Dispersion, crystallization kinetics, and parameters of Hoffman-Lauritzen theory of polypropylene and nanoscale calcium carbonate composite

Kinetics of Crystallization and Thermal Degradation of an Isotactic Polypropylene Matrix Reinforced with Graphene/Glass-Fiber Filler

Polypropylene composites reinforced with a filler mixture of graphene nanoplatelet-glass fiber were prepared by melt mixing, while conventional composites containing graphene nanoplatelet and glass fiber were prepared for comparative reasons. An extensive study of thermally stimulated processes such as crystallization, nucleation, and kinetics was carried out using Differential Scanning Calorimetry and Thermogravimetric Analysis. Moreover, effective activation energy and kinetic parameters of the thermal decomposition process were determined by applying Friedman’s isoconversional differential method and multivariate non-linear regression method. It was found that the graphene nanoplatelets act positively towards the increase in crystallization rate and nucleation phenomena under isothermal conditions due to their large surface area, inherent nucleation activity, and high filler content. Concerning the thermal degradation kinetics of polypropylene graphene nanoplatelets/glass fibers composites, a change in the decomposition mechanism of the matrix was found due to the presence of graphene nanoplatelets. The effect of graphene nanoplatelets dominates that of the glass fibers, leading to an overall improvement in performance.

Isoconversional Kinetics of Polymers: The Decade Past

This article surveys the decade of progress accomplished in the application of isoconversional methods to thermally stimulated processes in polymers. The processes of interest include: crystallization and melting of polymers, gelation of polymer solutions and gel melting, denaturation (unfolding) of proteins, glass transition, polymerization and crosslinking (curing), and thermal and thermo-oxidative degradation. Special attention is paid to the kinetics of polymeric nanomaterials. The article discusses basic principles for understanding the variations in the activation energy and emphasizes the possibility of using models for linking such variations to the parameters of individual kinetic steps. It is stressed that many kinetic effects are not linked to a change in the activation energy alone and may arise from changes in the preexponential factor and reaction model. Also noted is that some isoconversional methods are inapplicable to processes taking place on cooling and cannot be used to study such processes as the melt crystallization.

A time to search: finding the meaning of variable activation energy

This review deals with the phenomenon of variable activation energy frequently observed when studying the kinetics in the liquid or solid phase. This phenomenon commonly manifests itself through nonlinear Arrhenius plots or dependencies of the activation energy on conversion computed by isoconversional methods. Variable activation energy signifies a multi-step process and has a meaning of a collective parameter linked to the activation energies of individual steps. It is demonstrated that by using appropriate models of the processes, the link can be established in algebraic form. This allows one to analyze experimentally observed dependencies of the activation energy in a quantitative fashion and, as a result, to obtain activation energies of individual steps, to evaluate and predict other important parameters of the process, and generally to gain deeper kinetic and mechanistic insights. This review provides multiple examples of such analysis as applied to the processes of crosslinking polymerization, crystallization and melting of polymers, gelation, and solid-solid morphological and glass transitions. The use of appropriate computational techniques is discussed as well.