Two researchers from the United States have created a series of flattened-nanotube reinforced thermoplastic composites, which have been fabricated as a function of buckypaper loading. The researchers incorporated a Parmax polymer resin, to increase and optimise the mechanical strength of these composites.
The research is a product of a commercial desire to produce materials that are multifunctional and lightweight, which can be incorporated into a wide range of technological and engineering applications.
Carbon nanotubes (CNTs) are known to possess a multitude of beneficial properties, including mechanical, electrical and thermal properties, which can be used to enhance other materials. However, the incorporation of CNT’s into a composite isn’t enough to ensure that these properties are transferred into the new material- this is mainly due the dis-orientation that a group of CNT molecules exhibit.
Buckypaper is a thin film of aggregated carbon nanotubes, or carbon nanotube grid paper, with a carbon-carbon bond strength twice that of diamond. Buckypaper also possesses a high thermal and electrical conductivity and a high mechanical strength.
Buckypaper normally suffers from the same orientation problems as free-standing nanotubes. Misalignment in buckypaper can weaken the mechanical strength and thermal conductivity, which is not ideal for composites. However, by mimicking a ‘brick-and-mortar’ structure of nacre (mother of pearl), the researchers have developed a method that can mass produce buckypaper composites, containing aligned nanotubes.
The researcher’s product is a multifunctional thermoplastic composite composed of an aligned buckypaper framework and a self-reinforcing polyphenylene resin polymer, known commercially as Parmax. The researchers not only created the composite, but also optimised the internal composition to produce a composite with the most beneficial properties.
After testing the mechanical strength, electrical and thermal properties, alignment, volume fraction and nanotube length, the researchers deduced the optimal buckypaper content in the composite to be 60 ± 5 wt%. Such a ratio was found to produce a tensile strength a Young’s modulus of 1145 MPa and 151 GPa, respectively.
The composites produced by the researchers are lightweight, have a high mechanical strength and outperform many structural/reinforcement materials currently available.
The high reinforcement properties of the composite are attributed to the strong intermolecular bonds between the flattened nanotubes and Parmax molecules. There is a strong network of π-π and π-CH interactions that facilitates a good dispersion and interfacial stress transferring- these are two mechanisms that allow for the effective reinforcement of a composite.
The π-CH interactions arise from the Parmax acting as a CH-donor, and the sp2 sidewall of the CNT acting as the conjugated structure donor- which is determined by both the orbital distance and orientation of the Parmax molecules.
There are many factors that contribute to the material’s excellent properties. The optimal ratio between the nanotubes and Parmax molecules provides a large contact area which provides the material with a high thermal power. The increased contact area arises during a step in the fabrication process, known as the ‘hot-press fabrication process’. The CNTs fold under heat, which reduces the spacing between the nanotubes and increases the CNT-Parmax interfacial contact area.
In addition to this, the composite exhibits a strong interfacial adhesion, due to π-stacking interactions, which increases the interfacial interaction between the two species. Parmax molecules are also rigid, with a high degree of aromaticity. The limited flexibility allows the aromatic rings in both the CNT and Parmax molecules to strongly interact, producing π-π stacking intermolecular forces.
The alignment of the nanotubes in the matrix also improves the load transfer, which enhances the electrical and thermal properties of the composite. It is the optimal combination of molecular interactions that provides this composite with multifunctional properties.
The production method of these composites is also scalable. This, coupled with the multifunctional properties that it exhibits, makes it a great choice for many applications. The scalability of the method gives these composites the potential to be used in commercial applications in the near future.
Li Z., Liang Z., Optimization of buckypaper-enhanced multifunctional thermoplastic composites, Scientific Reports, 2017, 7, 42423