Chain branching detection by Cole–Cole modeling of rheological properties changes during PET mechanical recycling

The detection of branched chains in thermally degraded thermoplastic polymers is far from simple, especially at a low conversion ratio, mainly because of the low sensitivity of commonly used laboratory analytical techniques. The objective of this article is to present an approach able to demonstrate the formation of such macromolecular structures during thermal degradation of molten PET at low oxygen partial pressures (typically for pressures lower than 9% of atmospheric pressure). Such ageing conditions are met during PET mechanical recycling by extrusion, and can be simply reproduced by sequential nitrogen/air tests in a rheometer cavity. The approach consists of a careful analysis of the complex shear modulus changes in a Cole–Cole diagram. Values obtained for Cole–Cole model parameter K′ indicate clearly an accumulation of branched chains in the melt with the time of residence in the reactor.