Kinetics of microwave‐assisted polymerization of ϵ‐caprolactone

The kinetics of polymerization of ?‐caprolactone (CL) in bulk was studied by irradiating with microwave of 350 W and frequency of 2.45 GHz with different cycle‐heating periods (30–50 s). The molecular weight distributions were determined as a function of reaction time by gel permeation chromatography. Because the temperature of the system continuously varied with reaction time, a model based on continuous distribution kinetics with time/temperature‐dependent rate coefficients was proposed. To quantify the effect of microwave on polymerization, experiments were conducted under thermal heating. The polymerization was also investigated with thermal and microwave heating in the presence of zinc catalyst. The activation energies determined from temperature‐dependent rate coefficients for pure thermal heating, thermally aided catalytic polymerization, and microwave‐aided catalytic polymerization were 24.3, 13.4, and 5.7 kcal/mol, respectively. This indicates that microwaves increase the polymerization rate by lowering the activation energy. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1450–1456, 2004     

Kinetics of devitrification of styrene-divinylbenzene copolymers

The dependence of glass temperature on the heating rate for styrene-divinylbenzene solvent modified network copolymers has been investigated by differential scanning calorimetry. The McMillan equation has been utilized to determine apparent activation energies of vitrification. Values of glass temperature do not show a linear function of the degree of crosslinking.     

手机SIM卡TG-FTIR热解实验研究

利用热重-红外分析仪（TG-FTIR）研究了手机SIM卡在不同升温速率下的热解行为,探讨了升温速率对热解参数及热解产物的影响。采用分布式活化能模型求解了热解活化能,探讨了活化能随转化率的变化规律。研究结果表明：手机SIM卡呈现一段热解,主要热解温区在350～500 ℃,最大失重速率为?62.57%/min,总失重率高达90%。随着升温速率的提高,热解初始温度和热解结束温度均增大,最大热解速率和对应的温度也都增大;热解活化能在170～204 kJ/mol变化,随转化率变化规律呈现先增大后减小再增大后逐渐减小的规律,在转化率0.2时达到最大值;主要热解产物为苯、烷烯烃等可燃成分,而且含有氯、氮等元素;升温速率对热解组分没有影响。     

Nonisothermal kinetics of wood degradation in supercritical methanol

The kinetic analysis of wood degradation in supercritical methanol has been studied by a nonisothermal weight loss technique. The weight loss data according to degradation temperature have been analyzed using two integral methods based on Arrhenius form to obtain the kinetic parameters, such as apparent activation energy and overall reaction order. The experiments were carried out for three heating rates of 5.2, 11.6 and 16.3 °C/min. It was found that there are the distinct mass changes over the temperature range of 260–370 °C for all three heating rates, and the weight loss curves were displaced to higher temperatures as increasing of heating rate. The activation energies of wood degradation in supercritical methanol were 73.5–74.5 kJ/mol and 45.2–48.8 kJ/mol, and the reaction orders were 0.59–0.64 and 0.25, depending on the mathematical approach taken in the analysis and the heating rate.     

Studies on the thermal stability and degradation kinetics of Pd/PC nanocomposites

Effect of heating rate, Pd content, and synthesis method on the thermal stability of the ex situ and in situ Palladium/polycarbonate (Pd/PC) nanocomposites was investigated. TEM images revealed discrete Pd nanoclusters of about 5 and 15 nm sizes for 1 and 2 vol % ex situ nanocomposites, respectively. However, agglomerated Pd nanoclusters were noticed in the in situ samples, irrespective of the Pd content. The ex situ Pd/PC nanocomposites showed high onset temperature (Ti) for thermal degradation of PC than the in situ and pure PC samples. Pd content and heating rates were found to have a positive influence on the Ti and Tm (temperature at the maximum degradation rate occurs) of the Pd/PC nanocomposites. Thermal degradation of the PC was found to follow the first‐order kinetics in the Pd/PC nanocomposites. The activation energies associated with the degradation were determined by using the Kissinger method. These activation energies are used to construct the Master decomposition curve (MDC) and weight–time–temperature (α–t–T) plots that describe the time‐temperature dependence of the PC pyrolysis in the Pd/PC nanocomposites. These constructed α–t–T plots were validated with the data from isothermal measurements. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of thermal treatment on the electrical properties of the sol–gel-derived (Zr,Sn)TiO4 thin films

The effect of heating temperatures on the electrical properties of sol–gel-derived (Zr,Sn)TiO₄ thin films deposited on a p-type (1 0 0) Si substrate was studied. The leakage currents of films with two different heating temperatures chosen to burn-out the solvent as a function of applied voltage were measured at different temperatures. The activation energies obtained from the Arrhenius plot of the leakage current density versus measured temperature for (Zr,Sn)TiO₄ films were then extracted. Additionally, microstructures of films with two different heating temperatures chosen to burn-out the solvent were analyzed by a conductive atomic force microscope (AFM) and an X-ray diffraction (XRD). Finally, the conductive mechanisms of leakage current and leakage current correlated to microstructures were also discussed.     

Stress relaxation of poly(1,4-dimethylene-trans-cyclohexyl suberate)

Stress relaxation experiments were conducted on poly(1,4-dimethylene-trans-cyclohexyl suberate) (MCS) as a function of preparation condition and temperature. Deconvolution of the stress relaxations provides relaxation times, which can be plotted as a function of temperature to obtain an activation energy for the relaxation process. For an MCS sample of M_n=24.8 K, MWD=2, the activation energy varies from 12.7-5.0 kcal mol^?1 with forming temperatures varying from 45–90°C. These activation energies are associated with different populations of tie molecules between lamellae. We believe that these activation energies reflect the reorientation process in the amorphous segments of the polymer during stress relaxation.     

Dielectric studies of the cure of epoxy matrix systems

Dielectric spectroscopy was used to monitor the curing process of two epoxy resin systems. The basic system (system I) consisted of DGEBA (a difunctional epoxy) and a polyamide in a 50–phr mixture. In addition, a comparative analysis was performed on a high–performance resin system (system II) used primarily in unidirectional composite applications. This system contained TGDDM (a tetrafunctional epoxy) and DDS (a tetrafunctional amine) in a 25–phr mixture. The dielectric data were obtained using a simple yet functional sample cell electrode designed and constructed in the laboratory. For system I, isothermal dielectric data were used to determine apparent activation energies for the temperature range from 22 to 70°C. The data showed that the activation energy was a function of temperature and increased as the temperature of the cure increased. This indicated that the reaction mechanism was also a function of temperature. For system II, data were collected between 140 and 190°C and an overall activation energy for that temperature range was determined. The overall activation energies for both systems, calculated using dielectric spectroscopy, compared favorably to those obtained using differential scanning calorimetry. Also, using a wider frequency range (240 Hz to 2 MHz), Argand diagrams were constructed and modeled with the Cole–Cole empirical equation for systems with a distribution of relaxation times. This justified the calculation of average relaxation times, which could then be related to the bulk physical properties of the polymer, such as viscosity. Modified Argand diagrams, where ε″ is plotted against ε′ at one frequency as a function of time, were also constructed, which aided in the understanding of the curing processes for these thermosetting systems.     

Thermogravimetric characteristics and pyrolysis kinetics of Giheung Respia sewage sludge

Sewage sludge acquired from Giheung Respia treatment facility was characterized and converted into gas, bio-oil and char by pyrolysis. The rate of conversion as a function of temperature was obtained from differential thermogravimetric analysis (DTG) for different heating rates. The activation energy calculated from data selected conversions shows that the activation energy decreased with increasing conversion up to 10%, steadily increased from 10 to 70%, and substantially increased from 70 to 90%. Depending on the level of conversion, the values of activation energies varied between 181 and 659 kJ/mol. The gas product obtained in the experiment at 450 °C, 20 min mainly included CO₂ (30%), CO (23%) and CH₄ (17%). The product yields of gas, oil and char were systematically studied by changing the pyrolysis temperature and residence time.     

果壳生物质燃烧特性与动力学分析

为充分利用果壳生物质废弃物,采用热重分析对油茶壳、核桃壳、澳洲坚果壳进行了燃烧实验研究,考察了不同升温速率下3种果壳生物质的燃烧特性及动力学参数。结果表明：3种果壳生物质燃烧特性不同,但燃烧特性参数均随升温速率升高而增大;随着升温速率的增加,着火点、燃尽温度、最大燃烧速率、平均燃烧速率及综合燃烧特性指数提高;10℃/min时,油茶壳、核桃壳、澳洲坚果壳综合燃烧特性指数分别为0.56×10^-7、1.18×10^-7、0.88×10^-7;3种果壳生物质的燃烧反应遵循一级反应动力学模型,相关系数（R²）均达0.93以上,低温阶段活化能为30.40~52.41 kJ/mol,高温阶段活化能为18.49~40.62 kJ/mol,低温阶段活化能均大于高温阶段。     

Application of the kinetic composite methodology to autocatalytic-type thermoset prepreg cures

Using a commercial epoxy/carbon fiber prepreg as a model system, cure kinetics of an autocatalytic-type reaction were analyzed by a general form of conversion-dependent function first proposed for degradation kinetics of polymers and composites. The characteristic feature of conversion-dependent function was determined using a reduced-plot method where the temperature-dependent reaction rate constant was analytically separated from the isothermal data. Assuming two elementary reaction mechanisms that were expressed by the nth order and autocatalytic kinetic models, they were combined with a composite methodology capable of predicting overall kinetic behavior. The activation energies were determined and favorably compared for both isothermal and dynamic-heating differential scanning calorimetry experiments in the temperature region for standard epoxy cures at 177°C (350°F). Finally, the proposed model equation demonstrated excellent predictive capability and broad applicability in describing various types of thermoset polymer cure for both isothermal and dynamic heating conditions. © 1993 John Wiley & Sons, Inc.     

Compensation effect observed in thermally stimulated depolarization currents analysis of polymers

Thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC) are performed on thermoplastic polyesters and dimethacrylate resins over temperature ranges covering the α and β relaxation regions. The noncooperative β relaxation is characterized by a continuous variation of activation energies as a function of temperature and follows the activated states equation with a zero activation entropy. The cooperative α relaxation shows a prominent maximum of the activation energies at the glass transition temperature. Compensation behavior is often observed by TSDC for many polymers in the glass transition temperature range. We show that this behavior is not systematic and that it appears for the α relaxation if the increase of the apparent activation enthalpy is strong and if the width of the glass transition is weak. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2716–2723, 1999     

Influence of carbonization conditions on the gasification of acacia and eucalyptus wood chars by carbon dioxide

Gasification rates of cubic shaped acacia and eucalyptus wood chars were measured thermogravimetrically in a carbon dioxide atmosphere at temperatures in the range 810–960 °C. The effects of wood species and carbonization conditions, such as temperature, heating rate and soaking time, were determined. Both reactivity and the activation energy for the gasification of wood chars were found to be strongly influenced by the carbonization conditions employed during their preparation and wood type. The reactivities of both the acacia and eucalyptus wood chars decreased with increasing preparation temperature; while the activation energy for their gasification increased. Slow carbonization (heating rate: 4 °C min⁻¹) led to the production of wood chars having lower reactivities and higher activation energies than those of the wood chars prepared under rapid carbonization (heating rate: 30 °C min⁻¹) at the same temperature. With increasing soaking time, at carbonization temperatures of 800 and 1000 °C, the reactivity of resulting wood chars was reduced. The results also show that the reactivities of acacia wood chars are higher than those of similarly prepared eucalyptus wood chars.     

双马来酰亚胺对苯并噁嗪树脂固化动力学的影响

以含烯丙基醚的双马来酰亚胺预聚体(AE-BMI)作为苯并噁嗪(BOZ)的改性剂,采用非等温差示扫描量热(DSC)法、Kissinger法、Crane法和β-T(升温速率-温度)外推法研究了AE-BMI/BOZ体系的固化动力学过程。结果表明:BOZ体系的凝胶温度为174.86℃、固化温度为210.95℃和后处理温度为222.44℃,AE-BMI/BOZ体系的凝胶温度为114.84℃、固化温度为199.75℃和后处理温度为227.64℃;两者的反应活化能分别为89.03、69.97 kJ/mol,反应级数分别为0.83、0.79。     

纤维缠绕用环氧树脂固化动力学研究

采用非等温DSC法对一种纤维缠绕用环氧树脂体系进行了固化动力学研究。基于不同升温速率下的测试数据,确定了固化工艺参数,建立了n级动力学模型,并比较了通过Kissinger方程和Ozawa方程得到的活化能。研究表明:该树脂体系凝胶化温度为89.44℃,固化温度为114.5℃,后处理温度为155.04℃;固化反应过程符合n级动力学模型。     

Comparative Kinetic Study and Microwaves Non-Thermal Effects on the Formation of Poly(amic acid) 4,4'-(Hexafluoroisopropylidene)diphthalic Anhydride (6FDA) and 4,4'-(Hexafluoroisopropylidene)bis(p-phenyleneoxy)dianiline (BAPHF). Reaction Activated by Microwave, Ultrasound and Conventional Heating

Green chemistry is the design of chemical processes that reduce or eliminate negative environmental impacts. The use and production of chemicals involve the reduction of waste products, non-toxic components, and improved efficiency. Green chemistry applies innovative scientific solutions in the use of new reagents, catalysts and non-classical modes of activation such as ultrasounds or microwaves. Kinetic behavior and non-thermal effect of poly(amic acid) synthesized from (6FDA) dianhydride and (BAPHF) diamine in a low microwave absorbing p-dioxane solvent at low temperature of 30, 50, 70 °C were studied, under conventional heating (CH), microwave (MW) and ultrasound irradiation (US). Results show that the polycondensation rate decreases (MW > US > CH) and that the increased rates observed with US and MW are due to decreased activation energies of the Arrhenius equation. Rate constant for a chemical process activated by conventional heating declines proportionally as the induction time increases, however, this behavior is not observed under microwave and ultrasound activation. We can say that in addition to the thermal microwave effect, a non-thermal microwave effect is present in the system.     

Messung der Selbstdiffusion im System Polystyrol-Benzol; Temperatur-, Konzentrations- und Molekulargewichtsabhängigkeit

Self-diffusion of benzene and polystyrene in the system benzene-polystyrene is studied by means of spin-echo-technique as function of temperature, concentration and polymer molecular weight. From the dependence of temperature the activation energies are calculated as function of concentration and molecular weight. The concentration dependence is discussed in terms of restricted diffusion and the Fujita Free-Volume-Theory. The computed free volume shows a significant maximum at 50°C. This effect may be attributed to a change in the mobility of the phenyl side groups of polystyrene and is vanishing with decreasing molecular weight. The free-volume-temperature curves are giving references to the glass transition.     

Combustion synthesis of ultra-high-temperature solid solutions (ZrxNb1-x)B2. Part 1: The mechanisms of combustion and structure formation

The self-propagating high-temperature synthesis (SHS) in Zr–Nb–B system has been thoroughly investigated with a focus on its macrokinetics and phase formation mechanisms. The activation energies were determined by analyzing the relationship between the combustion rate and temperature. For compositions NbB₂–40%ZrB₂ and NbB₂–50%ZrB₂, the increase in the initial temperature has resulted in a significant change in combustion rate's dependence on temperature, signifying a possible shift in the reaction mechanisms. The quenched combustion front technique (QCF) method in conjunction with thermodynamic and ab initio calculations were used to analyze the sequence of phase formation events, including the formation of niobium and zirconium borides through gas transport reactions involving volatile boron suboxide BO in the heating zone, followed by the emergence of a zirconium-boron melt in the combustion zone and formation of main fraction of niobium and zirconium borides. Interactions between the primary niobium and zirconium borides and the melt result in the formation of solid solutions; however, at sub-optimal combustion conditions multiple non-equilibrium solid solutions are retained in the products. To address this issue, a machine learning model was developed, attaining coefficients of determination (R²) of 89% for combustion temperature and rate predictions, thus enabling the fine-tuning of macrokinetic parameters of the SHS process in the system.     

煤焦二氧化碳气化动力学研究(Ⅱ)非等温热重法

利用热失重仪以恒升温速率（非等温）法研究煤焦-CO2气化反应，并用Ozawa法和单一升温速率法来求解动力学参数。结果表明：Ozawa法和单一升温速率法求解得到的动力学参数差异大；单一升温速率法求得的不同升温速率下表观活化能和指前因子呈线性相关性；表现活化能随升温速率增加呈线性减小。     

THERMALLY ACTIVATED PARAMETERS OF SELF-ADHESION IN ACRYLIC PRESSURE-SENSITIVE ADHESIVE-LIKE NETWORKS

We have conducted an experimental study of acrylic pressure-sensitive adhesive-like networks (PSA-LN) with the goal of establishing deconvolution of interfacial and bulk processes in adhesion. For contact adhesion testing, four types of cylindrically shaped samples were created, each synthetically modified to attain distinct bulk and surface properties. Introduction of small amounts of polar comonomers during synthesis increased the intrinsic adhesion energy, G _o , from 65 mJ/m² for an unmodified acrylic PSA-LN to 129, 158, and 218 mJ/m² for PSA-LNs modified with 10 wt% of acrylic acid, amino acrylate, and acrylonitrile comonomers, respectively. Following a reversed trend, the critical rate of separation, ν* (below which deadhesion is an intrinsically interfacial process) was more than halved from 496 nm/s for the unmodified PSA-LN to 201, 188, and 212 nm/s for those modified with the same three comonomers (same order, respectively). The adhesion energy was found to be dependent upon the crack propagation rate and sensitive to it at all rates of interfacial separation, including those below the critical crack propagation rate, ν*. In addition, the dependence of the adhesion energy on crack propagation rate was measured at three temperatures. It was found that ν* displayed an Arrhenius dependence on temperature from which an activation energy could be calculated. Those activation energies, as a function of chemical composition, are compared with activation energies derived from shift factors determined from measurements of bulk modulus as a function of temperature. A direct correlation between those activation energies was noted.