Observations and Origin of Interfacial Heterogeneities in High-Performance Engineering Thermoplastics During Additive Manufacturing via Synchrotron X-ray Diffraction

Heterogeneity in extrusion-based 3D-printed thermoplastic polymers is generally both challenging to control and harmful to mechanical performance. Abrupt changes in material properties at the interfaces between roads are of particular interest due to the prevalence of poor inter-road adhesion. To gain insight into the origins of heterogeneities in 3D-printed parts, we report a combination of synchrotron-based in situ X-ray microdiffraction and infrared pyrometry to map the formation and evolution of crystallinity in polyether ether ketone (PEEK) in a series of single-road-width, two-road-tall prints. In all cases, the in situ maps show that crystallinity in the second road takes longer to form but reaches a higher degree of order compared to that of single-road prints. Both trends are attributed to the observation from IR pyrometry that the first road cools faster than the second, which is itself attributed to the higher thermal conductivity of the (copper) print bed compared to that of the first road. Beyond these differences, detailed analysis of crystallization kinetics suggests that crystallization in the first and second roads occurs in distinct kinetic regimes. Most remarkably, we find that the first road undergoes significant cold crystallization as the second road is deposited on top of it, leading to a sharp and persistent crystallinity gradient between the first and second roads. We show that this observation pertains not only to the two-road prints examined here but also to variations in crystallinity observed previously in multi-layered, multi-road prints. Specifically, cold crystallization induced by newly deposited roads in adjacent, already-deposited roads appears to be a general phenomenon and may contribute to poor inter-road and interfacial adhesion.