Mechanical Stretching Induced Alignment

Although CNT fibers and films spun from CNT aerogel using FCCVD technique resulted in good alignment, this in-line alignment method is not efficient due to low stretching ratio and the degree of alignment is not sufficient to approach the excellent properties of individual CNTs. During the polymer fiber extrusion process, mechanical stretching was applied to straighten the polymer chain and increase the crystalline area, improving the mechanical properties.[114] Similarly, it was also found to be efficient enough as a post alignment method to achieve ultrahigh degree of alignment degree of CNTs with controllability. Many research groups have used this mechanical stretching method to align the CNT bundles.[13a, 76b, 93, 96] Liang and coworkers et al.[13a, 93] reported a direct mechanical stretching method, as shown in

Figure 11a, where randomly oriented CNTs will become aligned under the stretching force

due to their entanglement and large aspect ratio. The electrical conductivity was improved from 4.2 × 104 S m-1 to 6.0 × 104 S m-1 at a 40% stretching ratio (Figure 11b), due to the better alignment, denser packing, and better contacts between neighboring nanotubes. Before the mechanical stretching process, the randomly oriented CNT films were wetted by a liquid solvent or polymer, which assisted the movement of entangled CNT bundles by acting as a lubricant under stretching.[93, 115] Liu et al.[76b] and Han et al.[96] reported a multi-step stretching and pressing method (Figure 11c), resulting in highly dense CNT sheets, which showed a considerably high volumetric density of 0.98 g/cm3 after two stretching-pressing processes. The degree of alignment was characterized by the intensity ratio of G-band peaks (IG‖/IG⊥) from polarized Raman spectroscopy, as shown in Figure 11d-f. The ratio increased

from ~1.1 for the as-prepared CNT sheet, to ~8.8 for the stretching–pressing process sheet, indicating an extremely high degree of nanotubes alignment.

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Figure 11. Mechanical stretching method to align CNTs. (a) Direct mechanical stretching to align nanotubes in the as-received random CNT film; (b) effects of

stretching ratio on electrical conductivity of aligned CNT film parallel to the alignment (stretching) direction (a,b) Reproduced with permission.[13a] Copyright 2009, Wiley- VCH Verlag GmbH & Co. KGaA. (c) Schematic illustration of multi-step stretching–

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Group. (d-f) The polarized Raman spectra of G‖ and G⊥ bands for the as prepared CNT

sheet (d), processed after stretching and pressing (e), and after second stretching and pressing (f).Reproduced with permission.[76b] Copyright 2014, Royal Society of Chemistry.

The benefit of CNT alignment is the improved electrical conductivity. With a variable-range- hopping (VRH) model, one can explain a portion of alignment induced high conductivity on the basis of simple geometrical considerations. Initially, one must assume that the intrinsic resistance of a nanotube should be much lower than the tube-tube contact resistances.

Considering there are fewer tube-tube contacts for a given length of an aligned sample in the alignment direction as compared with the random sample in any specified direction, the resistance should be lower. This translates to the difference in hopping rate between the alignment direction of the aligned sample and any other specified direction in the random sample.[116] As shown in Figure 12a, the ability of electrons to hop at the interfaces/ends between neighboring CNTs is crucial in determining the electrical resistance of the fiber. Fewer interfaces/ends and shorter distances at the interfaces help to reduce the electron loss during the hopping process,[117] resulting in a high conductivity, which is why the aligned and densely packed structure within fibers and films matters in determining the electrical

conductivity. The effect of alignment on the electrical and thermal conducitivity was also observed experimentally by Chen et al. (Figure 12b-d)[116b]

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Figure 12. (a) A model of electron hopping within a conducting fiber made from aligned

multi-walled carbon nanotubes (MWCNTs). The dashed line indicates the transport pathway of electrons Reproduced with permission.[117] Copyright 2017, Nature Publishing Group. (b- c) Anisotropy in (b) thermal and (c) electrical transportation properties for the CNT array with orderly aligned MWCNTs and the CNT network with randomly aligned MWCNTs. (d) Schematic illustration of the anisotropic transportations within the two types of MWCNT frameworks. For the MWCNT array, the thermal and electrical transportations along the MWCNT alignment are dominated by the intrinsic transportation properties along the MWCNTs, while the ones vertical to the MWCNT alignment are dominated by the joints of the cross-linked MWCNTs. For the MWCNT network, the transportation properties are dominated by the joints of the cross-linked MWCNTs along the two orthogonal directions b- d) Reproduced with permission.[116b] Copyright 2018. Elsevier.

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Understanding the dynamic structure-property relationship between the electrical conductivity and the structure of CNT network during mechanical stretching would be beneficial. However, correlating the structural changes to the performance improvements, such as electrical conductivity and mechanical strength, would be challenging considering the directly observed parameters of structural evolution including bundle size and alignment degree change are not direct indicators of electrical or mechanical performance. Basic indicators, such as load transfer and electron transport efficiency (or band structure change), are more directly related to mechanical and electrical properties, respectively. Once the relationships between structural parameters, such as bundle size and alignment, and basic indicators of properties, including electrical and mechanical performance, become more obvious and acceptable, a precise mathematical model may be derived.

Figure 13 schematically shows the structural evolution of the CNT network under stretching

and the corresponding SEM/TEM images.[37] Although the stretching induced structural change model was conducted[37] and the relationships between stretching ratio and electrical conductivity were also reported by Chen et al.,[13a] the specific mechanism to account for the electrical conductivity improvement after high mechanical stretching is still absent. The in- situ structure-property relationship characterization during stretching (i.e. IFSS change or, contact resistance change when bundles are merging together) is urgent and still challenging.

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Figure 13. (a) SEM image of an unstretched film. (b) Typical SEM image of the film after 8-

10% strain before broken. (c) SEM image at the breaking point. (d) Macroscopic SEM image near the breaking point. The film is going to be split. (e) Schematic diagram of the structure deformation process during stretching. (f) Schematic diagram of one simplified mesh and its morphology after 33% stretching. (g) Drawing a (10,10) SWCNT from a (10,10) bundle (upper two images), and the interbundle slipping within a 17 nm wide (10, 10) shared bundle. It is easy to count out that the area of the slipping surface is about 10% of the total outer surface for the SWcNTs in a semibundle. (h) Low-magnification TEM image for some typical stretched SWCNT meshes. All panels reproduced with permission.[37] Copyright 2007, American Chemical Society.