Variation of the Effective Activation Energy Throughout the Glass Transition

Summary: An advanced isoconversional method has been applied to determine the effective activation energies (E) for the glass transition in polystyrene (PS), poly(ethylene terephthalate) (PET), and boron oxide (B₂O₃). The values of E decrease from 280 to 120 kJ · mol⁻¹ in PS, from 1 270 to 550 kJ mol⁻¹ in PET, and from 290 to 200 kJ mol⁻¹ in B₂O₃. It is suggested that a significant variation in E should be observed for the fragile glasses that typically include polymers.

Variation in the effective activation energy of PS, PET, and B₂O₃ with temperature.     

Elastic Moduli and Activation Energies for an Epoxy/m-XDA System by DMA and DSC

The influence of the resin/diamine ratio on the properties of the system diglycidyl ether of bisphenol A (BADGE n=0/m-xylylenediamine) (m-XDA) was studied. Variation of this ratio resulted in significant effects on the cure kinetics and final dynamic mechanical properties of the product material. The study was made in terms of storage modulus (E′), vss modulus (E″) and molecular mass between cross-links (M_c) at different ratios. Two geometries (cylindrical and rectangular) were considered. The influence of temperature was studied through the activation energy (E_a>), which depends on the epoxy/amine ratio and the geometry of the samples. Glass transition temperatures (T_g>) and glass transition temperatures for thermosets with null degree of conversion (T_go>) were determined by DSC. T_g> decreases when amounts of curing agent greatly in excess of the stoichiometric composition were used. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrogen bond effect of azido polyurethane elastomer by dynamic mechanical analysis

Hydrogen bond effects in azido polyurethane elastomers (APUE) have been studied by dynamic mechanical analysis (DMA) and the results show that the hydrogen bond effect has stronger temperature dependence. The activation energy of hydrogen bond dissociation (E_a) and the hydrogen bond density (vs/V) have been evaluated from the elastic modulus–temperature relationship. The calculated E_a in this work is much higher than the reported values of normal polyurethane elastomer (PUE). The values of E_a are 81.3, 68.1, 53.3, and 42.3 kJ/mol at 150, 110, 50, and 20 Hz, respectively, for PUE‐1 (CPPB/HDI trimer elastomer); 94.6, 75.8, 48.4, and 36.9 kJ/mol at 150, 110, 50, and 20 Hz, respectively, for PUE‐2 (APPB/HDI trimer elastomer); 82.1, 74.4, 59.8, and 46.5 kJ/mol at 150, 110, 50, and 20 Hz, respectively, for PUE‐3 (APPB/HDI trimer/EG elastomer); 145, 124, 88.0, and 75.5 kJ/mol at 150, 110, 50, and 20 Hz, respectively, for PUE‐4 (APPB/HDI trimer/BD elastomer); and 72.2, 64.3, 49.8, and 39.9 KJ/mol at 150, 110, 50, and 20 Hz, respectively, for PUE‐5 (APPB/HDI trimer/HD elastomer). The DMA estimations are semiquantitative for it ignores other physical crosslinking effects and the results give relative order of vs/V and E_a. The values of vs/V of crosslinked APUE (PUE‐3, PUE‐4, and PUE‐5) are much higher than PUE‐2. The test frequency could affect the values of vs/V and higher frequency would minify the difference of the values of vs/V for two given temperatures. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2841–2851, 2006     

可控交联聚醚醚酮的动态力学性能

动态力学分析技术(DMA)是研究聚合物性能的重要方法之一．动态力学实验可以检测聚合物的玻璃化转变温度和次级松弛过程,直接与聚合物的链结构和聚集态结构密切相关,聚合物的化学组成、支化和交联、结晶和取向、增塑和共混等结构因素的变化,都与分子运动状态的变化密切相关,而分子运动的变化又能灵敏地反映在动态力学性能上,     

Mechanical Properties and Dynamic Mechanical Analysis of Thermoplastic-Natural Fiber/Glass Reinforced Composites

The mechanical properties and dynamic behavior of thermoplastic composites based on polypropylene/glass fibers and polypropylene/natural fibers (i.e. kenaf, hemp, flax) are presented. A survey is given on some aspects, crucial for the use of these composites in structural and non-structural components such as their vibration-damping response, in relation to the composite compaction level and the manufacturing procedure. In order to investigate a wide vibration frequency range, including acoustic frequencies, different testing techniques, both with forced and free vibrations, were applied. A comparison between natural fiber and glass fiber reinforced laminates is presented. Compaction levels, allowing to obtain the best compromise between mechanical performance and damping response, are investigated.     

反应性聚碳酸酯/环氧树脂体系的玻璃化转变行为及力学性能

增韧;反应性聚碳酸酯/环氧树脂体系的玻璃化转变行为及力学性能     

橡胶活性官能团对增韧环氧树脂动态力学性能的影响

用动态扭摆法测试聚丙烯酸丁酯橡胶增韧环氧树脂的动态力学行为,研究在环氧树脂低固化度和高固化度时,橡胶活性官能团种类(环氧基官能团与羧基官能团)和数量(官能度)对其影响。研究体系中橡胶玻璃化转变温度(Tg)的移动大小,与橡胶和基体树脂健合程度之间的关系。     

改性聚乙二醇修饰高交联度不饱和聚酯网络结构的动态力学分析

合成了一系列含有反应活性端基的改性聚乙二醇,并用其对BMC(团状模塑料)专用的高交联度不饱和聚酯进行增韧.结果表明,含有反应性马来酸酐端基的聚乙二醇参与了不饱和聚酯的固化反应,可在交联网络中构成不同长度的柔性链段,在显著提高不饱和聚酯的韧性的同时,基本保持了材料的模量及其它力学性能.用动态力学分析(DMA)对不饱和聚酯交联网络结构进行了系统研究.     

聚丙撑碳酸酯／环氧树脂体系的力学性能

研究了聚丙撑碳酸酯／环氧树酯固化产物的动态力学性能和力学性能，考察了ＰＰＣ的分子量、含量对力学谱和力学性能的影响．结果表明，随ＰＰＣ含量增加，ｔａｎδ半峰宽增加，ＰＰＣ含量在２０～３０份时力学性能达最佳值，ＰＰＣ的分子量对力学性能的影响不大，但对动态力学和相区尺寸稍有影响。     

Characteristics of Each Bonded Layer of a Polymer-Based Magnetic Tape Using Rule-of-Mixtures and Dynamic Mechanical Analysis

A rule-of-mixtures approach is used with dynamic mechanical analysis (DMA) to determine the viscoelastic characteristics of individual layers of a fourth-generation advanced magnetic tape used for archival storage. Such tapes are comprised of three layers: a 0.92 µm thick poly(ester-urethane)-based front coat that binds the magnetic particles together, a 5.00 µm thick poly(ethylene naphthalate) substrate, and a protective 0.64 µm thick back coat with cellulose nitrate as a constituent. The storage modulus, loss modulus, and loss tangent determined for each layer are discussed in terms of their influence on magnetic tape properties and characteristics.     

Dynamic Mechanical Behavior of PP/PET/MAPP Blends Filled with Glass Beads

PP/PET/MAPP blends have been filled with 50 wt% of glass beads. The orientation of the PP crystalline phase, the crystallization behavior and the dynamic mechanical response of these materials have been analyzed. The dynamic mechanical response is strongly affected by the presence of the glass beads, being possible to detect the effect of PET and MAPP on the storage modulus and loss factor values. Moreover, the alpha relaxation of the composites is visibly affected by thermal treatments.     

Activation energies for the thermodegradation process of an epoxy-diamine system

The study of the degradation of a polymer is important because it can determine the upper temperature limit, the mechanism of a solid-state process, and the life-time for this system. Since the behavior of thermosets is affected by the selection of the curing cycle, it is important to investigate the changes which take place during the thermal degradation of these materials when a change on the sequence of time and temperature is introduced during the curing reaction. In this work, the thermal degradation of two epoxy systems diglycidyl ether of bisphenol A (BADGE n=0)/1, 2 diamine cyclohexane (DCH) cured through different sequences of time and temperature was studied by thermogravimetric analysis in order to determine the reaction mechanism of the degradation processes, and also to check the influence of the curing cycle on this mechanism. Values obtained using different kinetic methods were compared to the value obtained by Kissinger’s method (differential method which do not require a knowledge of the n-order reaction mechanism), and to that obtained through Flynn–Wall–Ozawa method in a previous work.     

一种未知胶粘剂的剖析

采用在高温碱性条件下,将高分子化合物化学降解为小分子化合物后进行分离的方法,对未知物各组分的傅里叶变换红外光谱和色谱-质谱联用的分析谱图进行解析,鉴定了未知物所含的高聚物组分和助剂组分,确定了未知胶粘剂的组成。     

化学反应活化能测定实验

通过测定反应温度和碘化钾浓度对反应速率的影响计算活化能,根据公式推导结果消除了试剂浓度对实验结果的影响。用Origin软件对实验数据进行拟合,从图形中得到反应速率、反应浓度、反应温度和活化能的直观对应关系。改进后的实验由单纯的验证性实验变成了自主设计综合性实验。     

Synergism and multiple mechanical relaxations observed in ternary systems based on benzoxazine,epoxy, and phenolic resins

The synergism in the glass‐transition temperature (T_g) of ternary systems based on benzoxazine (B), epoxy (E), and phenolic (P) resins is reported. The systems show the maximum T_g up to about 180 °C in BEP541 (B/E/P = 5/4/1). Adding a small fraction of phenolic resin enhances the crosslink density and, therefore, the T_g in the copolymers of benzoxazine and epoxy resins. To obtain the ultimate T_g in the ternary systems, 6–10 wt % phenolic resin is needed. The molecular rigidity from benzoxazine and the improved crosslink density from epoxy contribute to the synergistic behavior. The mechanical relaxation spectra of the fully cured ternary systems in a temperature range of −140 to 350 °C show four types of relaxation transitions: γ transition at −80 to −60 °C, β transition at 60–80 °C, α₁ transition at 135–190 °C, and α₂ transition at 290–300 °C. The partially cured specimens show an additional loss peak that is frequency‐independent as a result of the further curing process of the materials. The ternary systems have a potential use as electronic packaging molding compounds as well as other highly filled systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1687–1698, 2000     

Determination of Silver in Plants by Flameless Atomic Absorption Spectrometry and Neutron Activation Analysis

Abstract

Three analytical techniques suitable for determining silver concentrations in plants are presented and compared. Graphite furnace atomic absorption spectrometry performed directly on sample digests was the most sensitive and convenient. Neutron activation analysis, measuring ^110mAg gave good reproducibility, but lower sensitivity. A cyclic activation scheme to generate and detect the short-lived isotope ¹¹⁰Ag was useful as a quick reconnaissance technique, but interference from ⁷⁶As reduced its effectiveness.

Data are presented on the silver content of terrestrial plants. Background silver concentrations for lichens and bryophytes collected from Wales, U.K. were <0.07μgg^?1. Samples collected from areas contaminated by derelict metal mines contained between 0.1-1.0μAgg^?1. The aerial portions of vascular species usually contained less silver than bryophytes growing on the same substrate. Fungi are shown to bioconcentrate silver to a greater extent than cadmium, copper or lead.     

对活化能相关问题的探析

在介绍阿仑尼乌斯速率方程基础上,从碰撞理论、过渡态理论和托尔曼统计等视角诠释活化能概念,认为仅以活化能大小作依据判断反应速率大小不严密,并从教学层面和试题角度进行了反思。     

Dynamic mechanical properties of epoxy/novolac system modified with reactive liquid rubber and carbon filler

This paper reports on the work carried out to evaluate the frequency dependent viscoelastic properties of epoxy/novolac compositons modified with a liquid reactive rubber and carbon filler. For epoxy systems modified with elastomer, three typical transitions were observed: the α-relaxation deeply related to the glass transition of epoxy, the β-transition of epoxy, and the glass transition of rubber appeared near to the β-relaxation of epoxy resin. Considering an Arrhenius equation, the activation energies of β-relaxation were estimated. In the region of glass transition and rubbery state the temperature dependence of the shift factor (α_T) was determined through Williams-Landel-Ferry (WLF) equation.     

Determination of Tellurium by Instrumental Photon Activation Analysis

Abstract

Tellurium has been determined at milligram and submilligram levels in the presence of uranium by a strictly instrumental photon activation ¹²¹ analysis procedure. The 575 keV gamma ray of Te was measured. At a decay time of three weeks, this gamma ray peak was interference-free. As little as 0.4 mg of tellurium was determined and the method evidences that sensitivity would increase with irradiation time.