Crystallization kinetics of a high-performance upcycled PEEK/PEKK/carbon fiber hybrid composite

The upcycling of aerospace composite waste through melt blending presents a promising pathway for circular polymer sustainability. However, effectively reusing these materials requires a fundamental mechanistic understanding of their phase transitions. This study examines the crystallization kinetics of an upcycled feedstock prepared by blending recycled carbon-fiber-reinforced PEKK into a virgin PEEK matrix. Isothermal calorimetry reveals a confinement-induced, two-step solidification mechanism: rapid primary spherulitic growth followed by spatially restricted secondary inter-lamellar infilling, which is well described by the sequential Hillier model. Under non-isothermal conditions, a practical processing limit appears at cooling rates (Formula presented), where the system bypasses the thermodynamic window for secondary crystallization due to matrix rigidification. Model-free isoconversional analysis maps the thermodynamic barriers, demonstrating a marked shift to strongly negative activation energies at high conversions ((Formula presented) ). This signifies a mechanistic transition to Hoffman and Lauritzen Regime III surface-nucleation-dominated crystallization at high supercoolings. These findings explain the structural phase distribution in PEEK/PEKK blends and define the processing windows necessary for manufacturing high-performance components from recycled aerospace composite.