, is found. The four phases, viz ., (i) desorbed expanded (DE), (ii) desorbed collapsed (DC), (iii) adsorbed expanded (AE), (iv) surface-attached globule (SAG), are found to meet at a multicritical point. These features are in agreement with those of an isotropic (or non-directed) polymer chain.     

Ab initio study of structural,elastic, and high-pressure properties of rubidium halides RbX (X = F,Cl, Br and I)

We present first-principle calculations on the structural, elastic, and high-pressure properties of rubidium halides compounds, using the pseudo-potential plane-waves approach based on density functional theory, within the generalized gradient approximation. Results are given for lattice constant, bulk modulus and its pressure derivative. The pressure transition at which these compounds undergo structural phase transition from NaCl-type to CsCl-type structure are calculated and compared with previous calculations and available experimental data. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's modulus and Poisson's ratio for ideal polycrystalline RbF, RbCl, RbBr, and RbI aggregates. We estimated the Debye temperature of these compounds from the average sound velocity.     

Ab initio studies of the structural,elastic, electronic and thermal properties of NiTi2 intermetallic

First principles calculations were performed in the framework of the density functional theory (DFT) using the Full Potential–Linear Augment Plane Wave method (FP–LAPW) within the generalized gradient approximation (GGA) to predict the structural, electronic, elastic and thermal properties of NiTi₂ intermetallic compound. By using the Wien2k all-electron code, calculations of the ground state and electronic properties such as lattice constants, bulk modulus, presure derivative of bulk modulus, total energies and density of states were also included. The elastic constants and mechanical properties such as Poisson’s ratio, Young’s modulus and shear modulus are estimated from the calculated elastic constants of the single crystal. Through the quasi-harmonic Debye model, the preasure and temperature dependences of the linear expansion coefficient, bulk modulus and heat capacity have been investigated. Finally, the Debye temperature has been estimated from the average sound velocity according to the predicted polycrystal bulk properties and from the single crystal elastic constants.     

Ab initio investigation of the structural,elastic and electronic properties of the anti-perovskite TlNCa3

We have investigated the structural, elastic and electronic properties of the anti-perovskite TlNCa₃ using ab initio calculations within the generalized gradient approximation and the local density approximation for the exchange–correlation potential. The lattice constant, bulk modulus, elastic constants and their pressure dependence, energy band structures, density of states and charge density distribution are calculated and analyzed in comparison with the available experimental and theoretical data. The bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, Lamé’s coefficients, average sound velocity and Debye temperature are numerically estimated for ideal polycrystalline TlNCa₃ aggregates in the framework of the Voigt–Reuss–Hill approximation. This is the first theoretical prediction of the elastic constants and their related properties for TlNCa₃ that requires experimental confirmation.     

Diblock copolymers adsorbed at a water-oil interface

We probe the conformation of a diblock copolymer layer adsorbed at the surface of water-in-oil emulsion droplets at various concentration of a molecular surfactant. The diblock copolymer is made of a hydrophobic polybutadiene part linked to a hydrophilic polyethylene oxide one. The measure provides the equilibrium thickness of the polymer layer that is obtained with two different techniques, i.e. dynamic light scattering and force measurements. The structure of the layer is shown to change from a “mushroom” conformation in which the adsorbed chains form independent Gaussian coils to a conformation where they interact and extend in the continuous phase. The transition from one regime to the other is progressive as the ratio surfactant/polymer bulk concentration varies. Received 5 May 2000 and Received in final form 6 July 2000     

Bulk Mechanical Behavior of Rootzone Sand Mixtures as Influenced by Particle Shape,Moisture and Peat

Mechanical properties such as bulk modulus, shear modulus, and failure stress of rootzone sand mixtures are some of the key parameters in understanding the load‐response of sands used in professional golf courses. According to the United States Golf Association (USGA) specifications, appropriate particle size distributions and their shape are important for preparing putting greens and bunker sands. Despite being an important parameter, the influence of sand particle shape on the bulk mechanical properties of the rootzone mixtures has not been studied systematically using a fundamental tester. Toward this end, bulk mechanical properties were measured using a low‐pressure cubical (true) triaxial tester. In this study, four of the commonly used basic shapes, i.e., round, subround, subangular, and angular sand particles comprising rootzone mixtures with sphagnum peat (organics) were tested at two different moisture levels, air‐dried and 30 cm moisture tension conditions. For all rootzone sand mixtures, an increase in bulk modulus was observed with increasing isotropic pressure. The failure stress values increased with the increase in mean pressure for air dried samples. In general, moisture increased compressibility of sands and decreased failure strength and shear modulus values. Peat had a dominant influence on the mechanical response of all four sand shapes. When peat and moisture were added, the rootzone mixture became most compressible and easier to fail, with noticeable changes in bulk mechanical properties.     

A convenient method to determine the bulk modulus of nanowires and its temperature dependence based on X-ray diffraction measurement

The bulk modulus of nanowires (NWs) and its temperature dependence were determined by a simple and convenient method based on temperature-dependent X-ray diffraction (XRD) measurement. It was found that the bulk moduli for Ni, Cu, and Ag NWs were much higher than that for their counterpart bulk materials in the temperature range from 25 °C to 800 °C and the influence of temperature on the bulk modulus for NWs was stronger than that for their counterpart bulk materials. A surface bond contraction model and the force–interatomic-distance curves were introduced to explain the experimental results.     

Optical switching studies of an azobenzene rigidly linked to a hexa-<Emphasis Type="Italic">peri</Emphasis>-hexabenzocoronene derivative in solution and at a solid–liquid interface

Arrays of test structures consisting of sub-150 nm wide beams were lithographically fabricated in poly(methyl methacrylate) (PMMA) and used to measure the elastic mechanical properties of the material. Capillary forces that arise during the drying of rinse liquids from the test structures caused the nanoscale polymer beams to deform. The initial capillary forces were defined by the test structure geometry, and the magnitudes of the forces were quantified using a two-dimensional Young–Laplace equation. The deformation of the nanostructured beams was measured experimentally and compared to a model based on continuum-level bending beam mechanics, thereby enabling the calculation of the Young’s modulus (E) of the material. For PMMA beams greater than 100 nm in width E was calculated to be 5.1 GPa at room temperature, which corresponds closely to the elastic modulus of bulk PMMA. The Young’s moduli of structures with dimensions less than 100 nm were measured to be less than the bulk value and the origin of the decrease is discussed in terms of dimension dependent properties and polymer degradation during fabrication. The polymer nanostructures also were determined to mechanically deform more readily with increasing characterization temperature. PACS 62.25.+g; 68.35.Gy; 81.16.Nd     

二元Ni-Al合金极端条件下的弹性和热力学性能：全电子准谐波近似 cited: 1)

通过在原子尺度上建模来研究Al、NiAl和Ni₃Al合金在极端高温和高压下的点阵常数、弹性常数、弹性模量、泊松比和弹性各向异性因子等性质．计算得到的弹性常数均满足相应的力学稳定条件．由于NiAl和Ni₃Al具有较高的B=G值,在0～30 GPa内都属于延展性材料．通过包含电子热运动对体系吉布斯自由能贡献的全电子准谐近似方法,得到了高温高压下Al、NiAl和Ni₃Al合金的热膨胀系数、体积模量、热容和熵等．计算值与已有的实验值和理论值符合较好     

Equation of state and compression effect on the bulk modulus of AlP,AlAs and AlSb compounds

The equation of state and the pressure effect on the bulk modulus of AlP, AlAs and AlSb are studied from the electronic theory of solids by using our presented binding force, although not reported experimentally. The obtained results of the pressure-volume relations involving the pressure- induced phase transition are useful to estimate the experimental data of these compounds. The obtained bulk modulus increases with the crystal volume compressed, and the pressure derivative of the bulk modulus for AlP, AlAs and AlSb is estimated theoretically.     

Mechanical properties of bulk and nanoscale TiO2 phases

The mechanical properties of bulk and nanoscale TiO₂ phases are examined with a view to assess the available bulk modulus and hardness data, and to understand the size-dependent behaviors. The bulk modulus values of thermodynamically stable bulk TiO₂ phases show a general correlation with Ti-O coordination number. As with the cotunnite-structured (OII) phase, it is likely that the seven-coordinated OI and eight-coordinated fluorite forms of TiO₂ are ultrahard substances. Of the nanoscale phases investigated thus far, nanocrystalline anatase displays the strongest size dependence of bulk modulus values, with possible stiffening behavior effected by incipient grain boundary amorphization under pressure. Nanocrystalline rutile and baddeleyite phases do not show appreciable size dependence in their compression behaviors.     

Plasmon energy, bulk modulus, electronic polarisability of AIBIIIC2VI and AIIBIVC2V ternary chalcopyrite semiconductors

A simple relation between the bulk modulus, plasmon energy, electronic polarisability are given for ternary chalcopyrite semiconductors. Bulk modulus has been evaluated from plasmon energy by proposing a linear relation between them. From bulk modulus electronic polarisability has been evaluated. The estimated values are in good agreement with the experimental values and earlier researchers.