Rheological study of the reaction kinetics in a poly(ethylene terephthalate) melt

Time dependent measurements of the zero shear viscosity are applied to study the thermal stability of a commercial poly(ethylene terephthalate). Firstly, the factors influencing the experimental results, like moisture content and type of atmosphere (air or nitrogen) are briefly reviewed. Secondly, the time dependent viscosity is studied on dry samples in nitrogen atmosphere at various temperatures, as these conditions are commonly used for the rheological material characterization. All viscosity curves show a characteristic maximum which strongly depends on time and temperature. This maximum reflects the changes in molar mass resulting from the various chemical reactions. A simplified kinetic model, based on one build-up and one degradation reaction, is derived and used to analyze the experimental data. This model enables to determine the activation energy of flow (65 kJ mol−1) and the activation energy resulting from the temperature dependence of the molar mass maximum (120 kJ mol−1). Kinetic constants and corresponding activation energies of the chemical reactions are determined as well. The comparison with literature shows that the activation energies are similar to those found for the polymerization process.