Polymerization Kinetics of Cyanate Ester Confined to Hydrophilic Nanopores of Silica Colloidal Crystals with Different Surface-Grafted Groups.

This study investigates the kinetics of confined polymerization of bisphenol E cyanate ester in the nanopores of the three types of silica colloidal crystals that differ in the concentration and acidity of the surface-grafted proton-donor groups. In all three types of pores, the polymerization has released less heat and demonstrated a very similar significant acceleration as compared to the bulk process. Isoconversional kinetic analysis of the differential scanning calorimetry measurements has revealed that the confinement causes not only a dramatic change in the Arrhenius parameters, but also in the reaction model of the polymerization process. The obtained results have been explained by the active role of the silica surface that can adsorb the residual phenols and immobilize intermediate iminocarbonate products by reaction of the monomer molecules with the surface silanols. The observed acceleration has been quantified by introducing a new isoconversional-isothermal acceleration factor Zα,T that affords comparing the process rates at respectively identical conversions and temperatures. In accord with this factor, the confined polymerization is 15-30 times faster than that in bulk.

All You Need to Know about the Kinetics of Thermally Stimulated Reactions Occurring on Cooling.

In this tutorial overview article the authors share their original experience in studying the kinetics of thermally stimulated reactions under the conditions of continuous cooling. It is stressed that the kinetics measured on heating is similar to that measured on cooling only for single-step reactions. For multi-step reactions the respective kinetics can differ dramatically. The application of an isoconversional method to thermogravimetry (TGA) or differential scanning calorimetry (DSC) data allows one to recognize multi-step kinetics in the form of the activation energy that varies with conversion. Authors' argument is supported by theoretical considerations as well as by experimental examples that include the reactions of thermal decomposition and crosslinking polymerization (curing). The observed differences in the kinetics measured on heating and cooling ultimately manifest themselves in the Arrhenius plots of the opposite curvatures, which means that the heating kinetics cannot be used to predict the kinetics on cooling. The article provides important background knowledge necessary for conducting successful kinetic studies on cooling. It includes a practical advice on optimizing the parameters of cooling experiments as well as on proper usage of kinetic methods for analysis of obtained data.

Thermodynamic behavior and polymorphism of 1-butyl-3-methylimidazolium hexafluorophosphate composites with multiwalled carbon nanotubes.

Based on room-temperature densities measured in this research for ionic nanofluids (INFs) with four ionic liquids (ILs), we conclude that evacuation is a necessary step to maximize the IL penetration into multiwalled carbon nanotubes (MWCNT). An improved procedure for reproducible preparation of INFs is proposed. Thermal behavior of five (1-butyl-3-methylimidazolium hexafluorophosphate + MWCNT) samples was studied by adiabatic calorimetry over the temperature range (78 to 370) K. The samples contained from 0.11 to 0.92 mass fraction of the nanophase. Their appearance changed from the fluid to the powder with increasing the MWCNT content. For the fluid samples, the specific heat capacity was found be an additive quantity of the specific heat capacities of the components for the crystal and liquid phases, and the temperatures of phase transitions did not change relative to the bulk values. For the powder-like sample with the highest IL content, a sigmoidal heat capacity curve was observed. Thus, the internal diameter of the studied MWCNT was small enough to switch from the isothermal melting process to the gradual transition from the crystal-like structures to the liquid-like ones.

Polyvinylpyrrolidone affects thermal stability of drugs in solid dispersions.

The present study explores the hypothesis that a polymer can affect the thermal stability of a drug in solid polymer-drug dispersions. The hypothesis is tested in a systematic fashion by combining isoconversional kinetic analysis with thermogravimetric measurements on several solid dispersions. Experimental systems involve three drugs: indomethacin (IMC), felodipine (FD), and nifedipine (ND) and their solid dispersions with polyvinylpyrrolidone (PVP). It is found that PVP stabilizes IMC but destabilizes FD and ND. Isoconversional kinetic analysis provides insights into the origin of the observed effects. The enhanced thermal stability of IMC in the PVP matrix is associated with an increase in the activation energy of the respective degradation process. A detrimental effect of the PVP matrix on the stability of FD and ND has been linked to a decrease in the activation energy and an increase in the preexponential factor, respectively. The molecular underpinnings of the observed effects are discussed. It is concluded that the effects in question are of relevance for drug performance and need to be taken into account in preformulation studies.

Kinetics of Thermal Polymerization Can Be Studied during Continuous Cooling.

It is demonstrated that differential scanning calorimetry can measure the kinetics of the thermally initiated polymerization during continuous cooling. The measurements are accomplished by switching from fast heating to much slower cooling. The study is exemplified by crosslinking polymerization (curing) of diglycidyl ether of bisphenol A epoxy and m-phenylenediamine taken in stoichiometric and nonstoichiometric ratios and measured under heating and cooling conditions. An advanced isoconversional method reveals that the reaction in the nonstoichiometric system follows the kinetics of the single-step type. Its activation energy is constant and the same for heating and cooling conditions. The stoichiometric system exhibits the multistep kinetics characterized by the dependencies of the activation energy on temperature that differ qualitatively for cooling and heating runs. The discovered differences emphasize the need for further systematic studies of the thermal polymerization during continuous cooling.

Melting kinetics of superheated crystals of glucose and fructose.

Glucose and fructose crystals undergo significant superheating during melting that allows one to study the kinetics of this process. Melting of both compounds has been studied by differential scanning calorimetry (DSC). The obtained data have been subjected to isoconversional kinetic analysis. The process has been determined to have unusually large values of the activation energy and preexponential factor that indicate that melting occurs by cooperatively breaking multiple hydrogen bonds. The experimentally determined activation energy of melting demonstrates a decrease with increasing temperature. The use of the nucleation and growth models has permitted deriving theoretical dependencies of the activation energy on temperature. Testing the theoretical dependencies against the experimental ones suggests that from either the statistical or physical viewpoint the melting kinetics should be parameterized by means of the growth model. This suggests that the mechanism of melting involves the growth of the stable melt nuclei that exist on the crystal surface below the equilibrium melting temperature.

Further insights into the kinetics of thermal decomposition during continuous cooling.

Following the previous work (Phys. Chem. Chem. Phys., 2016, 18, 32021), this study continues to investigate the intriguing phenomenon of thermal decomposition during continuous cooling. The phenomenon can be detected and its kinetics can be measured by means of thermogravimetric analysis (TGA). The kinetics of the thermal decomposition of ammonium nitrate (NH4NO3), nickel oxalate (NiC2O4), and lithium sulfate monohydrate (Li2SO4·H2O) have been measured upon heating and cooling and analyzed by means of the isoconversional methodology. The results have confirmed the hypothesis that the respective kinetics should be similar for single-step processes (NH4NO3 decomposition) but different for multi-step ones (NiC2O4 decomposition and Li2SO4·H2O dehydration). It has been discovered that the differences in the kinetics can be either quantitative or qualitative. Physical insights into the nature of the differences have been proposed.

Discovering the kinetics of thermal decomposition during continuous cooling.

The research presented in this paper is devoted to the intriguing phenomenon of thermal decomposition that takes place during continuous cooling after being initiated by heating to higher temperature. This paper describes the principles of detecting this phenomenon and measuring its kinetics. As one of the possible ways, the process can be detected and its kinetics can be measured by means of differential scanning calorimetry provided that cooling is performed several hundred times slower than heating. By way of illustration, the thermal degradation of isotactic polystyrene and thermal dehydration of lithium sulfate monohydrate have been studied upon cooling and heating. The kinetics of both processes have been analyzed by means of the isoconversional methodology. For polystyrene, the kinetics of degradation upon cooling and heating have been similar. The thermal dehydration of lithium sulfate monohydrate has revealed that cooling kinetics differ significantly from the kinetics measured upon heating. It is proposed that such differences should be observed in multi-step processes whose activation energy varies with reaction progress.