Molecular characterization and rheological properties of modified poly(ethylene terephthalate) obtained by reactive extrusion

The use of a tetrafunctional epoxy-based additive to modify the molecular s tructure of poly(ethylene terephthalate) (PET) was investigated with the ai m of producing PET foams by an extrusion process. The molecular structure a nalysis and shear and elongation rheological characterization showed that b ranched PET is obtained for 0.2, 0.3 and 0.4 wt% of a tetrafunctional epoxy additive. Gel permeation chromatography (GPC) analysis led to the conclusi on that a randomly branched structure is obtained, the structure being inde pendent of the modifier concentration. The evolution of shear and extension al behavior as a function of molecular weight (M-w), degree of branching, a nd molecular weight distribution (MWD) were studied, and it is shown that a n increase in the degree of branching and M-w and the broadening of the MWD induce an increase in Newtonian viscosity, relaxation time, flow activatio n energy and transient extensional viscosity, while the shear thinning onse t and the Hencky strain at the fiber break decrease markedly.