Hoffman-Lauritzen parameters for non-isothermal crystallization of poly(ethylene terephthalate) and poly(ethylene oxide) melts

Summary By applying an advanced isoconversional method to DSC data one can evaluate a dependence of the effective activation energy (the temperature coefficient of the growth rate) on the relative extent of melt crystallization. The conversion dependence can further be converted into a temperature dependence and parameterized in terms of the Hoffman-Lauritzen equation. For poly(ethylene terephthalate) (PET) we observe a transition from regime I to II. Poly(ethylene oxide) (PEO) crystallization appears to begin in regime II and then undergoes 2 consecutive changes that however cannot be clearly interpreted as regime III. The K_g and _e parameters obtained for regime I and II (PET) and regime II (PEO) are consistent with the respective parameters reported for isothermal crystallization.     

Advanced isoconversional method

A nonlinear algorithm has been suggested to increase the accuracy of evaluating the activation energy by the integral isoconversional method. A minor modification of the algorithm has made it possible to adapt the isoconversional method for an arbitrary variation of the temperature. This advanced isoconversional method allows for trustworthy estimates of the activation energy when the thermal effect of a reaction makes the temperature of a sample deviate from a prescribed heating program.     

Artificial Neural Networks for Pyrolysis,Thermal Analysis,and Thermokinetic Studies: The Status Quo

Artificial neural networks (ANNs) are a method of machine learning (ML) that is now widely used in physics, chemistry, and material science. ANN can learn from data to identify nonlinear trends and give accurate predictions. ML methods, and ANNs in particular, have already demonstrated their worth in solving various chemical engineering problems, but applications in pyrolysis, thermal analysis, and, especially, thermokinetic studies are still in an initiatory stage. The present article gives a critical overview and summary of the available literature on applying ANNs in the field of pyrolysis, thermal analysis, and thermokinetic studies. More than 100 papers from these research areas are surveyed. Some approaches from the broad field of chemical engineering are discussed as the venues for possible transfer to the field of pyrolysis and thermal analysis studies in general. It is stressed that the current thermokinetic applications of ANNs are yet to evolve significantly to reach the capabilities of the existing isoconversional and model-fitting methods.     

Isoconversional Kinetic Analysis of Thermally Stimulated Processes in Polymers

Summary: Isoconversional kinetic analysis involves evaluating a dependence of the effective activation energy on conversion or temperature and using this dependence for making kinetic predictions and for exploring the mechanisms of thermally stimulated processes. The paper discusses major results obtained by the authors in the area of the isoconversional analysis of polymer kinetics over the past decade. It provides a brief introduction to isoconversional methods and surveys the impact made by isoconversional analysis in several application areas that include kinetic predictions, thermal degradation, crosslinking (curing), glass transition, and glass and melt crystallization. It is concluded that isoconversional analysis has been used broadly and fruitfully because it presents a fortunate compromise between the single‐step Arrhenius kinetic treatments and the prevalent occurrence of processes whose kinetics are multi‐step and/or non‐Arrhenius.

An isoconversional method applies the Arrhenius equation to a narrow temperature region, ΔT related to a given extent of conversion.     

Isoconversional Approach to Evaluating the Hoffman–Lauritzen Parameters (U* and Kg) from the Overall Rates of Nonisothermal Crystallization

Summary: An equation has been derived that correlates the temperature coefficient of the growth rate with the temperature dependence of the effective activation energy of the overall crystallization rate, which can be measured by differential scanning calorimetry. The dependence is evaluated by using an advanced isoconversional method and is parameterized in terms of the Hoffman–Lauritzen equation. The parameters obtained for the nonisothermal crystallization of the poly(ethylene terephthalate) melt are consistent with the parameters reported for isothermal crystallization.

The fit of the equation derived here is shown for data corresponding to the dependence of the effective activation energy on average temperature (fit = solid line).     

Synthesis and thermodynamic functions of barium cerate co-doped with erbium and indium

1. Download : Download high-res image (128KB)
2. Download : Download full-size image

     

On the Origin of Heterogeneous Diffusion in Glassy Poly(alkyl methacrylates)

The diffusion of small molecules through amorphous polymers proceeds by the thermally activated jumps whose rate changes from one region to another due to polymer heterogeneity. Little is known at present about the origin of diffusion heterogeneity. The oxidation of triplet nitrene and quenching of phenanthrene phosphorescence have been used to study the movement of molecular oxygen on a nanometer length scale in glassy poly(ethyl methacrylate) and poly(n‐butyl methacrylate) far below the glass transition temperature. It has been found that, as temperature increases, the jump rates in all regions increase by the same factor. This finding points to the equality of the activation energies of oxygen jumps in different regions of the polymers. We conclude that the correlated elastic displacements of polymer chains inside regions about several nanometers in size provide molecules jumps. Due to the large size of these regions, the activation energy of jump does not depend on local polymer structure. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1097–1104     

Heat capacity by differential scanning calorimetry and thermodynamic functions of BaCe0.8Gd0.1Y0.1O2.9 in the temperature range of 166–790 K

Journal of Thermal Analysis and Calorimetry - In the present study, the heat capacities of barium cerate doped by gadolinium and yttrium oxides were measured for the first time in the temperature...     

Some aspects of mathematical statistics as applied to nonisothermal kinetics

Restrictions of the simplest use of correlation and regression analysis to obtain a single-valued solution to the inverse kinetic problem are considered. The Coats-Redfern method is suggested as a version of nonlinear regression analysis to increase the unambiguity of the solution.

---

Zusammenfassung Die Beschränkungen der einfachsten Anwendung der Korrelations- und Regressionsanalyse zur eindeutigen Lösung des inversen kinetischen Problems werden erörtert. Die Coats-Redfern-Methode wird als eine Version der nichtlinearen Regressionsanalyse zur Erhöhung der Eindeutigkeit der Lösung vorgeschlagen.

---

. - .