Atomistic understanding of cyclic heating and quenching of Polyether Ether Ketone (PEEK) via rapid pyrolysis

Polyether Ether Ketone (PEEK) is an engineered thermoplastic widely adopted in additive manufacturing due to its exceptional mechanical and chemical stability. However, PEEK’s robust thermal stability challenges conventional recycling strategies (i.e. pyrolysis, catalysis), leading to inefficient end-of-life management of the material. Despite Flash Joule Heating (FJH) offers a rapid and energy-efficient route to upcycle thermoplastic waste into valuable graphitic carbons, PEEK’s conversion pathway and structural evolution under ultrafast heating remain poorly understood. In this study, reactive molecular dynamics (ReaxFF-MD) simulations combined with FJH experiments were conducted to elucidate the transformation mechanism of pre-carbonised PEEK, with particular emphasis on the role of cyclic quenching. The experimental results had shown the 10-Cycles condition exhibits the lowest I D /I G ratio and a refined graphitic (002) reflection in XRD. The cyclic quenching acted as an effective kinetic control for improving the structural ordering of PEEK-derived carbon relative to the single DC-FJH. The ReaxFF-MD simulations further revealed that PEEK initially undergoes rapid ether cleavage and carbonyl linkages during the heat-up/isothermal stage, producing aromatic radicals, olefinic fragments, and gaseous products, while the graphitic order develops during quenching through edge fusion and ring closure that stabilise sp² networks. Compared to DC-FJH, the 5-Cycles heating-quenching scheme promoted 51% increment in six-membered carbon rings, demonstrating that the carbonisation can be promoted via cyclic heating. Increasing from 5 to 10 cycles further increased the number of rings by approximately 23%, reflecting the progressive defect healing and network reconstruction under repeated quenching.