Melt-state degradation mechanism of poly (ether ketone ketone): the role of branching on crystallization and rheological behavior

Abstract

In the melt-state, poly (ether ketone ketone) (PEKK) undergoes structural changes to the polymer backbone due to its high melting temperature. In this work, the thermal degradation mechanism of PEKK in the melt-state in air is identified using a combination of molecular dynamics (MD) simulations, X-ray photoelectron spectroscopy (XPS), and rheology. Ether linkages in the PEKK backbone are shown to have the lowest bond dissociation energies and preferentially undergo chain scission compared to ketone linkages. Upon chain scission, the formed radical species undergo hydrogen abstraction, chain transfer, and a coupling reaction, resulting in altered viscoelastic properties. Dynamic rheological behavior of PEKK thermally aged in the melt-state display viscoelastic characteristics of long-chain branched polymers. Results indicate that the PEKK rheological properties are highly dependent on time in the melt-state as well as angular frequency. Increased exposure times in the melt-state result in increased complex viscosity and a reduction in the degree and rate of crystallization. Thermal history has a significant impact on thermomechanical properties in the melt-state and during crystallization.