Enhancement of Polyethylene Terephthalate Rheological Properties via Chain Extension and Branching: A Design of Experiment Approach

Upgrading recycled polyethylene terephthalate (PET) for high-value applications requires enhanced viscoelastic properties that are difficult to achieve with single chain extenders. This study investigates the synergistic effect of pyromellitic dianhydride (PMDA, tetrafunctional) and 1,3-phenylene-bis-2-oxazoline (PBO, bifunctional) on virgin and recycled PET using the Design of Experiment methodology. Real-time rheological monitoring, combined with a heuristic kinetic analysis, enabled the identification of the reaction mechanisms and their evolution over time. Subsequently, the linear viscoelasticity analysis of the reacted samples revealed that the dual-additive system led to an increase of two orders of magnitude in zero-shear viscosity (compared to one order for PMDA alone) and three orders of magnitude in relaxation times, reaching approximately 20 s. Critical differences between virgin and recycled PET responses are identified, with recycled material showing enhanced properties at high additive concentrations due to higher carboxyl end-group content. These findings provide a systematic approach for upgrading postconsumer PET to meet industrial performance requirements for foaming and blow molding applications.