Multiple Molecular-Level Processes in the Evolution of Residual Strain in 3D Printed Poly(ether ether ketone)

Abstract

The processing that occurs during material extrusion (ME) 3D printing imparts residual stress in printed parts, which is released during annealing and manifests as an irreversible thermal strain. Our group seeks material-based solutions that control the release of residual stress through molecular-level processes, decrease the magnitude of observed strain, and produce structures that can be annealed without loss of structural fidelity. This work describes an examination of the release of residual stress in 3D printed monoliths made from poly(ether ether ketone) (PEEK) filaments using nanographene sheets as an additive, in situ illumination with an appropriate wavelength light source, and thermal annealing close to the melting temperature. The extent of strain release of the PEEK monoliths is far suppressed relative to that of those made from poly(lactic acid) for all formulations. Moreover, there is a consistent pattern of a transition from positive-to-negative residual strains with annealing time that is independent of the annealing temperature and material formulation. This is interpreted to indicate that multiple competing processes drive the development of residual stress in the ME-printed PEEK samples. Short time relaxation of molecular alignment compels macroscopic expansion, while increased polymer crystallization at longer times forces macroscopic contraction due to densification. These results, therefore, provide crucial insight into molecular-level processes that guide macroscopic structural changes and will inform the development of PEEK annealing protocols to form robust structures by ME without sacrificing structural integrity.