Letter | Published:

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

Naturevolume 561pages226230 (2018) | Download Citation

Abstract

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 properties5,6,7,8,9,10,11, 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.

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Data availability

The datasets generated and analysed during this study are available from the corresponding authors on reasonable request. Source Data for Figs. 24 and Extended Data Figs. 16 and 8 are provided with the online version of the paper.

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Acknowledgements

We thank G. Ghazaryan and the Kunststoff Ausbildungs- und Technologie-Zentrum for support with filament extrusion, the D-MATL X-ray Platform for access to the diffractometer, and J. Vermant, K. Feldman and N. Bahamonde for discussions. The Swiss Competence Center for Energy Research (SCCER - Capacity Area A3: Minimization of Energy Demand), ETH Foundation grant SP-MaP 01-15, SNSF Project 200021_156011 and consolidator grant BSCGIO_157696 are acknowledged for supporting this research.

Author information

Affiliations

  1. Complex Materials, Department of Materials, ETH Zürich, Zürich, Switzerland

    • Silvan Gantenbein
    • , Kunal Masania
    • , Wilhelm Woigk
    •  & André R. Studart
  2. Soft Materials, Department of Materials, ETH Zürich, Zürich, Switzerland

    • Jens P. W. Sesseg
    •  & Theo A. Tervoort

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Contributions

T.A.T. and A.R.S. conceived the idea together with S.G. and K.M.; S.G., W.W., K.M. and J.S. carried out the experimental work; S.G., K.M., W.W., T.A.T. and A.R.S. carried out the analysis and co-wrote the paper.

Competing interests

The authors have filed patent application EP18179376 relating to this work.

Corresponding authors

Correspondence to Kunal Masania or Theo A. Tervoort or André R. Studart.

Extended data figures and tables

  1. Extended Data Fig. 1 Comparison between the mechanical properties of different FDM feedstock materials.

    a, Young’s modulus as a function of print orientation for LCP, PLA and PEEK. b, Tensile strength of LCP, annealed LCP, PLA and PEEK. The strength of LCP samples is comparable to that of one of the strongest printable polymers (PEEK) at high orientation angles, and to our knowledge surpasses all other tested materials for print orientations below 20°. Error bars indicate the standard deviation for 5–10 measurements at each data point; see Source Data for details. Source data

  2. Extended Data Fig. 2 Values of the Hermans’ orientation factor for the LCPs.

    a, b, Hermans’ orientation factors as a function of nozzle diameter (a) and layer height (b). Thinner samples have an increased skin-to-core material ratio and thus a higher orientation factor. Source data

  3. Extended Data Fig. 3 Effect of annealing time on the ultimate strength of printed LCP filaments and parts.

    a, Strength (σ) as a function of annealing time (t) for single printed filaments. The experimental data suggest that the reaction is initially controlled by the diffusion of water out of the filament (σ t0.5) and later on by the intrinsic reaction kinetics (σ t). b, Tensile strength as a function of annealing time for printed parts. The annealing kinetics follow the same trend as in the case of single filaments. Error bars indicate the standard deviation for 5–10 measurements at each data point; see Source Data for details. Source data

  4. Extended Data Fig. 4 Strength of LCP parts of different thicknesses as a function of print orientation.

    The observed decrease in tensile strength for increasing print orientations follows the decay in elastic modulus predicted from classical laminate theory (Fig. 3a). The lower strength at high orientation angles is also explained by a transition in failure mode from print filament fracture to interface fracture. Error bars indicate the standard deviation for 5–10 measurements at each data point; see Source Data for details. Source data

  5. Extended Data Fig. 5 Effect of print parameters on the Young’s modulus of horizontally printed LCP parts.

    a, Tensile modulus and strength as a function of printed layer height. Lower layer heights lead to higher mechanical properties owing to the higher fraction of aligned skin relative to the material height. b, Tensile modulus and strength as a function of nozzle temperature (TN). For temperatures higher than 300 °C, the tensile properties along the printing direction (0°) follows the behaviour observed for the Young’s modulus of vertically printed filaments (Fig. 2g). The lower values obtained for the Young’s modulus and strength at TN< 300 °C probably result from poorer material flow and poorer print filament adhesion at lower nozzle temperatures. The tensile properties perpendicular to the print direction (90°) increase at higher temperatures, which inidicates an improved adhesion between hot printed filaments. Error bars indicate the standard deviation for 5–10 measurements at each data point; see Source Data for details. Source data

  6. Extended Data Fig. 6 Increased load transfer between filaments after annealing.

    a, Representative curve of shear stress versus strain shows that the shear strength of the LCP increases by 75% after thermal annealing for 96 h owing to the enhanced inter-filament adhesion. The shear strength data were obtained from ±45° tensile samples. b, The Young’s modulus measured along the printing direction is found to slightly increase with longer annealing times. Error bars indicate the standard deviation for 5–10 measurements at each data point; see Source Data for details. Source data

  7. Extended Data Fig. 7 Directional OHT sample.

    a, Print lines are guided around the hole, resembling a wood knot hole. The stress-shielded area next to the hole is reinforced with 90° filaments. b, The extensive damage to the same sample without catastrophic failure during tensile testing illustrates the high fracture toughness of the hierarchically structured architecture.

  8. Extended Data Fig. 8 Recyclability of printed LCP material.

    a, Melt flow index (MFI) of pristine pellets as well as recycled printed parts and recycled annealed printed parts (five measurements per material). The melt flow index quantifies the fluidity of the material at the indicated temperature and load. Higher indices indicate low viscosity. Non-annealed printed samples are readily recyclable owing to their high melt flow index. Although the higher molecular weight of annealed samples markedly reduces their fluidity at regular printing temperatures, this material is potentially recyclable if hydrolysis reactions are used to decrease its molecular weight and thus recover processability. Error bars indicate the standard deviation for 5 measurements at each data point; see Source Data for details. b, c, Examples of pellets (b) and recycled printed samples (c) as feedstock material. Source data

  9. Extended Data Fig. 9

    Geared direct drive extruder with all-metal V6 hotend on an Ultimaker 2+.

  10. Extended Data Table 1 Classical laminate theory parameters used for print filament layer heights between 0.05 mm and 0.20 mm

Source data

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DOI

https://doi.org/10.1038/s41586-018-0474-7

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