Increasing the isotropy of 3D printed poly(ether ether ketone) using a combination of bimodal polymer blends and post-process thermal annealing

Tough, semi-crystalline polymers such as poly(ether ether ketone) (PEEK) have recently gained popularity as Material Extrusion (ME) 3D printing feedstocks, but exhibit significant anisotropy when printed using fused filament fabrication (FFF). In this study, we seek to improve interlayer adhesion in PEEK FFF samples by combining the use of bimodal molecular weight polymer blends and post-process thermal annealing. Optimization of the loading of a secondary low molecular weight additive (LMWA) in the blend and identifying effective annealing temperatures leads to increases in the transverse tensile strength of PEEK prints by 250%. We interpret this strength improvement to be a product of tailoring the distribution of polymer crystallinity to induce trans-layer crystallization, which is facilitated by the addition of the LMWA. Thermal and optical analyses of annealed blend samples indicate that crystal structures are homogenously distributed within and between printed layers. The spatial distribution of crystals, and the resulting mechanical properties of the printed samples, are enhanced with incorporation of the LMWA and prescribed post-deposition annealing. The formulation and annealing protocols described herein represent a rational method to control the crystallization in FFF printed samples from crystalline polymers to maximize layer adhesion and improve print isotropy.