A COMPARATIVE-STUDY OF RHEOLOGICAL EVOLUTION AND CRYSTALLIZATION KINETICS OF POLY(ETHYLENE-TEREPHTHALATE)

The rheological behaviour of poly(ethylene terephthalate) (PET) is stu died anisothermally at constant cooling rates (1 to 5-degrees-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-degrees -C under vacuum). The modulus eta of the complex dynamic viscosity an d 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 eta. This fact can be explained by the higher sensitivity of G' to the increasing num ber of entanglements in the early nucleation process. As the starting temperatures for eta 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 mean s of an experimental design. A second-order relation is then establish ed and gives extrapolated starting temperature values for eta at low (or zero) shearing rates and frequencies in accordance with the d.s.c. values. Linear relationships between the starting temperatures of eta , G' and d.s.c. measurements, respectively, and the square root of th e cooling rate are finally observed. As they show quite similar limiti ng temperature at zero cooling rates, it is assumed that a change of t he crystallization or the nucleation process occurs at this point.