Characterization of thermally annealed PEEK and CFR-PEEK composites: Structure-properties relationships

The structure-properties correlation of neat and carbon fiber reinforced (CFR) PEEK, subjected to thermal annealing, was investigated. The polymer and its composites (30% w/w carbon fibers, PAN or pitch based) underwent annealing treatments at 200, 225, 250, 275 and 300°C. The resulting morphology was investigated by Differential Scanning Calorimetry (DSC), Fourier Transform InfraRed (FTIR) spectroscopy, Wide Angle X-ray Diffraction (WAXD) and Small Angle X-ray Scattering (SAXS), while the flexural peak load from a bending test and the reduced elastic modulus from nanoindentation measurements were chosen as markers of the mechanical properties. The morphological investigation showed that annealing induces an increase in crystallinity, by a combination of thickening of the existing lamellae and nucleation and growth of new, thinner lamellae. Although the mechanical properties showed highly significant differences depending upon the reinforcement type, we were able to induce a significant increase (form 9 to 12%) in the flexural peak load with the annealing treatments, regardless of the material formulation. The reduced elastic modulus of all PEEK and CFR PEEK formulations also increased with increasing the annealing temperature. In summary, this investigation provides an evidence of how appropriate thermal treatments can be used to tune the mechanical properties of both PEEK and CFR PEEK.