A comparative study of rheological evolution and crystallization kinetics of poly(ethylene terephthalate)

Abstract

The rheological behaviour of poly(ethylene terephthalate) (PET) is studied anisothermally at constant cooling rates (1 to 5°C min −1) during crystallization from the melt. Drying conditions are first set up to keep the properties of the polymer constant (6 h at 150°C under vacuum). The modulus η∗ of the complex dynamic viscosity and the elastic component G′ of the complex elastic modulus are studied as functions of temperature in the linear domain and show a sigmoidal shape during the crystallization process. The starting temperature of each phenomenon is dependent on the cooling rate, and the values for G′ are always higher than the corresponding values for η∗. This fact can be explained by the higher sensitivity of G′ to the increasing number of entanglements in the early nucleation process. As the starting temperatures for η∗ are close to the values observed by differential scanning calorimetry (d.s.c.), an exhaustive study of this parameter is performed under various shearing conditions and interpreted by means of an experimental design. A second-order relation is then established and gives extrapolated starting temperature values for η∗ at low (or zero) shearing rates and frequencies in accordance with the d.s.c. values. Linear relationships between the starting temperatures of η∗, G′ and d.s.c. measurements, respectively, and the square root of the cooling rate are finally observed. As they show quite similar limiting temperature at zero cooling rates, it is assumed that a change of the crystallization or the nucleation process occurs at this point.