Although individual carbon nanotubes (CNTs) are superior as constituents to polymer chains, the mechanical and thermal properties of CNT fibers (CNTFs) remain inferior to synthetic fibers due to the failure of embedding CNTs effectively in superstructures. Conventional techniques resulted in a mild improvement of target properties while degrading others. Here, a Double-Drawing technique is developed to rearrange the constituent CNTs in both mesoscale and nanoscale morphology. Consequently, the mechanical and thermal properties of the resulting CNTFs can simultaneously reach their highest performances with specific strength ~3.30 N/tex (4.60 GPa), work of rupture ~70 J/g, and thermal conductivity ~354 W/m/K, despite starting from low-crystallinity materials (I_G:I_D~5). The processed CNTFs are more versatile than comparable carbon fiber, Zylon and Dyneema. Based on evidence of load transfer efficiency on individual CNTs measured with In-Situ-Stretching-Raman, we find the main contributors to property enhancements are the increasing of the effective tube contribution, in addition to the known optimization on CNTs alignment and stacking.

CNT fibers preparation: Continuous CNTFs were fabricated using the floating catalyst method (1, 2), and supplied by Tortech Nanofibers Ltd. The produced CNT aerogels from a tube furnace were mechanically pulled out, densified by acetone, and spun continuously winded. Although a small tension force is applied during the spinning process to obtain a preferential alignment along fiber axis, the anisotropic ratio is always within 0.85 (3).

Enhancing CNTF with the Double-Drawing process: (i) Immersing and first drawing: The raw CNTF is fixed at its lower end inside a dropping funnel, and its upper end fixed on a spin rotor. The CNTF is straightened out but without a pre-tension. After being immersed in chlorosulfonic acid (CSA), CNTF is firstly drawn to a specific ratio (η_CSA); (ii) Poisson Tightening: After the immersing solvent being changed into chloroform, the CNTF is immediately further drawn by η_PT. (iii) Rinsing: after the Double-Drawing processes, the CNTF is successively rinsed in water and acetone, and finally vacuum dried.

Linear Density measurement: The LD is measured based on Direct Single-fiber Weight Determination, following ASTM D1577−07(2018) OPTION B. The weight of a CNTF with length ~100 mm is measured with a Sartorius SE2 Ultra-micro balance. We find it worthy to notice the importance of accurately measurement of LD, because the susceptibility of fiber’s tenacity to LD. The frequently used Vibroscopic method in reports is abandoned here, because of the potential serious underestimation of LD, if the “stiffness correction” was overlooked (ASTM D1577−07(2018) OPTION C − the standard for the Vibroscopic method). More discussion can be found in S2.

Tensile test: The CNTFs are tested with the Single-Fiber Testers (Textechno FAVIMAT with Load cell of 210 cN and delicately aligned clamps (4 mm hard rubber), the force resolution of ~0.0001 cN, the displacement resolution of 0.1 μm). The CNTFs are tested with gauge length of 10 mm, stretching speed of 1 mm min⁻¹ and pretension of 0.1 cN tex⁻¹. Every sample is tested for 3 specimens to guarantee the repeatability of results. Stretching speeds of 0.2, 2 and 5 mm min⁻¹, and gauge length of 20 mm have been tried to generate similar results.

Thermal and Electrical conductivity measurement: The thermal conductivity of CNTF along fiber axis is performed with a homemade measuring apparatus based on a self-heating method (4). The electrical conductivity of CNTFs along fiber axis is measured in air at room temperature (1 atm, 25–27 ℃, relative humidity: 40±3% RH) by a homemade testing stage using the four-electrode method and steady-state method (5).

In-Situ Stretching Raman: The suspended CNTFs are ends fixed onto a manual stretching stage to detect the Raman signal with HORIBA HR800 micro-Raman spectroscopy. We excite the Raman G’ mode with linearly polarized laser, and only collect the scattered radiation in the parallel polarization with a Glan Polarizer, so that only iCNTs with their axis close to parallel with the laser polarization can be detected. For the ZZ/XX configuration (6), the polarizations of both incident and scattered photons are parallel/perpendicular to the axis of CNTFs, offering the strain distribution of CNTs along/normal to the fiber axis;

Wide Angle X-ray Diffraction (WAXD)/Small-Angle X-ray Scattering (SAXS): The CNTFs with different processing are studied using a small and wide-angle diffractometer (Molecular Metrology SAXS system) equipped with a sealed microfocus tube (MicroMax−002+S) emitting Cu Kα radiation (wavelength of 0.1542 nm), two Göbel mirrors, and three pinhole slits. CNTFs with diameter of 18−50 µm were suspended onto a holder perpendicular to the beam and measured at ambient temperature. All the raw data are analyzed by SAXSGUI. For data analysis of WAXD, the sharp equatorial reflections at q ~1.8 Å⁻¹ corresponding to the scattering from (002) reflection of the inter-layer spacing of a few walled CNT, and to a higher order reflection of the hexagonal packing of parallel CNTs. Both possibilities are due to the planes perpendicular to the CNT axis. To obtain the azimuthal profile of (002) scattering, the intensity is integrated in the range of 1.6−1.9 Å⁻¹. With the increase of the alignment, two peaks emerge in the azimuthal profile around the preferred alignment, from which the Full Width Half Maximum is obtained. For data analysis of SAXS, the integrating range is 0.04−0.1 Å⁻¹ to obtain the azimuthal profile of scattering.

Other characterization: The cross-sections of CNTFs are fabricated with FIB (FEI Helios 600i). The cross-section is firstly cut with Gallium ions with current of 9 nA (30 kV) and then finely polished under current of 0.79 nA, after which the SEM of cross-sections are conducted by electron beam of FIB. The SEM for other CNTFs is conducted on a TESCAN MIRA3. HRTEM is conducted on an FEI Talos F200X TEM working under 80 kV to reduce the damage to CNTs.

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