Research ArticleMATERIALS SCIENCE

Three-dimensional printing of functionally graded liquid crystal elastomer

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Science Advances  25 Sep 2020:
Vol. 6, no. 39, eabc0034
DOI: 10.1126/sciadv.abc0034
  • Fig. 1 DIW printing of LCE with tailorable thermomechanical properties.

    (A) Schematic illustration of the setup of DIW printing of LCE. The LCE ink is heated up to the temperature T and extruded out of the nozzle with an inner diameter d. The nozzle tip moves at a speed of V during the printing, and the distance between the nozzle tip and build plate is h. Because of the shear stress generated through the extrusion process, liquid crystal mesogens are initially aligned along the printing path. After certain period of time, the extruded LCE filament gradually cools down to room temperature and a core-shell structure forms in the filament. The outer shell of the filament cools down much faster than the inner core. As a result, well-aligned liquid crystal mesogens in the outer shell are temporarily fixed by high viscosity of the material, while the mesogens have enough time to reorientate to a polydomain state in the inner core. (B) Molecular structure of uncrosslinked liquid crystal oligomer in the printing ink. (C) DSC traces of LCE ink and cured LCE. (D) Viscosity of the ink as a function of shear rate at different temperatures. (E) Polarized optical microscope (POM) images of LCE filaments printed at different temperatures. Scale bars, 0.5 mm. Photo credit: Zijun Wang, UCSD.

  • Fig. 2 Processing-property relationship of printed LCE sheets.

    (A) Optical images and (B) maximal actuation strain (εa) of printed LCE sheets obtained with different printing parameters. The printed sheet has a rectangular shape with a size of 30 mm by 10 mm by h (distance between the nozzle and build plate). (C) Actuation stress of a printed sheet as a function of temperature with a fixed strain (0%). The measured sheet has a rectangular shape with sizes of 30 mm by 10 mm by 0.2 mm or 30 mm by 10 mm by 0.8 mm. (D) Uniaxial tensile test results of the LCE sheets printed with different parameters. The measured sheet has a rectangular shape with sizes of 30 mm by 10 mm by 0.2 mm or 30 mm by 10 mm by 0.8 mm. The strain rate is set to be 0.1 min−1 during the mechanical measurements. Photo credit: Zijun Wang, UCSD.

  • Fig. 3 3D-printed active morphing discs with functionally graded LCE.

    (A) to (D) are four circular discs with LCE filaments printed circumferentially. Actuation strain of LCE filaments is homogeneous in (A) and with customized gradient in (B) to (D). The magnitude of actuation strain in each disc is represented by the darkness of the blue color in the sketch. When the discs are immersed in hot water of 90°C, the thermally induced deformation of the four discs is dramatically different from each other. FEA simulations are conducted to simulate the deformed shape. Vertical displacement field is represented by different colors. Scale bar, 20 mm. Photo credit: Zijun Wang, UCSD.

  • Fig. 4 3D-printed LCE bilayer structures with six petals.

    Fluorescent dye RhB is added to the ink for printing these structures. All photos are taken under 365-nm UV illumination. (A) Each petal is composed of two layers of LCE with different printing paths but the same printing parameters. The angle between the two printing paths in two layers is 90°. When the bilayer structure is immersed in hot water of 90°C, all petals twist. (B to D) The printing paths of two layers of petals are the same (along the length direction). In addition, for the bottom layer, the LCE is printed with minimal actuation strain. For the top layer of a petal, the actuation strain is homogeneous in (B) but with customized gradient in (C) and (D). When the bilayer structures are immersed in hot water of 90°C, their bending morphologies are distinct from each other in (B) to (D). FEA simulations are conducted to calculate the deformed shapes of the printed bilayer structures. The stress field is represented by different colors. The gradient printing strategy increases the design space for active morphing structures. Scale bars, 20 mm. Photo credit: Zijun Wang, UCSD.

  • Fig. 5 Active auxetic lattice structures composed of functionally graded LCE.

    In the design (A), the serrated beams have a maximum actuation strain of around 20% (indicated in deep blue), and the vertical beams do not contract (indicated in light blue). (B) The serrated beams have negligible actuation strain, and the vertical beams have a maximum actuation strain of around 20%. (C) Relation between calculated Poisson’s ratio and true strain for the structure in (A). At room temperature (R.T.), the Poisson’s ratio of the structure is negative, which increases with the applied stretch. At high temperature, the Poisson’s ratio of the structure is positive and insensitive to the applied stretch. (D) Relation between measured Poisson’s ratio and true strain for the structure in (B). At room temperature, the Poisson’s ratio of the structure is negative with small stretch and gradually increases to a small positive value with the increase of the stretch. A notable decrease of the Poisson’s ratio occurs when the temperature is increased to 100°C. The rectangular unit (in red) is marked to calculate true strain in loading direction (εx) and in transverse direction (εy). L = 11.55 mm, H = 16.8 mm, t = 1.7 mm, initial θ = 69.44°. Scale bar, 30 mm. Photo credit: Zijun Wang, UCSD.

  • Fig. 6 Demonstration of mitigating stress concentration near the interface between an active LCE tube and a glass plate.

    (A) Homogeneous LCE tube printed on a rigid glass plate. When the tube is heated to 94°C, the tube detaches from the glass plate. (B) LCE tube with gradient properties printed on a rigid glass plate. The tube stays attached to the glass plate when the temperature is increased to 94°C. (C) FEA simulations of the stress field of the tubes at 94°C with homogeneous properties (left) and graded properties (right). The stress field is represented by different colors. Scale bar, 20 mm. Photo credit: Zijun Wang, UCSD.

Supplementary Materials

  • Supplementary Materials

    Three-dimensional printing of functionally graded liquid crystal elastomer

    Zijun Wang, Zhijian Wang, Yue Zheng, Qiguang He, Yang Wang, Shengqiang Cai

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