Isothermal crystallization of Poly(ether ether ketone)/carbon fiber composites

Abstract

The quiescent crystallization kinetics of Poly(ether ether ketone) (PEEK) carbon fiber composites are highly relevant to polymer processing techniques that operate no shear or low shear conditions, such as 3D printing and automated fiber placement. This study investigates the isothermal crystallization kinetics of neat PEEK and its carbon fiber counterparts. We analyzed one commercial grade with 30 wt% carbon fiber and two lab-compounded grades with lower carbon fiber contents (5 and 15 wt%) using X-ray Micro Computed Tomography (μCT) and calorimetry technologies. μCT analyzed the volume fractions of PEEK resin, carbon fibers, and voids formed during processing. The carbon fiber content was also determined based on the volumetric fraction of each component. Using differential scanning calorimetry (DSC) and fast scanning calorimetry (FSC), the overall crystallization kinetics were extracted for neat PEEK and its carbon fiber composites over a wide range of crystallization temperatures from 160 °C to 330 °C. All kinetics data were fitted well using the Hoffman-Lauritzen model to extract values for U^∗, K₀, and K_G. The results indicate that the energy barriers associated with chain segment mobility U^∗ and nucleation K_G do not significantly change with the presence of carbon fiber. However, K₀, associated with the nucleation constant, decreases linearly with increasing non-resin volume fraction. Morphological investigations using scanning electron microscopy (SEM) and Fast Scanning Calorimetry - Atomic Force Microscopy (FSC-AFM) demonstrate the presence of weak surface nucleation and impingement effects from carbon fiber on PEEK resin crystallization. Based on these observations, we propose a simple mathematical model to describe the crystallization peak time of fiber-reinforced thermoplastic composites, in which fibers and voids primarily contribute to the slowdown of crystal growth.