2. Methods
Carbon nanotubes, with their unique shapes and characteristics, are being considered for various applications. A carbon nanotube has a tubular shape of one-dimensional nature which can be grown through a nano metal particle catalyst. More specifically, carbon nanotubes can be synthesized by arc discharge or laser ablation of graphite. In addition, carbon nanotubes can be grown by a chemical vapor deposition technique. With the chemical vapor deposition technique, there are also variations including plasma enhanced and so forth. Carbon nanotubes can also be formed with a frame synthesis technique similar to that used to form fumed silica. In this technique, carbon atoms are first nucleated on the surface of the nano metal particles. Once supersaturation of carbon is reached, a tube of carbon will grow. Regardless of the form of synthesis, and generally speaking, the diameter of the tube will be comparable to the size of the nanoparticle. Depending on the method of synthesis, reaction condition, the metal nanoparticles, temperature and many other parameters, the carbon nanotube can have just one wall, characterized as a single walled carbon nanotube, it can have two walls, characterized as a double walled carbon nanotube, or can be a multi-walled carbon nanotube. The purity, chirality, length, and defect rate can vary. Very often, after the carbon nanotube synthesis, there can occur a mixture of tubes with a distribution of all of the above, some long, some short. Some of the carbon nanotubes will be metallic and some will be semiconducting.
One aspect of the present study is directed to mechanical functionalization of carbon fibers processed in situ with molten polymers to create reactive bonding sites at the ends of the fibers. The reactive sites react with the polymer to chemically bond the carbon fibers to the polymer. This can be achieved with a variety of carbon fibers, including single or multi-walled carbon nanotubes and standard micron sized carbon fibers. It works well in conjunction with a variety of polymers that possess chemical groups having double bonds or various tertiary carbon bonds. Similar observations of good bonding at sites of broken covalent graphite and graphene bonds have been made while mechanically exfoliating graphite into graphene in situ with polymers. The fibers are broken or cut while in molten polymers during melt processing, and this can be achieved either by having a specially designed cutting tool in the melt processing equipment, or through high shear in the melt processing, or by a combination of the two. The opening up of new fiber ends by breaking or cutting the fibers while surrounded by liquid polymers introduces dangling bonds having unfilled valences which provide reactive sites on the fiber ends, which represent sites for strong bonding, such as covalent bonding, by the polymers having the attributes mentioned above. The resulting solid composites have improved mechanical properties upon cooling, and the optimal fiber length, and, subsequently, cost will be greatly reduced by this bonding. In one aspect, the present study provides a high efficiency mixing method to transform a polymer composite that contains carbon fibers into broken carbon fibers having reactive ends or edges, by compounding in a batch mixer or extruder that imparts repetitive, high shear strain rates. The method is low cost to produce a carbon fiber-reinforced polymer matrix composites that offers numerous property advantages, including increased specific stiffness and strength, enhanced electrical and thermal conductivity, and retention of optical transparency. Furthermore, these properties are tunable by modification of the process, vide infra. In some cases, an inert gas or vacuum may be used during processing. Other advantages of in situ carbon fiber breaking are that it avoids handling size reduced carbon fibers, and also avoids the need to disperse them uniformly in the polymer matrix phase. Superior mixing produces finer composite structures and very good particle distribution.
It should be understood that essentially any polymer inert to carbon fibers or nanotubes and capable of imparting sufficient shear strain to achieve the desired carbon fiber breakage may be used in the method of the present study. Mechanical functionalization of carbon fibers within a polymer matrix may be accomplished by a polymer processing technique that imparts repetitive high shear strain events to mechanically break the carbon fibers within the polymer matrix. A succession of shear strain events is defined as subjecting the molten polymer to an alternating series of higher and lower shear strain rates over essentially the same time intervals so that a pulsating series of higher and lower shear forces associated with the shear strain rate are applied to the carbon fibers in the molten polymer. Higher and lower shear strain rates are defined as a first higher, shear strain rate that is at least twice the magnitude of a second lower shear strain rate. After high-shear mixing, the mechanically size reduced carbon fibers are uniformly dispersed in the molten polymer, are randomly oriented, and have high aspect ratio. Graphite microparticles are also added to the molten polymer and are mechanically exfoliated into graphene via the succession of shear strain events. The amount of graphite added to the molten polymer can be an amount up to and including the amount of carbon fibers and nanotubes added. The shear strain rate within the polymer is controlled by the type of polymer and the processing parameters, including the geometry of the mixer, processing temperature, and speed in revolutions per minute. The required processing temperature and speed for a particular polymer is determinable from polymer rheology data given that, at a constant temperature, the shear strain rate is linearly dependent upon speed in revolutions per minute. Polymer rheology data collected for a particular polymer at three different temperatures provides a log shear stress versus log shear strain rate graph.
A polymer-carbon nanotube composite differs from a conventional carbon-fiber composite in that there is a much higher interface area between reinforcing carbon and polymer matrix phases. It has been proposed that introducing a uniform distribution of carbon nanotubes into a polymer matrix should yield property enhancements that go beyond that of a simple rule of mixtures [39, 40]. The challenge is to take full advantage of the exceptional properties of carbon nanotubes in the composite material. Carbon nanotubes are considered to be ideal reinforcing material for polymer matrices because of their high aspect ratio, low density, remarkable mechanical properties, and good electrical and thermal conductivity [41, 42]. One of the matrices that has been studied is commercially important epoxy [43, 44]. However, property improvements have not been significant to date, apparently due to poor interfacial carbon nanotube-polymer bonding and severe carbon nanotube agglomeration [45, 46]. These obstacles have now been overcome by utilizing a new processing route that involves high-shear mixing in a molten polymer to induce de-agglomeration and dispersal of carbon nanotubes, while enhancing adhesive bonding and covalent bonding by creating new sites on the carbon nanotubes to which the polymer chains can bond. An attempt is also being made to increase impact energy absorption by forming a biphasic composite, comprising a high fraction of strong carbon nanotube-reinforced epoxy particles uniformly dispersed in a tough epoxy matrix. A carbon nanotube consists of a sheet of hexagonal-bonded carbon atoms rolled up to form a tube. A single-walled carbon nanotube comprises a single layer of this tubular structure of carbon atoms. However, the structure of a multi-walled carbon nanotube is still open to some debate. In one model, a multi-walled carbon nanotube is imagined to be a single graphene sheet rolled up into a scroll. In another model, a multi-walled carbon nanotube is considered to be made of co-axial layers of helically-aligned carbon hexagons, with matching at the joint lines, leading to a nested-shell structure. In yet another model, a combination of scroll-like and nested-shell structures has been proposed.
A small percentage of carbon nanotubes can improve the robustness of the material without significantly compromising the elastomeric properties. Increases in mechanical strength properties reduce blade edge tears and substantially extend blade life due to edge wear. Low percentage additions of carbon nanotubes can also significantly increase electrical and thermal conductivity. Enhanced electrical conductivity can dissipate charge accumulation at the blade edge due to rubbing against the photoreceptor and air breakdown from the accumulation of charged toner at the blade edge. Enhanced thermal conductivity can aid heat dissipation due to friction at the blade-photoreceptor interface. Polymer properties such as electrical conductivity can be enhanced by incorporating therein a combination of carbon fibers or carbon nanotubes. Additionally, carbon nanotubes can prevent delamination and provide structural stability in polymer composites. Because carbon nanotubes have uniquely high strength to mass ratio, intrinsic light weight, thermal conductivity, electrical conductivity, and chemical functionality, and can prevent delamination and provide structural stability in polymer composites, they can impart these properties to polymers when effectively combined therewith. Though carbon nanotubes have extraordinary mechanical properties, their ability to strengthen polymers and epoxies is limited by the strength of interfacial bonding. As a result, when incorporated into polymeric resin without cross-linking or functionalization, they lack the ability to transfer loads across the structure. Carbon nanotubes can be functionalized via covalent or non-covalent bonding, to either the ends of the nanotubes or to the sidewalls. Covalent functionalization often requires beginning with modified tubes, such as fluorinated nanotubes, or with purified tubes where defect sites in the carbon nanotubes are produced by oxidation. Because these modifications often result in the disruption of the bonds along the tubes themselves, covalent functionalization can degrade the mechanical and electrical properties of the nanotubes and, thus, is not ideal for all applications. Non-covalent functionalization to the sidewalls of carbon nanotubes can be attained by exploiting the van der Waals and pi-pi bonding between the pi electrons of the carbon nanotubes and that of a polyaromatic molecule. This type of functionalization results in higher degrees of functionalization as the entire length of the carbon nanotube can be functionalized rather than just the ends and specific active sites. Like end-functionalization, non-covalent functionalization also opens up the possibility for tailoring the functionalization via the choice of molecule. For the purpose of polymerizing the carbon nanotube to a polymer resin or epoxy, in one embodiment, a polymerizable ligand comprising a polyaromatic molecule with an appropriate polymerizable group such as a vinyl, styryl, or amino group can be non-covalently bonded to the carbon nanotubes.
It is known that increases in elastic modulus and strength of epoxy-carbon nanotube composite resulted from making small additions of carbon nanotubes to polymer matrices. While Van der Waals bonding dominates interactions between carbon nanotubes and polymers, adhesion in some carbon nanotube composites also occurs via covalent bonds, which plays a role in reinforcement of carbon nanotube composites. Measurements by atomic force microscopy of the pull-out force necessary to remove a given length of an individual multi-walled carbon nanotube embedded in copolymers demonstrate covalent bonding between the outer layer of a multi-walled carbon nanotube and the polymer matrix. The polymer matrix in the near vicinity to the interface behaves differently than the polymer in the bulk, which is attributed to the outer diameter of a carbon nanotube having the same magnitude as the radius of gyration of the polymer chain. Because of the tendency of carbon nanotubes to agglomerate, difficulty of aligning them in the matrix and often poor load transfer, attempts are made to produce composites using different polymer matrix phases. The present study provides remarkable improvements in stiffness and strength of a carbon nanotube-reinforced epoxy composite. The composites are characterized by an increase in impact energy absorption. Processing parameters which achieve superior mechanical properties and performance are provided herein.
Incremental additions of carbon nanotubes to the molten epoxy are necessary to produce a composite that contains a high fraction of carbon nanotubes. It takes about one hour to ensure that mixing parameters remain as stable as possible. The rapid increase in melt viscosity during mixing is attributed to chemical bonding between dispersed carbon nanotubes and epoxy polymer matrix. After completion of the mixing process, the composite material is extracted from the barrel at the mixing temperature. Upon cooling to ambient temperature, the material became hard and brittle. This is further evidence for chemical bonding between dispersed carbon nanotubes and epoxy matrix. Larger samples of carbon nanotube-reinforced epoxy can be prepared using an integrated high shear mixing and injection molding apparatus. American Society for Testing and Materials standard test bars can be fabricated and evaluated for mechanical properties. Preliminary tests performed on small samples indicate significant improvements in stiffness and strength.