Migration and transformation of the polybutadiene phase in ABS during Fused deposition Modeling: Controlling mechanical anisotropy

• First reported migration of PB phase to interlayers during FDM, a key mechanism exacerbating anisotropy. • Dual content-dependent PB evolution mechanisms: agglomeration at low content vs. cross-linking at high content. • Interlayer strength and toughness show a non-monotonic trend, peaking at an intermediate PB content (20–25 wt%). • Establishes a “process-induced microstructure” paradigm for designing isotropic polymeric materials in additive manufacturing. Acrylonitrile-butadiene-styrene (ABS) is extensively utilized in Fused Deposition Modeling (FDM), yet the persistent mechanical anisotropy of printed parts remains a major obstacle. Moving beyond the conventional focus on processing parameters, this work unveils the decisive role of the intrinsic microstructure, specifically the content-dependent transformation and redistribution of the polybutadiene (PB) phase, in governing the performance of FDM-fabricated ABS. Through a systematic investigation of ABS with varying PB contents (10–30 wt%), we demonstrate that the FDM process actively drives a significant microstructural rearrangement of the PB phase. At low PB contents, shear-induced agglomeration prevails, forming weak interlayer boundaries. Conversely, at high PB contents, thermo-oxidative cross-linking dominates, embrittling the material and severely hindering interlayer diffusion. Most significantly, we report for the first time the preferential migration and segregation of the PB phase towards the interlayer regions, a phenomenon that fundamentally exacerbates anisotropy. As a direct consequence, the interlayer bonding strength and impact toughness exhibit a distinct non-monotonic dependence on PB content, revealing an optimal performance window at intermediate levels (20–25 wt%). This study establishes a new “process-induced microstructure-property” paradigm, providing a scientific foundation and critical guidelines for the rational design of high-performance, isotropic polymeric materials in additive manufacturing.