Imbibition and rheology of polymer-matrix nanoporous metal composites: Towards extrusion-based 3D printing

When combined with additive manufacturing, nanoporous metals can be shaped into multifunctional and interdigitated 3D circuits with hierarchical diffusional pathways, critical for optimizing electrochemical storage devices. In an attempt to develop a method for extrusion-based additive manufacturing of nanoporous metals and their composites, this paper examines a new polymer matrix composite reinforced with spherical nanoporous copper powders (PMC-NP-Cu) synthesized by dealloying at high fill fractions (>45 vol%), that can be integrated with fused filament fabrication (FFF). The role of capillary imbibition of the polymer into the metal's nanopores was investigated by examining their wettability, rheology, and density as a function of polymer's molecular weight (MW) particularly for a range of radius of gyration (R_g) between 2 and 20 nm comparable to its average pore size (22 nm). It was found that the melt viscosity of PMC-NP-Cu with the shortest M_w departed from classical viscosity scaling (i.e., η ∞ M_W¹) which is attributed to the polymer imbibition into nanopores and composite densification. For molecular weights equal to 5 and 200 kg/mol, it was found that the volume fraction of the reinforcing phase was 16% and 8% higher respectively when imbibition was accounted for, resulting in an increase of two orders of magnitude in its zero-shear viscosity. These results correlated to the extrudability tests of PMC-NP-Cu filaments at higher packing factors (i.e., 47 vol%) for FFF. The insight of this study might be beneficial to integrate nanoporous metals into binder-based 3D printing technology to fabricate interdigitated battery electrodes and multifunctional 3D printed electronics.