Three-dimensional printing of hierarchical liquid-crystal-polymer structures.

Fibre-reinforced polymer structures are often used when stiff lightweight materials are required, such as in aircraft, vehicles and biomedical implants. Despite their very high stiffness and strength1, such lightweight materials require energy- and labour-intensive fabrication processes2, exhibit typically brittle fracture and are difficult to shape and recycle3,4. This is in stark contrast to lightweight biological materials such as bone, silk and wood, which form by directed self-assembly into complex, hierarchically structured shapes with outstanding mechanical properties5-11, and are circularly integrated into the environment. Here we demonstrate a three-dimensional (3D) printing approach to generate recyclable lightweight structures with hierarchical architectures, complex geometries and unprecedented stiffness and toughness. Their features arise from the self-assembly of liquid-crystal polymer molecules into highly oriented domains during extrusion of the molten feedstock material. By orienting the molecular domains with the print path, we are able to reinforce the polymer structure according to the expected mechanical stresses, leading to stiffness, strength and toughness that outperform state-of-the-art 3D-printed polymers by an order of magnitude and are comparable with the highest-performance lightweight composites1,12. The ability to combine the top-down shaping freedom of 3D printing with bottom-up molecular control over polymer orientation opens up the possibility to freely design and realize structures without the typical restrictions of current manufacturing processes.

Control of Particle Properties in Thermally-Induced Precipitation of Polyetherimide.

The feasibility of thermally-induced phase separation and crystallization for the production of semi-crystalline polyetherimide (PEI) microparticles from an amorphous feedstock has been reported recently. Here, we investigate process parameter dependencies for designing and control of particle properties. A stirred autoclave was used to extend the process controllability, as the applied process parameters, e.g., stirring speed and cooling rate, were adjusted. By increasing the stirring speed, the particle size distribution was shifted to larger values (correlation factor ρ = 0.77). Although, the enhanced droplet breakup, induced by the higher stirring speed, led to the formation of smaller particles (ρ = -0.68), broadening the particle size distribution. The cooling rate showed a significant influence on the melting temperature, reducing it with a correlation factor of ρ = -0.77, as confirmed by differential scanning calorimetry. Lower cooling rates led to larger crystalline structures and enhanced the degree of crystallinity. The polymer concentration mainly affected the resulting enthalpy of fusion, as an increased polymer fraction enhanced the latter (correlation factor ρ = 0.96). In addition, the circularity of the particles was positively correlated to the polymer fraction (ρ = 0.88). The structure assessed via X-ray diffraction, was not affected.