Research ArticleCONDENSED MATTER PHYSICS

Liquid crystal elastomer shell actuators with negative order parameter

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Science Advances  12 Apr 2019:
Vol. 5, no. 4, eaaw2476
DOI: 10.1126/sciadv.aaw2476
  • Fig. 1 Schematic representation of LCs with varying order parameter.

    (A) Organization of mesogens (represented by ellipsoids) for order parameter varying from 〈P2〉 = −1/2 to 〈P2〉 = +1. In the isotropic phase, n is still required as a reference direction for calculating 〈P2〉, but its orientation is arbitrary. (B) Comparison of the conventional way of making LCEs with uniaxial stretching (top), leading to 〈P2〉 > 0 and birefringence Δn > 0 (assuming rod-shaped mesogens), and our in-plane isotropic stretching (bottom), yielding LCE shells with 〈P2〉 < 0 and Δn < 0.

  • Fig. 2 POM images of radially aligned LCE shells with positive (A to C) and negative (D to F) order parameter.

    The top row shows a shell with Δn > 0 (and 〈P2〉 > 0) produced with only dithiols in the precursor mixture, while the bottom row shows a shell with Δn < 0 (and 〈P2〉 < 0) produced with tetra- and dithiols in the precursor mixture. The shells are observed (A and D) in transmission without analyzer, (B and E) between crossed polarizers (indicated by P and A), and (C and F) between crossed polarizers with a λ plate inserted (its optic axis is indicated with a white line). In C, the upper left and lower right corners shift to orange, whereas the upper right and lower left corners shift to blue, confirming Δn > 0. In F this color contrast is reversed: the upper left and lower right corners shift to blue, whereas the upper right and lower left corners shift to orange. This confirms that the shell in D to F has Δn < 0.

  • Fig. 3 Simulation and transmission POM investigation of thermal response of 〈P2〉 < 0 LCE shell cross-linked at 60°C.

    The shell is spherical only at high temperature where it has been cross-linked, whereas it wrinkles and buckles upon cooling, to expand the surface area as 〈P2〉 gets increasingly negative. Upon heating, the shell contracts in area and expands in thickness, as expected for 〈P2〉 < 0 approaching 〈P2〉 = 0. (α1 to α4) Simulation images show stress-induced buckled regions when the inner volume is slightly offset from the center. (a to h) POM images without analyzer, (A to H) between crossed polarizers, and (i to viii) between crossed polarizers with λ plate inserted (optic axis indicated by white line).

  • Fig. 4 Schematics of 〈P2〉 < 0 shell fragments and POM investigation of thermal response.

    (A) The shell fragment is close to a spherical cap shape in the ground state, gradually transforming into an ellipsoid upon heating. (B) A self-closing ribbon cut near the equator folds and twists upon heating. (C) A nonclosing spiral-shaped ribbon fragment shows strong curling and twisting when heated. In all three cases, Δn decreases strongly on heating, approaching zero, in correlation with the shape morphing. A λ plate is inserted during all experiments (optic axis indicated by white line). Movie S5 shows the full actuation cycle for each fragment.

  • Fig. 5 Actuation of macroscopic 〈P2〉 < 0 disk.

    (A) The cylindrical disk, which was compressed along its symmetry axis between the two polymerization stages, is measured at room temperature before actuation. (B) Above the clearing point, at 130°C, the disk is markedly thicker, while it has contracted in the disk plane, opposite to the behavior of a 〈P2〉 > 0 LCE.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/4/eaaw2476/DC1

    Fig. S1. LCE precursor molecules and reaction mechanism for conventional 〈P2〉 > 0 LCEs.

    Fig. S2. Schematic representation and micrograph of actual shell production.

    Fig. S3. Schematic representation of actuation in radial shells of LCE with negative and positive order parameters.

    Fig. S4. POM investigation of thermal response of pristine 〈P2〉 < 0 LCE shell with radial director.

    Fig. S5. POM investigation of shell actuation.

    Fig. S6. Fluorescence confocal microscopy images of a shell with an opening.

    Fig. S7. Transmission POM investigation of thermal response of 〈P2〉 < 0 LCE shell fragment.

    Fig. S8. Fluorescence confocal microscopy images of a shell fragment.

    Fig. S9. LCE shells were modeled using ABAQUS finite element software.

    Fig. S10. LCE shell fragment in the shape of twisted ribbon.

    Fig. S11. Macroscopic disk with negative mesogen order parameter.

    Movie S1. The video shows thermal actuation cycles (25° to 75°C) of a pristine shell UV cross-linked at 35°C, recorded with a first-order waveplate, crossed polarizers, and one polarizer in the light path, respectively.

    Movie S2. Thermal actuation of a shell UV cross-linked at 35°C with a hole at the top, immersed in glycerol.

    Movie S3. The video shows actuation of a fragment cut from a shell UV cross-linked at 35°C.

    Movie S4. Thermal actuation of a shell UV cross-linked at 60°C.

    Movie S5. Complex modes of actuation observed by cutting a shell UV cross-linked at 35°C into different topological objects such as a cap shape, a self-closed ribbon, and a long spiral stripe shape.

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. LCE precursor molecules and reaction mechanism for conventional 〈P2〉 > 0 LCEs.
    • Fig. S2. Schematic representation and micrograph of actual shell production.
    • Fig. S3. Schematic representation of actuation in radial shells of LCE with negative and positive order parameters.
    • Fig. S4. POM investigation of thermal response of pristine 〈P2〉 < 0 LCE shell with radial director.
    • Fig. S5. POM investigation of shell actuation.
    • Fig. S6. Fluorescence confocal microscopy images of a shell with an opening.
    • Fig. S7. Transmission POM investigation of thermal response of 〈P2〉 < 0 LCE shell fragment.
    • Fig. S8. Fluorescence confocal microscopy images of a shell fragment.
    • Fig. S9. LCE shells were modeled using ABAQUS finite element software.
    • Fig. S10. LCE shell fragment in the shape of twisted ribbon.
    • Fig. S11. Macroscopic disk with negative mesogen order parameter.
    • Legends for Movies S1 to S5

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). The video shows thermal actuation cycles (25° to 75°C) of a pristine shell UV cross-linked at 35°C, recorded with a first-order waveplate, crossed polarizers, and one polarizer in the light path, respectively.
    • Movie S2 (.mp4 format). Thermal actuation of a shell UV cross-linked at 35°C with a hole at the top, immersed in glycerol.
    • Movie S3 (.mp4 format). The video shows actuation of a fragment cut from a shell UV cross-linked at 35°C.
    • Movie S4 (.mp4 format). Thermal actuation of a shell UV cross-linked at 60°C.
    • Movie S5 (.mp4 format). Complex modes of actuation observed by cutting a shell UV cross-linked at 35°C into different topological objects such as a cap shape, a self-closed ribbon, and a long spiral stripe shape.

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