Thermal behavior of annealed organic glasses

The influence of annealing processes on the thermal behavior of organic glasses in the glass-transition interval has been investigated and analyzed quantitatively. In detailed annealing studies of atactic polystyrene and Aroclor 5460, the absorption of thermal energy superposed on the increase in the specific heat at the glass transition, observed with suitably chosen heating rates, was followed by the differential thermal method. It is concluded that the absorption of thermal energy observed under these conditions parallels the extent of molecular relaxation that has taken place during the annealing period. It is not necessary to postulate a first-order process to account for the energy absorption. Moreover, such a postulate leads to severe conceptual difficulties regarding the development of crystallinity in crystallizable materials. The areas and the shapes of the endotherms are considered in terms of the original physical properties of the quenched glasses and the anticipated equilibrium properties. Relationships between the extent of energy absorption and time-dependent processes such as volume relaxation are discussed.     

Isothermal and isobaric PVT-measurements of atactic polymers

It is evidenced that due to the kinetic character of the glass transition as a freeze in process, PVT measurements extended over the glass transition range depend not only on the thermal history but also on the pressure acting during the formation of the polymeric glasses. As a consequence metastable glasses are formed which show during heating of the glassy polymer through the glass transition range volume relaxation zones, characterized by a retarded increase or even decrease of the volume. The width of the relaxation zone increases with increasing pressure and depends additional on the mode of operation used during the PVT measurements. In the same time a pressure induced shift of the glass temperature to higher temperatures is observed, the shift being the greater the stiffer the polymer, i.e. the higher the glass temperature of the polymer at atmospheric pressure. Due to the metastable character of polymeric glasses the evaluation of universal equations of states is thus not ingenious for polymeric glasses, because the deduced EOS will be valid only for that given glass characterized by a well defined thermal and pressure history. Additionally the EOS is influenced by the unknown time dependent aging and relaxation processes within polymeric glasses.     

“Vanishing” structural effects of temperature in polymer glasses close to the glass-transition temperature

Positron annihilation lifetime (PAL) measurements were used for observation of structural effects of temperature in polystyrene (PS), super-cross-linked polystyrene networks (CPS), and in polyimides (PI) below and in the vicinity of glass-transition temperature T_g. “Vanishing” of these structural effects in the repeating cycles of the temperature controlled PAL experiments due to the slow relaxation processes in different conditions and details of chemical structure is demonstrated. Obtained results illustrate complex, dependent on thermal history, inhomogeneous character of the glass structure. In fact, structure of some polymer glasses is changing continuously. Calculations of the number density of free volume holes in these conditions are discussed.     

Densification of silica glass at ambient pressure

We show that densification of silica glass at ambient pressure as observed in irradiation experiments can be attributed to defect generation and subsequent structure relaxation. In our molecular dynamics simulations, defects are created by randomly removing atoms, by displacing atoms from their nominal positions in an otherwise intact glass, and by assigning certain atom excess kinetic energy (simulated ion implantation). The former forms vacancies; displacing atoms and ion implantation produce both vacancies and "interstitials." Appreciable densification is induced by these defects after equilibration of the defective glasses. The structural and vibrational properties of the densified glasses are characterized, displaying resembling features regardless of the means of densification. These results indicate that relaxation of high free-energy defects into metastable amorphous structures enriched in atomic coordination serves as a common mechanism for densification of silica glass at ambient pressure.     

Volume relaxation and related dynamic mechanical property changes during physical aging of poly-ether-ether-ketone (peek)

Non-equilibrium glasses, including polymer glasses, spontaneously tend to approach the equilibrium state. The time-dependent phenomena occurring in glassy polymers below the glass transition are commonly referred to as physical aging. Since the physical aging results primarily in a volume relaxation (or densification), our efforts have been focused on the measurements of the time-dependent behavior of the specific volume. The dependence of the storage and loss moduli upon aging have been related to the progressive loss of mobility associated with the volume relaxation. A Williams-Watts type equation has been adopted to model the volume relaxation data at different temperatures, and a numerical procedure has been developed in order to predict the kinetics of volume relaxation subjected to complex thermal histories.     

Microscale fish bowls: a new class of latex particles with hollow interiors and engineered porous structures in their surfaces

Microscale fish bowls, hollow particles with engineered holes in their surfaces, were prepared using two different methods. In the first method, commercial latex beads suspended in water were swollen with a good solvent of the polymer, followed by freezing with liquid nitrogen and evaporation of the solvent below 0 degrees C. While one big hole was generated when the amount of solvent used for the swelling was relatively low, small holes could be produced in the outer surface of each bowl by increasing the degree of swelling. The porosity and pore structure show a similar dependence on the degree of swelling for both amorphous and semicrystalline polymers even though they are supposed to exhibit different phase behaviors during the freezing and solvent evaporation processes. In the second method, a polymer emulsion in water was prepared and then frozen with liquid nitrogen, followed by solvent evaporation below 0 degrees C. The porosity and pore structure could be controlled by adjusting the concentration of the polymer solution used to prepare the emulsion. As for encapsulation, the bowl-shaped particles could be transformed back into solid beads via thermal annealing at a temperature near the glass transition temperature of the polymer or by adding a good solvent of the polymer to the colloidal suspension. In a proof-of-concept experiment, microscale fish bowls were fabricated from poly(caprolactone), quickly loaded with a fluorescent dye, and sealed through thermal annealing. The encapsulated dye could then be slowly released in a phosphate buffered saline, suggesting their potential use as a new class of microscale capsules for drug delivery.     

Simulation of intermediate order in polymeric glasses

The glass phase of most polymers is typically considered amorphous. However, many commercially important polymer glasses do contain a subtle level of structural order intermediate between the crystalline and truly amorphous phases. These include a number of commercially important polymers. This “Intermediate Order” is distributed homogeneously throughout the glass phase and is therefore quite different than the morphology of semi-crystalline materials. Typical computer simulation methods can fail to accurately reproduce the structure of polymers with intermediate order, because they are often based on the assumption that the polymer structure is truly amorphous. Given the subtle nature of this order, computer simulation is important in understanding this structural order and the unique material properties commonly associated with it. We outline the critical issues required for the successful simulation of polymer glasses with intermediate order, and discuss the results of some continuing research designed to improve the accuracy of such simulations.     

聚合物电致发光二极管热特性研究

采用ANSYS有限元分析软件中热分析结构单元,对聚合物电致发光二极管(PLED)在光强为1000cd/m2时的热特性进行模拟,获得其温度场、热流分布及温度梯度的分布图,从仿真结果知PLED器件的最高温度为45.968℃,处于PFO-BT发光层,最低温度为45.95℃,处于石英玻璃基底末端.计算得出聚合物发光器件总热阻为1305℃/W,聚合物发光层至石英玻璃基底末端热阻为1℃/W.通过改变PLED器件输入功率、基底材料以及基底厚度3个参数,分别模拟得出其对PLED器件热特性的影响,仿真结果表明器件最高温度TH与输入功率P显现良好的线性关系;不同基底材料对器件温度影响小,负极端为器件主要散热通径;当基底厚度不断增加时,PLED器件最高温度随着增加,而最低温度不断减少,器件总热阻基本不变,发光层至石英基底末端热阻线性增大.     

Kissinger equation versus glass transition phenomenology

The applicability of the Kissinger equation for the evaluation of apparent activation energy corresponding to glass transition kinetics is examined. Theoretically simulated data based on the generally accepted Tool–Narayanaswamy–Moynihan model were used to represent relevant cases of structural relaxation behavior. The values of the apparent activation energy determined by the Kissinger equation were, despite the linearity of the dependencies, in major disagreement with the original values of ?h ^* used for the simulation of the source data. Furthermore, a large dependence of the ?h _Kis ^* evaluation (performed using the Kissinger equation) on the thermal history of the glass was found. The latter represents an unacceptable systematic error in the methodology, implying the incorrectness of the Kissinger equation usage for the evaluation of “glass transition activation energy”. This study addresses the currently widespread (incorrect) usage of the Kissinger equation for the above-mentioned purpose.     

Thermally stimulated current studies of thermotropic polyesters with polymethylene spacers (I): The glass transition and relaxation behavior of anisotropic and isotropic glasses

Thermally stimulated current (TSC) and relaxation map analysis (RMA) were used to study the glass transitions and relaxation phenomena of the anisotropic and isotropic glasses for the semiflexible polyesters with various polymethylene spacers (n = 7 ˜ 10). TSC analysis indicated that the glass transition temperatures (T_g) of polymers at around 40 ˜ 50°C exhibited an apparent even-odd behavior not only for the anisotropic glasses but also for the isotropic glasses. The T_g of the isotropic glass for the polymer with even n value was observed to be higher than those for the two neighboring polymers with odd n values. The anisotropic glass for the polymer with even n value had a lower T_g than those for the two neighboring polymers with odd n values. The lower value was attributed to the configuration and orientation effects on the behavior of polarization. RMA revealed that the relaxation modes of the investigated polymers were also influenced by the configuration and orientation effects. The dipolar relaxation of the anisotropic glass for the polymer with odd n value occurred at a higher temperature and had a lower entropy (enthalpy) of activation than that of the isotropic glass for the same polymer due to the orientation effect. However, an inverse relation was found to occur for the polymer with even n value, which came from the trans configuration of the even polymethylene spacers. Finally, the thermokinetic properties evaluated by RMA (e.g., the final state after depolarization) correlated quite well with the results obtained by TSC. © 1995 John Wiley & Sons, Inc.     

Calorimetric study and modeling of molecular mobility in amorphous organic pharmaceutical compounds using a modified Adam-Gibbs approach

The purpose of this study is to provide a quantitative characterization of the thermal behavior of amorphous organic pharmaceutical compounds across their glass transition temperature, and to assess their molecular mobility as a function of temperature and time by combining theoretical simulations with experimental measurements using differential scanning calorimetry. A computational approach built on the Boltzmann superposition principle of nonexponential decay and the Adam-Gibbs theory of entropic-dependent structural relaxation is presented. The heat capacities of the crystalline and amorphous forms are incorporated into the simulation in order to accurately assess the entropic fictive temperature as functions of temperature and time under any arbitrary set of experimental conditions. Using this method, we evaluated properties of the glass former, D and T0, and the nonexponentiality index beta, for amorphous salicin, felodipine, and nifedipine, by fitting the simulated glass transition profile with the experimentally determined heat capacity across the glass transition region. From this fit, the evolution of the relaxation time of the model compounds following any thermal cycle, including heating, cooling, and isothermal holds can then be estimated a priori. This study reveals the profound and inextricable effect of thermal history on the molecular mobility of the amorphous materials, and the ability of the glass to undergo fast changes in its molecular motions over an aging process even at low temperatures.     

A computational tool for the prediction of crystalline phases obtained from controlled crystallization of glasses

An automatic tool (named CLUSTER) for the prediction of the most probable crystal phases that can separate from glasses has been developed. The program analyzes the output of molecular dynamics simulations of glasses or glass ceramics, systematically sampling the ratios of the ions in different portions of the simulation box and comparing them to the stoichiometric ratio of compositionally equivalent crystalline phases retrieved from a crystal structure database. The efficacy of the similarity index elaborated has been judged by comparing the results obtained with the crystal phases identified by XRD analysis after thermal treatment in a series of multicomponent potential bioactive glasses and glass ceramics for which the advantages of rational-designed erosion-controlled release is straightforward.     

氟代硼酸锂玻璃的分子动力学模拟

用分子动力学模拟方法,计算了氟代硼酸锂玻璃的电导率.研究的温度范围高于和近于玻璃转变温度,共模拟了七个体系,研究的成分大致覆盖了能形成玻璃的区域.所得极限电导率、活化能以及电导率随温度的变化与实验数据符合得相当好.以往的研究认为快离子传导的典型特征是仅有一种离子发生迁移,我们的模拟表明氟离子对电导也有较大贡献.用活化能数据可顺利解释这个三元系各体系的电导率相对高低问题.     

Spectroscopic and thermal studies of silicate-phosphate glass

The structure of silicate-phosphate glass containing the different amounts of phosphorus, magnesium and calcium cations, acting as macroelements was examined by FTIR, XRD and thermal (DTA/DSC) methods. It has been found that in the structure of silicate-phosphate glass modified by an addition of Mg²⁺ and Ca²⁺ there are formed domains characterized by certain degree of ordering of the units present in their composition, while the structure of the newly formed domains is similar to the structure of the crystal compounds formed during crystallization of these glasses. The changing character of domains structure may be the reason of different chemical activity of glass acting as glassy fertilizers.     

Phase composition and structure of sodium aluminophosphate based glass materials depending on their synthesis conditions

The effect of the method of synthesis on the structure of aluminophosphate glasses intended for immobilization of high level waste from uranium–graphite channel reactor spent nuclear fuel reprocessing has been studied by X-ray powder diffraction and Fourier transform IR spectroscopy. Slow cooling in a furnace, both spontaneous cooling in a switched-off furnace and cooling simulating the conditions of glass cooling in their disposal canisters, or additional heat treatment (annealing) led to partial crystallization of glasses with segregation of different modifications of aluminum orthophosphate. For orthoand pyrophosphate based glasses with similar anionic motifs, their phase composition and degree of crystallization depend not so much on the chemical composition of a sample as on its heat treatment history.     

Shear softening and structure in a simulated three-dimensional binary glass

Three-dimensional model binary glasses produced by quenching from a range of liquid temperatures were tested in shear over a range of strain rates using molecular-dynamics techniques. Tests were performed under constant volume and constant pressure constraints. The simulations revealed a systematic change in short-range order as a function of the thermal and strain history of the glass. While subtle signs of differences in short-range order were evident in the pair distribution function, three-body correlations were observed to be markedly more sensitive to the changes in structure. One particular structural parameter, the number of aligned three-atom clusters, was analyzed as a function of the degree of supercooling, the strain and the strain rate. The glasses quenched from the supercooled liquid regime were observed to contain an initially higher number of such clusters, and this number decreased under shear. Those quenched from high-temperature equilibrium liquids contained lower numbers of such clusters and these increased or remained constant under shear. The glasses quenched from the supercooled liquid regime showed higher strength, more marked shear softening, and an increased propensity toward shear localization. The evolution of this structural parameter depended both on its initial value and on the imposed shear rate. These results were observed to hold for simulations performed under both constant density and constant pressure boundary conditions.     

Molecular Modeling of the Chain Structures of Polybenzoxazines

IntroductionThe benzoxazine polymer could be deemed asan alternative to traditional phenolics because ofthe similar main chain structures.A Novolac typeof phenolic resin has the unit,Ar—CH2 — withsome Ph—CH2 N( R)—CH2 — structure,where Phdenotes the phenolic group.In addition to theadvantages owned by ordinary phenolics,such ashigh temperature resistance,dimensional stability,good electrical properties,flame retardance,andlow smoke generation,this family ofpolybenzoxazines provides t…     

Amorphous phase and interphase organization of syndiotactic polypropylene

Materials prepared by syndiotactic polypropylene of different stereochemistry (62 ÷96% r pentads) undergo transitions in the crystal phase, which depend on the thermomechanical history. The interfaces and the amorphous phase may be affected by the crystallite organization and the stereoregularity of the material. MAS NMR techniques provided a tool for characterizing the arrangement and the mobility of the polymer chains in the inter-crystallite region. Chemical shifts, line-width and relaxation parameters were determined as a function of recording temperature and glass transition temperature of the material. The same parameters were followed in the presence of a plasticizer. The different degree of intimacy of the phases was established by following the diffusion of magnetization among the hydrogen atoms of the amorphous and the crystalline phase. Defective regions and boundary regions were also identified and showed a large amount of conformations in the trans arrangement; the relaxation times indicate the chains being rigid and intimately mixed with the crystalline phase.     

Molecular dynamics simulation study of glass transition in hydrated Nafion

The thermal transition of Nafion is studied using a molecular dynamics simulation through a chemically realistic model. Static and dynamic properties of polymer melts with different water contents are investigated over a wide range of temperatures to obtain viscometric and calorimetric glass transition temperatures. The effect of cooling rate of the simulation on the glass transition of the hydrated polymer is also examined within the well‐known Williams–Landel–Ferry (WLF) equation. Variation of relaxation times versus temperature shows a fragile‐to‐strong transition. The hydration level has a significant impact on the static and dynamic properties of the polymer chains and water molecules confined in nanometric spaces between polymer chains. The results of this study are useful to predict the behavior of Nafion for various applications including fuel cells, sensors, actuators, and shape memory devices at different temperatures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 907–915     

Molecular dynamics simulation on devitrification: isothermal devitrification and thermodynamics of PbF2 glasses

The vitrification and devitrification features of lead fluoride are investigated by means of molecular dynamic simulations. The influence of heating rate on the devitrification temperature as well as the dependence of the glass properties on its thermal history, i.e., the cooling rate employed, is identified. As expected, different glasses are obtained when the cooling rates differ. Diffusion coefficient analysis during heating of glass and crystal, indicates that the presence of defects on the glassy matrix favors the transition processes from the ionic to a superionic state, with high mobility of fluorine atoms, responsible for the high anionic conduction of lead fluoride. Nonisothermal and isothermal devitrification processes are simulated in glasses obtained at different cooling rates and structural organizations occurring during the heat treatments are clearly observed. When a fast cooling rate is employed during the glass formation, the devitrification of a single crystal (limited by the cell dimensions) is observed, while the glass obtained with slower cooling rate, allowing relaxations and organization of various regions on the glass bulk during the cooling process, devitrifies in more than one crystalline plane.