Three-dimensional printing of biologically inspired composites from liquid crystal polymers

Abstract

Lightweight biological materials such as bone, silk and wood demonstrate complex hierarchically structured shapes with outstanding mechanical properties by utilising directed self-assembly to grow structures. Here1, we demonstrate a bioinspired 3D printing approach to create lightweight structures with hierarchical architectures, complex geometries and unprecedented stiffness and toughness. Using the self-assembly of liquid crystal polymer molecules combined withorienting the molecular domains with the print path, we can reinforce the polymer according to the expected mechanical stresses. The resulting material is recyclable and leads to stiffness, strength and toughness that outperform state-ofthe- art 3D printed polymers by an order of magnitude and rival even high-performance composites. Thermotropic LCPs form nematic liquid crystalline domains when molten. These domains can be aligned in the FDM nozzle along the extrusion direction. After exiting the nozzle, solidification stops the thermal reorientation of the molecules due to the cessation of flow resulting in a skin-core morphology. The maximal Young’s modulus and strength were achieved when printing parallel (0°) to the loading direction (17 GPa stiffness and 400 MPa strength). The dependence of the modulus on the print direction can successfully be predicted using a classical laminate theory approach. It is possible to adapt the local material architecture to the specific loading conditions that are applied to the part by designing the filament deposition direction during 3D printing. We show 3D printed objects with stressadapted print line architecture with mechanical properties that are much stronger than state-of-the-art 3D printed polymers and rival the highest performance lightweight materials using a readily available polymer and a commercial desktop printer. In addition, we present a new approach to combine conventional 3D printing with in-situ spinning of fibres. By controlling the volume of molten polymer at the nozzle tip and rapidly translating the printhead, we are able to spin fibres with diameters down to 20 um, reaching tensile strength and Young’s modulus as high as 2.6 GPa and 208 GPa, respectively. These values are comparable to those of carbon fibres, which require energy-intensive fabrication processes. By spin-printing complex-shaped structures with unusual fibre-reinforcing architectures, we show that this new manufacturing platform opens new opportunities for the design and fabrication of lightweight parts combining recyclability and high mechanical performance.