Research ArticleMATERIALS SCIENCE

High-contrast and reversible polymer thermal regulator by structural phase transition

See allHide authors and affiliations

Science Advances  13 Dec 2019:
Vol. 5, no. 12, eaax3777
DOI: 10.1126/sciadv.aax3777
  • Fig. 1 Structural phase transition in crystalline PE nanofibers.

    (A) Highly ordered all-trans conformation of an orthorhombic PE crystal before the phase transition. Inset in (A) shows an aligned and ordered PE molecular chain, where r, α, and θ represent bond length, bond angle, and dihedral angle, respectively. (B) Combined trans and gauche conformation after the phase transition, which corresponds to a rotationally disordered hexagonal phase. Inset in (B) shows a PE molecular chain with random segmental rotations. (C) Transmission electron microscopy (TEM) micrograph of a typical crystalline PE nanofiber sample. Scale bar, 200 nm. (D) Selected area electron diffraction (SAED) pattern of the PE nanofiber taken using low-dose TEM at cryogenic temperature. The arrow in (D) indicates the c axis.

  • Fig. 2 High-contrast and reversible polymer thermal regulator based on PE nanofibers.

    (A) False-colored SEM micrograph of a suspended platinum resistance thermometer microdevice. A PE nanofiber crosses its heating and sensing islands. Scale bar, 10 μm. (B) Temperature-dependent thermal conductance G(T) of nanofiber #1 from 320 to 455 K. An abrupt and reversible thermal conductance change is observed around 440 K due to the structural phase transition. (C) Thermal switching ratio f of PE nanofibers compared with the solid-solid or solid-liquid transition in some existing materials. The values are summarized in table S1. The associated references are as follows. nCB (31); VO2 (26); K, Zn, Cd, and Hg (32); c-Se (33); Ni-Mn-In alloy (13); C6H14 (34); Sn, In, and LiNO3 (15); C/C16H34 composite (17); Ge2Sb2Te5 (27); azobenzene polymers (14). (D) Multiple on/off thermal cycles of nanofiber #2 around the phase transition temperature of 440 K.

  • Fig. 3 Thermal stability and temperature limit of the phase transition in PE nanofibers.

    (A) Thermal conductance of nanofiber #3 before and after holding the specimen at 450 K for 10 hours. The thermal conductances of this nanofiber in the heating and the cooling processes overlap and, thus, show the complete reversibility when the temperature is ~10 K higher than the phase transition temperature. (B) High-temperature stability of nanofiber #4. The nanofiber shows the partial phase transition at ~430 K when the temperature reaches 530 K. This phase transition disappears, and the crystalline PE nanofiber becomes amorphous when the temperature increases up to 560 K.

  • Fig. 4 Micro-Raman measurements and MD simulations of PE nanofibers before and after the phase transition.

    (A) Temperature-dependent micro-Raman measurements of a PE nanofiber. All Raman spectra are normalized with respect to the intensity of the Raman peak at ~1128 cm−1 at 300 K. (B and C) Phonon dispersions of the orthorhombic phase and the hexagonal phase of a PE nanofiber. The less blurry (cleaner) lines of the phonon dispersion in (B) suggest less phonon scattering. (D) Corresponding vibrational power spectra at the reduced wave vector value of k = 0.33. (E) Corresponding dihedral angle θ distributions.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/12/eaax3777/DC1

    Supplementary Text

    Table S1. Data of the switching ratios of the thermal switches in Fig. 2C.

    Table S2. Data of the switching ratios of multiple samples.

    Table S3. Thermal conductivity, volumetric heat capacity, phonon group velocity, and phonon life time.

    Fig. S1. Determination of Gon and Goff.

    Fig. S2. Thermal conductance measurement of a PE microfiber in two heating/cooling cycles.

    Fig. S3. Heat flow versus temperature bias at different heating rates.

    Fig. S4. Multiple temperature sweeps of a PE nanofiber.

    Fig. S5. The vibrational power spectra at three wave vectors before and after the phase transition of crystalline PE nanofibers.

    Fig. S6. Thermal conductivity of PE from 300 to 500 K.

    Fig. S7. Spectral energy density of PE at different k-points.

    Fig. S8. Phonon dispersion and group velocity of PE.

    Fig. S9. Volume as a function of temperature when fibers are under stretching stress.

    Fig. S10. Thermal contact resistance between the PE nanofiber and one suspended island as a function of the axial thermal conductivity of the PE nanofiber.

    Fig. S11. The height map of a suspended nanofiber measured using atomic force microscopy.

    Fig. S12. Raman spectra in the temperature range from 300 to 420 K.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Table S1. Data of the switching ratios of the thermal switches in Fig. 2C.
    • Table S2. Data of the switching ratios of multiple samples.
    • Table S3. Thermal conductivity, volumetric heat capacity, phonon group velocity, and phonon life time.
    • Fig. S1. Determination of Gon and Goff.
    • Fig. S2. Thermal conductance measurement of a PE microfiber in two heating/cooling cycles.
    • Fig. S3. Heat flow versus temperature bias at different heating rates.
    • Fig. S4. Multiple temperature sweeps of a PE nanofiber.
    • Fig. S5. The vibrational power spectra at three wave vectors before and after the phase transition of crystalline PE nanofibers.
    • Fig. S6. Thermal conductivity of PE from 300 to 500 K.
    • Fig. S7. Spectral energy density of PE at different k-points.
    • Fig. S8. Phonon dispersion and group velocity of PE.
    • Fig. S9. Volume as a function of temperature when fibers are under stretching stress.
    • Fig. S10. Thermal contact resistance between the PE nanofiber and one suspended island as a function of the axial thermal conductivity of the PE nanofiber.
    • Fig. S11. The height map of a suspended nanofiber measured using atomic force microscopy.
    • Fig. S12. Raman spectra in the temperature range from 300 to 420 K.

    Download PDF

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article