Small but strong: A review of the mechanical properties of carbon nanotube–polymer composites
Introduction
Since their discovery in 1991, carbon nanotubes have generated huge activity in most areas of science and engineering due to their unprecedented physical and chemical properties. No previous material has displayed the combination of superlative mechanical, thermal and electronic properties attributed to them. These properties make nanotubes ideal, not only for a wide range of applications [1] but as a test bed for fundamental science [2].
In particular, this combination of properties makes them ideal candidates as advanced filler materials in composites. Researchers have envisaged taking advantage of their conductivity and high aspect ratio to produce conductive plastics with exceedingly low percolation thresholds [3]. In another area, it is thought that their massive thermal conductivity can be exploited to make thermally conductive composites [4]. However, probably the most promising area of composites research involves the mechanical enhancement of plastics using carbon nanotubes as reinforcing fillers.
The idea of using pseudo one-dimensional fillers as a reinforcing agent is nothing new: straw has been used to reinforce mud bricks since about 4000 BC. In more recent times, fibres made from materials such as alumina, glass, boron, silicon carbide and especially carbon have been used as fillers in composites. However, these conventional fibres have dimensions on the meso-scale with diameters of tens of microns and lengths of order of millimetres. Their mechanical properties are impressive with carbon fibres typically displaying stiffness and strength in the ranges 230–725 GPa and 1.5–4.8 GPa, respectively [5]. In recent years carbon nanofibres have been grown from the vapor phase with diameters of order of 100 nm and lengths between 20 and 100 μm. These small dimensions mean they have much higher surface area per unit mass than conventional carbon fibres allowing much greater interaction with composite matrices. They also tend to have impressive mechanical properties with Young’s modulus in the range 100–1000 GPa and strengths between 2.5 and 3.5 GPa [6].
However the ultimate mechanical filler material must be carbon nanotubes. Nanotubes can have diameters ranging from 1 to 100 nm and lengths of up to millimetres [7]. Their densities can be as low as ∼1.3 g/cm3 and their Young’s moduli are superior to all carbon fibres with values greater than 1 TPa [8]. However, their strength is what really sets them apart. The highest measured strength for a carbon nanotube was 63 GPa [9]. This is an order of magnitude stronger than high strength carbon fibres. Even the weakest type of carbon nanotubes have strengths of several GPa [10].
However a large amount of work will have to be done before we can really make the most of the exceptional mechanical properties of carbon nanotubes. In this paper we will explore the progress that has already been made to this end. First we will review the properties of carbon nanotubes and the theory of fibre reinforcement. This will lead us to a study of the system requirements in order to achieve reinforcement. The techniques used in the literature to produce polymer–nanotube composites will be reviewed before we look at what levels of reinforcements have actually been achieved. Finally we will discuss the advances made so far and study what needs to be done in the future.
Section snippets
Properties of nanotubes
There are two main types of nanotubes available today. Single walled nanotubes (SWNT) [11], [12] consist of a single sheet of graphene rolled seamlessly to form a cylinder with diameter of order of 1 nm and length of up to centimetres. Multi-walled nanotubes (MWNT) consist of an array of such cylinders formed concentrically and separated by 0.35 nm, similar to the basal plane separation in graphite [13]. MWNTs can have diameters from 2 to 100 nm and lengths of tens of microns.
Single walled
Theory of fibre reinforced composite materials
A great deal of theoretical work has been carried out since the 1950s with the aim of modelling the mechanical properties of fibre reinforced composites. While some of these models [42] are quite sophisticated, we will consider two of the simplest and most common. These are the rule of mixtures and the Halpin–Tsai equations.
In the simplest possible case a composite can be modelled as an isotropic, elastic matrix filled with aligned elastic fibres that span the full length of the specimen. We
System requirements for mechanical reinforcement
There are four main system requirements for effective reinforcement. These are large aspect ratio, good dispersion, alignment and interfacial stress transfer. The effects of aspect ratio have been dealt with in the previous section. Dispersion is probably the most fundamental issue. Nanotubes must be uniformly dispersed to the level of isolated nanotubes individually coated with polymer. This is imperative in order to achieve efficient load transfer to the nanotube network. This also results in
Solution processing of composites
Perhaps the most common method for preparing polymer nanotube composites has been to mix the nanotubes and polymer in a suitable solvent before evaporating the solvent to form a composite film. One of the benefits of this method is that agitation of the nanotubes powder in a solvent facilitates nanotube de-aggregation and dispersion. Almost all solution processing methods are variations on a general theme which can be summarised as
- 1.
Dispersion of nanotubes in either a solvent or polymer solution
Mechanical properties of polymer nanotube composites
In this section we will discuss results from the literature on mechanical properties of polymer nanotube composites. Many studies have been published each with a different focus. However, the common theme seems to have been enhancement of Young’s modulus. In order to attempt to compare different studies we will use the Young’s modulus reinforcement, dY/dVf, as a yardstick. In most papers this quantity is not quoted so we have attempted to evaluate it from the data presented. As this procedure
Discussion
Shown in Table 6 is a summary of the mechanical results discussed in the previous sections for each composite preparation type. The values shown represent the mean and median reinforcement values. In addition the maximum value reported in each section is shown for composites based on both SWNTs and MWNTs. Finally the maximum reported values for modulus and strength are also shown.
The most obvious result here is that composites based on chemically modified nanotubes show the best results. This
Conclusion
In conclusion, much progress has been made over the last few years in composite reinforcement using carbon nanotubes. Reinforcement levels are creeping towards those levels predicted by theory. In addition, many researchers are branching out with the fabrication of novel structures by innovative processing techniques. Some of these materials, such as coagulation spun fibres have even challenged state of the art materials such as Kevlar. Most importantly, the overriding problems of dispersion
Acknowledgement
The authors would like to thank Milo Shaffer and Brian Grady for useful discussions. In addition, we are grateful to Leslie Carpenter for proofreading the manuscript and providing many useful comments. Financial help from the Embark Initiative, operated by the Irish Research Council for Science, Engineering and Technology (IRCSET) is appreciated (JNC).
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