Crystallization in ultra-thin polymer films: Morphogenesis and thermodynamical aspects

We present a computer model for polymer crystallization in ultra-thin films where chains are considered as dynamical units. In our model chains can change their internal state of order by cooperative motions to improve thermodynamic stability. The interplay between reorganization, enthalpic interactions and the morphology of crystals enables us to explain many properties of growth, morphogenesis and melting of polymer lamellae. We emphasize the relation between the thermodynamic stability of non-equilibrium crystals and morphological features which are beyond the average thickness of the lamellae. In particular, we show that melting of polymers is preceded by reorganization processes and the stability of polymer crystals is not necessarily related to the structure formed at the crystallization point. The simulations allow for the determination of some non-equilibrium properties such as the internal energy and the non-equilibrium heat capacity. We show that multiple-peak melting endotherms result from morphological transformations. The results of our computer simulations are compared with AFM observations in ultra-thin polyethyleneoxide films.     

Thermal treatment and analysis

Heating and/or cooling of substances is one of the oldest and basic methods for preparing materials with defined properties. This always leaves a definitive fingerprint of the thermal history. Beside knowing the structure we need to specify such materials by their thermodynamic behaviour, i.e., stability/metastability, phase relations and transitions, particularly establishing corresponding characteristic points. All this can be based on ordinary thermodynamics but its validity must be approved for non-equilibrium conditions of temperature changes where equilibrium and kinetic effects overlap. The slower the phase transition proceeds the greater is the deviation of the system state (kinetic curve) from its equilibrium state (equilibrium background). This makes possible to locate the actual phase boundary between two states investigated, resulting in the so-called kinetic phase diagrams. Most of modern technologies are intentionally based on non-equilibrium phenomena in order to create metastable/nonstable phases of specific properties. In this sense thermal analysis is understand as the method for determining the sample state on the basis of the sample interactions with the surroundings whose intensive parameters are controlled. Temperature is here considered as a basic parameter that connects all thermophysical measurements and thermal treatments. ICTA-TA Award lecture     

Isothermal and non-isothermal melting of the binary solution inside an emulsion

The aim of the present paper is to study the melting process inside an emulsion using the non-equilibrium model of microscopic heat transfer between the emulsifying medium and the dispersed droplet of binary solution (DSC). DSC experiments are used to validate the numerical results. The effects of the heating rate, mass fraction of the dispersed saline binary solution, initial mass fraction of the solute and the sample mass on the kinetics of the melting process are examined.     

Non-equilibrium thermodynamics and kinetic theory of gas mixtures in the presence of interfaces

A broad range of the boundary value problems of the kinetic theory of gases and gas mixtures is considered based on kinetic theory and non-equilibrium thermodynamics. The interrelation of the kinetic theory and non-equilibrium thermodynamics is discussed. The balance equations at the interface are obtained for the case of the boundary layers with peculiar properties. Procedures for deriving the boundary conditions for slightly rarefied gas mixtures are outlined. The problems of calculating slip coefficients are discussed. The specificity of the kinetic effects in the boundary conditions is shown. A set of general relations related to gas mixture flows in capillaries is deduced. The possibility of non-equilibrium kinetic effects in the form of a paradoxical distribution of non-equilibrium temperature is shown. Methods of non-equilibrium thermodynamics are used to obtain the phenomenological equations describing the thermophoresis and diffusiophoresis of particles and cross phenomena. The growth and evaporation of droplets is considered based on kinetic theory and non-equilibrium thermodynamics.     

Shear-induced lamellar ionic liquid-crystal foam

In a recent paper we reported an experimental study of two N-alkylimidazolium salts. These ionic compounds exhibit liquid crystalline behaviour with melting points above 50°C in bulk. However, if they are sheared, a (possibly non-equilibrium) lamellar phase forms at room temperature. Upon shearing a thin film of the material between microscope slides, textures were observed that are strikingly similar to liquid (wet) foams. The images obtained from polarising optical microscopy (POM) were found to share many of the known quantitative properties of a two-dimensional foam coarsening process. Here we report an experimental study of this foam using a shearing system coupled with POM. The structure and evolution of the foam are investigated through the image analysis of time sequences of micrographs obtained for well-controlled sets of physical parameters (sample thickness, shear rate and temperature). In particular, we find that there is a threshold shear rate below which no foam can form. Above this threshold, a steady-state foam pattern is obtained where the mean cell area generally decreases with increasing shear rate. Furthermore, the steady-state internal cell angles and distribution of the cell number of sides deviate from their equilibrium (i.e. zero-shear) values.     

Melting of nascent and thermally treated super-high molecular weight polyethylene

The melting of nascent and thermally treated super-high molecular weight polyethylene (SHMWPE) is investigated by means of differential scanning calorimetry (DSC). Higher melting temperatures and enthalpies of the nascent and annealed samples are observed. The melting temperatures and enthalpies of melt crystallized SHMWPE is lower and depends on the temperature of thermal treatment. All melting properties are explained by presuming that the lamellar structure contains a high concentration of entangled tie macromolecules in amorphous regions formed during polymerization. It was supposed that the concentration of the entanglements and the stressed tie molecules are changed with the temperature of the thermal pretreatment.     

Thermal stability of aliphatic polytmides

Results on the preparation and thermoanalytical investigation of aliphatic bismaleamic acids and bismaleimides are presented. Correlations were established between the chemical structure and the thermal properties. The melting point and the thermal stability of the bismaleimides decrease as the number of carbon atoms in the structure increases. Chemically imidized samples have significantly higher thermal stability, which is almost independent of the chemical structure. Thermal polymerization begins just after the melting of the materials. A thermal fragmentation scheme is proposed, based on the results of mass spectrometry.     

Mechano-chemical stability of gold nanoparticles coated with alkanethiolate SAMs

Molecular dynamics simulations are used to probe the structure and stability of alkanethiolate self-assembled monolayers (SAMs) on gold nanoparticles. We observed that the surface of gold nanoparticles becomes highly corrugated by the adsorption of the SAMs. Furthermore, as the temperature is increased, the SAMs dissolve into the gold nanoparticle, creating a liquid mixture at temperatures much lower than the melting temperature of the gold nanoparticle. By analyzing the mechanical and chemical properties of gold nanoparticles at temperatures below the melting point of gold, with different SAM chain lengths and surface coverage properties, we determined that the system is metastable. The model and computational results that provide support for this hypothesis are presented.     

Melting behavior of drawn films from polyethylene single-crystal mats

Drawing of mats of linear polyethylene single crystals prepared from dilute solution is possible at temperatures above about 90°C. The structure and properties of the drawn specimens are much different from those ordinary drawn bulk polymer. Drawn mats have been investigated by differential scanning calorimetry. The characteristic experimental results are: (a) a broad melting curve, (b) considerable superheating depending on the rate of heating, (c) constancy of the melting point and the heat of fusion with annealing, (d) deviation from the relation between the heat of fusion and the density obtained for the drawn bulk specimens, (e) appearance of two melting peaks in samples annealed at temperatures above about 130°C. These results imply that the structure of the drawn mat is characterized by a larger number of the tie chains connecting the neighboring crystals (the structure postulated in earlier papers) than is the case in ordinary drawn bulk polymer. It can be concluded that the transformation of a fringed micellar type of structure to the folded lamellar structure may be difficult during annealing unless crystals melt and then recrystallize during cooling.     

Effect of molecular symmetry on melting temperature and solubility

Molecular symmetry has a pronounced effect on the melting properties and solubility of organic compounds. As a general rule, symmetrical molecules in crystalline form have higher melting temperatures and exhibit lower solubilities compared with molecules of similar structure but with lower symmetry. Symmetry in a molecule imparts a positive amount of residual entropy in the solid phase (i.e., more possible arrangements leading to the same structure). This means that the entropy of a crystal of symmetric molecules is greater than the entropy of crystal of a similar, but non-symmetric molecule. An analysis is presented relating the enthalpy, entropy and temperature of melting for an idealised system of structural isomers of different molecular symmetries. The analysis presented helps explain why often, yet not always, the crystal of a more symmetric molecule, which has greater entropy to start (closer to that of the liquid), also exhibits a greater gain in entropy upon melting, compared with the crystal of a less symmetrical molecule. The residual entropy due to molecular symmetry has the direct effect of reducing the entropy gain upon melting (a negative effect). However, molecular symmetry also exerts indirect effects on both the entropy and enthalpy of melting. These indirect effects, imposed by the condition of equilibrium melting, are positive, such that it is the balance between the direct and indirect effects what determines the value observed for the entropy of melting of the symmetric molecules. When the indirect effect of molecular symmetry is greater than its direct effect, the observed entropy gain upon melting of the more symmetrical molecule is greater than that of a less symmetrical one.     

一种支氏液晶共聚酯的合成

在聚对苯二甲酸乙二醇酯与对羟基苯甲酸 (HBA)形成的共聚酯 (PET HBA)分子链中 ,引入具有分形结构的单体———三羟基苯 (TOP) ,以降低其熔点 ,改善加工性能 .考察乙酰化时间、缩聚时间、压力、TOP和HBA加入量对新型分形共聚酯的对数比浓粘度的影响规律 ,以及TOP和HBA加入量对新型分形共聚酯的熔点和液晶清亮点的影响 .TOP的加入能使PET HBA共聚酯的熔点下降 10℃以上 ,而液晶清亮点没有变化 ,拓宽了液晶区域     

Efficient method to include nuclear quantum effects in the determination of phase boundaries

We developed a methodology to assess nuclear quantum effects in phase boundaries calculations that is based on the dynamical integration of Clausius-Clapeyron equation using path integral simulations. The technique employs non-equilibrium simulations that are very efficient. The approach was applied to the calculation of the melting line of Ne in an interval of pressures ranging from 1 to 3366 bar. Our results show a very good agreement with both experimental findings and results from previous calculations. The methodology can be applied to solid and liquid phases, without limitations regarding anharmonicities. The method allows the computation of coexistence lines for wide intervals of pressure and temperature using, in principle, a single simulation.     

Transport properties of 2F <==> F2 in a temperature gradient as studied by molecular dynamics simulations

We calculate transport properties of a reacting mixture of F and F(2) from results of non-equilibrium molecular dynamics simulations. The reaction investigated is controlled by thermal diffusion and is close to local chemical equilibrium. The simulations show that a formulation of the transport problem in terms of classical non-equilibrium thermodynamics theory is sound. The chemical reaction has a large effect on the magnitude and temperature dependence of the thermal conductivity and the interdiffusion coefficient. The increase in the thermal conductivity in the presence of the chemical reaction, can be understood as a response to an imposed temperature gradient, which reduces the entropy production. The heat of transfer for the Soret stationary state was more than 100 kJ mol(-1), meaning that the Dufour and Soret effects are non-negligible in reacting mixtures. This sheds new light on the transport properties of reacting mixtures.     

A method to estimate the Gibbs free energy of non-equilibrium alloys by thermal analysis

A method has been developed to estimate the Gibbs free energy $ \left( {G_{\text{S}}^{\text{NE}} } \right) $ of the non-equilibrium solid alloys with multicomponents based on differential scanning calorimetry (DSC) analysis. In this method, the DSC curves of the non-equilibrium and equilibrium alloys during heating up to fully melting and those of the alloys during solidifying were measured. Then the thermal effects of the solid phase transformations from non-equilibrium to equilibrium states and the equilibrium solidification could be calculated. By evolving the traditional equal-G curve principle to equal-G point, the Gibbs free energy of the equilibrium solid alloy with multicomponents could be obtained on condition that the free energy of the liquid alloy was known. Considering the thermal effects of the solid phase transformations from non-equilibrium to equilibrium states, the Gibbs free energy value of the non-equilibrium alloys with a given composition could be achieved although the phase constitution of the equilibrium solid alloys and the Gibbs free energy of each phase were not known, and the calculation errors could be reduced by dividing the alloys into many infinitesimal virtual pure metals. The Gibbs free energy of the non-equilibrium Al?CSi?CMn alloys was calculated by using this method, confirming the validity of this method.     

Melting properties of molten salts and ionic liquids. Chemical homology,correlation, and prediction

The application of the concept of chemical homology to describe melting properties of molten salts and ionic liquids (ILs) is analyzed. This concept was used several years ago to correlate and predict properties of solids and more recently to correlate melting temperatures of ILs. To analyze the characteristics of the extended method, this is first applied to melting properties of organic substances for which abundant data are available. The method is extended to analyze its applicability for properties of molten salts and ILs such as glass transition temperature, heat of melting, and entropy of melting. The foundation of the chemical homology concept is revised, and the difficulties for extending the method to correlate and predict melting properties of ILs are presented. Despite the difficulties, the homology concept can still be used with some conditions and limitations that are analyzed in this article. Several correlations are proposed.     

Artificial Supramolecular Pumps Powered by Light

The development of artificial nanoscale motors that can use energy from a source to perform tasks requires systems capable of performing directionally controlled molecular movements and operating away from chemical equilibrium. Here, the design, synthesis and properties of pseudorotaxanes are described, in which a photon input triggers the unidirectional motion of a macrocyclic ring with respect to a non-symmetric molecular axle. The photoinduced energy ratcheting at the basis of the pumping mechanism is validated by measuring the relevant thermodynamic and kinetic parameters. Owing to the photochemical behavior of the azobenzene moiety embedded in the axle, the pump can repeat its operation cycle autonomously under continuous illumination. NMR spectroscopy was used to observe the dissipative non-equilibrium state generated in situ by light irradiation. We also show that fine changes in the axle structure lead to an improvement in the performance of the motor. Such results highlight the modularity and versatility of this minimalist pump design, which provides facile access to dynamic systems that operate under photoinduced non-equilibrium regimes.     

Model of partial crystallization and melting derived from small-angle X-ray scattering and electron microscopic studies on low-density polyethylene

A temperature-dependent small-angle x-ray scattering and electron microscopic study on a sample of low-density polyethylene affords a determination of the structure changes in a heating and cooling cycle and suggests a new model of partial crystallization and melting. The analysis of SAXS data is based upon some general properties of the electron-density correlation function. Electron micrographs are obtained from stained sections γ irradiated at elevated temperatures and are analyzed quantitatively by statistical means. According to the model proposed here the thickness distribution in the amorphous layers, rather than that of the crystalline regions, is the essential factor governing the crystallization and melting behavior. The temperature-dependent changes in this thickness distribution provide a natural explanation for the large reversible changes in long-spacing.