Polymer nanocomposites that are revolutionising materials science

Identifying and selecting the right materials for manufacturing products requires taking various material properties into consideration. The “best” material identified must possess certain properties that distinguish it from others. Graphene, the strongest and thinnest materials known by far, and polymer nanocomposites (PNCs), polymers embedded with inorganic nanoparticles, have attracted significant attention from scientists and nations around the world.

Materials science has evolved considerably in the past 50 years. The discipline has gone beyond metallurgy and is now focused more on non-metallic materials. To Prof. Robert YOUNG, Emeritus Professor of Polymer Science and Technology at The University of Manchester (UoM) and Senior Fellow at PAIR, improvements in scientific equipment have enabled many previously unthinkable discoveries in materials science. Prof. Young sees material science inherently as an interdisciplinary subject, and the wide range of techniques in the discipline can address many real-world problems.

 
Prof. Robert YOUNG

Graphene: A one-atom-thick carbon material that can change the world

Graphene, a single layer of carbon atoms arranged in a hexagonal pattern, was “discovered” in 2004 by a research group at the UoM. Countries around the world are racing to scale up the mass production of graphene-based products. What is so special about graphene that there is such great global interest in graphene applications?

There are four major factors. First, graphene has record-high levels of stiffness and strength. Second, it is a good conductor of electricity and heat. Third, graphene sheets and fibres are flexible because they are very thin and can therefore bend easily. This is an advantage for graphene-based wearables and clothing, but it can be a disadvantage for structures because any bending and folding of graphene layers can make the transfer of stress ineffective, causing structures to fail and break. Fourth, graphene is a good candidate for barrier applications like food packaging. Thin films of graphene have large surface areas and are impermeable.

These electrical and mechanical properties suggest that graphene has a high application potential. In fact, the material has already been used in some niche products in sports, such as HEAD tennis racquets and Vittoria racing cycle tyres, as well as luxury watches such as the Richard Mille ultra-light-weight RM50-03 graphene watch, which the brand produces jointly with McLaren F1 and the UoM. The aerospace industry is exploring the application of graphene in aircraft. Airbus is working on the use of graphene as an additive to the outside coatings of A350 aircraft for protection from lightning strikes, as well as for de-icing, so that planes can be warmed up through resistive heating in graphene without any use of de-icing fluid. The biomedical sector has been looking at the use of graphene for drug delivery. The small size of graphene means that it can pass through cell walls to deliver drugs.

The construction industry has interest in graphene application, too. It is reported that adding graphene to concrete can enhance the material’s compressive strength by 20%. However, this application does not seem practically viable because graphene is an expensive material.

Breaking the limits and solving the puzzles of miracle materials

What are some major barriers that need to be overcome so that graphene can be used more extensively?

First, the processing and fabrication of polymer nanocomposites present a challenge. Polymers, generally speaking, can be easily moulded to produce various components. To achieve optimal mechanical properties, at the nanoscale, graphene nanofiller particles have to be uniformly dispersed in polymer matrix. However, in the case where these nanofiller particles are added to polymers, graphene particles tend to form clusters and clumps. The molten polymer becomes more viscous, and this makes processing difficult. New processing methods such as 3D printing may provide solutions to this limitation.

The second issue lies in the mechanics. The record-high stiffness and strength of graphene cannot be achieved when graphene is incorporated into polymer matrix to form composites. Effective stress transfer requires flakes that can stretch into large films, deform and transfer stress. However, graphene has a finite size, and therefore its ability to transfer stress is limited. Furthermore, since the nanofiller is much stiffer and stronger than the matrix, the interface between the nanofiller and polymer matrix will always break before the nanofiller does.

Third, graphene has random orientation and waviness, that is, the tendency to fold and twist. This is bad for stress transfer. To obtain good mechanical properties for fibres, all graphene particles must be aligned in one direction. The random orientation and waviness of two-dimensional (2D) graphene nanofillers can cause the mechanical properties of the nanocomposites to deteriorate. After all, it is the geometry of a fibre rather than the inherent properties of constituent materials that impacts the fibre’s stiffness and strength.

My team at UoM has conducted a major review study, which was published in Nano Materials Science, on how the mechanical properties of polymer-based nanocomposites are controlled by the geometry of the reinforcement. We developed and unified theories and came to some very interesting, and sometimes surprising, conclusions, unveiling how the structure, size, geometry and distribution of nanoparticles can affect the properties of nanocomposites. Graphene has a very high level of stiffness, but if it is put together with polymers, the resulting composite does not exhibit such properties.

Miss the boat in the graphene race? It is never too late for 2D materials research.

Europe is taking the lead in graphene research and development. In Hong Kong, no formal research organisation with an exclusive focus on graphene research has been formed thus far. What is your advice to PolyU if the University is to seize the opportunities in the booming graphene market?

The National Graphene Institute in the United Kingdom, which was opened in 2015, and the Graphene Flagship, a European Union scientific initiative which was set up in 2012 and has now finished, have both produced exciting basic research. Over the past 20 years, research on graphene has been extended to “other 2D materials”. These include transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN), which is also known as “white graphite”. Both materials can readily be exfoliated into monolayers and possess some properties that are akin to those of graphene.

In my opinion, it does not matter if universities have missed the boat on graphene. New institutes dedicated to 2D materials research could be set up. I personally suggest consolidating and embedding research on 2D materials in existing schools and departments. Research approaches to graphene and 2D materials are similar. Embedding this research in existing units provides greater flexibility and versatility for universities to grow and change research directions as they develop.

Are there any novel classes of materials other than graphene which you think may have great potential for application and research?

High-entropy alloys (HEAs) may be one possibility. Unlike most alloys which comprise two elements mixed together, HEAs include five or more elements, and the resulting alloys possess properties that are completely different from the constituent elements.

Many researchers are now working on the scientific concepts of HEAs. There are over 100 elements on Earth, and new HEAs can be formed from many different possible combinations of elements. This is what makes HEAs an exciting field for research.

Graphene was always there: Interdisciplinary techniques enable discoveries of the invisible

How important is “interdisciplinary collaboration” in bringing new solutions to graphene or polymer nanocomposites research and application? What implications does this importance have for university or research development?

World-class research these days requires the utilisation of a wide range of state-of-the-art experimental techniques. Graphene is one good example. It was only the availability of identification techniques, including the atomic force microscope (AFM) and high-resolution transmission electron microscope (TEM), that enabled the “discovery” of graphene 20 years ago.

I am amazed by how materials science has developed over my 50-year career in the field. Revolutions of new and improved scientific equipment in the past 50 years have enabled many previously unthinkable discoveries, such as making individual atoms “visible” under TEMs. Back then, materials science was essentially metallurgy with minor activities focusing on non-metallic materials such as ceramics and polymers. Now, however, materials science is a massive interdisciplinary field in its own right, addressing diverse problems with different experimental techniques. Chemists and physicists tend to use a few experiment techniques in their fields, but materials scientists employ an interdisciplinary approach through the use of techniques from different disciplines.

Should PolyU establish a new department or a research institute for materials science, it is important that the university pool the equipment in various faculties and schools together to enable resource sharing at a university level. This provides two advantages, one being cost-effectiveness and the other being the availability of professional technical support. Interdisciplinary research really requires many different instruments, as well as professionals who know how to use these instruments and can understand and communicate scientific problems across fields.

Synergising disciplines for optimum sports tech and clothing solutions

Currently, PAIR has 18 constituent research units, each focusing on a niche area of research. In what ways do you think that different units can collaborate to bring forth advanced functional materials or polymer nanocomposites?

One possibility is the synergy among intelligent wearables, future textiles, and advanced manufacturing for better sports technology. Sports is one area with high potential for graphene applications. Carbon-plated running shoes are now worn by marathon runners who smash world records. Soccer jerseys worn by professional players are in fact made of very sophisticated fibres. More interdisciplinary collaborations for sports should be explored.

Another possible development is to bring research on resources engineering for carbon neutrality, smart cities, advanced manufacturing and technologies into closer relationship with one another. One technology which I can think of is recycling plastic bottles for clothing. Making new bottles from recycled polyester bottles is difficult and not practically viable because it requires selecting used bottles that are transparent and clean enough for production. However, the use of recycled bottles for clothing may avoid these practical issues and be a potential way out.

You are on the International Academic Advisory Committee of the Research Institute for Intelligent Wearable Systems (RI-IWEAR). Can you give some advice on the research directions and work of the Institute?

RI-IWEAR might consider research on wearable dielectric generators which can be induced by movement to generate energy. Some of the present work on novel materials such as underwater glue and bone grafts might consider the integration of biomimicry. Many creatures in nature such as mussels and barnacles exploit water adhesion, and therefore these properties could be studied to determine how they can contribute to bio-inspired innovations.

Last but not least, the Institute can focus on generating a small number of outputs in high-impact publications and expanding networks for technology transfer through its industrial advisory board, spin-out companies and outreach at trade fairs.