Structural studies and polymorphism in amorphous solids and liquids at high pressure

When amorphous materials are compressed their structures are expected to change in response to densification. In some cases, the changes in amorphous structure can be discontinuous and they can even have the character of first-order phase transitions. This is a phenomenon referred to as polyamorphism. Most evidence for polyamorphic transitions between low and high density liquids or analogous transformations between amorphous forms of the same substance to date has been indirect and based on the changes in thermodynamic and other structure-related properties with pressure. Recent studies using advanced X-ray and neutron scattering methods combined with molecular dynamics simulations are now revealing the details of structural changes in polyamorphic systems as a function of pressure. Various "two state" or "two species" models are used to understand the anomalous densification behaviour of liquids with melting curve maxima or regions of negative melting slope. Thermodynamic analysis of the two state model leads to the possibility of low- to high-density liquid transitions caused by differences in bulk thermodynamic properties between different amorphous forms and on the degree of cooperativity between low- and high-density structural configurations. The potential occurrence of first-order transitions between supercooled liquids is identified as a critical-like phenomenon. In this tutorial review we discuss the background to polyamorphism, incorporating the experimental observations, simulation studies and the two-state models. We also describe work carried on several systems that are considered to be polyamorphic.     

Molecular dynamics simulation of planar elongational flow at constant pressure and constant temperature

Molecular dynamics simulations of liquid systems under planar elongational flow have mainly been performed in the NVT ensemble. However, in most material processing techniques and common experimental settings, at least one surface of the fluid is kept in contact with the atmosphere, thus maintaining the sample in the NpT ensemble. For this reason, an implementation of the Nose-Hoover integral-feedback mechanism for constant pressure is presented, implemented via the SLLOD algorithm for elongational flow. The authors test their procedure for an atomic liquid and compare the viscosity obtained with that in the NVT ensemble. The scheme is easy to implement, self-starting and reliable, and can be a useful tool for the simulation of more complex liquid systems, such as polymer melts and solutions.     

Anharmonicity and the elementary event of plastic deformation of amorphous polymers and glasses

The critical displacement of an excited atom (group of atoms) corresponding to the maximum in the interatomic attraction force plays an important part in the elementary event of plastic deformation of glassy solids. As a result of considerable departure of the excited kinetic unit from the equilibrium position and the nonlinearity of the interatomic interaction force, the microdeformation in the elementary event turns out to be a function of the degree of anharmonicity (Grüneisen parameter).     

Brillouin scattering in amorphous polymeric solids

Brillouin scattering of laser light has been used to study the temperature dependence of phonon velocity in a variety of amorphous polymeric systems, particularly internally and externally plasticized methacrylates. Discontinuities in the temperature coefficient of the hypersound velocities are observed at the glass transition temperatures (T_g). This phenomenon is related to changes in the temperature behavior of the specific volume accompanied by corresponding discontinuities in certain second-order thermodynamic quantities. This method was also used to examine the temperature dependence of the Landau-Placzek ratio. This ratio is relatively large in polymer systems and appears to be independent of temperature in the region of the glass transition, provided that there are no internal strains in the sample at the temperature of measurement. Evidence is presented which suggests that the abrupt changes in this ratio at T_g reported by earlier workers were due to kinetic effects related to the relaxation of internal strains above T_g, and the results of recent studies by other investigators, both corroborating and supplementing the present work, are reviewed.     

Molecular dynamics at constant pressure and temperature

Methods are discussed for generating by molecular dynamics isobaric-isoenthalpic, NPH, isochoric-isothermal, NVT, and isobaric-isothermal, NPT, ensembles. Andersen's constant-pressure method is reformulated so that the ensemble rather than the scaled system is directly calculated. Four constant-temperature schemes were considered. Two involve the addition of a stochastic collision term to the molecular trajectories. The Andersen method and a stochastic dynamics approach were examined. The latter employed a velocity damping term in addition to the random force. Two other methods employed uniform velocity scaling applied to all molecules. The NPT algorithm induces a transition to the dilute phase for a Lennard-Jones fluid in the spinodal region (p^* = 0.5, T^* = 1.28) of the phase diagram. The thermodynamic equivalence of the ensembles is demonstrated by long calculations of the chemical potential of Lennard-Jones states by the particle insertion method. The internal energy, pressure, constant volume and pressure specific heats, adiabatic compressibilities, pair radial distribution functions and self-diffusion coefficients are also evaluated. Only for second-order thermodynamic quantities is there evidence of an ensemble dependence.     

Structural and electrophysical aspects of phenolphthalein coloration in plastic deformation under high pressure

The influence exerted on the process of phenolphthalein coloration by the electrophysical conditions under which its pressure treatment is carried out was studied.     

Thermogravimetric analysis of the aluminum-polypropylene mixtures after plastic deformation under high pressure

The thermogravimetric analysis of the aluminum-polypropylene mixture of varied composition was performed to determine the effect of plastic deformation under the high pressure. It was found that a loss of mass by the mixed samples up to 400°C is due to decomposition of a polymer and the mass gain above 500°C, due to aluminum oxidation.     

Analysis of local rearrangements in chains during simulation of the plastic deformation of glassy polymethylene

A molecular-dynamics simulation of the low-temperature (～100 K below T _g) plastic deformation of glassy polymethylene (PM) was conducted. A model system consisting of 64 chains containing 100 CH₂ groups (the united-atoms approach) in each computational cell with periodic boundary conditions was considered. The behavior of 32 such cells was considered. Each cell was subjected to an active isothermal uniaxial compression at a constant temperature of T _def = 50 K to a strain of ? = 30%. An analysis showed that the inelastic deformation of glassy PM proceeded via nonaffine displacements (“gliding”) of chain fragments comprising 11–13 sites -CH₂-. These displacements are correlated and directed mainly along chain axes. Only a small number of conformational rearrangements occur in chains during the deformation of the material. Conformational transitions add only small additional displacements to nonaffine atomic transformations. A free-volume analysis using Voronoi-Delaunay tessellation in the deformed polymer did not show its relation to local plastic rearrangements.     

Molecular simulation with variable protonation states at constant pH

A new method is presented for performing molecular simulations at constant pH. The method is a Monte Carlo procedure where trial moves consist of relatively short molecular dynamics trajectories, using a time-dependent potential energy that interpolates between the old and new protonation states. Conformations and protonation states are sampled from the correct statistical ensemble independent of the trial-move trajectory length, which may be adjusted to optimize efficiency. Because moves are not instantaneous, the method does not require the use of a continuum solvation model. It should also be useful in describing buried titratable groups that are not directly exposed to solvent, but have strong interactions with neighboring hydrogen bond partners. The feasibility of the method is demonstrated for a simple test case: constant-pH simulations of acetic acid in aqueous solution with an explicit representation of water molecules.     

Computer simulation of transformations in solids

Recent developments in molecular dynamics (MD) and Monte Carlo (MC) methods enable us to fruitfully investigate transformations in solids by employing appropriate potentials. The possibility of varying both the volume and the shape of the simulation cell in these simulation techniques is especially noteworthy. In this article we briefly describe some of the highlights of the recent MD and MC methods and show how they are useful in the study of transitions in monatomic solids, ionic solids, molecular solids (especially orientationally disordered solids), and glasses. The availability of reliable pair potentials will undoubtedly make these methods more and more useful for studying various aspects of condensed matter in the years to come.     

The mechanism and a slip model for the initial plastic deformation of amorphous polyethylene under uniaxial tension

The mechanical behaviors of a polyethylene (PE) bulk consisting of amorphous molecular chains under uniaxial tension have been explored using molecular simulations. The stress–strain relationship and the plastic deformations of the PE bulk have been analyzed. Two deformation stages were found in the stress–strain curve, the elastic stage with a straight linear part of the curve and the plastic stage with a flat sawtooth‐like part. The Young's modulus calculated from the elastic part is in good agreement with experimental results. Some key parameters such as the energy variations in different terms reveal that the interchain slip should be chiefly responsible for the initial plastic deformations of amorphous PE under uniaxial tension. In order to address how this slip influences the plastic deformations, the mechanical details of a single chain have been elucidated when it was pulled out from two PE clusters consisting of regular and amorphous chains, respectively. The interchain slip, found as the basic movement style, is responsible for the movement of the stretched chain. Both the critical slip force and the critical slip length have been found in these two cases. For the straight chain pulled out from the cluster with regular chains, the critical slip force is about 1.81 nN and the critical slip length is about 40 polymerization degrees. While for the chain in the amorphous cluster, the critical force is about 0.86 nN and the critical length is almost the same. Based on the simulation results, a meso slip model has been deduced to explain the behaviors of the amorphous PE bulk under uniaxial tension. With reference to the slip model of single crystals and polycrystals a constitutive relation was obtained by considering the Young's modulus, the equivalent slip stress and the average orientation parameters of each chain. The comparison of the results from the constitutive relation and the simulations proves that this model does well in predicting the mechanical behaviors of amorphous PE under uniaxial tension in general. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 986–998     

Normal structural bonding and defects in covalent amorphous solids

Although the 8-N rule of covalent bonding is generally obeyed in amorphous semiconductors, well-defined defect centers exist and these control the electronic properties of the solids. The defects have two distinct reasons for their presence—they can arise from either strains upon material preparation or thermodynamic considerations. The strain-related defects characterize those amorphous solids in which the average coordination number is larger than approximately 2.4; their concentration is ordinarily very sensitive to the preparation techniques. In contrast, thermodynamically induced defects arise because of their low creation energy, and a minimum concentration characterizes any given material. These ideas have led to a resolution of several major puzzles with regard to the electronic properties of the two major classes of amorphous semiconductors—chalcogenide glasses and amorphous silicon-based alloys. Pure amorphous silicon is overconstrained and has large defect densities, but these can be reduced by many orders of magnitude if the material is alloyed with monovalent atoms such as hydrogen or fluorine. On the other hand, amorphous As₂Se₃ always contains a high defect density, for thermodynamic reasons. In addition to the concentration of defects present in a given material, its electronic properties depend critically also on the nature of these defects. In particular, the sign of the effective correlation energy of the defect with the lowest creation energy is of the utmost importance.     

Radiation telomerization of tetrafluoroethylene in chlorinated telogens at a constant reactor pressure

The kinetics of the radiation-initiated telomerization of tetrafluoroethylene (TFE) in the solutions of telogens (butyl chloride, chloroform, methylene chloride, and carbon tetrachloride) at a constant monomer pressure in the reactor was studied. In the course of the Γ-irradiation of the reaction mixture, TFE telomers containing repeatedly soluble and insoluble fractions were formed. In chloroform, methylene chloride, and carbon tetrachloride, the rapidly occurring process of the synthesis of insoluble telomer made the main contribution to the total yield of products. For the solutions of TFE in butyl chloride, the rates of formation of soluble and insoluble telomers were almost the same. The average chain length and the thermal stability of the resulting telomers were evaluated.     

Thermodynamic integration at constant pressure from the nonlocal Einstein crystal

The Gibbs free energy of a periodic, d-dimensional crystalline assembly can be estimated using thermodynamic integration at constant pressure from a nonlocal Einstein crystal. The method is demonstrated using a two-dimensional, harmonic crystal with hexagonal symmetry for which the isobaric Gibbs function can be derived analytically.     

Internal structure rebuilding reactions of crystalline and amorphous solids

Structural mechanism of the internal thermal reactions of dehydration and dehydroxylation as of solids well as crystallization of their amorphous products and glasses are considered. They are multi-stage processes, realized through small translative displacements of polymerized anion network elements and diffusional shift of cations. It is the way to equilibrium state of the lowest energy consumption and the least energetic barrier to overcome.

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Zusammenfassung Es werden Strukturmechanismen von internen thermischen Reaktionen von Dehydratation und Dehydroxylierung sowie der Kristallisierung der entstehenden amorphen Reaktionsprodukte betrachtet. Dabei handelt es sich um mehrstufige Prozesse, die über kleine translative Verschiebungen von polimerisierten Elementen des Anionennetzwerkes und durch Diffusionsverschiebung von Kationen ablaufen. Dies ist der Weg zum Gleichgewichtszustand mit dem geringsten Energieverbrauch und der niedrigsten, zu überwindenden Energiebarriere.

     

Structural changes induced by the plastic deformation of carbon fibers at elevated temperatures

Summary The changes in a number of parameters characterizing the microstructure of carbon fibers have been studied as a function of the strain rate at elevated temperatures. The results indicate that there are essentially three types of deformation processes, an irreversible unwrinkling of the ribbon-shaped carbon layers at low strain rates, unwrinkling accompanied by gliding of groups of ribbons at medium strain rates and gliding of individual ribbons at high strain rates. The intrinsic mechanical quality of the fibers as characterized by the elongation on fracture seems to be best preserved by a deformation process which does not involve gliding.

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Zusammenfassung Es wurde die ?nderung einer Reihe von Parametern, die die Mikrostruktur von Kohlenfasern charakterisieren, als Funktion der Streckgeschwindigkeit bei h?heren Temperaturen untersucht. Die Ergebnisse zeigen, da? im Wesentlichen drei Typen von Verformungsprozessen vorliegen: Eine irreversible Gl?ttung von bandf?rmigen Kohlenstoffschichten bei kleinen Streckgeschwindigkeiten, eine Gl?ttung verbunden mit Gleiten von Gruppen von B?ndern bei mittleren Dehnungsgeschwindigkeiten und das Gleiten einzelner B?nder bei hohen Streckgeschwindigkeiten. Die Festigkeit der Fasern, wie sie durch die Bruchdehnung charakterisiert ist, bleibt am besten bei einem Deformationsproze? erhalten, der keine Gleitung bedingt.

     

Structural aspects of deformation of amorphous polymers

Experimental and theoretical data on the inelastic deformation of amorphous glassy polymers were analyzed. The decisive role of direct structural methods in determination of the deformation mechanism of glassy polymers was established. A new mechanism of deformation and thermally stimulated recovery of strained glassy polymers was considered on the basis of structural data analysis.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 1–6, January, 2005.