2. Methods
The method of preparation can encompass dissolving a polymer in a solvent while maintaining the temperature of the solvent at a level high enough to prevent precipitation of the polymer out of the solvent; sonicating a plurality of carbon nanotubes in a solvent; mixing the dissolved polymer and the sonicated carbon nanotubes; and sonicating the mixture for a sufficient period of time to disperse the carbon nanotubes throughout the polymer to produce a subject nanocomposite in solvent. The composite is optionally further processed by spin coating the nanocomposite and solvent onto a substrate. The spin coating step evaporates the solvent so that the nanocomposite is deposited as a coating on the substrate. The solvent can also be removed by heating the nanocomposite under a vacuum, thereby removing the solvent. The nanocomposite can then be molded into a device or structure. Optionally, the solvents used to dissolve the polymer and to sonicate the carbon nanotubes are the same. Preferably, the solvent is cyclohexyl chloride.
Stable suspensions of carbon nanotubes are achieved in water with the use of surfactants, and non-covalent and covalent attachment of polymers. Covalent functionalization is a valuable approach to the preparation of carbon nanotube materials, as controlled compositions and reproducible properties may be obtained. The incorporation of carbon nanotubes into polymer matrices results in composites that exhibit increased thermal stability, modulus, strength, and electrical and optical properties. The polymer is dissolved in cyclohexyl chloride at 110 °C to make a solution. Carbon nanotubes are sonicated in N,N-dimethylformamide using a Branson Sonifer for one hour. The dispersion is placed in a vacuum oven at 80 °C to remove the solvent. The N,N-dimethylformamide treated carbon nanotubes are then dispersed in the polymer solution via sonication for 6 hours. The carbon nanotube-polymer mixture is placed in a warm beaker lined with TEFLON film, the solvent is allowed to evaporate at room temperature for 12 hours, and the composite is then placed in a vacuum oven at 80 °C to remove any residual solvent. The dried composite with carbon nanotubes is compression molded for analysis. Pieces are placed between KAPTON film and stainless-steel plates and pressed for 6 minutes at 6000 pounds of pressure at a temperature of 246 °C. A neat polymer is prepared in the same manner. After processing, the melt temperature for the neat and composite sample is measured.
The composites comprise carbon nanotubes that are incorporated into the matrix of a polymer. Advantageously, the carbon nanotubes can be single-walled, multi-walled, or a combination thereof. Advantageously, carbon nanotubes are 100 times stronger than steel, exhibit excellent electrical and mechanical strength, and are light in weight. Due to their weight, carbon nanotubes are thought to be ideal fillers in a polymer matrix in order to produce a composite with improved thermal properties, as well as with enhanced electrical and mechanical properties. The polymer utilized in the nanocomposites comprises a plurality of repeating hydrocarbon units that exhibits solubility in organic solvents. Preferably, the solvents are cyclohexane, cyclohexyl chloride, and cyclohexene. More preferably, the solvent is cyclohexyl chloride. The polymer composites are then characterized by measuring their physical, melt rheological, and electrical properties. Carbon nanotubes have a powerful effect on the melt rheology, increasing the low shear viscosity dramatically. Escalating the carbon nanotube concentration also increased the flexural and tensile moduli, decreased the elongation, and increased the electrical conductivity. Polymerization is generally preferred as the method of dispersing the carbon nanotubes. The electrical conductivity is in some cases quite high, approaching that of metal strips. In addition, the electrical conductivity is pressure sensitive.
Such acid derivatized carbon nanotubes can be added to and subsequently copolymerized with precursors of polymers including but not limited to monomeric precursors to polyamides, polyesters, polyimides, or polyurethanes [61, 62]. For example, the acid derivatized carbon nanotubes can be contacted with a diacid and a diamine and the resultant pre-polymer product polymerized to form a carbon nanotube-polymer composite, or the acid derivatized carbon nanotubes can be contacted with a diacid or a diester, and a diol and the resultant pre-polymer product polymerized to form a carbon nanotube-polymer composite [63, 64]. Following contact of the acid derivatized carbon nanotubes with the polymer precursors, the pre-polymer product of such contact may be filtered, washed, and dried [65, 66]. For example, this procedure would be appropriate for treatment of the salt precipitate when using polyamide precursors [67, 68]. In addition, for the formation of other pre-polymer products, water or alcohol may be removed such as in the formation of pre-polymers.
One of the problems with blending any filler into a molten polymer is the difficulty of dispersing the individual particles into the polymer matrix. High shear mixing is usually employed for this purpose, but certain fillers, such as carbon nanotubes, offer a special problem because they require dispersal at the angstrom or nanometer level, and they have a particularly strong Van der-Waals affinity for each other. These characteristics make it especially difficult to effectively disperse clusters of carbon nanotubes, and then keep them dispersed in the polymer matrix. Indeed, the quality of dispersion is usually the limiting factor when engineering these composites. To measure the electrical resistance across these composites, a circular disk of 0.6-inch diameter and 0.06-inch thickness is first compression molded. The measurement is carried out by heating the polymer to 180 °C in a 2 inch by 2 inches by 0.05-inch mold, applying about 600 pounds per square inch pressure to the mold, waiting 6 minutes, then cooling under pressure for another 8 minutes to 60 °C, where the hardened plaque is removed. The circular disk is then cut from this larger plaque. To each side of this disk is then smeared a small amount of micronized silver paste. The brass disk is then pressed against the silver paste on each side of the disk, and the two metal plates with sample sandwiched between are then pressed together by means of a strong spring-loaded wood-gluing clamp. These two brass plates, which are soldered to a wire, are then connected to a voltmeter.
Electrical resistance, measured in ohms, is then taken as the two metal disks are being pressed together. Sometimes the force is varied to obtain maximum contact, and duplicates are obtained to achieve a consistent result. A blank determination is also made in which the composite is omitted, to determine the electrical resistance of the other parts of the circuit. This electrical resistance is small, usually 0.2 ohms at most, compared to sample measurements of many orders of magnitude larger. The electrical resistance measured in this way is then converted into a standard resistivity by multiplying by the area of the disk and dividing by the path length. Electrical conductivity as a function of pressure is determined by placing a sample of the composite between two metal plates, which are then inserted in a hydraulic press between insulating barriers. Electrical resistance between the plates is then monitored as the force exerted by the press is increased.
Scanning electron microscopy characterization is performed on a JEOL 7401-F with energies greater than approximately 6 keV in secondary electron imaging mode with a working distance of 2-8 mm. Electrical conductivity is measured using the four-probe method with metal electrodes attached to the ends of cylindrical samples. The amount of current transmitted through the sample during measurement is 80 mA, and the voltage drop along the sample is measured over distances of 2 to 6 mm. Seven or more measurements are taken on each sample, and results are averaged. Mechanical properties are studied by indentation in an MTS XP Nanoindenter with a Berkovich diamond tip. A series of both continuous and partial load-unload indents is carried out in laboratory air at room temperature. The loading rate is continuously adjusted to keep a constant representative strain rate. For every cycle, the unloading rate is kept constant and equal to the maximum loading rate of the cycle. The Oliver-Pharr method is used to analyze partial load-unload data in order to calculate the indentation elastic modulus as a function of the indenter penetration.