Welding Adjacent Layers in Additively Manufactured Polypropylene via Expansion Annealing

Material extrusion additive manufacturing (MEX-AM) provides a promising alternative to traditional manufacturing methods, as it is accessible, cost-effective, and allows for rapid generation of complex structures. However, a critical limitation of MEX-AM is poor interlaminar adhesion between successive layers, which leads to highly anisotropic mechanical properties and weakness in the build (out-of-plane) direction. Particularly for semicrystalline materials, crystallization can occur rapidly upon material deposition, further restricting the successful welding of subsequent filament traces. In this study, we demonstrate a post-print annealing strategy that directly addresses the weak interlayer properties of semicrystalline 3D-printed polymers. Our approach uses a solvent swelling treatment that selectively penetrates and expands the amorphous domains of printed specimens while preserving their semicrystalline structure. This process promotes enhanced chain entanglement, tie-chain formation, and cocrystallization across interlaminar interfaces, leading to significantly improved mechanical performance along the build direction. In a model system of semicrystalline polypropylene (PP), tensile specimens printed in the build direction exhibit an ∼605% increase in elongation at break and an ∼596% increase in toughness upon expansion annealing. This work establishes a promising, generalizable platform for strengthening 3D printed semicrystalline polymers and advancing the performance of MEX-AM parts.