(Nanowerk Spotlight) Material scientists and engineers are excited by the possibilities for creating super-strong, high-performance polymer composite materials using carbon nanotubes. Since carbon nanotubes (CNTs) are five times less dense than steel and approximately 30 times stronger this makes them the ultimate mechanical filler for reinforcing polymers, with very low densities, and Young moduli (the measurement of stiffness of a material) superior to all other carbon fibers.
Currently, all existing methods of fabricating CNT-polymer composites involve quite complicated, expensive, time-demanding processing techniques such as solution casting, melting, molding, extrusion, and in situ polymerization. In all of these techniques, nanotubes must either be incorporated into a polymer solution, molten polymer or mixed with the initial monomer before the formation of the final product (e.g. yarn, ribbon or film). In addition, these methods can not be applied in the case of insoluble or temperature sensitive polymers, which decompose without melting.
Kevlar is a well known high-strength polymer with a variety of important applications – think bullet-proof vests and car armor plating. However, Kevlar is not soluble in any common solvent and, having no melting point, decomposes above 400°C. As a result, Kevlar fibers must be produced by wet spinning from sulphuric acid solutions.
Researchers in Ireland have now found a way to develop a new effective post-processing technique which would allow to incorporate carbon nanotubes into already formed polymer products, such as for example Kevlar yarns.
"Our approach is new and different because it enables the incorporation of nanotubes into already formed polymer yarns or fibres by swelling them in carbon nanotube suspensions," Yurii Gun'ko tells Nanowerk.
SEM images of Kevlar fibres before (left) and after (right) the treatment in nanotube suspension (scale bar is 20 µm). (Image: Dr. Yurii Gun'ko, Trinity College Dublin)
Gun'ko, an associate professor in inorganic chemistry at Trinity College Dublin, and a member of the College's Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), together with his colleagues at CRANN, have found that the swelling of Kevlar in a CNT suspension in N-methylpyrrolidon (NMP) resulted in new Kevlar-nanotube composites with improved mechanical properties.
The core of the findings by Gun'ko's team is the development of new swelling under ultrasound approach for the fabrication of new reinforced composites. In their experiments, commercially sourced Kevlar yarns were placed in stable suspensions of multiwalled CNTs in a selected organic solvent. These Kevlar-nanotube mixtures were processed using an ultrasonic bath for an optimized period of time at ambient temperature. This processing resulted in swelling of Kevlar and in an uptake of nanotubes inside Kevlar fibres.
Gun'ko says that, when they performed mechanical testing of these Kevlar-CNT composites, they found considerable increases in all mechanical parameters of the nanocomposite material compared to the original Kevlar fibers, e.g.: Young’s modulus, from 115 to 207 GPa; strength, from 4.7 to 5.9 GPa; strain at break, from 4.0 to 5.4%; toughness, from 63 to 99 J/g.
These improvements have been achieved at only 1 - 1.75 wt% of carbon nanotube content. This can be considered a quite significant advancement in the area of nanotube-polymer composites.
The CRANN team believes that there will be a number of possible important applications for their new technique.
"It is clear that our new approach of incorporating nanomaterials into polymer macromaterials by swelling could be expanded and utilized for many other nanosystems and polymer materials," says Gun'ko. "For example, it can be used to incorporate various nanoparticles, nanotubes, nanowires etc. inside pre-fabricated polymer fibres, yarns, films, ribbons etc. These can open up completely new opportunities in the large and important area of polymer nanocomposites."
He also points out that one of the promising likely applications for their technique is the production of conductive CNT-polymer composites (films and wires), which could be potentially used as electrodes in flexible displays, electronic paper, solar cells and in different electronic devices.
According to Gun'ko, another very promising area of future research involves the reinforcement of various polymer fibres and films using carbon nanotubes as reinforcing additives. Potential applications of new ultra-strong polymer-nanotube materials would include bullet-proof vests, protective clothing, high-performance composites for aircraft and automotive industries (e.g. seat belts, cables, reinforcement of tires, break linings, bumpers, etc.)
The researchers caution that there will be several challenges in further development of their approach. "First of all, stable suspensions of nanoparticulate materials must be available" says Gun'ko. "This will require the preparation of new functionalized nanomaterials, which can form stable colloidal suspension in selected solvents. Secondly, more detailed studies on polymer swelling processed will be necessary. In particular, swelling under ultrasound of selected polymers must be carefully investigated. Thirdly, new solvent recycling approaches must be explored in order to make our technology commercially viable and environmentally friendly. Finally, potential nanotoxicity of new technology and new polymer nanocomposite must be investigated and addressed."
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