Polymer-Grafted WS₂ Nanocomposites: from Edge-Site Passivation to Melt-Stable Fused Granular Fabrication

Layered transition metal dichalcogenides (TMDs) are widely regarded as chemically inert nanofillers in polymer composites. Here, we demonstrate that this assumption fails under melt-processing conditions. We show that pristine WS₂ nanopowders act as heterogeneous catalysts for polyester chain scission during melt extrusion, inducing a catastrophic, 10-fold reduction in molecular weight, severe loss of melt viscosity, printing failure, and brittle mechanical behavior at filler loadings as low as 0.2 wt %. To suppress this unexpected catalytic activity, we develop an edge- and defect-selective functionalization strategy for WS₂ based on covalent carboxylation and hydroxylation, followed by surface-initiated ring-opening polymerization of ε-caprolactone. Spectroscopic and microscopic analyses (XPS, XRD, HAADF-STEM, and EELS) demonstrate that polymer grafting is confined to edge and defect sites, while preserving the multilayer 2H-WS₂ lattice. When incorporated into a polycaprolactone (PCL) matrix and processed by large-format fused granular fabrication, polymer-grafted WS₂ nanostructures exhibit stable melt rheology, excellent printability, and substantial mechanical reinforcement, with Young's modulus and tensile strength increases up to 45% and 65%, respectively, without loss of ductility. Crucially, polymer grafting effectively passivates catalytically active WS₂ edge and defect sites, preventing melt-induced polymer degradation. These findings provide direct experimental evidence that exposed edge sites in layered nanomaterials can actively catalyze polymer degradation under melt-processing conditions and establish edge-site passivation as a general design principle to mitigate chemically driven polymer degradation in polyester-based systems during melt processing, with the magnitude of the effect depending on the chemical susceptibility of the host polymer.