Melting and vitrification of Lennard-Jones spheres

Melting and vitrification processes of different types of mixtures of Lennard-Jones spheres are considered; mechanisms and geometrical models of transition from the solid to liquid state are defined and established.     

Melting and homogeneous crystallization of a Lennard-Jones system

Melting and homogeneous crystallization in a Lennard-Jones system of 10,976 atoms in a model box with periodic boundary conditions were investigated by the molecular dynamics method in an NVE ensemble. Crystal melting occurs by arbitrary generation and growth of local defects transformed into regions of a disordered phase. These defects gradually span the entire space of the sample, absorbing the residual islands of crystal. Homogeneous crystallization of a liquid starts with generation of crystal nuclei which grow into defective crystals. The resulting crystal varies in structure between different realizations of the model. Face-centered cubic (fcc) structures prevail. A hexagonal close packing (hcp) structure is present on the boundaries of fcc regions and arises from disordering in alternation of atomic planes. Multiple twinning of the fcc structure is observed, and aggregates with fivefold symmetry have been found.     

Melting of Lennard-Jones clusters in confined geometries

The melting of Lennard-Jones (argon) clusters of various size (N = 500 to 10000 atoms), confined in a rigid matrix, is studied by molecular dynamics simulations. For spherical clusters we show the existence of a cluster size below which the dependence of the melting temperature cannot be described by the classical Gibbs-Thompson equation. We also provide evidence of the formation of a quasi-liquid layer at the surface of mesoscopic clusters. A good agreement is found between the theoretical model due to Celestini et al. and the simulation results obtained in this work. The melting of an ellipsoidal cluster is also investigated. We observe, in agreement with recent experimental and theoretical work, that the thickness of the molten layer is larger in the region of higher local curvature.     

Melting line of the Lennard-Jones system, infinite size, and full potential

Literature estimates of the melting curve of the Lennard-Jones system vary by as much as 10%. The origin of such discrepancies remains unclear. We present precise values for the Lennard-Jones melting temperature, and we examine possible sources of systematic errors in the prediction of melting points, including finite-size and interaction-cutoff effects. A hypothetical thermodynamic integration path is used to find the relative free energies of the solid and liquid phases, for various system sizes, at constant cutoff radius. The solid-liquid relative free energy and melting temperature scale linearly as the inverse of the number of particles, and it is shown that finite-size effects can account for deviations in the melting temperature (from the infinite-size limit) of up to 5%. An extended-ensemble density-of-states method is used to determine free energy changes for each phase as a continuous function of the cutoff radius. The resulting melting temperature predictions exhibit an oscillatory behavior as the cutoff radius is increased. Deviations in the melting temperature (from the full potential limit) arising from a finite cutoff radius are shown to be of comparable magnitude as those resulting from finite-size effects. This method is used to identify melting temperatures at five different pressures, for the infinite-size and full potential Lennard-Jones system. We use our simulation results as references to connect the Lennard-Jones solid equation of state of van der Hoef with the Lennard-Jones fluid equation of state of Johnson. Once the references are applied the two equations of state are used to identify a melting curve. An empirical equation that fits this melting curve is provided. We also report a reduced triple point temperature T(tr)=0.694.     

The Melting Process of Acetylsalicylic Acid Single Crystals

Crystallisation is generally regarded as a nucleation — growth mechanism of a solid phase and often studied using thermo chemical methods. The present work postulates an analogy to melting processes, looking at melting as nucleation — growth of a liquid phase. The melting process of acetylsalicylic acid single crystals was investigated by DSC measurements under isothermal conditions. The fraction of material molten after a certain time period, α(t), was calculated by integrating the DSC curves. The resulting kinetic curves were fitted using the Avrami-Erofeev equation: –ln(1–α)=kt ⁿ, where parameter n was analysed. According to established methods, functions I('2')=[t('2')]/[t('2')+t('3')]100% and I('3')=[t('3')]/[t('3')+t('2')]100% were introduced, where t('2') and t('3') is the absolute time of consumption two- and three-dimension nuclei growth, respectively. Applying correlation analysis, relationships between two- or three-dimensional growth and the independent variables describing the single crystals (for strictly definite trajectories into the space of sizes) were found. Particular correlations were:a) Two-dimensional growth is a function of the total surface area of the crystal, S, and of the surface area of the (ac)-face, S _ac; b) Three-dimensional growth is a function of S/M (where M is the mass of the single crystal). It is also a function of S _ac/M and of S. The obtained experimental data are explained by the ‘layer’ structure of crystals of acetylsalicylic acid. This revised version was published online in August 2006 with corrections to the Cover Date.     

Elastic Organic Crystals as Bioinspired Hair-Like Sensors

One of the typical haptic elements are natural hairy structures that animals and plants rely on for feedback. Although these hair sensors are an admirable inspiration, the development of active flow sensing components having low elastic moduli and high aspect ratios remains a challenge. Here, we report a new sensing approach based on a flexible, thin and optically transmissive organic crystal of high aspect ratio, which is stamped with fluorescent dye for tracking. When subjected to gas flow and exposed to laser, the crystal bends due to exerted pressure and acts as an optical flow (hair) sensor with low detection limit (≈1.578 m s⁻¹) and fast response time (≈2.70 s). The air-flow-induced crystal deformation and flow dynamics response are modelled by finite element analysis. Due to having a simple design and being lightweight and mechanically robust this prototypical crystal hair-like sensor opens prospects for a new class of sensing devices ranging from wearable electronics to aeronautics.     

Lennard-Jones Elastic Moduli by Liquid Structure Integral Equations and Molecular Dynamics Computer Simulations

Abstract

The infinite frequency shear modulus, G∞, and compressional modulus, K∞, of the Lennard-Jones, LJ, fluid have been determined over essentially the whole phase diagram at densities below the solid-fluid coexistence line using PY, HNC, and Rogers and Young (RY) closures of the Ornstein-Zernike relation. At low density PY is best at reproducing simulation G∞, and K∞, whereas close to the coexistence line, above the critical temperature, the RY closure is best and is remarkably accurate. Agreement is poorest for all three closures below T_c in the liquid phase.     

针形颗粒二氧化钛纳米薄膜的制备

使用醋酸修饰的溶胶-凝胶法制备了一种由针形颗粒构成的二氧化钛纳米薄膜.研究发现,可以通过控制溶胶-凝胶过程的反应条件控制所制备的纳米薄膜的形貌.对这种形貌的形成机理进行了讨论.     

Temperature Modulated DSC of Irreversible Melting of Nylon 6 Crystals

A new method is presented to analyze the irreversible melting kinetics of polymer crystals with a temperature modulated differential scanning calorimetry (TMDSC). The method is based on an expression of the apparent heat capacity, , with the true heat capacity, mc_p, and the response of the kinetics, . The present paper experimentally examines the irreversible melting of nylon 6 crystals on heating. The real and imaginary parts of the apparent heat capacity showed a strong dependence on frequency and heating rate during the melting process. The dependence and the Cole-Cole plot could be fitted by the frequency response function of Debye's type with a characteristic time depending on heating rate. The characteristic time represents the time required for the melting of small crystallites which form the aggregates of polymer crystals. The heating rate dependence of the characteristic time differentiates the superheating dependence of the melting rate. Taking account of the relatively insensitive nature of crystallization to temperature modulation, it is argued that the ‘reversing’ heat flow extrapolated to ω → 0 is related to the endothermic heat flow of melting and the corresponding ‘non-reversing’ heat flow represents the exothermic heat flow of re-crystallization and re-organization. The extrapolated ‘reversing’ and ‘non-reversing’ heat flow indicates the melting and re-crystallization and/or re-organization of nylon 6 crystals at much lower temperature than the melting peak seen in the total heat flow. This revised version was published online in July 2006 with corrections to the Cover Date.     

Highly Elastic Organic Crystals for Flexible Optical Waveguides

The study of elastic organic single crystals (EOSCs) has emerged as a cutting‐edge research of crystal engineering. Although a few EOSCs have been reported recently, those suitable for optical/optoelectronic applications have not been realized. Here, we report an elastic crystal of a Schiff base, (E)‐1‐(4‐(dimethylamino)phenyl)iminomethyl‐2‐hydroxyl‐naphthalene. The crystal is highly bendable under external stress and able to regain immediately its original straight shape when the stress is released. It displays bright orange–red emission with a high fluorescence quantum yield of 0.43. Intriguingly, it can serve as a low‐loss optical waveguide even at the highly bent state. Our result highlights the feature and utility of “elasticity” of organic crystals.     

The Isothermal Melting Kinetics of Ultra-High Molecular Weight Polyethylene Crystals

The isothermal melting behaviors of ultra-high molecular weight polyethylene (UHMWPE) with different entangled states (i.e., nascent and melt-crystallized samples) are studied. For two kinds of UHMWPE samples, the result shows that the relative content of survived crystals (X_s) exponentially decreases with time and reaches a constant value. It is suggested that such a melting behavior is related to the observed nonlinear growth of crystals induced by the kinetically rejected entanglements accumulated at the growth front. Additionally, the exponential decay of X_s with time provides a characteristic melting time (τ) for the melting process. Compared to the melt-crystallized UHMWPE, the τ value of nascent UHMWPE is generally longer even in a higher temperature range, which is mainly because the former has a larger entanglement density difference. Furthermore, these observations demonstrate that UHMWPEs with different entangled states have an analogous melting mechanism since they exhibit a similar melting activation energy (≈1300 kJ mol⁻¹).     

Crystal Adaptronics: Mechanically Reconfigurable Elastic and Superelastic Molecular Crystals

Mechanically reconfigurable molecular crystals—ordered materials that can adapt to variable operating and environmental conditions by deformation, whereby they attain motility or perform work—are quickly shaping a new research direction in materials science, crystal adaptronics. Properties such as elasticity, superelasticity, and ferroelasticity, which are normally related to inorganic materials, and phenomena such as shape‐memory and self‐healing effects, which are well‐established for soft materials, are increasingly being reported for molecular crystals, yet their mechanism, quantification, and relation to the crystal structure of organic crystals are not immediately apparent. This Minireview provides a condensed topical overview of elastic, superelastic, and ferroelastic molecular crystals, new classes of materials that bridge the gap between soft matter and inorganic materials. The occurrence and detection of these unconventional properties, and the underlying structural features of the related molecular materials are discussed and highlighted with selected prominent recent examples.     

Mechanical Effects of Elastic Crystals Driven by Natural Sunlight and Force

Flexible crystals that can capture solar energy and convert it into mechanical energy are promising for a wide range of applications such as information storage and actuators, but obtaining them remains a challenge. Herein, an elastic crystal of a barbiturate derivative was found to be an excellent candidate, demonstrating plastic bending behavior under natural sunlight irradiation. ¹H NMR and high-resolution mass spectrum data of microcrystals before and after light irradiation demonstrated that light-induced [2+2] cycloaddition was the driving force for the photomechanical effects. Interestingly, the crystals retained elastic bending even after light irradiation. This is the first report of flexible crystals that can be driven by natural sunlight and that have both photomechanical properties and elasticity. Furthermore, regulation of the passive light output direction of the crystals and transport of objects by applying mechanical forces and light was demonstrated.     

Electrical Properties of the Swollen Particles of Elastic Gels

The distributions of ion concentrations and electric potential at an interface between macromolecular associates and external solutions were calculated by solving nonlinear equations of the theory of electrolytes. The calculated and experimental results were compared.     

Ferroelectric Particles in Nematic Liquid Crystals with Soft Anchoring

A theoretical evaluation of the electric Freedericksz transition threshold and saturation field is proposed for a liquid crystals composite with ferroelectric particles. Existing models consider a strong anchoring of nematic molecules on the glass support of the cell, but in this paper a soft molecular anchoring of molecules on the glass support and also on the ferroelectric nanoparticle’s surface is assumed. Thus, a finite saturation field was obtained in agreement with real systems. Calculations are made for planar configuration of positive dielectric anisotropy liquid crystals. The results are compared with data obtained on similar systems from different publications and the differences are discussed.     

Light-Driven Crawling of Molecular Crystals by Phase-Dependent Transient Elastic Lattice Deformation

The light-driven crawling of a molecular crystal that can form three phases, (α, β, and γ) is presented. Laser irradiation of the molecular crystal can generate phase-dependent transient elastic lattice deformation. The resulting elastic lattice deformation that follows scanning irradiation of a laser can actuate the different phases of molecular crystal to move with different velocity and direction. Because the γ phase has a large Young's modulus (ca. 26 GPa), a force of 0.1 μN can be generated under one laser spot. The generated force is sufficient to actuate the γ-formed molecular crystals in a wide dimensional range to move longitudinally at a velocity of about 60 μm min⁻¹, which is two orders of magnitude faster than the α and β phases.     

Lennard-Jones流体中胶体粒子间耗尽势的密度泛函理论研究

基于Roth, Evans和Dietrich有关耗尽势的密度泛函理论研究了Lennard-Jones (L-J)流体中胶体粒子间的耗尽势. 通过计算胶体稀溶液中两个胶体粒子在不同条件下的耗尽势, 进一步分析了L-J流体的相关因素对耗尽势的影响. 结果表明, L-J流体分子的体积分数、胶体粒子与溶剂分子的尺寸比率、L-J流体分子间的相互作用以及胶体粒子与流体分子之间的弱相互作用等因素均可对胶体粒子间耗尽势产生显著影响. 研究结果可为实验上调控胶体粒子间的相互作用提供可能的线索.     

Surface Fixing Effect of Vacuum Evaporated Particles on Polyethylene Single Crystals

Summary: The recrystallization behavior of high density polyethylene (HDPE) single crystals with vacuum evaporated metal chromium or carbon on their surface has been investigated by transmission electron microscopy (TEM) and electron diffraction (ED) techniques. The results indicate that the particle‐coated HDPE single crystals can retain their single crystal structure after complete melting and subsequent recrystallization, with the heat‐treated temperature as high as 200 °C. This phenomenon is attributed to a surface fixing effect of vacuum evaporated Cr or carbon on the single crystals.

A bright‐field electron micrograph and the corresponding electron diffraction pattern of recrystallized Cr‐coated HDPE single crystals.