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Since graphene was isolated by a group of physicists from Manchester University, UK in 2004, interest in graphene research throughout the world has skyrocketed.  This huge activity stems from graphene’s unusual and extraordinary electrical, thermal, and mechanical properties.  Professor Geim, who was instrumental in the separation of graphene, recently commented, “Graphene is a wonder material with many superlatives to its name”.  Why such glorification of graphene as a material?  Because it is the thinnest known material in the universe and its strength is the highest ever measured¹. Prior to its separation into platelets, graphene was a controversial material and the subject of much speculation.  Many believed that graphene could not exist as a freestanding sheet, and yet it was studied theoretically for over 6o years. The results of this intense work over the years have been comprehensively documented in an article by Geim and Novoselov².  Particularly noteworthy is the research, at MIT, of Gene Dresselhaus and Mildred Dresselhaus who began work with graphite (multi-layered graphene) several decades ago.  The results, until 1980, of the Dresselhaus team on graphite intercalated compounds have been described by these authors themselves³.  Today graphene’s unique structure allows for a wide spectrum of applications in a variety of fields while giving researchers an unprecedented opportunity for fundamental physical science. Picture on the top left show false-color 3-D rendered TEM image of isolated hydrogen atoms (purple-tipped) and an isolated carbon atom (red-tipped) on a graphene membrane ("Courtesy Zettl Research Group, Lawrence Berkeley National Laboratory and University of California at Berkeley"). This article aims to capture and convey in a few words the excitement provided by some of the interesting trends observed in research on graphene and graphene-based polymer nanocomposites (GPNC).

 

 

Background

Invented in 1564, the familiar pencil-lead is graphite, a three dimensional allotrope of carbon. Graphite has a layer structure and is anisotropic.  As a result of this structure, one carbon layer can slide over another layer making graphite a valuable lubricant in addition to a material familiar to all school children.  If the planar, hexagonal arrangements of carbon atoms are stacked together, the result is graphite.  These individual building blocks are nothing but graphene layers.  In other words, graphene is a flat single layer of carbon atoms (sp² bonded) packed into a two-dimensional honeycomb lattice (hexagonal arrangement).  Additionally, graphene can be rolled into a cylindrical one-dimensional carbon nanotube or can be wrapped up into zero-dimensional fullrenes.

Graphene is an atomic-scale honeycomb lattice made of carbon atoms (Coutesy: Dr Thomas Szkopek http://www.ece.mcgill.ca/~ts7kop/images/graphene_xyz.jpg )

Prior to 2004, whenever scientists tried to slice layers from graphite as thin as was then possible, they ended up with 100 or more layers of graphene together.  Andre Geim and Kostya Novoselov of Manchester University used a micromechanical cleavage technique to separate a single sheet from the three-dimensional graphite.  They put an infinitesimal amount of graphite between two layers of cellophane tape and peeled the tape apart; this process allowed them to whittle the graphite down to a single layer known as graphene⁴.  The process employed was conceptually very simple, but it allowed for a consideration of what one could expect from graphene in terms of real world applications. This was not a puzzle given the research that had already been done on exfoliated graphite and with carbon nanotubes.  The main challenge was how to produce a large quantity of graphene in a cost efficient manner?  That’s where the race begins!

Graphene production methods

The current literature describes several techniques for graphene production.  Each method has its own benefits and related drawbacks.  Another trend is whether one wishes to synthesize defect free graphenes (purity) or graphenes with defects (containing oxygen species onto the surface).   The drawback of graphene materials with defects is the loss of some of the interesting properties of graphene.  On the other hand, defects could provide numerous application opportunities.  Therefore, not only the quantity of graphene but the types of applications dictate graphene’s preparative methods.  Well practiced methods of making graphene today are mechanical exfoliation, chemical exfoliation, solvo-thermal reduction, and chemical vapour deposition (CVD) or a combination of these.  An overview of current synthetic trends for producing graphene has been reviewed recently⁵.   However, we like to draw the readers’ attention to some key synthetic work on graphene that has been taking place around the world.  
To produce graphene in bulk quantities, Princeton University researchers led by Prud’homme and Aksay successfully worked out a method through thermal expansion of graphite oxide which provided single functionalized graphene sheets⁶.   Separately, J.N. Coleman’s group in Trinity College, Dublin in Ireland produced graphene, utilizing their earlier strategy where they avoided aggregation of carbon nanotubes by the use of liquids whose surface energy matched that of nanotubes and yielded a stable dispersion of graphene^7,8.  They termed the graphene production technique as liquid-phase exfoliation of graphite.  Czech and Greek researchers used a similar technique to produce a set of colloidal dispersions of solubilised graphenes⁹.  To avoid harsh oxidation chemistry (using liquids such as DMF or NMP), Rice University scientists recently claimed that they had obtained a high yield of homogeneous graphene dispersions by using ortho-dichlorobenzeneas as the solvent¹⁰.  Researchers from China utilized worm-like exfoliated graphite to make graphene sheets¹¹ .    Looking ahead, another group of researchers electrochemically reduced exfoliated graphite oxides at a graphite electrode and reported the work as a green approach to the synthesis of graphene¹².  Bucking the graphitic precursor trends, John A. Stride¹³ of the University of New South Wales, in Sydney, Australia allowed common laboratory reagents ethanol and sodium to react to produce an intermediate which was then pyrolized to give a fused array of graphene sheets only to be sonicated to disperse and separate.  Interestingly, because of graphene’s promising applications in the electronics area, Ajayan’s group at RPI focused their attention on cutting processes to controlling graphene’s shapes¹⁴.  Meanwhile, Martin Pumera of the National Institute for Materials Science of Tsukuba, in Ibaraki, Japan is leading the development of electrochemical enzyme biosensors using graphene¹⁵ .  All of these works have implications to graphene based polymer nanocomposites (GPNC).

Graphene based Polymer Nano-composites

Graphite is cheap and abundant in nature.  Property wise, graphite is superior to clay and therefore provides a unique opportunity for polymer reinforcement.  The key, however, is to exfoliate graphite’s layer structure and utilize it as a nano-reinforcement.   For the past 20 years, researchers have incorporated, intercalated, exfoliated, or expanded graphite platelets into polymers to produce nanocomposite materials.  Undoubtedly, the separation of graphene (a single and a flat layer of graphite) as a nano-material has opened-up new vistas as well as challenges for polymer nanocomposites research.

The main challenge in making GPNCs is to disperse the individual graphene sheets in the polymer matrix. Stankovitch and Dikin’s work showed that it was, indeed, possible to prepare a well dispersed homogeneous mixture of graphene nano sheets¹⁵ in polymer.  Furthermore, the researchers found that the percolation threshold of graphene in polystyrene-graphene composites was close to 0.1 vol% which was three times lower than that of any other two-dimensional filler.  A team of researchers from Northwestern UniversityPrinceton University (New Jersey), and from the University of Texas at Austin further confirmed the homogeneous dispersion by creating functionalized graphene-PMMA nanocomposites that even competed against single-walled carbon nanotube-PMMA composites¹⁶.  In fact, these works, along with earlier work of Ilhan and Prud’homme⁶, resulted in a patented technology that Vorbeck Materials is exploiting in their commercial products.   Recently, Vorbeck and BASF announced that they are developing dispersions of highly conductive graphene for producing electrically conductive coating and compounds, especially for the electronics industry¹⁷.   Yongsheng Chen’s group studied EMI shielding of graphene/epoxy composites and found a low percolation threshold of 0.52 volume%.  The group showed that these composites could be used as lightweight and effective EMI shielding materials¹⁸.   Rodney Ruoff reviewed¹⁹ different chemical methods to produce graphene and chemically modified graphenes (CMG).  His work has resulted in a start-up “Graphene Energy” which is exploiting the market for ultracapacitors where the CMG could be used to make electrodes.Another graphene nanoplatelet supplier is Angstron Materials.  This company is following Bor Jang’s (Wright State University, Dayton, Ohio) work who recently patented highly conductive nano-scaled graphene plate nanocomposites²⁰.   This group is actively pursuing and patenting its work for varieties of graphene based polymer nanocomposites (GPNC).  Another active group in this arena is L.T. Drzal’s group from Michigan State University.  The work of this group is well documented in the literature^21,22.  Drzal’s work provided the basis of XG Sciences product lines. 

After the isolation of free standing graphenes, the floodgate of ideas to produce graphene and use them in real life applications has been opened.  Advantages of graphenes over carbon nanotubes stem from easy access to the graphitic precursor material, the cost, and the scalable method.  Creation of several start-ups confirms the commercial potential of graphene based nanocomposites (GPNC), and different applications are rapidly becoming a reality. 

References

1.    A.K. Geim, Science, 324, pp. 1530-1534 (2009)
2.    A.K. Geim and K.S. Novoselov, Nature Mater. 6, pp. 183-191 (2006)
3.    M.S. Dresselhaus and G. Dresselhaus, Advances in Physics, 30, pp. 139-326 (1981)
4.    K.S Novoselov et al. Science, 306, p.666 (2004)
5.    M. Jacoby, Chemical and Engineering News, pp. 14-20, March 02, 2009.
6.    R.K. Prud’homme, I.A. Aksay et al. Chem. Mater., 19, pp 4396-4404 (2007) 
7.    J.N. Coleman et al. Nat. Nanotechnol., 3, pp. 563-568 (2008)
8.    J.N. Coleman et al. J. Am. Chem. Soc., 131, pp. 3611-3620 (2009)
9.    A.B. Bourlinos et al. Small, 5, pp. 1841-1845 (2009)
10.   A.R. Barron, J.M. Tour et al., Nanoletters, to appear (2009)
11.   W. Gu et al., J. Mater. Chem., 19, pp. 3367-3369 (2009)
12.   X-H. Xia et al., ACS Nano, to appear 2009
13.   J.A. Stride et al. Nature Nanotechnology, 4, pp. 30-33 (2009)
14.   P.M. Ajayan et al. Advanced Materials, 21, pp.1-5 (2009)
15.   M. Pumera et al., Chemistry – A European Journal, Published Online: 10 sep 2009
16.   S. Stankovich, D.A. Dikin et al., Nature, 442, pp. 282-286 (2006)
17.   T. Ramanathan et al. Nature Nanotechnology, 3, pp. 327-331 (2008)
18.   http://www.vorbeck.com/news/basf1.html site accessed on September 19, 2009
19.   Y. Chen et al. Carbon, 47, pp. 922-925 (2009)
20.   S. Park and R.S. Ruoff, Nature Nanotechnology, Published Online March 29, 2009. DOI:10.1038/NNANO.2009.58
21.   US Patent 7566410, issued on July 28, 2009
22.   L.T. Drzal et al., Carbon, 45, pp. 1446-1452 (2007)
23.   S. Kim and L.T. Drzal, Composites Part A: Applied Sci. & Manufacturing, DOI: 10.1016/j.compositesa.2009.05.002

Rakesh K. Gupta

Dr. Rakesh Gupta is George B. and Carolyn A. Berry Professor and Department Chairman of Chemical Engineering at West Virginia University where he has been teaching since 1991. He has also taught at the State University of New York at Buffalo and worked for Monsanto. Dr. Gupta earned his B. Tech. Degree from the Indian Institute of Technology in Kanpur, India, and his Ph.D., also in chemical engineering, from the University of Delaware. His research is focused on polymer rheology, polymer processing and polymer composites. He has published 100 peer reviewed journal papers, 60 conference papers and 10 book chapters on these topics. He also holds two U.S. patents. He is the author of Polymer and Composite Rheology, the coauthor of Fundamentals of Polymer Engineering and the co-editor of Polymer Nanocomposites Handbook.

Prithu Mukhopadhyay

Dr. Mukhopadhyay is scientist with IPEX Technologies Inc., Quebec, Canada.  He earned his doctorate in polymer chemistry from the Indian Institute of Technology, Kharagpur, India.  He has spent 6 years as a research scientist at the Ecole Polytechnique of University of Montreal, Canada.
Dr. Mukhopadhyay is a senior member of Society of Plastics Engineers (SPE), member of American Chemical Society (ACS), and Division members of Polymer Chemistry and Polymeric Materials: Science & Engineering (PMSE). He is a past Chairman of New Technology Committee (NTC) and a past Chairman of the Publication Committee of Society of Plastics Engineers.

He has authored numerous research publications and technical articles in polymers and plastics journals and has chaired many technical programs and sessions in international conferences. Dr. Mukhopadhyay is the founding editor of the Plasticstrends site.