Rheological characterization of random poly(butylene isophthalate/terephthalate) copolymers

Random poly(butylene isophthalate/terephthalate) (PBI/PBT) copolyesters of various composition and molecular weight were synthesized and characterized in terms of dilute solution viscosity, molecular weight and thermal properties. From the results of measurements carried out with a capillary rheometer at different temperatures (in the range 200-240 degrees C) on molten samples, the flow curves eta-gamma were obtained; these curves permitted to extrapolate the Newtonian viscosity eta(0), which was correlated with molecular weight. At a fixed temperature, samples with the same weight-average molecular weight and different composition showed a Newtonian viscosity increasing as the content of PET comonomeric units is increased. Furthermore, the activation energy for melt flow, calculated using an Arrhenius-type equation, was found to be slightly affected by composition only. (C) 1997 Elsevier Science Ltd.