WO2008030038A1

WO2008030038A1 - Pre-treatment method of carbon nanotube for carbon nanotube/polymer composite, manufacturing method for carbon nanotube/polymer composites and carbon nanotube/polymer composites using the carbon nanotube

Info

Publication number: WO2008030038A1
Application number: PCT/KR2007/004286
Authority: WO
Inventors: Woo-Nyon Kim; Mi-Sun Han; Ji-Woong Chung
Original assignee: Korea University Industry and Academy Cooperation Foundation
Priority date: 2006-09-05
Filing date: 2007-09-05
Publication date: 2008-03-13
Also published as: KR20080021967A

Abstract

Provided is a pretreatment method of carbon nanotubes for carbon nanotube/polymer composite, a method for preparation of a carbon nanotube/polymer composite using the same, and a carbon nanotube/polymer composite prepared therefrom. The pretreatment method of carbon nanotubes in accordance with the present invention includes 1) sonicating carbon nanotubes in a hydrogen peroxide (H O ) solution at a temperature of 3O0C to 7O0C for 50 to 130 min; 2) adjusting a pH of the product of Step 1 to a desired range; and 3) freeze-drying the product of Step 2. The pretreatment method of the present invention increases the dispersibility of carbon nanotubes, and enables production of a carbon nanotube/ polymer composite having excellent dispersibility, electrical properties and rheological properties. Further, the carbon nanotube/polymer composite of the present invention has excellent electrical conductivity and electromagnetic wave-shielding effect.

Description

Description PRE-TREATMENT METHOD OF CARBON NANOTUBE FOR

CARBON NANOTUBE/POLYMER COMPOSITE, MANUFACTURING METHOD FOR CARBON NANOTUBE/POLYMER COMPOSITES AND CARBON NANOTUBE/POLYMER

COMPOSITES USING THE CARBON NANOTUBE Technical Field

[1] The present invention relates to a method for pretreatment of carbon nanotubes for carbon nanotube/polymer composite. More specifically, the present invention relates to a method for pretreatment of carbon nanotubes for carbon nanotube/polymer composite, which enables production of the carbon nanotube/polymer composite having excellent dispersibility, electrical properties and rheological properties; a method for preparation of a carbon nanotube/polymer composite using the same; and a carbon nanotube/polymer composite prepared therefrom. Background Art

[2] Since the first discovery of carbon nanotube (CNT) synthesis through the arc discharge process by Sumio Iijima, such a new carbon material has received a great deal of attention, and intensive research and study on CNT have been actively undertaken by numerous domestic and foreign research groups and institutions. Carbon nanotubes, which were first discovered in 1991 by Iijima, have excellent physical properties such as mechanical strength 200-fold higher than that of steel, superior modulus of elasticity, thermal resistance that can withstand a high temperature of 2800⁰C under vacuum conditions, thermal conductivity approximately two-fold higher than that of diamond and capability to transfer electric current about 1000-fold higher than that of copper, and are therefore evaluated to have a very high applicability in all fields of engineering.

[3] In general, the carbon nanotubes are carbon materials having a diameter of 1 to 100 nm, a length of several nm to several tens of D and consequently a high aspect ratio, the carbon nanotubes are a rolled up sheet of a planar honeycomb lattice in which each atom is connected via a strong chemical bond to its three adjacent atoms. There are various kinds of CNT which may be classified into single- walled carbon nanotube(SWNT) and multi-walled carbon nanotubes(MWNT), depending upon the number of wall constituting nanotube. If the number of wall is two or more, CNT is classifed into multi- walled carbon nanotube, and if the numer of wall is one, it is classifed into single- walled carbon nanotube. [4] Due to their high surface area and aspect ratio, the carbon nanotubes also have an advantage of achieving a percolation threshold (a minimum content of the carbon nanotube at which a carbon nanotube/polymer composite exhibit electrical conductivity) in electrical properties even with the addition of a much lower amount of the carbon nanotue, as compared to conventional carbon fibers. The conventional carbon fibers have a percolation threshold of 8 to 20% by weight as their normal electrical properties, whereas use of the carbon nanotubes leads to a decrease in a percolation threshold. The percolation threshold of carbon nanotubes was reported to be 2% by weight in the study of Potschke on polycarbonate (European Polymer Journal 40 (2004) 137-148). But, this value is not so small either.

[5] Further, due to high electrical conductivity (EC), high thermal stability, high tensile strength and high restoration ability, the carbon nanotubes are used as additives for a variety of composite materials. For preparing functional composite materials to which carbon nanotubes are added, it is important to disperse bundles of the carbon nanotubes in a solvent effectively. For example, the preparation of a polymer composite material with carbon nanotubes essentially requires homogeneous dispersion of the carbon nanotubes in a polymer matrix. However, there has been a problem that the carbon nanotubes has a very low dispersivity in the solvent, due to a long length and strong interactive attraction between the carbon nanotubes.

[6] As an approach to overcome the problems and disadvantages as described above, extensive and intensive studies and experiments have been made to find a method to mix carbon nanotubes and polymer materials by imparting dispersibility to the carbon nanotubes via chemical and physical pretreatment processes.

[7] As an example of such a method known hitherto in the art, there is a method of primarily enhancing the dispersibility of carbon nanotube materials by oxidizing the carbon nanotube's surface with a strong acid solution such as sulfuric acid, hydrochloric acid, nitric acid, or the like. However, this method has a problem associated with a difficulty in disposing by-products becasue of the strong acid solution. Further, thermal drying of the carbon nanotube treated by the stron acid results in insufficient dispersibility of carbon nanotubes.

[8] Further, besides the method of imparting the desired dispersibility to the carbon nanotube materials by means of surface modifications of the carbon nanotubes as described above, a method of preparing a carbon nanotube/polymer composite having excellent dispersibility originally has not been investigated sufficiently. Disclosure of Invention Technical Problem

[9] Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for pre treating carbon nanotubes, thereby increasing dispersibility of the carbon nanotubes for a carbon nanotube/polymer composite. [10] It is another object of the present invention to provide a method for preparing a carbon nanotube/polymer composite having excellent dispersibility, electrical and rheological properties. [11] It is a further object of the present invention to provide a carbon nanotube/polymer composite having excellent dispersibility, electrical properties and rheological properties

Technical Solution [12] In accordance with an aspect of the present invention, the above and other objects can be accomplished by a method for pretreating carbon nanotubes for a carbon nanotube/polymer composite, comprising 1) sonicating carbon nanotubes in a hydrogen peroxide (H O ) solution; 2) adjusting a pH of the product of Step 1 to a desired range; and 3) freeze-drying the product of Step 2. [13] In one embodiment of the present invention, sonication of the carbon nanotubes in

Step 1 may be carried out at a temperature of 3O⁰C to 7O⁰C for 50 to 130 min. [14] In another embodiment of the present invention, the carbon nanotubes may be single- walled carbon nanotubes (SWNTs) or multi-walled carbon nanotubes (MWNTs). [15] In a further embodiment of the present invention, the pH of Step 2 may be adjusted to the range of 5.0 to 7.0. [16] In accordance with another aspect of the present invention, there is provided a method for preparing a carbon nanotube/polymer composite, comprising sonicating the carbon nanotubes prepared by the aforesaid pretreatment method and a polymer in a solvent. [17] In one embodiment of the present invention, the solvent may be any one selected from the group consisting of water, tetrahydrofuran (THF), dimethylformamide

(DMF), toluene, ethanol, and any combination thereof.

[18] In another embodiment of the present invention, the polymer may be polycarbonate.

[19] In a further embodiment of the present invention, a content of the carbon nanotubes may be in the range of 0.5 to 2.0% by weight, based on the weight of polycarbonate. [20] In accordance with a further aspect of the present invention, there is provided a carbon nanotube/polymer composite prepared by the aforesaid method for preparing a carbon nanotube/polymer composite.

Advantageous Effects

[21] A method for pretreatment of carbon nanotubes for a carbon nanotube/polymer composite in accordance with the present invention increases the dispersibility of carbon nanotubes, and enables production of a carbon nanotube/polymer composite having excellent dispersibility, electrical and rheological properties. Further, the carbon nanotube/polymer composite of the present invention has excellent electrical conductivity and electromagnetic wave-shielding effect.

Brief Description of the Drawings [22] FIG. 1 is a view showing a chemical structure of multi- walled carbon nanotubes

(MWNTs) which were acid- treated using hydrogen peroxide (H O ); [23] FIG. 2 is an infrared (IR) spectrum for carbon nanotubes of Example 1 and

Comparative Example 2; [24] FIG. 3 is a graph showing thermogravimetric analysis (TGA) results for carbon nanotubes of Example 1, Comparative Examples 1 and 2; [25] FIG. 4 is a series of a field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM; JEM-2000EX/T) images for carbon nanotubes of Example 1 and Comparative Examples 1 and 2; [26] FIG. 5 is a graph showing electrical conductivity (EC) of carbon nanotube/polymer composite of Example 2, Comparative Examples 3 and 4; [27] FIG. 6 is a graph showing storage modulus of carbon nanotube/polymer composite of

Example 2-(4), Comparative Examples 3- (4) and 4-(l); and [28] FIG. 7 is a graph showing viscosity of carbon nanotube/polymer composite of

Example 2-(4), Comparative Examples 3- (4) and 4-(l).

Best Mode for Carrying Out the Invention [29] Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings. [30] In one aspect of the present invention, a method for pretreating carbon nanotubes for a carbon nanotube/polymer composite comprises 1) sonicating carbon nanotubes in a hydrogen peroxide (H O ) solution at a temperature of 3O⁰C to 7O⁰C for 50 to 130 min;

2) adjusting a pH of the product of Step 1 to a desired range; and 3) freeze-drying the product of Step 2. [31] By sonication of carbon nanotubes in the hydrogen peroxide (H O ) solution at a temperature of 3O⁰C to 7O⁰C for 50 to 130 min in Step 1, it is possible to oxidize the carbon nanotubes with H O solution, such that the carbon nanotubes have a suitable

2 2 length via introduction of substituents. Further, application of ultra sonic waves can also provide disentanglement of the randomly entangled carbon nanotubes. Concurrent treatment of hydrogen peroxide and sonication may render the carbon nanotubes to have dispersibility by shortening the length of the carbon nanotubes and maintaining a constant distance between the carbon nanotubes. Further, as shown in FIG. 1, the above oxidation treatment leads to substitution of carbon atoms in carbon nanotubes with carboxyl and hydroxyl groups, and therefore functional groups of the oxidized carbon nanotubes may increase the repulsive force between the carbon nanotubes to thereby improve the dispersibility of carbon nanotubes. If the sonication temperature is lower than 3O⁰C, this may result in poor oxidation of carbon nanotubes. On the other hand, if the sonication temperature is higher than 7O⁰C, this may result in excessive oxidation of carbon nanotubes. Further, if the sonication time is shorter than 50 min, this may also lead to insufficient oxidation of carbon nanotubes. On the other hand, if the sonication time is longer than 130 min, this may result in excessive oxidation of carbon nanotubes.

[32] The mixture solution obtained from Step 1 may be allowed to stand at room temperature to thereby lower a temperature thereof, followed by centrifugation. Hydrogen peroxide was removed from the thus-separated mixture solution to which distilled water was then added to make a mixture, followed by centrifugation to separate the solution. In this manner, pH adjustment of Step 2 can be carried out to reach a pH which is close to neural pH. The removal of hydrogen peroxide via the pH adjustment process of Step 2 can prevent deterioration of electrical or rheological properties of carbon nanotubes which may result from excessive oxidation of carbon nanotubes. The excessive oxidation of carbon nanotubes leads to a decreasing length of individual carbon nanotubes, which in turn disadvantageously results in aggregation of carbon nanotubes. Preferably, the pH of solution in Step 2 is adjusted to the range of 5.0 to 7.0. If the pH is lower than 5.0, residual hydrogen peroxide may cause the aforementioned problems. Achieving the pH exceeding 7.0 should disadvantageously use an additional basic solvent.

[33] Next, the product obtained from Step 2 is freeze-dried, which can therefore solve the problem of poor dispersibility suffered by conventional thermal drying. When the conventional thermal drying is adopted, carbon nanotubes continue to undergo oxidation to thereby decrease the length of carbon nanotubes, due to the remaining hydrogen peroxide which was not completely evaporated in thermal drying, and the carbon nanotube-carbon nanotube distance decreases due to thermal evaporation of the solvent, which consequently leads to excessively high van der Waals attraction in a sol vent-removed narrow space, thereby resulting in re-aggregation or entanglement of carbon nanotubes. Therefore, use of the aforementioned freeze-drying method instead of the conventional thermal drying method can prevent excessive oxidation and re- aggregation of carbon nanotubes, thereby improving the dispersibility of carbon nanotubes. Further, unlike thermal drying, the freeze-drying also provides better dispersibility of carbon nanotubes when nanotubes are subsequently dissolved in the solvent, due to formation of interparticle pores which arises from sublimation of liquid which was present between particles. [34] As the carbon nanotubes, there may be employed any kind of carbon nanotubes including single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs), and the like. MWNTs are economically preferred.

[35] In another aspect, the present invention provides a method for preparing a carbon nanotube/polymer composite, comprising sonicating the carbon nanotubes prepared by the aforesaid pretreatment method and a polymer in a solvent. Since the freeze-dried carbon nanotubes prepared by the aforesaid pretreatment method have excellent dis- persibility, sonication of the carbon nanotubes and polymer in the solvent enables production of the carbon nanotube/polymer composite where carbon nanotubes are uniformly dispersed in the polymer.

[36] Examples of the solvent that can be used in the present invention may include water, tetrahydrofuran (THF), N,N-dimethylformamide (DMF), toluene, ethanol, and the like. Preferred is tetrahydrofuran. This is because a zeta potential of the carbon nanotubes prepared by the aforesaid pretreatment method has a low value in the tetrahydrofuran solvent and therefore the carbon nanotubes have high repulsive force therebetween, thus providing excellent dispersibility.

[37] There is no particular limit to the polymer that can be used in the present invention.

Polycarbonate may be advantageously used in terms of strength. The content of the carbon nanotubes may be in the range of 0.5 to 2.0% by weight, based on the weight of polycarbonate. If the content of the carbon nanotubes is lower than 0.5% by weight, deficiency of the carbon nanotubes may bring about the failure of formation of pathways between the carbon nanotubes, through which electricity conducts. On the other hand, the content of the carbon nanotubes is higher than 2.0% by weight, it is not desirable because an increase in electrical conductivity is insignificant despite increasing amounts of carbon nanotubes to be used. Further, according to the present invention, a percolation threshold of multi- walled carbon nanotubes (MWNTs )/polycarbonate is 0.5% by weight in terms of the electrical conductivity, thus having superior electrical properties, as compared to 5.0% by weight for non- pretreated MWNTs and 2.0% by weight for MWNTs which were heat-dried following oxidation.

[38] With reference to the research result of Potschke obtained through a conventional rheological experiment on polycarbonate, the percolation threshold of carbon nanotubes was 2.0% by weight. However, according to the present invention, the percolation threshold of carbon nanotubes is 0.5% by weight. Therefore, it can be seen that electrical conduction takes place even with use of a very tiny amount of carbon nanotubes, exhibiting superior effects as compared to a conventional prior art. It is considered that the reason for the above-mentioned excellent electrical properties is because excellent dispersibility of carbon nanotubes in accordance with the present invention ensures uniform dispersion of carbon nanotubes in the polymer, and the carbon nanotubes have a length suitable to form carbon nanotube-carbon nanotube networks even at a low content of carbon nanotubes, thereby forming pathways through which electricity can conduct.

[39] Owing to the aforementioned properties, the carbon nanotube/polymer composite can exhibit good electrical conductivity even at a low content of carbon nanotubes as well as excellent rheological properties such as storage modulus and viscosity.

[40] In a further aspect, the present invention is directed to a carbon nanotube/polymer composite prepared by the aforesaid method for preparing a carbon nanotube/polymer composite. For the aforementioned reasons, the thus-prepared carbon nanotube/ polymer composite employs the carbon nanotubes which were appropriately oxidized to exhibit excellent dispersibility, whereby the carbon nanotubes are uniformly dispersed in the polymer, and a moderate length of the carbon nanotubes leads to formation of carbon nanotube-carbon nanotube networks even at a low content of carbon nanotubes in the carbon nanotube/polymer composite, thereby providing excellent electrical and rheological properties. Mode for the Invention

[41] EXAMPLES

[42] Now, the present invention will be described in more detail with reference to the following Examples. These examples are provided only for illustrating the present invention and should not be construed as limiting the scope and spirit of the present invention.

[43]

[44] Example 1 : Freeze-drying pretreatment of carbon nanotubes treated with hydrogen peroxide and sonication

[45] 1.5 g of multi- walled carbon nanotubes (MWNTs) (length: 10 to 15 nm, diameter: 10 to 20 nm, and purity: more than 97 wt%, available from JEIO Co., Ltd., Incheon, Korea) was added to 500 mL of hydrogen peroxide, and the mixture was subjected to sonication at a frequency of 40 kHz and a temperature of 5O⁰C for 90 min. The thus- obtained mixture solution was allowed to stand at room temperature for 1 hour to thereby lower a solution temperature, followed by centrifugation at 3000 rpm for 1 hour. After centrifugation was complete, the supernatant was discarded, and distilled water was added to the residue which was then centrifuged at 3000 rpm for 10 min. This procedure was repeated three times. In this manner, a pH of the mixture solution was adjusted to 6.0. The thus-treated carbon nanotubes were then dried in a freeze- dryer (VD-500F, Taitec, Japan) at a pressure of 3 torr and a temperature of -6O⁰C for 72 hours, thereby obtaining carbon nanotubes for a carbon nanotube/polymer composite in the form of a powder.

[46]

[47] Comparative Example 1 : Thermal-drying pretreatment of carbon nanotubes treated with hydrogen peroxide and sonication

[48] 1.5 g of multi- walled carbon nanotubes (MWNTs) (length: 10 to 15 nm, diameter: 10 to 20 nm, and purity: more than 97 wt%, available from JEIO Co., Ltd., Incheon, Korea) was added to 500 mL of hydrogen peroxide, and the mixture was subjected to sonication at a frequency of 40 kHz and a temperature of 5O⁰C for 90 min. The thus- obtained mixture solution was dried in an oven at 12O⁰C for 48 hours to thereby obtain modified MWNTs.

[49]

[50] Comparative Example 2: Non-pretreated carbon nanotubes

[51] 1.5 g of multi- walled carbon nanotubes (MWNTs) (length: 10 to 15 nm, diameter: 10 to 20 nm, and purity: more than 97 wt%, available from JEIO Co., Ltd., Incheon, Korea) was prepared.

[52]

[53] Example 2: Preparation of carbon nanotube (Treeze-driedVpolycarbonate composite

[54]

[55] Example 2-m

[56] Relative to 9.97 g of polycarbonate (PC 201 15; number average molecular weight:

11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea), 0.03 g of multi- walled carbon nanotubes (MWNTs) of Example 1 was added to be 0.3% by weight in conjunction with the polycarbonate to 300 mL of tetrahydrofuran, and the mixture was subjected to sonication at a frequency of 40 kHz and a temperature of 6O⁰C for 6 hours, followed by drying at 8O⁰C for 12 hours and hot pressing at 26O⁰C to thereby prepare a carbon nanotube/polycarbonate composite in the form of a film.

[57]

[58] Example 2-(D

[59] A carbon nanotube/polycarbonate composite was prepared in the same manner as in

Example 2-(l), except that 0.05 g of multi-walled carbon nanotubes (MWNTs) of Example 1 was used to be 0.5% by weight, relative to 9.95 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[60]

[61] Example 2-(3)

[62] A carbon nanotube/polycarbonate composite was prepared in the same manner as in Example 2-(l), except that 0.07 g of multi-walled carbon nanotubes (MWNTs) of Example 1 was used to be 0.7% by weight, relative to 9.93 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[63]

[64] Example 2-(4)

[65] A carbon nanotube/polycarbonate composite was prepared in the same manner as in

Example 2-(l), except that 0.1 g of multi- walled carbon nanotubes (MWNTs) of Example 1 was used to be 1.0% by weight, relative to 9.9 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[66]

[67] Example 2-(5)

[68] A carbon nanotube/polycarbonate composite was prepared in the same manner as in

Example 2-(l), except that 0.2 g of multi-walled carbon nanotubes (MWNTs) of Example 1 was used to be 2.0% by weight, relative to 9.8 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[69]

[70] Example 2-(6)

[71] A carbon nanotube/polycarbonate composite was prepared in the same manner as in

Example 2-(l), except that 0.3 g of multi-walled carbon nanotubes (MWNTs) of Example 1 was used to be 3.0% by weight, relative to 9.7 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[72]

[73] Example 2-(l)

[74] A carbon nanotube/polycarbonate composite was prepared in the same manner as in

Example 2-(l), except that 0.5 g of multi-walled carbon nanotubes (MWNTs) of Example 1 was used to be 5.0% by weight, relative to 9.5 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[75] [76] Example 2-f 8)

[77] A carbon nanotube/polycarbonate composite was prepared in the same manner as in

Example 2-(l), except that 0.7 g of multi-walled carbon nanotubes (MWNTs) of Example 1 was used to be 7.0% by weight, relative to 9.3 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[78]

[79] Comparative Example 3: Preparation of carbon nanotube (heat-driedVpolvcarbonate composite

[80]

[81] Comparative Example 3-(I)

[82] Relative to 9.97 g of polycarbonate (PC 201 15; number average molecular weight:

11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea), 0.03 g of multi- walled carbon nanotubes (MWNTs) of Comparative Example 1 was added to be 0.3% by weight in conjunction with the polycarbonate to 300 mL of tetrahydrofuran, and the mixture was subjected to sonication at a frequency of 40 kHz and a temperature of 6O⁰C for 6 hours, followed by drying at 8O⁰C for 12 hours and hot pressing at 26O⁰C to thereby prepare a carbon nanotube/polycarbonate composite in the form of a film.

[83]

[84] Comparative Example 3-(2)

[85] A carbon nanotube/polycarbonate composite was prepared in the same manner as in

Comparative Example 3-(l), except that 0.05 g of multi-walled carbon nanotubes (MWNTs) of Comparative Example 1 was used to be 0.5% by weight, relative to 9.95 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[86]

[87] Comparative Example 3-(3)

[88] A carbon nanotube/polycarbonate composite was prepared in the same manner as in

Comparative Example 3-(l), except that 0.07 g of multi-walled carbon nanotubes (MWNTs) of Comparative Example 1 was used to be 0.7% by weight, relative to 9.93 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[89]

[90] Comparative Example 3-64) [91] A carbon nanotube/polycarbonate composite was prepared in the same manner as in

Comparative Example 3-(l), except that 0.1 g of multi-walled carbon nanotubes (MWNTs) of Comparative Example 1 was used to be 1.0% by weight, relative to 9.9 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[92]

[93] Comparative Example 3-(5)

[94] A carbon nanotube/polycarbonate composite was prepared in the same manner as in

Comparative Example 3-(l), except that 0.2 g of multi-walled carbon nanotubes (MWNTs) of Comparative Example 1 was used to be 2.0% by weight, relative to 9.8 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[95]

[96] Comparative Example 3-(6^

[97] A carbon nanotube/polycarbonate composite was prepared in the same manner as in

Comparative Example 3-(l), except that 0.3 g of multi-walled carbon nanotubes (MWNTs) of Comparative Example 1 was used to be 3.0% by weight, relative to 9.7 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[98]

[99] Comparative Example 3-(7)

[100] A carbon nanotube/polycarbonate composite was prepared in the same manner as in Comparative Example 3-(l), except that 0.5 g of multi-walled carbon nanotubes (MWNTs) of Comparative Example 1 was used to be 5.0% by weight, relative to 9.5 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[101]

[102] Comparative Example 3-(S)

[103] A carbon nanotube/polycarbonate composite was prepared in the same manner as in Comparative Example 3-(l), except that 0.7 g of multi-walled carbon nanotubes (MWNTs) of Comparative Example 1 was used to be 7.0% by weight, relative to 9.3 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea). [104]

[105] Comparative Example 4: Preparation of carbon nanotube/polvcarbonate composite

[106]

[107] Comparative Example 4- (I^s)

[108] Relative to 9.9 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea), 0.1 g of multi- walled carbon nanotubes (MWNTs) of Comparative Example 2 was added to be 1.0% by weight in conjunction with the polycarbonate to 300 mL of tetrahydrofuran, and the mixture was subjected to sonication at a frequency of 40 kHz and a temperature of 6O⁰C for 6 hours, followed by drying at 8O⁰C for 12 hours and hot pressing at 26O⁰C to thereby prepare a carbon nanotube/polycarbonate composite in the form of a film.

[109]

[110] Comparative Example 4- (2^s)

[111] A carbon nanotube/polycarbonate composite was prepared in the same manner as in Comparative Example 4-(l), except that 0.3 g of multi-walled carbon nanotubes (MWNTs) of Comparative Example 2 was used to be 3.0% by weight, relative to 9.7 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[112]

[113] Comparative Example 4-(3)

[114] A carbon nanotube/polycarbonate composite was prepared in the same manner as in Comparative Example 4-(l), except that 0.5 g of multi-walled carbon nanotubes (MWNTs) of Comparative Example 2 was used to be 5.0% by weight, relative to 9.5 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[115]

[116] Comparative Example 4- (4)

[117] A carbon nanotube/polycarbonate composite was prepared in the same manner as in Comparative Example 4-(l), except that 0.7 g of multi-walled carbon nanotubes (MWNTs) of Comparative Example 2 was used to be 7.0% by weight, relative to 9.3 g of polycarbonate (PC 201 15; number average molecular weight: 11,000, weight average molecular weight: 30,000, and glass transition temperature: 156.6⁰C, available from LG Chem, Ltd., Seoul, Korea).

[118]

[119] Experimental Example 1 : Bonding properties of carbon nanotubes with functional group substitution

[120] Using an infrared spectrometer (Perkin-Elmer FT-IR), peak analysis of carbon nanotubes of Example 1 and Comparative Example 2 was carried out. Samples were prepared by adding potassium bromide to each of carbon nanotubes of Example 1 and Comparative Example 2 and then compressing the mixture. 32 scan cycles were carried out at a scan rate of 2 cm^" and averaged to calculate transmittance (%).

[121] The results thus obtained are shown in FIG. 2.

[122] Referring to FIG. 2, it can be seen that Example 1 exhibits new peaks at 3300 cm , 1730 cm , and 1220 cm , which did not appear in Comparative Example 2. Functional groups corresponding to individual peaks are hydroxyl (-OH), carbonyl (-CO), and a carbon-carbon (-C-C) single bond, respectively. These results represent that new functional groups can be formed by a carboxyl (-COOH) group substituted by acid treatment of multi-walled carbon nanotubes (MWNTs), and oxidation of carbon- carbon double bonds of MWNTs leads to replacement of carbon-carbon double bonds with carbon-carbon single bonds. Therefore, it can be seen that carbon atoms of the carbon nanotubes were substituted with carboxyl groups by the present invention.

[123]

[ 124] Experimental Example 2: Thermogravimetric analysis of oxidized carbon nanotubes

[125] Using a thermogravimetric analyzer (TGA), weight loss (%) was measured with elevation of a temperature upto 6O⁰C for carbon nanotubes of Example 1 and Comparative Examples 1 and 2.

[126] The results thus obtained are shown in FIG. 3.

[127] As shown in FIG. 3, it can be seen that the weight loss (%) increases in the order of Comparative Example 2, Example 1 and Comparative Example 1. That is, treatment of hydrogen peroxide exhibited a high weight loss (%), and thermal drying leads to a high weight loss (%) as compared to freeze-drying treatment. These results are considered to be due to decomposition of oxidation-substituted hydroxyl (-OH) groups. Accordingly, it is understood that thermal drying leads to the highest oxidation, whereas freeze-drying leads to less oxidation. These results also represent that multi-walled carbon nanotubes (MWNTs) undergo carboxyl substitution through oxidative reaction by hydrogen peroxide.

[128]

[129] Experimental Example 3: Morphological analysis of carbon nanotubes

[130] A field emission scanning electron microsope (FE-SEM) and transmission electron microscope (TEM; JEM-2000EX/T) images were taken for carbon nanotubes of Example 1 and Comparative Examples 1 and 2, respectively.

[131] The results thus obtained are shown in FIG. 4. Left: FE-SEM image and Right: TEM image. [132] Referring to FIG. 4, it can be seen that the carbon nanotubes of Example 1 and

Comparative Example 1 have a shorter length than those of Comparative Example 2, and exhibit the non-smooth surface morphology due to breakage of carbon-carbon bonds in outer walls of the carbon nanotubes. These results are considered due to oxidation of the carbon nanotubes of Example 1 and Comparative Example 1 by the action of hydrogen peroxide. Further, it was confirmed that the carbon nanotubes of Example 1 have a moderate length as compared to those of Comparative Example 1 and maintain a carbon nanotube-carbon nanotube distance to some extent, whereas the carbon nanotubes of Comparative Example 1 have a too short length, thereby resulting in aggregation of the carbon nanotubes. This is because conventional thermal drying is accompanied by continued oxidation of the carbon nanotubes during a drying process, thus decreasing the length of carbon nanotubes, which consequently leads to high van der Waals attraction in a narrow space, thereby resulting in re- aggregation or entanglement of carbon nanotubes. However, according to the present invention, it is possible to appropriately control the length of carbon nanotubes and disentangle the randomly entangled carbon nanotubes. As a result, it can be seen that the present invention enables excellent dispersion of carbon nanotubes in a polymer matrix.

[133]

[134] Experimental Example 4: Zeta potential of carbon nanotubes

[135] Each 0.01 g of carbon nanotubes of Example 1 and Comparative Examples 1 and 2 was dispersed in 80 mL of distilled water (pH 7.0), and zeta potentials of carbon nanotubes were measured by a solution dispersion technique using ultrasonic waves. The results thus obtained are given in Table 1 below.

[136] Further, 0.01 g of carbon nanotubes of Example 1 was dispersed in 80 mL of tetrahydrofuran (THF), dimethylformamide (DMF), toluene and ethanol, respectively and zeta potentials of carbon nanotubes were measured by a solution dispersion technique using ultrasonic waves. The results thus obtained are given in Table 2 below.

[137]

[138] Table 1 [Table 1] [Table ]

[139]

[140] Table 2 [Table 2] [Table ]

[141] [142] As can be seen from Table 1, the zeta potential of Example 1 has a large negative value, as compared to that of Comparative Example 1 or 2. Such a result confirms that the zeta potential of the carbon nanotubes prior to oxidation has a value of -5.61 mV, representing some repulsive force between the carbon nanotubes. The Zeta potentials, however, vary significantly, depending upon drying manners to be employed after oxidation. It can be said that this is because when it is desired to remove hydrogen peroxide by thermal drying after treatment of carbon nanotubes with hydrogen peroxide, the residual hydrogen peroxide due to incomplete evaporation brings about continued oxidation of carbon nanotubes, which consequently leads to an excessive reduction in the length of carbon nanotubes and excessively high van der Waals attraction in a solvent-removed narrow space, thereby causing re- aggregation or entanglement of carbon nanotubes.

[143] Further, as can be seen from Table 2, according to measurement results of the zeta potential of carbon nanotubes in accordance with the present invention in various solvents, it can be said that carbon nanotubes have good dispersibility because they exhibit repulsive force therebetween in all of the solvents. The lowest zeta potential particularly in a tetrahydrofuran (THF) solvent represents excellent dispersibility of carbon nanotubes. From the above results, it can be seen that the dispersibility of carbon nanotubes is excellent particularly upon the use of tetrahydrofuran as a solvent in the preparation method of the carbon nanotube/polymer composite.

[144] [145] Experimental Example 5: Electrical conductivity of carbon nanotube/polymer composite

[146] Using a conductive graphite paint, four thin gold wires (purity of 99% and thickness of 0.05 mm) were attached to each surface of carbon nanotube/polymer composite of Examples 2-(l) to 2-(8), Comparative Examples 3-(l) to 3-(8), and Comparative Examples 4- (2) to 4- (4), and then the conductivity of carbon nanotube/polymer composite was measured by a 4-probe method. The results thus obtained are shown in FIG. 5.

[147] Referring to FIG. 5, it can be seen that carbon nanotubes of Comparative Example 4 exhibit a low electrical conductivity as compared to those of Example 2 and Comparative Example 3, and has an electrical percolation threshold of 5.0% by weight. Further, it can be seen that carbon nanotubes of Example 2 and Comparative Example 3 exhibit a similar value in electrical conductivity at a content of more than 2.0% by weight, whereas Example 2 exhibits a high electrical conductivity in a region of 0.5 to 2.0% by weight, as compared to Comparative Example 3, and Example 2 has an electrical percolation threshold of 0.5% by weight, whereas Comparative Example 3 has an electrical percolation threshold of 2.0% by weight, thus representing that the carbon nanotube/polymer composite in accordance with the present invention exhibits superior electrical conductivity. This is because of the following reasons. Achieving good electrical conductivity requires the formation of pathways between the carbon nanotubes, through which electricity can conduct via carbon nanotube-carbon nanotube interconnection. Non-oxidation of carbon nanotubes leads to poor dispersibility of carbon nanotubes to thereby result in entanglement and aggregation of carbon nanotubes and therefore uniform formation of carbon nanotube pathways needs a large amount of carbon nanotubes. Further, since thermal drying suffers from continued oxidation of carbon nanotubes leading to excessive decrease of the length of carbon nanotubes, larger amounts of carbon nanotubes are needed to form carbon nanotube pathways through which electricity can conduct, as compared to freeze-drying.

[148] Further, the carbon nanotube/polymer composite in accordance with the present invention exhibits a superior electromagnetic wave-shielding effect even at a low content of carbon nanotubes, because the electromagnetic wave-shielding effect is proportional to the electrical conductivity.

[149]

[150] Experimental Example 6: Rheological properties of carbon nanotube/polvmer composite

[151] For Example 2-(4), Comparative Example 3-(4) and Comparative Example 4-(l), rheological properties (such as storage modulus and viscosity) of each carbon nanotube/polymer composite were measured using an Advanced Rheometric Expansion System (ARES, Rheometric Scientific, Inc., Piscataway, NJ, USA). The storage modulus and viscosity of the composite at a frequency sweep from 0.1 to 100 rad/s were measured at 26O⁰C under dry nitrogen. The results thus obtained are shown in FIG. 6 and 7.

[152] Referring to FIG. 6, it can be seen that the storage modulus of carbon nanotube / polymer composite of Example 2- (4) exhibits a higher value as compared to those of Comparative Example 3-(4) and Comparative Example 4-(l). Further, it can be seen through FIG. 7 that the viscosity of carbon nanotube/polymer composite exhibits the same pattern as shown in the storage modulus.

[153] This is because the expression of high-viscosity and storage modulus requires the formation of carbon nanotube networks via carbon nanotube-carbon nanotube interconnection. But, non-oxidation of carbon nanotubes leads to low dispersibility of carbon nanotubes to thereby result in entanglement and aggregation of carbon nanotubes and therefore uniform formation of carbon nanotube networks is not achieved. Further, thermal drying suffers from continued oxidation leading to an excessive reduction in the length of carbon nanotubes, thus resulting in a failure to form sufficient amounts of carbon nanotube networks. Industrial Applicability

[154] As apparent from the above description, a method for pretreatment of carbon nanotubes for a carbon nanotube/polymer composite in accordance with the present invention increases the dispersibility of carbon nanotubes, and enables production of a carbon nanotube/polymer composite having excellent dispersibility, electrical and rheological properties. Further, the carbon nanotube/polymer composite of the present invention has excellent electrical conductivity and electromagnetic wave- shielding effect.

Claims

[1] A method for pretreating carbon nanotubes for a carbon nanotube/polymer composite, comprising:

1) sonicating carbon nanotubes in a hydrogen peroxide (H O ) solution;

2) adjusting a pH of the product of Step 1 to a desired range; and

3) freeze-drying the product of Step 2.

[2] The method according to claim 1, wherein sonicating of the carbon nanotubes in

Step 1 is carried out at a temperature of 3O⁰C to 7O⁰C for 50 to 130 min.

[3] The method according to claim 1, wherein the carbon nanotubes are single- walled carbon nanotubes (SWNTs) or multi- walled carbon nanotubes (MWNTs).

[4] The method according to claim 1, wherein the pH of Step 2 is in the range of 5.0 to 7.0.

[5] A method for preparing a carbon nanotube/polymer composite, comprising sonicating the carbon nanotubes prepared by the method of any one of claims 1 to 4 and a polymer in a solvent.

[6] The method according to claim 5, wherein the solvent is selected from the group consisting of water, tetrahydrofuran (THF), dimethylformamide (DMF), toluene, ethanol, and any combination thereof.

[7] The method according to claim 5, wherein the polymer is polycarbonate.

[8] The method according to claim 7, wherein a content of the carbon nanotubes is in the range of 0.5 to 2.0% by weight, based on the weight of polycarbonate.

[9] A carbon nanotube/polymer composite prepared by the method of claim 5.