Plasticization of a Diels–Alder Covalent Adaptable Network for Fused Filament Fabrication

Fused filament fabrication (FFF) is a 3D printing technique used to manufacture thermoplastic objects that offers users nearly unlimited design freedom compared with traditional subtractive and formative methodologies. The most significant barriers prohibiting its more widespread adoption are the reduced mechanical properties compared to injection or compression-molded counterparts and the large degree of mechanical anisotropy due to the layer-by-layer additive process, where strength between layers is developed through the reptation and entanglement of polymer chains. Herein, a highly cross-linked covalent adaptable network (CAN) based on the Diels–Alder reaction between furan and maleimide functionalities is blended with flexible and tough polycaprolactone (PCL) to afford a dynamic semi-interpenetrating polymer network (semi-IPN) that can be spooled for use on an unmodified, commercially available desktop FFF 3D printer. By leveraging the thermoreversible nature of the Diels–Alder adducts, strong covalent bonds can form across the interfaces between deposited tracks. Tensile testing is performed on specimens fabricated using 0 and 90° raster angles, causing stress to be transferred either along the tracks or across the interlayer welds, respectively. The semi-IPN exhibits a degree of mechanical anisotropy of 11% without requiring specialized hardware or postprint processing steps, a marked improvement over about 60% in the typical polylactic acid used in FFF. The addition of the PCL makes the toughness of the blend 100× that of pure CAN, and the 3D-printed objects do not contain void space between tracks, making them fully dense.