Research ArticleLIQUID CRYSTALS

Liquid crystalline cellulose-based nematogels

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Science Advances  18 Aug 2017:
Vol. 3, no. 8, e1700981
DOI: 10.1126/sciadv.1700981

Figures

  • Fig. 1 Fabrication of cellulose-based nematogels.

    (A) Structure of CNFs initially obtained in an aqueous dispersion. Inset: Chemical structure of cellulose before (left) and after (right) the oxidation process. (B) A hydrogel with interlinked CNFs was formed upon cross-linking of the individual fibers into a network, which can be aligned by unidirectional shearing before cross-linking. (C) Organogel obtained via the replacement of water with isopropanol or other organic solvents. (D) Nematogel obtained via the substitution of the organic solvent with an LC such as 5CB in nematic phase at room temperature. (E) Transmission electron microscopy (TEM) image of CNF2 negatively stained by 2–wt % phosphotungstic acid solution. (F to H) Scanning electron microscopy (SEM) images of unaligned CNF2 aerogel (F) and aligned CNF2 aerogels (G and H), with 0.12 volume % (G) and 0.6 volume % of CNFs (H), coated with a thin layer of gold and observed at a low voltage of 5 kV.

  • Fig. 2 Optical properties of nematogels and their use in fabricating stimuli-responsive devices.

    (A to F) Photographs of nematogel films at different temperatures (indicated in the top right corners) corresponding to nematic [CNF1 (A) and CNF2 (D)] and isotropic [CNF1 (B and C) and CNF2 (E and F)] phases of the 5CB infiltrating the cellulose gel network. The photographs (C and F) were obtained for nematogel samples placed between two crossed polarizers. (G) Schematic of a flexible nematogel LC cell fabricated by confining the nematogel film between two plastic films with transparent ITO electrodes facing inward. Photographs of the flexible nematogel cell (CNF2-5CB) in nematic phase (H) and isotropic phase (I), with the electrodes used to apply fields for electro-optic characterization, such as that shown in (J) and (L). (J) Transmission of 1-mm-thick hydrogel, organogel, and nematogel and 30-μm-thick CNF2-5CB nematogel with and without electric field, as indicated in the legend. (K) Transmission of CNF2-5CB nematogel versus temperature, showing that the paranematic-to-isotropic transition takes place at TPNI ≈ 38.5°C; for reference, the temperature of nematic-isotropic transition for a pristine 5CB TNI ≈ 35.3°C is indicated using a dashed red vertical line. (L) Transmission of CNF2-5CB nematogel versus voltage at different temperatures, showing that the critical field Ec ≈ 3.0 V/μm varies only weakly with temperature. Scale bars, 1 cm.

  • Fig. 3 Characterization of response times associated with electric switching of nematogels.

    (A and B) Transmission versus time curves used to characterize rising time (A) and falling time (B) of CNF2-5CB nematogel at different temperatures and applied voltages indicated in the legends. The inset in (A) shows the coordinate system and the physical model of the nematogel cell of thickness d, with a CNF-compartmentalized LC domain characterized by geometric parameters a and b; the uniform director at no fields and the CNF fibers are shown co-aligned. The experimental data (scatter symbols) are fitted by the corresponding results emerging from the physical model (solid lines). (C) Response times of CNF2-5CB nematogel versus temperature, where τrising and τfalling were measured using a field of 7.8 V/μm. (D) Response time versus voltage characterized at 31.5°C.

  • Fig. 4 Characterization of mechanical properties of nematogels.

    (A) Tensile elastic modulus versus temperature of CNF2-5CB nematogel. The storage modulus G′ (black open squares) and the loss modulus G″ (red open squares) are plotted using the left and right vertical axes, respectively. (B) Stain-stress relation along and perpendicular to Ns.

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