Physical-chemical characterization of a crystallinity gradient in a carbon black-filled PEEK laminate manufactured by laser-assisted tape placement

Laser-assisted tape placement (LATP) of thermoplastic composites (TPC) enables the rapid production of laminates. However, during layup, higher cooling rates are expected on the bonded interphases between two plies (in the vicinity of the interply), potentially resulting in more amorphous regions, although this has not been experimentally demonstrated in existing literature. Given the impact of crystallinity on the mechanical properties of TPC, an investigation of the crystallinity gradient through the thickness of a laminate manufactured by LATP is of critical importance. However, this requires using a technique able to assess a degree of crystallinity at microscale, which generally involves heavy-duty methods or specific post-processing. Therefore, various techniques have been employed in this study to highlight a through-thickness crystallinity gradient by several ways in a carbon black-filled PEEK (CB/PEEK) laminate. Qualitative techniques, such as laminate's cross-section observation by Polarized Optical Microscopy (POM), and by Scanning Electron Microscopy (SEM) of cryofractured and chemical etched surfaces, revealed the presence of less-crystalline interphases. Additionally, the use of nanoindentation and micro-FTIR (Fourier-Transform Infrared) techniques enabled the measurement of a drop in micromechanical properties and intensity band ratio I₁₃₀₅/I_{1277, 1280} at the interphases revealed by the previous methods. Since these properties are proportional to the degree of crystallinity, nanoindentation and micro-FTIR allow for an indirect evaluation of local crystallinity. Finally, these findings were correlated with Differential Scanning Calorimetry (DSC) analysis on thin in-plane microtome-cut sections. It showed variations of the degree of crystallinity from 31 % to 18 %, which is in accordance with the results from the different techniques employed before.