Thermal transformations of selected transition metals oxyhydroxides

We investigated the features of the glass transition relaxation of two room temperature ionic liquids using DSC. An important observation was that the heat capacity jump, that is the signature of the glass transition relaxation, shows a particularly strong value in this type of new and promising materials, candidates for a range of applications. This suggests a high degree of molecular mobility in the supercooled liquid state. The study of the influence of the heating rate on the temperature location of the glass transition signal, allowed the determination of the activation energy at the glass transition temperature, and the calculation of the fragility index of these two ionic glass-formers. It was concluded that this kind of materials belong to the class of relatively strong glass-forming systems. This revised version was published online in July 2006 with corrections to the Cover Date.     

Anharmonicity and fragility in lithium borate glasses

Since the physical properties of lithium borate glasses xLi₂O-(1-x)B₂O₃ (0<x< 0.28) vary over a wide range with the composition, this binary system is particularly suitable for studying the relationship between vibrational anharmonicity and fragility. The density, the linear expansion coefficient, the longitudinal and transverse ultrasonic velocities and their respective temperature coefficients of the velocities are measured, from which the vibrational anharmonicity in lithium borate glasses is evaluated with the help of the Grüneisen parameter at the Debye cut-off frequency and the Anderson-Grüneisen parameter: these two parameters plotted vs. composition have the same characteristics with minima at x≈0.08. The fragility is evaluated from the temperature width of the glass transition; the fragility also shows a minimum at x≈0.08. The presence of minima at x≈0.08 is ascribable to the fact that the crosslinking density between six-membered rings in the glass reaches a maximum at this composition. We show that the anharmonic parameters strongly correlate with the fragility metrics. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparison of the fragility index of different eudragit polymers determined by activation enthalpies

The present study aimed to apply fragility index (m) of polymers in the determination of the optimal amount of plasticizer in polymer films. The fragility index of different Eudragit polymers (RS, RL, EPO) was assessed by differential scanning calorimerty (DSC), applying the Arrhenius connection (logq–1/T _g). The fragility of Eudragit EPO films proved to be the highest, while in the case of RS and RL, the increase of the alkyl-chain length caused the increase of fragility. Studying the effect of plasticizer (triethyl citrate, TEC) on the m value of Eudragit RL and RS films, a near linear reduction of the fragility index could be observed between 5–30% TEC concentration, but above 30%, this value leveled out to constant.     

An investigation of glasses for capillary chromatography

An investigation was conducted of various glasses, other than soda lime or borosilicate, for use in glass capillary gas chromatography. The work has uncovered some unique chromatographic qualities in the use of potash soda lead and fused silica glasses as materials for making glass capillary columns. The fused silica proved to be an ideal material for capillary column construction, being inherently more inert than glass containing metal oxides. It has been shown that through the use of thin wall capillary tubing of high flexibility many of the mechanical problems associated with glass capillary columns, such as fragility and column straightening, can be avoided.     

Intramolecular proton transfer in the isomerization of hydroxyacetone: Characterization based on reaction force analysis and the bond fragility spectrum

The mechanism of isomerization of hydroxyacetone to 2-hydroxypropanal is studied within the framework of reaction force analysis at the M06-2X/6-311++G(d,p) level of theory. Three unique pathways are considered: (a) a step-wise mechanism that proceeds through the formation of the Z-isomer of their shared enediol intermediate, (b) a step-wise mechanism that forms the E-isomer of the enediol, and (c) a concerted pathway that bypasses the enediol intermediate. Energy calculations show that the concerted pathway has the lowest activation energy barrier at 45.7 kcal mol⁻¹. The reaction force, chemical potential, and reaction electronic flux are calculated for each reaction to characterize electronic changes throughout the mechanism. The reaction force constant is calculated in order to investigate the synchronous/asynchronous nature of the concerted intramolecular proton transfers involved. Additional characterization of synchronicity is provided by calculating the bond fragility spectrum for each mechanism.     

Thermal behaviour of two anti-inflammatory drugs (celecoxib and rofecoxib) and slow relaxation dynamics in their amorphous solid state. Comparison between the dynamic fragility obtained by dielectric spectroscopy and by thermostimulated currents

This study of the thermal behaviour of two glass formers showed that rofecoxib has a good ability to vitrify while celecoxib displays a moderate glass forming ability. On the other hand, celecoxib forms an instable glass while rofecoxib displays moderate glass stability. The α-relaxation of the two amorphous drugs was studied by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). The values of the dynamic fragility obtained by the two techniques are in good mutual agreement and suggest that both drugs are moderately fragile glass formers. These values are however considerably lower than those obtained by dielectric relaxation spectroscopy (DRS) and published in the literature. In the present work we will compare the values of the dynamic fragility obtained by TSDC and DRS for a set of ~30 glass-forming liquids, in order to evaluate their relative suitability for the determination of the m-index. The observed differences are discussed and interpreted. Finally, secondary mobility modes were detected by TSDC in both celecoxib and rofecoxib. The kinetic parameters are identical in both cases and similar to those found by DRS for the slow-b relaxation of celecoxib. These secondary relaxations, that are precursors of the main relaxation on its lower temperature side, are believed to be Johari-Goldstein relaxations.     

立方非线性产品包装损坏边界的三维曲面表示

用三次Ｂ样条函数配点法，求得在矩形波冲击下具有立方非线性产品包装的冲击谱。讨论了软、硬非线性及外界激励幅值对产品损坏边界的影响。提出用损坏边界曲面来描述非线性产品包装的安全区域，并作出了相应的损坏边界曲面．     

Dielectric relaxation spectroscopy in poly(hydroxyethyl acrylates)/water hydrogels

The electrical and dielectric properties of poly(hydroxyethyl acrylate), PHEA, hydrogels were studied by means of dielectric relaxation spectroscopy in wide ranges of frequencies (5–2 × 10⁹ Hz), temperatures (173–363 K) and water contents (0.065–0.46, g of water per gram of dry material). The secondary dipolar mechanisms (γ and β_sw) and the dc conductivity mechanism were studied in detail by analyzing the dielectric susceptibility data within the complex permittivity formalism, the modulus formalism, and the complex impedance formalism. For both mechanisms molecular mobility was found to increase with increasing temperature or increasing water content (T-f-h superposition principle). The energy parameters and the shape parameters of the response were determined for both mechanisms at several water contents and temperatures. The temperature dependence of dc conductivity was found to change from Vogel-Tamman-Fulcher (VTF) type to Arrhenius type at water contents of ca. 0.30. At water contents lower than about 0.30 the hydrogels are homogeneous whereas at higher water contents a separate water phase appears. In terms of the strong/fragile classification scheme our results suggest that the PHEA hydrogels are fragile systems. ©1995 John Wiley & Sons, Inc.     

Effect of Alkyl Side Chain Length on Relaxation Behaviors in Poly(n-alkyl Acrylates) and Poly(n-alkyl Methacrylates)

Dynamic mechanical analysis (DMA) is used to investigate the effect of alkyl side chain length on the relaxation behavior of poly(n-alkyl acrylates) (PnAA) and poly(n-alkyl methacrylates) (PnAMA) above the glass transition temperature (T_g). Master curves and shift factors (log a_T) were obtained using the time–temperature superposition (TTS) principle. The log a_T curves of PnAA and PnAMA exhibit a dynamic crossover from one Vogel–Fulcher–Tammann–Hesse (VFTH) equation to another above T_g. The corresponding temperature was designated as the dynamic crossover temperature (T_c). It is found that T_c/T_g and the apparent activation energy (E_g) increases, e whereas the fragility index (m) decreases with increasing alkyl side chain length. Further analysis shows that m ∝ T_g, E_g∝ , and E_g∝ m² for both PnAA and PnAMA.     

基于显示连通贝叶斯网络的机场塔台地震易损性分析

将基于性能的多维易损性分析方法,结合显示连通贝叶斯网络,应用于机场塔台的多维易损性分析。考虑地震激励的不确定性,通过非线性时程分析获得结构响应数据;将塔台结构分为三个层次,每个层次按包含的层数分为相应的子层次。根据功能特性确定子层次的评价指标和极限状态,建立服从多元对数正态分布的概率地震需求模型;考虑各种极限状态之间的相关性,建立极限状态方程,确定失效域,通过蒙特卡洛法求得构件的超越概率;建立塔台结构的显示连通贝叶斯网络模型,利用层次分析法获得中间节点的条件概率表,利用MATLAB进行贝叶斯网络的推理计算,实现从单一层次的易损性到整体易损性的推理。