Functional carbon nanofiber yarn

Abstract

Functional nanofiber yarns are provided that have increased hydrophilicity and more than double the water absorption properties. Processing methods for producing such functional nanofiber yarns are also provided.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to U.S. Provisional Application No. 63 / 402,185, filed August 30, 2022, which is incorporated herein by reference in its entirety.

[0002] The present disclosure relates generally to nanofibers. In particular, the present disclosure relates to the functionalization, manufacture, and applications of such modified carbon nanofiber yarns and yarn bundles. [Background technology]

[0003] Carbon nanofibers are elongated cylinders made of carbon arranged in a remarkable hexagonal lattice structure, interchangeable with carbon nanotubes (CNTs). They possess a wide range of excellent electronic, thermal, and structural properties, making them suitable for a variety of applications, including, but not limited to, field emitters, conductors, transistors, energy storage, and fibers and textiles with varying strengths, biomedical applications, and industrial applications. However, carbon nanofibers are generally hydrophobic, due to their surface's inability to repel water and liquids, and they have a strong tendency to aggregate, primarily due to van der Waals forces. Therefore, carbon nanofibers may not perform well where liquid absorption properties are desired, and alternative materials may be sought for such applications. Summary of the Invention

[0004] According to aspects of the present disclosure, treated nanofiber yarn bundles are provided that retain the yarn bundle structure and have higher water absorption.

[0005] According to aspects of the present disclosure, the treated nanofiber yarn bundles have more than two or four times the water absorption capacity of untreated or pristine nanofiber yarns or yarn bundles.

[0006] The present disclosure will be further described in the following detailed description with reference to the several drawings, which are mentioned as non-limiting examples of preferred embodiments of the present disclosure, and in which like characters represent like elements throughout the several views of the drawings. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram of CNT synthesis by chemical vapor deposition (CVD), according to an exemplary embodiment. [Figure 2] FIG. 1 is a schematic diagram of continuous CNT sheet production by a drawing method, according to an exemplary embodiment. [Figure 3] FIG. 1 is a schematic diagram of continuous CNT yarn production by drawing and spinning, according to an exemplary embodiment. [Figure 4] 1 is a flowchart illustrating a method for subjecting a CNT yarn bundle to an acid treatment, according to an example embodiment. [Figure 5] 1 is a flowchart illustrating a method for subjecting a CNT yarn bundle to ozone treatment, according to an example embodiment. [Figure 6] 1 is a graph showing increased water absorption in treated CNT yarn bundles according to an example embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Through one or more of its various aspects, the embodiments and / or specific features or subcomponents of the present disclosure are intended to demonstrate one or more of the advantages specifically mentioned above and below.

[0009] Carbon nanofiber overview Carbon nanofibers, or CNTs, are long tubes with small diameters, typically measured in nanometers. They generally have high aspect ratios, with length-to-diameter ratios in the range of over 1000:1, and are composed of one or more graphene sheets rolled into a concentric structure. Each graphene sheet is considered a wall. Single-walled CNTs (SWCNTs) are made from a single graphene sheet, double-walled CNTs are made from two graphene sheets, and multi-walled CNTs (MWCNTs) have multiple graphene sheets.

[0010] Many CNT synthesis methods have been developed depending on the type of product, precursor, heat source, reaction time, temperature, and atmosphere. The most common methods include, but are not limited to, arc discharge, electrolysis, laser ablation, chemical vapor deposition (CVD), flame synthesis, and mechanothermal. Well-known CVD methods are classified into plasma-assisted PE-CVD, aerosol CVD (AACVD), water-added WA-CVD, oxygen-added CVD, and catalytic CVD.

[0011] The CVD method utilizes acetylene (C2H2), ethylene (C2H4), or other hydrocarbons as the carbon source in a reaction chamber at temperatures ranging from 350 to 1,000°C using a catalyst. The resulting CNTs vary depending on the actual reaction temperature, reaction time, catalyst selection, catalyst density, and carbon source.

[0012] In the CVD reactor, the catalyst can be suspended in the reactor during synthesis (FC-CVD). The nanofibers are then collected and made into yarns by dispersing the CNTs in a liquid using a dispersant or surfactant, followed by extruding the dispersed mixture into a conventional solution for continuous spinning.

[0013] Variations on the above yarn spinning method include, but are not limited to, the use of superacids in the spinning solution and the use of polymeric ethylene glycols in the CNT dispersion.

[0014] An alternative, commonly employed CVD synthesis method begins with catalyst deposition on a substrate, such as a silicon wafer, by electron beam (E-beam) deposition. The substrate can also include stainless steel or aluminum disposed on an underlying silicon (Si) wafer or other ceramic substrate. Besides E-beam deposition, other catalyst deposition methods include, but are not limited to, sputtering, electrochemical methods, atomic layer deposition, laser-assisted CVD, and plasma-assisted CVD. An exemplary catalyst includes iron on a buffer layer of silicon dioxide or aluminum oxide on top of the substrate.

[0015] Upon completion of the CVD process, the substrate is shown to have what is often referred to as a forest of individual nanofibers aligned perpendicular to one another with one end of the nanofiber attached to the substrate. The fact that these CNTs are maintained in such a vertically aligned position is believed to be primarily due to van der Waals forces and entanglement between adjacent nanofibers, given the small diameters of these individual CNTs, ranging from 0.3 nm to 100 nm.

[0016] The CNT forest can ultimately be spun into CNT yarn as described in WO2007 / 015710 and incorporated herein.

[0017] Briefly, an attachment is attached to or near the sidewall of the CNT forest. The attachment is continuously pulled away from the CNT forest, with the angle between the stretching direction and the alignment direction of the nanofibers in the CNT forest ranging from approximately 90° to 5°, to form a CNT sheet. The CNT sheet is twisted around the axis of the stretching direction to form a nanofiber yarn, which is then wound onto a reel.

[0018] CNT yarns may also be made by other methods known in the literature. They may be commercially available from a variety of suppliers.

[0019] G / D ratio of CNT As mentioned above, carbon nanofibers are synthesized by incorporating carbon from various carbon source materials into one of several carbon allotropes, including, but not limited to, diamond, graphite, fullerenes, and graphene. From a molecular perspective, these materials are all composed entirely of carbon-carbon (C-C) bonds, and the different orientations of these bonds within a given material represent vastly different materials with distinguishable properties. Raman spectroscopy is a commonly applied, non-destructive tool well suited to characterizing the intermolecular interactions and molecular bonding of carbon materials. Its measured vibrational frequencies are highly sensitive to the orientation of the C-C bond and the weights of the atoms at either end of that bond, representing one or more carbon allotropes.

[0020] The nanofibers, which are essentially rolled-up graphene sheets, exhibit a Raman spectrum characteristic of graphene at 1590 cm -1 G band at 1340 cm -1 The latter is a defect-induced feature due to dislocation defects. The amorphous graphite on the nanofiber surface may also affect this D band of the test sample.

[0021] The intensity ratio of both D / G bands (I D / I G ) quantitatively reflects the defect state of the nanofiber and also indicates the degree of crystallinity. D / I G A smaller value of I compared to a larger value indicates better crystallinity or fewer defects. D / I G The change in value correlates with structural or microstructural changes in the CNT walls that manifest as functional or property changes in the CNT material.

[0022] Raman spectrometers are commercially available, ranging from handheld to benchtop models. B&W Tek's i-Raman Plus scans the sample at a power of 100 and an acquisition time of 60,000 ms, capturing 1590 cm -1 and 1350cm -1was the exemplary spectrometer selected to measure the peak values ​​of G and D, respectively. G / I D Calculate the ratio.

[0023] CNT hydration Carbon nanotubes are generally hydrophobic. Their surfaces repel water and have no affinity for liquids, and CNTs tend to aggregate, primarily due to van der Waals forces. Studies measuring the contact angles between single CNTs and various liquids using atomic force microscopy and the Wilhelmy method have shown that the more polar the liquid, the worse the hydration. Surfactants are frequently used to disperse CNTs in solution and create CNT suspensions.

[0024] CNT yarns appear to inherit the same hydrophobic properties. The poor contact and interaction between individual CNT fibers or bundles of CNT fibers (CNT yarns) and other materials or matrix materials limits the uses and applications of CNT yarns.

[0025] As an illustrative example, 100 individual CNT yarns, each approximately 15 μm in diameter, produced by drawing from a CNT forest on a substrate and then twisting and untwisting were bundled to form a single CNT bundle or a pretwisted CNT bundle, where the individual CNT yarns within the CNT bundle are substantially parallel to the central axis of the CNT bundle. The bundles were further processed to modify the surface morphology and bundle function for desired properties and applications, as described below.

[0026] Each nanofiber yarn bundle may have the same length, for example, 2 cm. The treated nanofiber yarn bundle was transferred to a silicone tube, which had an inner diameter of 1 / 16 inch (1 / 16") or an inner diameter generally larger than the diameter of the nanofiber yarn bundle or the aggregate of nanofiber yarn bundles. The initial weight was measured and recorded. One end of the nanofiber yarn bundle and the silicone tube were inserted into a water container with a constant depth of 1 cm in a closed room for three consecutive days to prevent water evaporation. The bottom of the container was at least 10 times larger than the total cross-sectional area of ​​the silicone tube to ensure a sufficient water supply during the water (or liquid) absorption test. Each silicone tube with a nanofiber yarn bundle was weighed and its initial weight was subtracted to calculate the water absorption amount.

[0027] Processing of CNTs The untreated CNT yarns and CNT yarn bundles retain their pristine surface and hydrophobic properties and are referred to herein as pristine yarns and yarn bundles.

[0028] The initial strands of CNTs can be treated with a chemical or chemicals with strong oxidizing potential to alter the surface and structure or microstructure of the CNT material to endorse new properties and functions. Depending on the chemical nature of the chemical, the chemical can be substantially pure, close to 100% pure, or have a percentage between 0 and 100%.

[0029] According to the present disclosure, an exemplary oxidizer can be one gas or at least two gases.

[0030] Exemplary strong oxidizing gases may include, but are not limited to, ozone and chlorine.

[0031] According to an exemplary embodiment, a CNT yarn bundle can be placed in a sealed process chamber. The chamber can then be filled with at least one selected oxidizing gas at a target flow rate for a predetermined period of time. Once the predetermined period has expired, the process chamber is purged with an inert gas or gas mixture, such as argon and / or nitrogen.

[0032] According to the present disclosure, treatment of CNTs can include applying strong oxidizing agents and removing such agents once the desired results are achieved.

[0033] Exemplary oxidizing agents may include, but are not limited to, strong liquid chemicals such as one or more strong acids. Additionally, certain acids may be mixed together in volume ratios, such as a 2 to 1 volume ratio. In chemistry, a strong acid is an acid that completely ionizes its target or substrate in aqueous solution.

[0034] Exemplary strong acids include, but are not limited to, nitric acid, sulfuric acid, perchloric acid, nitric acid, perchloric acid, and a combination of nitric acid and sulfuric acid, a combination of sulfuric acid and potassium dichromate, and a combination of sulfuric acid and potassium permanganate.

[0035] For liquid-based treatments, according to exemplary embodiments, the CNT yarn bundle is immersed in the intended solution for a predetermined period of time until the desired effect is achieved, and then removed. The treated yarn bundle is then rinsed thoroughly with deionized water (DI water) and dried in air, in a heated oven, or in a vacuum chamber.

[0036] FIG. 4 is a flow chart illustrating a method for subjecting a CNT yarn bundle to an acid treatment, according to an example embodiment.

[0037] In step S401, a carbon nanotube forest was synthesized on a silicon wafer by standard chemical vapor deposition using an iron catalyst and acetylene gas as the carbon source. In step S402, the CNT forest was drawn into a CNT sheet. In step S403, the CNT sheet was twisted into a CNT yarn having a predetermined diameter. The predetermined diameter may be 15 μm with a standard deviation of 1 μm. In step S404, the CNT yarn was wound onto a spool. In one example, the spool may have 100 loops of CNT yarn. In step S405, the CNT yarn spool was cut to form a CNT yarn bundle or multiple CNT yarn bundles, which may be referred to as a nanofiber yarn bundle or multiple nanofiber yarn bundles.

[0038] In step S406, the CNT yarn bundle is treated with a strong acid mixture. According to an exemplary embodiment, the strong acid may include, but is not limited to, at least nitric acid (HNO) and sulfuric acid (HSO). For example, the strong acid mixture may be formed by mixing 70% nitric acid with 98% sulfuric acid. Furthermore, the nitric acid and sulfuric acid may be mixed in a 2:1 volume ratio (i.e., 2 parts nitric acid to 1 part sulfuric acid). However, embodiments of the present disclosure are not so limited; that is, the strong acid mixture may include, but is not limited to, perchloric acid, nitric acid, sulfuric acid, potassium dichromate, or potassium permanganate. While a mixture of acids is disclosed herein, embodiments of the present disclosure are not so limited and may utilize a single acid solution. In one example, the single acid solution may have different chemical concentration levels.

[0039] In step S407, the CNT yarn bundle was immersed in the strong acid mixture for a predetermined time. According to exemplary embodiments, the predetermined time period may be at least 30 minutes, 300 minutes, or less than 24 hours. However, embodiments of the present disclosure are not so limited; the predetermined time period may be any time period that results in a targeted increase in hydrophilicity in as little as 10 minutes. The targeted hydrophilicity may be at least two or four times the inherent hydrophilicity of the untreated or pristine CNT yarn bundle while maintaining the structural integrity of the CNT yarn bundle. The CNT yarn bundle may be immersed in the strong acid mixture for a predetermined time period with occasional agitation.

[0040] In step S408, the treated CNT yarn bundle is removed from the acid mixture, immersed in a water bath, and rinsed thoroughly with distilled water.

[0041] FIG. 5 is a flow chart illustrating a method for subjecting a CNT yarn bundle to ozone treatment, according to an example embodiment.

[0042] In step S501, a carbon nanotube forest was synthesized on a silicon wafer by standard chemical vapor deposition using an iron catalyst and acetylene gas as the carbon source. In step S502, the CNT forest was drawn into a CNT sheet. In step S503, the CNT sheet was twisted into a CNT yarn having a predetermined diameter. The predetermined diameter may be 15 μm with a standard deviation of 1 μm. In step S504, the CNT yarn was wound onto a spool. In one example, the spool may have 100 loops of CNT yarn. In step S505, the CNT yarn spool was cut to form a CNT yarn bundle or multiple CNT yarn bundles, which may be referred to as a nanofiber yarn bundle or multiple nanofiber yarn bundles.

[0043] In step S506, the CNT yarn bundle is placed in a treatment chamber, which is then sealed. In step S507, the treatment chamber is filled with a selected oxidizing gas. According to an exemplary embodiment, the oxidizing gas may be formed from 20% ozone gas and 80% nitrogen. However, embodiments of the present disclosure are not limited thereto; other ozone content oxidizing gases may be utilized. In step S508, the CNT yarn bundle may be exposed to the oxidizing gas for a predetermined period of time. In one example, the predetermined period may have a minimum of 10 minutes, 30 minutes, or 300 minutes, or a maximum of 24 hours. However, embodiments of the present disclosure are not limited thereto; the predetermined period may be any time that results in a targeted increase in hydrophilicity. The targeted hydrophilicity may be at least two or four times the inherent hydrophilicity of the untreated or pristine CNT yarn bundle while maintaining the structural integrity of the CNT yarn bundle.

[0044] In step S509, upon expiration of a predetermined period of time (e.g., upon completion of the ozone treatment), the treatment chamber is purged with an inert gas. In one example, the inert gas may include, but is not limited to, nitrogen, argon, etc. In step S510, the treatment chamber is returned to an atmospheric environment, and the treated CNT yarn bundle is quickly removed from the treatment chamber and placed in a storage location, preferably a sealed storage location filled with an inert gas.

[0045] Effect of CNT treatment A group of CNT yarn bundles is treated with a strong acid mixture for 300 minutes, as detailed in the description of FIG. 4 above.

[0046] The other two groups of CNT yarn bundles are treated with ozone gas for 30 minutes and 300 minutes, respectively, as detailed in the description of FIG. 5 above.

[0047] All three treated CNT yarn bundle groups showed I D / I G There is a significant change in the ratio, and as shown in Table 2, there is also a significant change in the wettability.

[0048] Table 1 shows an increase in G and D peak values ​​in all treatment groups with three samples per group. After converting these peak values ​​to ratios, the mean I G / I D The value decreases from 1.77 for the pristine CNT yarn bundle to 1.51 for the 30 min ozone-treated yarn bundle, to 1.39 for the 300 min ozone-treated yarn bundle, and to 1.30 for the acid-treated yarn bundle. As the treatment conditions become more aggressive, I G / I D The values ​​decrease more significantly, indicating a greater structural change in the CNT walls. [Table 1]

[0049] Table 2 shows the water absorption rates for all CNT yarns before and after strong acid and ozone treatments. Figure 6 shows the same results in a bar graph. The weight gain, or percent water absorption, increases from 1.47% to 2.68%, 4.32%, and 6.68% for the initial group, the 30-minute ozone treatment group (ozone L-treated CNT yarn bundle), the 300-minute ozone treatment group (ozone H-treated CNT yarn bundle), and the acid-treated ozone group (acid-treated CNT yarn bundle), respectively. Harsher treatments tend to enhance water absorption in the treated nanofiber yarn bundles, resulting in greater hydrophilicity. The water absorption capacity of the CNT yarn can be doubled or quadrupled by the most aggressive treatments, from an initial increase of 10 mg of water per mg of CNT yarn bundle to 19 mg, 30 mg, and 47 mg for the 30-minute ozone-treated CNT yarn group, the 300-minute ozone-treated CNT yarn group, and the acid-treated CNT yarn group, respectively. [Table 2]

[0050] Increasing the hydrophilicity of carbon nanotubes allows the nanostructures to retain more exogenous enzymatically active and inactive small molecules, as well as larger molecules with slower release properties.

[0051] Exemplary molecules include, but are not limited to, biomolecules, proteins, growth hormones, recombinant proteins, small molecules, therapeutic drugs, chemicals, or gas molecules.

[0052] Nanofiber yarn bundles with improved hydrophilic sidewalls may support novel applications in various technological fields and industries, such as lithium intercalation, gas storage, sustained release of large active agents, novel composite materials, and biotechnology and medical devices (implantable and ex vivo).

[0053] The nanofiber yarns may first be treated to improve their hydrophilicity. Bundles of the treated yarns may then be twisted together to form twisted nanofiber yarn bundles. The twisted nanofiber yarn bundles may then be further untwisted to produce false-twisted nanofiber yarn bundles, as detailed above.

[0054] As the hydrophilicity of the nanofiber yarn surface changes, the van der Waals forces between the nanofibers also change. The reduction in van der Waals forces can lead to loosely packed and false twisted nanofiber yarn bundles, limiting the usefulness and applications of such materials. However, loosely packed nanofiber yarn bundles may offer opportunities and / or advantages for the migration or penetration of foreign substances across the nanofiber yarn bundle itself.

[0055] Further Considerations The foregoing description of embodiments of the present disclosure has been presented for purposes of illustration and is not intended to be exhaustive or to limit the scope of the claims to the precise form disclosed. Those skilled in the art will recognize that many modifications and variations are possible in light of the above disclosure.

[0056] The language used herein has been selected primarily for purposes of readability and instruction, and the language may not have been selected to describe or limit the subject matter of the present disclosure. Accordingly, the scope of the present disclosure is intended to be limited not by this Detailed Description, but rather by any claims that issue in an application based thereon. Accordingly, the disclosure of the present embodiments is intended to illustrate, but not limit, the scope of the invention, which is set forth in the following Claims.

[0057] While the present invention has been described with reference to certain exemplary embodiments, it is understood that the words used are words of description and illustration, rather than words of limitation. Changes may be made within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the disclosure in its aspects. While the present invention has been described with reference to particular means, materials, and embodiments, the invention is not intended to be limited to the particulars disclosed. Rather, the invention covers all functionally equivalent structures, methods, and uses that are within the scope of the appended claims.

[0058] The illustrations of the embodiments described herein are intended to provide a general understanding of various embodiments. The figures are not intended to serve as a complete description of all elements and features of apparatus and systems that utilize the structures or methods described herein. Many other embodiments may be apparent to those skilled in the art upon reviewing the present disclosure. Since other embodiments may be utilized and derived from the present disclosure, structural and logical substitutions and changes may be made without departing from the scope of the present disclosure. Moreover, the illustrations are merely representative and may not be drawn to scale. Certain proportions within the illustrations may be exaggerated, while other proportions may be minimized. Therefore, the present disclosure and figures should be considered illustrative and not limiting.

[0059] One or more embodiments of the present disclosure may be referred to herein, individually and / or collectively, by the term "invention," merely for convenience and without any intention to intentionally limit the scope of the present application to any particular invention or inventive concept. Additionally, while specific embodiments have been illustrated and described herein, it will be understood that any subsequent arrangements designed to achieve the same or similar purpose may be substituted for the specific embodiment shown. The present disclosure is intended to cover any and all subsequent adaptations or variations of the various embodiments. Combinations of the foregoing embodiments, as well as other embodiments not specifically described herein, will be apparent to those skilled in the art upon reviewing this description.

[0060] The Abstract of the Disclosure is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Additionally, the foregoing Detailed Description may group or describe various features together in a single embodiment for the purpose of streamlining the disclosure. The disclosure should not be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter may be directed to less than all features of any of the disclosed embodiments. Accordingly, the following claims are incorporated into the Detailed Description, with each claim standing on its own as defining separately claimed subject matter.

[0061] The subject matter disclosed above should be considered illustrative and not limiting, and the appended claims are intended to cover all such modifications, extensions, and other embodiments that fall within the true spirit and scope of the present disclosure. Accordingly, to the maximum extent permitted by law, the scope of the present disclosure should be determined by the broadest permissible interpretation of the following claims and their equivalents, and should not be limited or constrained by the foregoing detailed description.

Claims

1. A nanofiber yarn bundle comprising at least two treated nanofiber yarns, The nanofiber yarn bundle, wherein the treating comprises exposing the nanofiber yarn bundle to an oxidizing agent and removing the oxidizing agent.

2. The nanofiber yarn bundle of claim 1 , wherein the treated nanofiber yarn bundle has increased water absorption compared to an untreated nanofiber yarn bundle.

3. 3. The nanofiber yarn bundle of claim 2, wherein the increased water absorption is at least 18 mg of water per mg of the treated nanofiber yarn bundle.

4. The nanofiber yarn bundle of claim 3 , wherein the oxidizing agent is at least one oxidizing gas or at least one strong acid.

5. 5. The nanofiber yarn bundle of claim 4, wherein the at least one oxidizing gas comprises ozone, chlorine, or a combination thereof.

6. 5. The nanofiber yarn bundle of claim 4, wherein the at least one strong acid comprises nitric acid, sulfuric acid, perchloric acid, dichromic acid, permanganic acid, or a combination thereof.

7. 10. The nanofiber yarn bundle of claim 1, wherein the nanotube yarn bundle is a carbon nanotube yarn bundle and the at least two treated nanofiber yarns have microstructurally modified carbon nanotube walls.

8. 1. A method for increasing the hydrophilicity of a yarn bundle, comprising: a) providing a nanofiber yarn bundle; b) selecting an oxidizing agent; c) exposing the nanofiber yarn bundle to the oxidizing agent; d) removing the nanofiber yarn bundle from the exposure to the oxidizing agent and removing the nanofiber yarn bundle from the oxidizing agent; The method comprising:

9. The method of claim 8 , wherein the oxidizing agent comprises at least one acid.

10. The method of claim 8 , wherein the oxidizing agent comprises a combination of at least two acids.

11. 11. The method of claim 10, wherein the two acids are mixed in a 2 to 1 ratio.

12. 11. The method of claim 10, wherein the two acids include at least one of nitric acid, sulfuric acid, and perchloric acid.

13. 11. The method of claim 10, wherein the combination comprises a mixture of nitric acid and sulfuric acid, a mixture of sulfuric acid and potassium dichromate, or a mixture of sulfuric acid and potassium permanganate.

14. 11. The method of claim 10, wherein the oxidizer comprises a mixture of 70% nitric acid and 98% sulfuric acid in a 2:1 volume ratio, respectively.

15. The method of claim 8 , wherein the oxidizer comprises at least one gas.

16. 16. The method of claim 15, wherein the gas is ozone or chlorine.

17. 9. The method of claim 8, wherein the nanofiber yarn bundle is exposed to the oxidizing agent for at least 300 minutes.

18. 9. The method of claim 8, wherein the nanofiber yarn bundle is exposed to the oxidizing agent for at least 30 minutes.

19. 9. The method of claim 8, wherein the nanofiber yarn bundle is exposed to the oxidizing agent for at least 10 minutes.

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