Advanced Isoconversional Method Research Articles

Calorimetric studies of crystallization for multi-component glasses of Se–Te–Sn–Ag (STSA) system using model-free and model-fitting non-isothermal methods

Silver-containing chalcogenide glasses are well-known candidates for technical applications as well as for the fundamental studies. In the present report, we have studied the kinetics of glass/crystal phase transformation in novel synthesized glasses of STSA system. For this, purpose, differential scanning calorimetric technique has been employed at four different heating rates (5, 10, 15, 20 K min−1). This paper explores the thermal analysis of calorimetric data using both advanced iso-conversional methods for the determination of effective activation energy as a function of extent of crystallization and classical non-isothermal methods for the determination of over-all crystallization activation energy. Iso-conversional methods, such as Kissinger–Akahira–Sunose method and Flynn–Wall–Ozawa method, have been used to study the variation of activation energy of crystallization and other kinetic parameters with extent of crystallization. Non-isothermal methods (Kissinger method, Augis–Bennett method, and Matusita–Sakka method) have been used to determine over-all activation energy of crystallization and other significant parameter of thermally activated crystallization. We have also explained the composition dependence of various kinetic parameters.

EFFECT OF POLY (N- VINYPYRROLIDONE) ON THE NON-ISOTHERMAL CRYSTALLIZATION KINETICS AND VISCOELASTIC PROPERTIES OF PVDF FILMS

Poly(vinylidene fluoride) (PVDF) and PVDF blends with various molecular weights of poly (N- vinylpyrrolidone) (PVP) films were prepared in dimethyl formamide through the solution casting method. Non-isothermal melt crystallization studies of PVDF films were carried out by cooling the molten samples at different temperatures using differential scanning calorimetry (DSC). The obtained films have been characterized by dynamic mechanical thermal analysis (DMTA). Crystallization kinetics of PVDF films were successfully described by the Jeziorney, Mo and Ziabicki models. The Ozawa equation was found to be invalid for describing the crystallization kinetics. Kinetic parameters such as t1/2, Zc and F(T) indicated that the crystallization rate decreased for PVDF/PVP films as compared to neat PVDF films and was affected by the molecular weight of PVP. The results based on Ziabicki's model revealed that the addition of PVP decreased the ability of PVDF to crystallize under non-isothermal melt crystallization conditions. The activation energy was calculated through Friedman and advanced isoconversional methods. Results showed that the addition of PVP to PVDF films caused an increase in activation energy. By comparing DMTA results of PVDF/PVP blends with neat PVDF films, it could be concluded that blending PVDF with PVP caused an increase in the glass transition temperature (Tg) while the storage modulus was decreased.

Comparative study on the kinetic behavior of neat benzyl thiirane ether and benzyl glycidyl ether reacted with polyether amine

The neat episulfide of benzyl thiirane ether (BTE) resin was synthesized by the reaction of benzyl glycidyl ether (BGE) epoxy resin and KSCN under the microwave atmosphere at low temperature. The ring opening kinetic behaviors of neat BGE and neat BTE with polyether amine (D230) as the hardener were comparatively investigated by non-isothermal differential scanning calorimetry(DSC), and analyzed by the model-fitting Málek method and the model free advanced iso-conversional method of Vyazovkin. The results indicated that both BGE/D230 and BTE/D230 systems fitted Šesták–Berggren model well. The activation energy of neat BTE/D230 system (43.2kJ/mol) was much lower than that of neat BGE/D230 system (58.7kJ/mol). And, the m value in the Šesták–Berggren kinetic model equation for BTE/D230 system (m=0.1902) is much higher than that for BGE/D230 system (m=0.0512), indicating BTE/D230 system has much more autocatalytic effect and/or a secondary reaction initiated by the sulfhydryl groups, which generated from the ring-open reaction of thiirane groups.

A detailed non-isothermal kinetic study of elephant grass pyrolysis from different models

Elephant grass (Pennisetum Purpureum) is a promising source of renewable energy. Thermal degradation of this biomass in an inert atmosphere was studied. Thermogravimetric analysis were carried out at different heating rates (5–50Kmin−1) and temperatures ranged from 473 to 773K. Two model-free (isoconversional) methods, Kissinger (KAS) and Flinn-Wall-Ozawa (FWO) were used to determine the activation energy. Both methods indicated that the activation energy was relatively constant until the conversion 0.6. A non-linear method (Vyazovkin advanced isoconversional method) was used to evaluate the activation energy behavior. From Vyazovkin advanced isoconversional method (VYA), the average activation energy until conversion 0.6 was 185.28±6.87kJmol−1. The activation energies obtained through VYA were utilized to determine the reaction mechanism using master plots parameters (Criado method). The results showed that the main process reaction mechanism could be characterized by two successive reactions, a diffusion followed by a reaction-order model. A model-based method (three-component mechanism) was proposed considering parallel reactions in the entire pyrolysis process. Activation energy for hemicellulose, cellulose and lignin ranged from 46.5 to 65.5kJmol−1, from 108.0 to 127.2kJmol−1 and from 45.6 to 53.5kJmol−1, respectively.

Isoconversional Kinetics of Nonisothermal Crystallization of Salts from Solutions.

In this study, differential scanning calorimetry (DSC) has been applied to measure the kinetics of nonisothermal crystallization of potassium nitrate and ammonium perchlorate from unsaturated and saturated aqueous solutions. DSC data have been analyzed by an advanced isoconversional method that demonstrates that the process is represented by negative values of the effective activation energy, which varies with the progress of crystallization. The classical nucleation model can be used to predict and understand the experimentally observed variation in the effective activation energy. The saturated and unsaturated solutions have demonstrated distinctly different crystallization kinetics. It is suggested that the unsaturated solutions undergo a change in crystallization mechanism from homogeneous to heterogeneous nucleation.

A new diagnostic when determining the activation energy by the advanced isoconversional method

The Advanced Isoconversional (AIC) method involves minimization of a function which uses conversion data at a minimum of three heating rates to determine an activation energy (Ea) at different values of conversion (α). Reactions that can be described by a single model give an Ea which is independent of α, while reactions which are described by more than one model give an Ea which varies with α. Shifts in the conversion curves due to error can lead to Ea vs. α trends which falsely indicate, or mask, true multi-model kinetics. It was found that the minimum value obtained during optimization of what we call the ‘Optimization Indicator’ (Ω) can indicate whether trends in Ea vs. α are likely to be error-derived in the case of single-model reactions or artifacts in the case of multi-model reactions. Ea and Ω for the calcination of CaCO3 are used to demonstrate the experimental application of this new diagnostic.

Reaction pathway and kinetics of CdO nanoparticles prepared from CdCO3 precursor using thermal decomposition method

The non-isothermal kinetics of CdO nanoparticles prepared from CdCO3 precursor using thermal decomposition method was investigated. A model-fitting Malek approach and a model-free advanced isoconversional method of Vyazovkin were applied to the analysis of the DSC and TGA data. The results showed that CdO nanoparticles prepared from CdCO3 followed an autocatalytic reaction. Sestak–Berggren model could favorably describe the studied reaction process. Moreover, the apparent activation energy of CdCO3 decomposition was calculated to be (119.19±9.97) kJ/mol and the explicit rate equation form of CdCO3 decomposition was established.

Thermoanalytical, magnetic and structural investigation of neutral Co(II) complexes with 2,2′-dipyridylamine and salicylaldehydes

Cobalt(II) complexes of 2,2′-dipyridylamine (dpamH) and substituted salicylaldehyde ligands (X-saloH, where X = 5-NO2, CH3, Br and Cl) with the general formula [Co(X-salo)2(dpamH)] (1–4), were synthesized and characterized by physicochemical methods and by spectroscopy (IR and UV–Vis). The octahedral geometry around Co2+ ion and the bidentate chelating mode of the anion X-salo− were proved by single-crystal X-ray diffraction analysis for the complexes [Co(5-CH3-salo)2(dpamH)] (2) and [Co(5-Cl-salo)2(dpamH)] (4). The variable-temperature (76–303 K) magnetic susceptibility measurements showed a paramagnetic nature of the complexes, in accordance with their molecular structure. The simultaneous TG/DTG–DTA technique was used to analyze their thermal behavior under inert atmosphere, with particular attention to determine their thermal degradation pathways, which was found to have a multi-step nature, accompanied by the release of the ligand molecules, as it is partially confirmed by analyzing the amount of gases evolved during heating under experimental conditions comparable with those of TG by using a TG–MS device. Finally, the decomposition kinetics, analyzed by applying the Vyazovkin’s advanced isoconversional method and referred to the elimination of the ligand molecules in the range 200–400 °C (process that was found to be common in all four complexes), confirmed the stability order assessed on the basis of the decomposition temperatures only.

The effect of a renewable fatty acid derivatives based epoxy diluent on the curing kinetics and thermal properties of epoxy/anhydride systems

The diglycidyl ether of dimer diol (DGEDD) was synthesized and the curing kinetics of diglycidyl ether of bisphenol-A/hexahydrophthalic anhydride/tris-(dimethylaminomethyl)phenol (DGEBA/HHPA/DMP-30) system with and without DGEDD as reactive diluent were investigated by non-isothermal DSC technique with Málek method. The dynamic mechanical properties and thermal stabilities of the cured systems with different contents of DGEDD were evaluated using DMTA and TGA, respectively. The results showed that the activation energy calculated by advanced isoconversional method for DGEBA/HHPA/DMP-30 system with DGEDD was slightly higher than that of DGEBA/HHPA/DMP-30 system without DGEDD, and Šesták-Berggren model can simulate well the curing reaction rates of both systems. DMTA showed that the storage moduli of all the cured systems were similar in glassy region and decreased with increasing DGEDD content in rubbery region, and Tg diminished with increasing DGEDD content. TGA showed that loading DGEDD had little impact on the thermal stabilities of the cured systems.

Study on curing kinetics of diglycidyl 1,2-cyclohexane dicarboxylate epoxy/episulfide resin system with hexahydro-4-methylphthalic anhydride as a curing agent

The curing kinetics of the hexahydro-4-methylphthalic anhydride (MHHPA)/diglycidyl 1,2-cyclohexane dicarboxylate (CY184) epoxy resin system and MHHPA/CY184 epoxy/episulfide resin system (containing 2 mass% DMP-30 as an accelerator) was comparatively investigated by non-isothermal differential scanning calorimetry with a model-fitting Malek method and a model-free advanced isoconversional method of Vyazovkin, and the curing behavior was discussed based on the proposed curing mechanism. The results indicated that both of the MHHPA/CY184 epoxy resin system and MHHPA/CY184 epoxy/episulfide resin system fitted Sestak–Berggren model. The activation energy of MHHPA/CY184 epoxy/episulfide resin system was lower than that of MHHPA/CY184 epoxy resin system, suggesting that the episulfide resin has higher reactivity and can accelerate the reaction. The value of m in the kinetic model equation in MHHPA/CY184 epoxy/episulfide resin system is much smaller than that in MHHPA/CY184 epoxy resin system, indicating, unlike the MHHPA/CY184 epoxy resin system, MHHPA/CY184 epoxy/episulfide resin system has much less autocatalytic effect.

On the Thermal Degradation of a Novel Epoxy-Based Nanocomposite Cured With Tryptophan as an Environment-Friendly Curing Agent

The thermal degradation kinetics of diglycidyl ether of bisphenol-A (DGEBA) cured with tryptophan (Trp) in the presence of silica nanoparticles (SiNP) was investigated by thermogravimetry analysis. The activation energies of the solid-state decomposition process were evaluated using the advanced isoconversional method. The dependence of conversion (degradation) on the temperature and activation energy was determined allowing the calculation of master plots. The experimental master plots agreed with the first-order (F1) kinetic function for both neat epoxy and nanocomposite in the conversion range of 0.45–0.85. Using the kinetic model and the calculated kinetic parameters, the times at half conversion (α = 0.5) were computed for different degradation temperatures. The kinetic analysis showed that the degradation rate of the epoxy nanocomposite was lower than that of the neat epoxy for conversions between 0.45 and 0.85. Therefore, we concluded that adding SiNP to DGEBA/Trp can improve the thermal stability in the conversion range of 0.45–0.85.

Influence of carbon nanofillers on the curing kinetics of epoxy-amine resin

Variation of the activation energy with conversion obtained by “advanced isoconversional method”.

Arrhenius parameters determination in non-isothermal conditions for mechanically activated Ag2O–graphite mixture

The non-isothermal kinetics of mechanochemical reduction of Ag2O with graphite was studied by DSC and TGA with a model of fitting Malek approach and a model-free advanced isoconversional method of Vyazovkin. To evaluate the kinetics parameters, Ag2O–graphite mixture of as-received and milled for 2 and 4 h samples were selected. Based on the results obtained by Vyazovkin method calculation, however, the difference between the maximum and minimum values of activation energy is less than 20%–30% of the average activation energy ((99.38±2.36) kJ/mol) and thermal decomposition of mechanically activated Ag2O for 2 h is a multi-step process. Moreover, the thermal decomposition of mechanically activated Ag2O–graphite powder activated for 4 h is a single-step process (the average activation energy=(93.68±2.26) kJ/mol). The kinetics modeling shows that the complexity of thermal decomposition of as-received Ag2O–graphite mixture is higher than that of the others. While, the autocatalytic tendency of as-received Ag2O–graphite mixture is lower than that of the others.

The Effect of Mechanical Activation on Non-isothermal Decomposition Kinetics of Ag2O–Graphite Mixture

The effect of mechanical activation for 0, 2 and 4 h using high-energy planetary ball mill on the thermal decomposition kinetics of Ag2O–graphite powder mixture, i.e., mechanochemical decomposition, is systematically investigated by thermogravimetric analysis and a differential scanning calorimeter (DSC) in terms of a model-free advanced isoconversional method of Vyazovkin. The DSC results showed the mechanochemical decomposition shifts the reaction temperature in the range of 140–436°C (for as-received sample) to 170–335°C and 161–250°C after 2 and 4 h milling, respectively. The results show that decomposition of as-received Ag2O–graphite mixture with the average activation energy of 118.93±3.95 kJ mol−1 consists of two steps. The first step is a complex process with the participation of at least of two mechanisms, and the second step is a single-step process. The thermal decomposition of mechanically activated Ag2O for 2 h is a multi-steps process with the average activation energy of 99.38 ± 2.36 kJ mol−1. Moreover, the thermal decomposition of mechanically activated Ag2O–graphite powder for 4 h is a single-step process with the average activation energy of 93.68± 2.26 kJ mol−1.

Kinetics analysis of non-isothermal decomposition of Ag2O-graphite mixture

The thermal decomposition kinetics of Ag2O-graphite mixture was systematically studied using a differential scanning calorimeter (DSC) in terms of model-fitting Malek approach, integral master plot method and model-free advanced isoconversional method of Vyazovkin. The results showed that the thermal decomposition occurred in two stages. The first step was the decomposition process of Ag2O to form porous silver particles as a complex process with the participation of at least two mechanisms, and the second step corresponded to the structural change from porous particles to silver bulk crystals as a single-step process.

Kinetics Study with Rigorous Nonlinear Methods for Thermal Decomposition of Polysaccharide Iron Complex

The advanced isoconversional and nonlinear model-fitting methods were used to calculate the activation energies of the thermal process of polysaccharide iron complex (PIC). The results of TG/TDG indicate that the processing temperature of the food should not exceed 223 °C when PIC is used as a food additive. The calculated results indicated the decomposition process involved two stages. The region I in stage II is a kinetically complex process, while stage I and region II of stage II are all single-step processes. The most probable mechanisms for stage I and region II of stage II are chemical reaction and contracting sphere. Meanwhile, the most probable mechanism functions for region I of stage II were determined by using nonlinear model-fitting method. The results of nonlinear model-fitting method showed the most probable mechanism of the parallel reactions for region I of stage II were nucleation and branching nuclei.

Curing Characterisation of Spruce Tannin-based Foams using the Advanced Isoconversional Method

The curing kinetics of foam prepared from the tannin of spruce tree bark was investigated using differential scanning calorimetry (DSC) and the advanced isoconversional method. An analysis of the formulations with differing amounts of components (furfuryl alcohol, glycerol, tannin, and a catalyst) showed that curing was delayed with increasing proportions of glycerol or tannins. An optimum amount of the catalyst constituent was also found during the study. The curing of the foam system was accelerated with increasing temperatures. Finally, the advanced isoconversional method, based on the model-free kinetic algorithm developed by Vyazovkin, appeared to be an appropriate model for the characterisation of the curing kinetics of tannin-based foams.

Comparative curing kinetics of 1,4-bis (4-diaminobenzene-1-oxygen) n-butane and 4,4′-bis-(diaminodiphenyl) methane with tetraglycidyl methylene dianiline systems

Aromatic amine curing agent with flexible unit in backbone, 1,4-bis (4-diaminobenzene-1-oxygen) n-butane (DDBE), was synthesized, and the structure was confirmed by FT-IR and 1H NMR. The curing kinetics of tetraglycidyl methylene dianiline (TGDDM, or AG80) using DDBE and 4,4′-bis-(diaminodiphenyl) methane (DDM) as curing agents, respectively, were comparatively studied by non-isothermal DSC with a model-fitting Malek approach and a model-free advanced isoconversional method of Vyazovkin. The dynamic mechanical properties and thermal stabilities of the cured materials were investigated by DMTA and TG, respectively. The results showed that the activation energy of AG80/DDBE system was slightly higher than that of AG80/DDM system. Sestak-Berggren model can generally simulate well the reaction rates of these two systems. DMTA measurements showed that the storage modulus of cured AG80/DDBE is similar to that of cured AG80/DDM at the temperature below glass transition temperature (T g) and lower than that of cured AG80/DDM at the temperature above glass transition temperature, while T g of cured AG80/DDBE is lower than that of cured AG80/DDM. TG showed that the thermal stabilities of these two cured systems are similar.

Kinetic research on low-temperature cure of epoxy adhesive

Low-temperature cure of epoxy adhesives was investigated by means of differential scanning calorimetry analysis (DSC) on both isothermal and dynamic curing process and isothermal and dynamic curing phenomenological autocatalytic models were established. For dynamic curing section, an advanced isoconversional method was taken into account for computing the minimum apparent activation energy Ea value for each value of α lying between 0.05 and 0.95 with a step of 0.01. The correlation of invariant apparent activation energy and pre-exponential factor was expressed by “compensation parameters” equation. The isothermal experimental results showed that curing at low-temperatures of 10–15°C did take place but it was difficult to reach complete reaction over a reasonable experiment time period because the curing process was significantly decelerated owing to the effects of material vitrification and diffusion control in the late curing stages. In order to match the calculated and measured data better and describe the cure in the later stages of reaction, a heating rate-dependent pre-exponential factor and diffusion control were taking into account. The modified modeling with the heating rate-dependent pre-exponential factor and diffusion control agreed well with experimental data. Moreover, analysis of nonlinear regression was carried out on the isothermal modeling, results showed that the nonlinear least squares fitting had a satisfactory effect.

Thermal degradation kinetics study of curcumin with nonlinear methods

The results of TG/DTG when curcumin was used as the food colouring agent indicated that the processing temperature of the food should not exceed 190°C. The decomposition process of curcumin involved two stages. The results of Eα values, determined by an advanced isoconversional method, showed that the two stages were both single-step processes. The most probable mechanisms of the two stages were estimated by using comparative and nonlinear model-fitting methods. The mechanisms obtained from the two methods are the same, which are the assumed random nucleation and its subsequent growth for stage I and one-dimensional diffusion for stage II, respectively. The values of pre-exponential factor A for both stages were obtained on the basis of Ea and g(α). Besides, some thermodynamic functions (ΔS≠, ΔH≠ and ΔG≠) of the transition state complexes for the two stages were also calculated.