Spatial control of crystallinity in additively manufactured PAEK: strategies for interlayer and intralayer functional grading

• This paper presents a framework for interlayer and intralayer crystallinity grading in PAEK composite structures. • Vertical interlayer crystallinity varied from 9 % to 49 % via thermal profile modulation. • Discrete horizontal gradients achieved by combining fast and slow PAEK materials. • Matrix-driven stiffness control eliminates the need for external adhesion bonding. This study presents a novel framework for manufacturing polyaryletherketone (PAEK) based functionally graded materials (FGMs) with controlled, stepwise stiffness gradients. While traditional material extrusion (MEX) gradients are often limited to vertical variations due to discrete layer stacking, we propose two strategies to achieve discontinuous interlayer or intralayer gradation using a commercial printer. First, we produced a stepwise interlayer (vertical) stiffness gradient using a single material (slow crystallising PAEK AM200) by actively modulating thermal profile to create distinct sections varying in crystallinity (ranging from 9 % to 49 %). Second, we achieved a discrete intralayer (horizontal) gradient by combining fast crystallising PEEK 450 (t 1/2 = 1.4 s) and slow crystallising PAEK AM200 (t 1/2 = 15 s) within the same layer, resulting in sharp material interfaces. By spatially controlling the crystallinity through both thermal parameters and material selection, this work unlocks new design possibilities for complex, high performance thermoplastic components.