WO2020231277A1

WO2020231277A1 - Functionalization of carbon nanotubes

Info

Publication number: WO2020231277A1
Application number: PCT/OM2019/050005
Authority: WO
Inventors: Muna AL-HINAI; Halima AL-HINAI; Myo Myint; Mohammed AL-ABRI; Ashraf AL-HINAI; Joydeep Dutta
Original assignee: Sultan Qaboos University
Priority date: 2019-05-14
Filing date: 2019-05-14
Publication date: 2020-11-19

Abstract

Methods of preparing functionalized carbon nanotubes (CNTs) and polymer composites thereof are provided. The CNTs are washed with an aqueous acidic solution of HCl. The washed CNTs are functionalized with a solution of acid such as H₂SO₄ and an oxidizing agent selected from KMnO₄, chromic acid, chromates or dichromate (like K₂Cr₂O₇, KCr₂O₄), vanadium pentoxide (V₂O₅), molybdenum trioxide (MoO₃), and bismuth molybdate. No additional heat is required for the oxidation. The resultant functionalized CNTs are treated with a solution of deionized H₂O and H₂O₂.

Description

FUNCTIONALIZATION OF CARBON NANOTUBES

Technical Field

The present disclosure relates to carbon nanotubes (CNTs) and more particularly, to preparation of functionalized CNTs using mild acidic methods.

Background

CNTs are allotropic forms of carbon with a cylindrical nanostructure. CNTs have a high level of one-dimensional p-electron conjugation, mechanical strength, flexibility, electrical conductivity, chemical, and thermal stability, which make CNTs very attractive for use in a variety of applications. Such applications include nanodevices, electronic devices, membranes, composite materials, field emission, conducting plastics, thermal conductors, conductive adhesives, thermal interface materials, structural materials, fibers, catalyst supports, filtration, ceramics, sensors, tissue engineering and fuel cells. However, due to a hydrophobic nature, CNTs tend to agglomerate in bundles. Such agglomeration limits the dispersion of CNTs in various solvents and polymeric membranes and therefore reduces the mechanical properties of composites containing CNTs. Accordingly, the production of such composites at industrial scale remains a challenge. To overcome this limitation, surface functionalization of CNTs has been explored to enhance the dissolution and chemical properties of CNTs. Currently, acidic surface treatment is a common method for CNTs functionalization, which involves the use of strong, concentrated acids like HNO3 and other such harsh conditions to functionalize CNTs on the surface.

Chinese patent, CN102108634 titled, “Preparation method of functionalized carbon,” discloses a method of preparing functionalized carbon fibre. However, the disclosed methods involve the use of strong acids and are time-consuming. Treatment with strong acids causes structural damage and unwanted fragmentation of the CNTs leading to loss of mechanical and electronic properties of CNTs, such as, aspect ratio.

Thus, there is an unmet need for preparing functionalized CNTs to preserve the structural integrity and electronic properties of CNTs and CNT based composites for various industrial applications within a reasonably short time period. Summary of the Disclosure

In one aspect of the present disclosure, a method of preparing functionalized CNTs is disclosed. CNTs are washed with an aqueous acidic solution of HC1. The washed CNTs are functionalized with a solution of an acid (like H2SO4) and an oxidizing agent selected from a group consisting of KMnCC, chromic acid, chromates or dichromate (like K2C1 2O7, KCnCC), vanadium pentoxide (V2O5), molybdenum trioxide (M0O3), and bismuth molybdate. The resultant functionalized CNTs are treated with a solution of deionized H2O and H2O2.

In one embodiment of the present disclosure, CNTs are washed with the aqueous acidic solution of 1 M HC1 for up to 2 hours. In another embodiment, CNTs are washed with the aqueous acidic solution of HC1 refluxed at 50 °C for up to 3 hours. In various embodiments of the present disclosure, the functionalization of the washed CNTs includes a solution of an acid (like H2SO4) at a concentration of up to 6 M and KMnCU at a concentration of up to 0.25 M. The washed CNTs are functionalized for up to 2 hours. In an embodiment the functionalization of washed CNTs occurs at a temperature between 35 to 40 °C. The functionalized CNTs are treated with a solution including deionized H2O and H2O2 at 90 °C for 30 minutes. The CNTs are multi, double or single-walled CNTs. An outer diameter of the CNTs is up to 30 nm and a length is more than 30 pm. In an alternate embodiment, the outer diameter of the CNTs is up to 10 nm and the length is up to 30 pm.

In some embodiments, functionalized CNTs are polymerized to form polymer composites. Further, the functionalized CNTs are fabricated with polyethersulfone (PES) to form PES- CNT membranes. The polymer composites have a tensile strength of about 4 MPa to about 6 MPa and Young's modulus of about 138 MPa to about 165 MPa.

The concentration of CNT loaded in the polymer matrix is an important aspect for the utility of resultant composites. Thus, in certain embodiments, the PES-CNT membranes are loaded with CNT component of about 4% to 10%.

Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

Brief Description of Drawings

FIG. 1 is a schematic diagram showing the steps for mild functionalization of CNTs; FIG. 2 shows the Fourier Transform Infrared (FTIR) spectra of: (a) untreated CNTs, (b) w- CNT washed with 1 M HCl(aq.), and (c) f-CNT-5 treated in (6 M H₂S0₄ + 0.25 M KMn0₄);

FIG. 3 shows the FTIR spectra of functionalized CNT at different acid concentrations: w- CNT washed with 1 M HCl(aq.), f-CNT-1 treated with (4 M H₂S0₄ + 4 M HN0₃), f-CNT- 2 treated with (18 M H₂S0₄ + 15 M HNO3 +0.725 M KMn0₄), f-CNT-3 treated in 18 M H₂S0₄ + 0.725 M KMn0₄), f-CNT-4 treated in (10 M H₂S0₄ + 0.42 M KMn0₄), and f-CNT- 5 treated in (6 M H₂S0₄ + 0.25 M KMn0₄);

FIG.4 shows Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectra of CNT-PES membranes;

FIG.5 shows viscosity measurements of CNT-PES samples at varying shear rate 0.1-100 s ¹ at 25 °C showing non-Newtonian behavior of PES solutions loaded with 4%-10% CNT;

FIG.6 shows Young’s Modulus and elongation at break of the PES and PES-CNT membranes;

FIG.7 shows ultimate strength and yield strength of PES and PES-CNT membranes;

FIG.8 shows stabilized pure water flux (Jo) of the PES-CNT membranes measured at 5.17 bar;

FIG. 9 shows BSA rejection and flux recovery of the membranes measured at 5.17 bar and 25 °C;

FIGS. 10A and 10B show schematic diagrams of types of interactions in PES-CNT mixtures at different CNTs loading that affect the viscoelasticity behavior of the mixtures: (a) PES unit block, (b) PES-2% CNT few CNT tubes interact with PES through H-bonding, (c) PES- 4% CNT more chance of H-bonding between PES and CNT, (d) PES-6% CNT at higher CNT loading CNT-PES interaction and CNT-CNT interactions induce the changes in PES- CNT mixture; and

FIG. 11 shows amplitude sweeps of CNT-PES mixtures at 1 Hz and 25 °C, in which G" is lose modulus indicating liquid property of the mixture and G' is storage modulus indicating solid property of the mixture. Detailed Description

Reference will now be made in detail to specific embodiments or features, examples of which are illustrated in the accompanying drawings. Wherever possible, corresponding or similar reference numbers will be used throughout the drawings to refer to the same or corresponding parts. Moreover, references to various elements described herein, are made collectively or individually when there may be more than one element of the same type. However, such references are merely exemplary in nature. It may be noted that any reference to elements in the singular may also be construed to relate to the plural and vice-versa without limiting the scope of the disclosure to the exact number or type of such elements unless set forth explicitly in the appended claim.

The present disclosure is related to methods for preparing functionalized CNTs and various composites including modified CNTs. The methods include preparation of functionalized carbon nanotubes (CNTs) by washing CNTs with an aqueous acidic solution of HC1. The method further includes functionalizing the washed CNTs with a lower concentration of reagent solution including an acid (like H₂SO₄) and an oxidizing agent selected from a group consisting of KMnCC, chromic acid, chromates or dichromate (like K₂C1₂O_7, KCnCC), vanadium pentoxide (V₂O₅), molybdenum trioxide (M0O₃), and bismuth molybdate. The oxidation may not require an external supply of heat or temperature. The functionalized CNTs are then treated with a solution including deionized H₂O and H₂O₂.

Throughout the application, where CNTs are referenced, CNTs shall include single- walled CNTs, double-walled CNTs, multi-walled CNTs, and mixed CNTs.

As used herein, “carbon nanotube” or“CNT” or“CNTs” refers to hollow structures composed primarily of carbon atoms. CNTs of the present disclosure are primarily include carbon which take the form of tubes, bundles, rods, sheets, cones, yams, plates, cylinders, wafers, disks, planes, slivers, granules, ellipsoids, wedges, polymeric fibers, natural fibers, buckytubes, small-diameter CNTs, fullerene tubes, tubular fullerenes, graphite fibrils, carbon nanofibers, and combinations thereof and other objects which have at least one characteristic dimension less than about 100 nm.

As used herein,“functionalization” or“derivatization” includes any type of general chemical protocol including a functionalizing agent suitable for attaching or replacing or substituting or transforming chemical moieties to/on the CNTs, and particularly to/on the ends and sidewalls of CNTs in either capped or opened state. The functionalization of CNTs generally occurs in acidic media.

As used herein,“functionalized CNTs” include CNTs whose surfaces are modified by a functionalizing agent to attaching or replacing or substituting or transforming chemical moieties associated therewith. The attachment can be uniform or non-uniform and can be on any surface of the CNTs.

As used herein,“oxidizing agents” include mild oxidizing agents. Examples of oxidizing agents include a mild oxidizing agent such as KMnCC, chromic acid, chromates or dichromate like K₂C1₂O_7, KC¾q₄, vanadium pentoxide (V₂O₅), molybdenum trioxide (M0O₃), bismuth molybdate and alike.

As used herein, “single- walled CNTs” or “SWNTs” include CNTs which are one dimensional and single-walled and include sheets of graphene rolled to form a hollow structure cylindrical tube of one atom thickness. “Double- walled” include CNTs which are single dimensional and double walled with one CNT nested into another CNT. As used herein,“multi-walled” or“MW CNTs” include CNTs which have multi-rolled layers of CNTs nested into each other in the shape of concentric cylinders.

As used herein, “polymer composite” or“composite” or“polymer nanocomposite” or “composite material” refers to a multi-phase material which has a polymer matrix integrated with reinforcing fillers resulting in synergetic mechanical properties otherwise lacking in the components of the composite.

As used herein, “polymer” refers to polysulfones including PES, fluoroelastomers, perfluoroelastomers, rubber, silicones, epoxy, polyetheretherketone, bismaleimide, polyethylene, polyvinyl alcohol, phenolic resins, nylons, polycarbonates, polyesters, polyurethanes, copolymers, silicone, polyketone, polyterpene, polyolefin, vinyl polymer, fluoropolymer, biopolymer or a combination comprising at least one of the foregoing resins.

As used herein,“Young’s modulus” or“modulus of elasticity” is the mechanical property that measures the stiffness of a material. It is equal to the longitudinal stress divided by strain. The SI unit of Young’s modulus is Pascal (Pa). Commonly used units include megapascals (MPa) and gigapascals (GPa or kN/mm²).

As used herein,“tensile strength” or“ultimate strength” is the maximum capacity of a material to withstand loads before failure, such as breaking and bending. As used herein,“aspect ratio” is the length-to-diameter ratio of the CNT sizes in different dimensions. The average aspect ratio of the nanotubes is calculated from the ratio of average length to average diameter. The aspect ratio is thus calculated as the average ratio of the highest to the lowest dimension over several similar CNTs.

The use of the terms“include,”“includes”,“including,”“have,”“has,” or“having” should be generally understood as open-ended and non-limiting unless specifically stated otherwise.

The use of the singular herein includes the plural (and vice versa) unless specifically stated otherwise.

The order of steps or order for performing certain actions is immaterial so long as the method disclosed in the present disclosure remains operable. Moreover, two or more steps or actions may be conducted simultaneously.

Due to high aspect ratio, CNTs display strong mechanical and thermal properties. As, CNTs tend to agglomerate in bundles due to hydrophobic nature, functionalization of CNTs has emerged as a common method to enhances the distribution of CNTs in solvents and composites. According to the present disclosure, the method includes functionalizing CNTs without causing structural damage and random fragmentation which results in the loss of aspect ratio, mechanical and electronic properties of CNTs.

The present disclosure as illustrated in FIG. 1 includes the method to functionalize CNTs by using mild oxidation agents in low concentrations without application of external heat as heat is generally evolved during the oxidation process and preserving the aspect ratio and mechanical properties of CNTs. The method also reduces the chances of peroxide formation during functionalization.

In an embodiment, CNTs which are commercially available are washed with an aqueous acidic solution of HC1 to remove any catalyst impurities from production. The washed CNTs are then dispersed in a solution including H2SO4 and KMnCC at low concentrations to functionalize the CNTs thus limiting structural damage to the washed CNTs. This treatment results in the functionalization of CNTs, generally with the formation of carboxylic groups (COOH).

Numerous variations on the present disclosed method exist and should be considered as alternative embodiments of the present disclosure. For example, in an embodiment of the present disclosure, the functionalization of CNTs occurs in cooled an acid (like H₂SO₄) in either ice or water bath at a temperature of around 5 °C. In yet another embodiment, the present disclosure includes the step of terminating the oxidation process by dilution with deionized water and the addition of H2O2. In an example, the termination step includes addition of 30% H2O2.

The present disclosure includes methods of functionalization of CNTs with reagents at a comparatively lower amount and concentration known previously. Thus, for example, in an embodiment, the amount and concentration of the reagents for functionalization and oxidation process steps is 2-15 times lower than known in the prior art. In some embodiments, the concentration of KMnC and that of H2O2 is at least three times less than the concentration and amount of the respective reagents known previously. In one example, the amount and concentration of H2SO4 is 10 times lower than known previously.

While most of the methods proposed in the prior art require utilizing heat and higher temperatures for initiating oxidation, the oxidation step of the present method is advantageous in initiating the oxidation without the requirement of any external supply of heat or specific temperature.

In one embodiment, H2O2 is used as a reducing agent that reduces KMnC and terminates the oxidation process at 90 °C for 30 mins. In another embodiment, treating the functionalized CNTs with the solution including deionized H2O and H2O2 at 90 °C for 30 minutes. A person skilled in the art will appreciate that the reaction conditions such as the concentration of H2O2, time, and temperature can be changed with respect to the used concentration of oxidants.

In an embodiment of the present disclosure, the method of functionalizing CNTs includes the step of oxidation at the temperature of the carboxylation reaction of up to 40 °C to control the degree of functionalization of CNTs.

In another embodiment of the present disclosure, the method includes washing the CNTs with the aqueous acidic solution of 1 M HC1 for up to 2 hours. In some embodiments, the CNTs are washed in the aqueous acidic solution of HC1 for 1 hour. In another embodiment, the washing time of CNTs is between 30 minutes to 1 hour. Further, CNTs are washed with the aqueous acidic solution of HC1 refluxed at 50 °C for up to 3 hours.

In an embodiment, the CNTs used for functionalizing are single-walled CNTs. In alternate embodiments, multi-walled CNTs (MWCNTs) and double-walled CNTs are used for the functionalizing reaction. The outer diameter of the CNTs is up to 30 nm and the length is up to 50 pm. The treatment method of the present disclosure can create smaller CNTs, and the outer diameter of the CNT is up to 10 nm and length is up to p30 m.

Another advantage of the present disclosure is to fabricate a polymer composite in which CNTs are dispersed within the polymer matrix. Yet another advantage is to fabricate polymer composite membranes with superior water flux, BSA rejection, and antibacterial activity. CNTs are mixed with PES to fabricate PES-CNT membranes. The polymer composites comprise a tensile strength of about 4 MPa to about 6 MPa and Young's modulus of about 138 MPa to about 165 MPa. In specific examples, the polymer composites and/or fabricated membranes display Young’s modulus between about 138 MPa to 163 MPa. The PES-CNT membranes comprise a CNT component of about 4% to 10%. Thus, the PES membranes fabricated with 8% CNT display a high Young’s modulus of about 149.1.

In some embodiments, the tensile strength of the composite fabricated with 8% CNT is up to 5.8 MPa. In yet another embodiment, the tensile strength of the composite is between 4 MPa and 5 MPa. The polymer composite and fabricated membranes display Young’s modulus up to 165 MPa and tensile strength up to 6 MPa. In one example, the composites of the present disclosure display Young’s modulus of 149.1 MPa and a tensile strength of 5.8 MPa with PES membranes loaded with 8% CNT.

Industrial Applicability

An objective of the present disclosure is to provide milder acidic treatment for CNT functionalization which is more economical and better for the environment and has safer practical usage.

An advantage of the present disclosure is to reduce the time of reaction and therefore limit the structural damage that occurs during the functionalization of CNTs. Another advantage of the present disclosure is to enable functionalization of CNTs at reduced reagent consumption to the concentrations while ensuring proper functionalization as desired. Thus, in an embodiment, a lower concentration of H2SO4 up to 6 M or KMnCC up to 0.25 M or total combined concentration is used for controlled carboxylation of the CNTs limiting structural damages to the CNTs and sound end-products. Yet another advantage of the present disclosure is that it reduces the time of reaction of keeping the CNTs in acidic conditions. Thus, in an embodiment of present disclosure, the time of reaction is up to 2 hours. An essentially economical advantage of the mild chemical functionalization is the lower consumption of chemicals, materials, and containers used to carry out the reaction. For example, the table below shows the differences in using mild acidic treatment compared with concentrated acids. The total reduction in cost is 67-75 %. Chemicals cost of CNTs treatment scaled up to 1 kg CNT using the method of present disclosure and two reference methods where concentrated acids are used. The prices from Sigma Aldrich for all the chemicals and Timesnano for MWCNTs are converted from Euro to US dollar (1 Euro= 1.13 US $, 12^th March 2019)

Another objective of the present disclosure is to produce reinforced polymer composites by mixing CNT with a variety of polymers for membrane production and other applications.

Another application of the present disclosure is the use of composites of the disclosure in drug delivery applications, and tissue engineering.

Examples and Experimentals

The following examples are provided to illustrate further and to facilitate the understanding of the present disclosure.

Example 1

Referring to FIG. 2, the method of the present disclosure is used to improve the performance of PES ultrafiltration membranes. PES membranes were fabricated and tested for water treatment.

Functionalization of CNTs

CNTs were treated in HC1 to remove impurities and catalysts remaining from production processes. CNTs (5 g) were dispersed in 1 M HCl(aq) (500 mL) with sonication for 30 minutes to de-bundle the CNTs. The composition was then refluxed at 75-80 °C for 3 hours under continuous magnetic stirring following which CNTs were filtered using sintered glass filter by vacuum filtration and washed repeatedly with warm water (50 °C). The filtered CNTs were then refluxed in deionized water at 50 °C overnight to remove acidic remnants. The composition was finally filtered and again washed repeatedly with deionized warm water and then dried in vacuum oven at 20 °C for 48 hours. As illustrated in FIG. 2, the carboxylation peaks appear as C=0 at 1714 cm ¹ and asymmetric -COOH at 1422 cm ¹ in FTIR spectrum after H₂SO₄/ KMnCF treatment as compared to the infrared spectra of untreated and washed CNTs.

Oxidation of CNTs with H2SO4 and KMnCC

Pre-washed CNTs (5 g) were mixed with the H2SO4 (6 M) in ice-water bath at 5 °C. The obtained black slurry was then sonicated for 30 minutes to disperse the CNTs. Following this, KMn0₄(s) (0.2414 g, 0.004551 mol) was slowly added under continuous stirring at controlled temperature of 35- 40 °C for 2 hours. To stop the reaction, FhCKl) and FhC iaq) were slowly added and the composition was refluxed at 90 °C for 30 minutes. After cooling the suspension to room temperature, CNTs were collected by centrifugation at 10,000- 30,000 rpm and then washed with deionized water repeatedly until neutrality was achieved. The solute consisting of the CNTs were then filtered out and subsequently dried in a vacuum oven at 50 °C for 48 hours.

CNT-PES membrane fabrication

Functionalized CNTs were dispersed in N-Methyl-2-pyrrolidone and then polyethersulfone was added under continuous magnetic stirring. Homogenous CNT-PES composition was formulated by mechanical dispersion using Qsonica probe sonicator at 850 W for 5 minutes and then further homogenized by stirring for 24 hours. The composition was sonicated for 30 minutes and then kept stagnant for 1 hour to remove air bubbles prior to membrane casting. Membranes were prepared using phase inversion method by spreading the PES- CNT composition on a glass slide to form a film which was polymerized in water bath at room temperature (~22 °C). The casting thickness was fixed at 240 pm for all the membranes.

Characterization of PES-CNT membranes As illustrated in FIG. 4, The functional groups of the CNTs and the PES-CNTs membrane surfaces were identified from the Fourier transform infra-red (FTIR) spectra. Attenuated total reflection (ATR) technique was used to collect infrared spectra in a FTIR spectrometer supplied by PerkinElmer (SpectraOne) recorded in the 4000-400 cm ¹ range with a resolution of 4 cm ¹ and an average of 32 scans were considered for each measurement.

Mechanical Properties of PES-CNT membranes

FIG. 6 and FIG. 7 show the mechanical properties (Young’s Modulus, Elongation at break, ultimate strength, and yield strength) of PES-CNT membranes. These properties were calculated and extracted from the measured true stress - true strain curves of the membranes.

Young’s modulus and Tensile Strength of membranes

Young’ s modulus and the other mechanical properties were slightly reduced by CNT loading in PES matrix (by maximum 6%) in the membranes fabricated during this study. The membranes fabricated with 8% CNT loading was found to yield the highest mechanical reinforcement with the Young’s modulus increasing from 137.4 MPa for untreated PES membrane to 149.1 MPa for the PES-8% CNT membrane. Ultimate strength and yield strength also increased from 4.7 to 5.8 MPa and from 2.5 to 2.9 MPa respectively. The membranes were also more elastic at break point as the elongation of the membrane increased by 2% compared to untreated PES membranes. These observations suggest that the CNT self- assembled in ordered structure within the PES matrix, but more detailed analysis needs to be carried out prior to conclusive assertion. The young’s modulus further increased to 163 MPa in PES-CNT composites fabricated with 10% CNT loading, but the membranes turn brittle and break at 9.5% elongation which is 50% lower than the elongation observed in PES-8% CNT membranes.

Water flux and Bovine Serum Albumin (BSA) rejection of the membranes

The water flux of the PES membranes which were blended with carboxylated CNT is shown in FIG. 8. Higher water flux was observed for the CNT- blended membranes, except for the 2% CNT- loaded composite membranes. The highest water flux was recorded for the PES membranes blended with 4% CNT that stabilized at 81.9 L.m^h ¹ compared to 24.0 L.m- 2h ¹ water flux recorded for untreated PES membrane. This membrane and the PES membrane loaded with 10% CNTs show unique flux performance including superior water flux, viscoelastic properties, and BSA filtration performances. It was observed from XPS results PES-4% CNT membrane has free carboxylic acid and functional groups at the surface that can attract more water molecules.

The rejection and water flux recovery after 30 minutes of BSA filtration and 30 minutes of water flow, respectively, is reported in FIG. 9. In the first 30 minutes, BSA rejection for the PES membrane was 85% and the rejection for CNT-blended membranes was in the range 55-96%. For all the membranes, BSA rejection was higher than 95% within 90 minutes of filtration. The increase in rejection with time indicates the formation of the deposited BSA layer that was rejected by the membrane inducing fouling. Except for the 2%-CNT membranes, CNT- blended membranes showed higher BSA rejection. Carboxylic groups in functionalized CNT enhanced the rejection of BSA molecules thus lowering the adsorption of BSA at the membrane surface and pores thus reducing biofouling.

CNTs were successfully oxidized by milder acidic treatments using H2SO4 and KMnCC mixture that improved the dispersion within the PES matrix upon probe sonication. By blending PES with the functionalized CNTs, the water flux was significantly enhanced. The highest water flux of the PES-CNT membranes was enhanced by ~ 70.6%. PES-4%CNT membranes due to their higher hydrophilicity compared to untreated PES membranes. Higher BSA rejection similarly was achieved up to 96.7%. The enhancement of the PES- CNT membranes performance was due to the presence of functional groups at the CNTs surface. The alignment of CNTs within PES and the orientation of the functional groups influence the increase in mechanical strength as well as the water flux and BSA rejection.

Example 2

Referring to FIG. 3, CNTs were functionalized in acidic treatment to enhance dispersion within PES matrix. Functionalization steps were followed as detailed in Example 1 above. To reduce the fragmentation and avoid nitration, HNO3 was removed from the process (f- CNT-3) while keeping concentrated 18 M H2SO4 with KMn0₄ as oxidants whereby the carboxylation peaks appear as C=0 at 1714 cm ¹ and asymmetric -COOH at 1422 cm ¹ in FTIR spectrum. H2SO4 concentration was further reduced to 10 M (f-CNT-4) and then to 6 M (f-CNT-5). By reducing the H2SO4 to 6 M in presence of KMn0₄, less CNT fragmentation was observed for similar levels of carboxylation.

CNT-PES mixture properties Polyethersulfone- CNT composites were prepared using f-CNT-5 which was selected as the optimized carboxylated CNTs with least tube damage. The rheological properties of PES- CNT compositions were studied to investigate the effect of CNTs functionalization and homogeneity with polyethersulfone. FIG. 5 shows the viscosity of CNT-PES mixtures at varying shear rates in the range 0.1-100 s ¹ at 25 °C. For low concentration of CNTs (< 4%) in admixture, a Newtonian behavior identical to the pristine PES solution is observed. Upon adding 4% CNT, the mixture is found to have non-Newtonian behavior. Shear thickening behavior is observed at low shear rates (0.1-0.3 s ¹) followed by shear thinning, when the shear rate is increased from 0.3 s ¹ to 100 s ¹. The overall viscosity is increased with the addition of CNTs in the polymer mixture. To further understand the rheological behavior of the solutions, dynamic rheological measurements are performed including amplitude sweep and frequency sweep analyses at 25 °C. The amplitude sweep curves at constant frequency (1 Hz) are shown in FIG. 11. All the analyzed mixtures have higher viscous modulus than the elastic modulus, which indicates that the PES-CNT polymer mixtures have more liquid property. Both PES and 2% CNT-PES behave as Newtonian solutions. Upon increasing the CNTs content to more than 4% loading, storage modulus was found to drop sharply with increasing shear stress. Though 4% CNT and 10% CNT in the polymer lead to higher moduli at low shear stress compared to the other compositions, the linearity of the curves changed at higher shear stress showing more pronounced viscoelastic behavior. The unique behavior of the 4% CNT composition can be explained from the charge compensation between the functionalized CNTs and the sulfone group in the PES chain. At 4% CNT loading, the effect of CNT- PES interaction was a dominant factor. By increasing the CNT up to 6% and 8%, the amount of -COOH groups increased inducing CNT-CNT interaction which enhance the dynamic elasticity of the composition. A substantially higher loss and storage moduli are achieved by adding 10% CNT to the PES solution.

Polyethersulfone- CNT compositions with different f-CNT (functionalized CNT) loading were prepared as illustrated in FIGS. 10A and 10B, and their viscosity and viscoelasticity were measured. Non-Newtonian behavior was observed with 4% or higher CNT loading in PES matrix showing shear thinning behavior as CNT loading was further increased. The prepared composition showed liquid like properties at the studied CNTs loading (0-10%) and no gelation was observed at low frequency oscillation. Storage modulus and loss modulus properties of the compositions increased generally with increasing CNTs loading, with emphasis that 4% CNTs and 10% CNTs in PES compositions were a unique characteristic related to -COOH groups and their interaction with sulfone groups at PES chains.

It is understood that the examples, embodiments and teachings presented in this application are described merely for illustrative purposes. Any variations or modifications thereof are to be included within the scope of the present application as discussed.

Claims

Claims What is claimed is:

1. A method of preparing functionalized carbon nanotubes (CNTs), the method comprising:

washing CNTs with an aqueous acidic solution of HC1;

functionalizing the washed CNTs with a solution comprising of an acid (like H2SO4) and an oxidizing agent selected from a group consisting of KMnCC, chromic acid, chromates or dichromate (like K2C12O7, KCnCC), vanadium pentoxide (V2O5), molybdenum trioxide (M0O3), and bismuth molybdate; and

treating the functionalized CNTs with a solution comprising deionized H2O and

H2O2.

2. The method of claim 1 further comprising, washing the CNTs with the aqueous acidic solution of 1M HC1 for up to 2 hours.

3. The method of claim 1 further comprising, washing the CNTs with the aqueous acidic solution of HC1 refluxed at 50 °C for up to 3 hours.

4. The method of claim 1 further comprising, functionalizing the washed CNTs with a solution comprising H2SO4 at a concentration of up to 6 M and KMnCC at a concentration of up to 0.25 M.

5. The method of claim 1 further comprising, treating the functionalized CNTs with the solution comprising deionized H2O and H2O2 at 90 °C for 30 minutes.

6. The method of claim 1 further comprising, functionalizing the washed CNTs for up to 2 hours.

7. The method of any one of the preceding claims, wherein the CNTs are multi, double or single- walled CNTs.

8. The method of claim 7, wherein an outer diameter of the CNTs is up to 30 nm and a length is up to 50 pm.

9. The method of claim 8, wherein the outer diameter of the CNTs is up to 10 nm and the length is up to 30 pm.

10. The method of any one of the preceding claims further comprising, polymerizing the functionalized CNTs to form polymer composites.

11. The method of any one of the preceding claims further comprising, fabricating the functionalized CNTs with polyethersulfone (PES) to form PES-CNT membranes.

12. The method of claims 10-11, wherein the polymer composites comprise a tensile strength of about 4 MPa to about 6 MPa.

13. The method of claims 10-11, wherein the polymer composites comprise Young's modulus of about 138 MPa to about 165 MPa.

14. The method of claim 11, wherein the PES-CNT membranes comprise a CNT component of about 4% to 10%.