Better composite materials using graphene
During the last few decades, a large number of novel materials have been produced, enabling the development of new products and the improvement of the properties of old ones. This development shows no signs of slowing down. Composite materials are produced by combining two or more materials to get a composite with desired properties. Graphene, a one-atom-thick lattice structure formed by carbon molecules is the thinnest possible material, and its discovery in 2004 brought the Nobel Prize in Physics for 2010 to Andre Geim and Konstantin Novoselov. The studies on graphene have multiplied exponentially, one sub-field being the use of graphene as a composite material. This research is also being carried out at the Aalto University Department of Biotechnology and Chemical Technology.
Post-docs as an asset
Nguyen Dang Luong, who started as a post-doctoral researcher at the department 18 months ago, has studied the production and properties of a composite material formed by polyimide and graphene. Last October, the first findings were published in the journal Polymer, becoming the fifth most read article in the online version of the journal soon after publication.
Professor Jukka Seppälä says that the Aalto University Research Assessment Bonus granted to the department has been put towards hiring new post-doc researchers. “They bring with them new know-how and have a positive impact on the results we achieve as well as on the other researchers in the department”, states Seppälä. The remarkable results achieved by Doctor Luong in a relatively short time bear witness to Seppälä’s assessment.
Production of composites
Polyimide is universally used as an encapsulation material in the electronics industry. This polymer is an electrical insulator, with good thermal stability and excellent mechanical properties counting as its other qualities. Insulation properties can however become a hindrance, as a static electric charge can form on an insulator, possibly damaging the electronics. Thus, adequate but not too high a conductivity is a desirable quality in polyimide, provided its other beneficial properties do not deteriorate.
The starting point for Luong’s research was to discover a way to make the raw material graphite, comprising stacked graphene planes, soluble in inorganic and organic solvents. “First, graphite needs to be turned into the more reactive graphite oxide, which is then treated with ethyl isocyanate to give us soluble isocyanate-modified graphite oxide (iGO)”, Nguyen Luong describes the central phase of the method.
After this, the synthesis of polyimide in the presence of iGO takes place and the resulting dispersion can be cast on the chosen substrate and dried. The result is an approximately 30-micrometre film, in which some of the multi-plane iGO flakes produce corresponding single-plane graphene oxide composite flakes.
The properties of the resulting polyimide/graphene composites have been tested using different graphene contents. Even at low levels (0.38-0.75 weight percent), notable improvements can be achieved, with the most important one being the rise in electrical conductivity to such a level that the problems stemming from static electricity can be avoided. The amount of graphene required is so small that the film obtained is close to transparent, a quality that is useful in various applications.
An improvement is also seen in the mechanical properties, such as tensile strength, which is better than in pure polyimide. “In the composite, graphene flakes are distributed evenly throughout the whole volume and orient themselves randomly”, Luong comments on the better values. In addition, tests indicate that a strong chemical bonding, a covalent bond, is formed between graphene and polymer, which would be a new finding in the field of graphene research. Follow-up studies will mainly focus on electrical properties as well as looking into the ageing of the material.
Composites have application potential
According to Professor Seppälä, the department focuses on basic research and on making new discoveries in that area but: ”of course we will still have one eye on possible new applications and on finding someone to develop further the results we have obtained.”
The polyimide/graphene composite has aroused interest in the electronics industry, but the department has also conducted research that has caught the attention of the paper industry. The domestic forest products industry has a strong interest in and research focus on microfibrillated cellulose, produced from wood pulp by disintegrating its structure down to the nano level.
The department has been the first to manufacture various nanocellulose/graphene composites, which can be used to produce paper that is not only significantly stronger than ordinary paper but also has conductive properties. It is exactly these kinds of innovations allowing a high degree of processing that the Finnish forest products industry is in need of.
Text by Jari Koponen, photo by Adolfo Vera