Imagine a future in which we wear clothing that is self-cleaning. Imagine painting your living-room wall to display a real-time image of another part of the world; or utilizing greenhouse gas to make value-added products; or designing surfaces that selectively destroy viruses and pathogenic bacteria; or a surgeon placing a removable stent that changes shape inside an artery of a patient’s body; the list goes on. What materials could provide all the properties necessary for these and other future applications? Plastics! My previous article1 covered a series of developments in specific areas of plastics technology—including advances in plastics nanocomposites, plastics electronics, the self-assembly process, fuel cells, tissue engineering, and high-throughput techniques. As these areas keep maturing, other areas where plastics may be used are gaining attention. This article highlights some of the current activities that have commercial and social implications, and also offers a glimpse into the future.
The “Nano-World”
How far will nanotechnology take us? Consider one example: spider silk, known to be among nature’s most advanced materials, is strong, yet elastic. Researchers at the Massachusetts Institute of Technology (MIT) (USA) have created elastomeric nanocomposites that rival spider silk2. Applications of nanocomposites might range from tear-resistant films to elastic fibers to biomedical devices. An excellent overview of nanoparticles by Pitkethly3 explains why they are interesting and how nanoparticles are made, including the driving force behind nanomaterials; the article also covers market segments and health, environmental, and ethical issues. Another entry to the “nano-world” is carbon tubes, popularly known as carbon nanotubes (CNTs). A must-read article on CNT-polymer composites provides a detailed account of CNTs, processing methods for composite reinforcement, and a look into the future4. The unique structure of CNTs and their properties could have a huge impact on a variety of technologies ranging from macroelectronics to solid-state lighting to organic solar cells to smart fabrics5. Research on organic photovoltaic cells (PV or solar cells) is now reaching a stage where polymer solar cells may rival silicon devices. Efforts are under way to enhance the efficiency and longevity of plastic solar cells6. Mixing quantum dots (nanocrystals) of lead selenide with conducting polymers such as MEH-PPV (phenylenevinylene) or polythiophenes have led to a ramp-up of electricity7,8. Using quantum dots, Professor Sargent’s group at the University of Toronto (Canada) has shown how to turn infrared power into electricity in a printable solar cell9. Konarka’s photovoltaic technology is unique in that it can use both visible and invisible light sources to pro-duce electricity. The technology is based on cold sintering the dye-sensitized (nanosized light-absorbing-dyecoated titanium dioxide) PV material to plastics. Currently, Evident’s proprietary EviDots™ (active throughout the solar spectrum) are being combined with Konarka’s conductive poly-mers to create ultrahigh-performance solar cells (hybrid solar cells) that might exceed the capabilities of today’s best silicon-based technologies. The quantum dot power plastic could be used for demanding energy, communications, and military applications. Professor Kamat’s group came up with a novel approach to use SWCNT (single-wall CNT) as a nanowire network (kind of a conducting scaffolding) to disperse titanium dioxide nanoparticles, which increased the photoconversion efficiency twofold. References are for interested readers10-13.
In a different type of application, scientists from University of California, Los Angeles, and Nanomix Inc. have demonstrated that CNT transistors fused with CO2-detecting polymers could determine CO2 con-centrations in both ambient and exhaled air14. Its application? A real life sensor that could monitor human breathing at a disaster site by detecting concentrations of CO2. Likewise, pressure-sensitive adhesives (PSA) based on CNT composites could find applications in electronics packaging and in assembly of displays15.
Creativity Abounds
Outside the nano-world, numerous exciting works are being carried out in numerous fields that might spur commercial applications in the near future. The competition to bring flexible electronics to market spurred Polymer Vision, a Dutch firm (a spin-off from Philips Electronics), to enter into an agreement with Telecom Italia to develop and launch the world’s first rollable display for mobile devices16. A spinoff from the University of Illinois (USA), Semprius Inc., a start-up based in Chapel Hill, North Carolina, USA, has developed a two-step process for making large-scale, high-performance electronic circuits that can be applied to a thin plastic-film surface. A major source of carbon is fossil fuels. New routes from bio-renewable resources to chemical intermediates to polymers are becoming an integral part of the sustainable-development strategy to conserve energy. Researchers at the University of Wisconsin (USA) have converted high-fructose corn syrups into a poly-mer precursor HMF (5-hydroxymethyl furfural) as an alternative to petroleum-based material used in many plastics17. Researchers at Cornell University (USA) have pioneered a zinc-based catalyst that could combine carbon dioxide and limonene oxide to make polylimonene carbonate18, a polymer produced by alternating copolymerization that has many characteristics of poly-styrene. The use of sugars and vitamin C to reduce the amount of copper catalyst could create new possibilities for an environmentally friendly catalyst system19. At the Center for Environmental Technology and Engineering of Massey University of New Zealand, work is being carried out on dairy-farm effluent with the objective of better understanding the transient stages of fermentation—in an effort to create more efficient bioplastics20. Much of today’s talk on bioplastics is about corn. Slowly but surely, researchers are exploiting the fierce “work ethic” of bacteria and using their genetic secrets21,22. Poly-(3 hydroxy-butyrate (PHB) is a very common energy-storage material in many microorganisms, such as Alcaligenes, Azotobacter, Bacilus, Nocardia, Pseudomonas, and Rhizobium. PHB belongs to the polyhydroxyalkanoates (PHA) group, a family of polyester. Its physical properties are comparable to those of polypropylene (PP). The difference is that PP shows the least degradation while PHB shows complete degradation. Producers of PHAs already vary their properties by playing with their feed. For example, when Ralstonia eutropha H16 is fed hydrogen or car-bon dioxide, it could make PHAs. Companies like Metabolix, Kaneka, Biomatera, and Biomer are actively pursuing research to make different grades of PHAs utilizing different bacterial hosts, feeding regimes, and co-feeding target monomers. Recently, the bacterium’s two chromosomes have been sequenced23 to identify 50 genes that are involved in producing PHAs. Taking advantage of polymers’ swelling character, engineers and doctors from Stanford University24 (USA) and from institutes in Germany partnered with GE Corporate Research to create a novel hydrogel called DuoptixTM, which can swell to a water content of 80%. The hydrogel is transparent and permeable to nutrients, which makes Duoptix a promising candidate for developing artificial corneas—an application that could give sight to the blind. Other similar applications of the hydrogel are being explored. Scientists are using polymers as building blocks for creative science. When charged polymeric coatings are applied on surfaces like spikes, they are able to poke holes to destroy the viral or the bacterial lipid membrane25. The process of creating ordered stiff surface wrinkles on a polymeric substrate is vital for various technological applications such as in biological sensors and in optical diffraction gratings. A team of scientists did just that by exposing a focused ion beam on a flat polydimethylsiloxane (PDMS) surface26. Another study27 focused on developing methods to prevent polymer films from buckling by controlling their surface topographies. Since the first published work on Atom Transfer Radical Polymerization (ATRP)28, it has become an active area of research. The beauty of ATRP is that it can chemically combine diverse monomers to produce specialized polymers. Consequently, one could develop surfaces that are hard on one side and soft on the other, or soft on top but sticky on the bottom29.
Tapping the Potential of Plastics for the Future
In the domain of nanotechnology, carbon-nanotube-based polymeric materials will herald an era of their own. Work will continue to focus on unique applications and in mass production of carbon nanotubes while safeguarding health and the environ-ment. The rivalry with silicon-based technology will intensify. New initiatives will focus on harvesting free, clean, and inexhaustible solar energy with greater efficiency. Flexible photovoltaics will capture both visible and infrared light, but the challenge will be to produce more power and manufacture photovoltaics on a commercial scale. RFID tracks life from creation to death. This technology is already widely in use and is responsible for saving of millions of dollars in logistic planning. Smart polymers will act as stimuli-responsive switches that will have a tremendous impact in bioengineering research. Attention in creating plastics will gradually shift from petroleum derivatives to plants and microorganisms. Developing additives for applications in stringent conditions will continue to challenge researchers, whether the additives are for synthetic polymers or for natural polymers and biopolymers. More and more design engineers will use biomaterials in their creativity. Ross Lovergrove’s design ethic (DNA–Design, Nature, and Art) will inspire up-and-coming industrial design engineers to take more innovative approaches. The use of polymers in space will be creative, and not just for space suits and robots with muscle; the challenge we’ll encounter is how to explore the terrain of Mars. Already, MIT researchers (Professor Dubowsky’s group) are investigating how a “swarm” of plastic mini-probes, powered by fuel cells, could hop, bounce, and roll across the Martian surface. These probes could carry sensors and cameras, and should able to communicate with other probes through a LAN and transmit the data to Earth.
Ultimately, regardless of where and how we employ plastics in the future, responsible initiatives are needed, those that keep in mind the bigger picture—a sustainable, safe, and healthy planet Earth.
References
1. P. Mukhopadhyay, Plastics Engineering, pp. 28-37, Sept. 2002. 2. S.M. Liff, N. Kumar, and G.H. McKinley, Nature Materials, 6 (1), pp. 76-83 (2007). 3. M.J. Pitkethly, Nanotoday, pp. 20-35, December 2004. 4. J.N. Coleman, U. Khan, W.J. Blau, and Y.K. Gun’ko, Carbon, 44, 5. G. Gruner, J. Materials Chemistry, 16, pp. 3533-39 (2006). 6. H. Larsen and L.S. Petersen, eds., RisO Energy Report 5, November 2006. 7. D. Qi, M. Fischbein, M. Drndic, and S. Selmic, Appl. Phys. Lett., 86, 093103 (2005). 8. R.D. Schaller, M. Sykora, J.M. Pietryga, and V.I. Klimov, Nano 9. T. Sargent in The Dance of Molecules— How Nanotechnology Is 10. Z. Zhu, D. Waller, R. Gaudiana, M. Morana, D. Muhlbacher, M. Scharber, and C. Brabec, Macromolecules (web release date: Feb. 17, 2007). 12. A. Kongkanand, R.M. Dominguez, and P.V. Kamat, Nano Lett., in 13. K.M. Coakley and M.D. McGehee, Chem. Mater., 16 (23), 4533- 42 (2004). 14. A. Star, T.-R. Han, V. Joshi, J.-C.P. Gabriel, and G. Gruner, 15. T. Wang, C.-H. Lei, A.B. Dalton, C. Creton, Y. Lin, K.A.S. Fernando, Y.-P. Sun, M. Manea, J.M. Asua, and J.L. Keddie, Advanced Materials, 18 (20), pp. 2730-34 (2006). 17. Y. Român-Leshkov, J.N. Chheda, and J.A. Dumesic, Science, 312, 1933 (2006). 18. C.M. Byrne, S.D. Allen, E.B. Lobkovsky, and G.W. Coates, J. Am. Chem. Soc., 126 (37), pp. 11404 - 05 (2004). 19. K. Matyjaszewski, W. Jakubowski, K. Min, W. Tang, J. Huang, W.A. Braunecker, and N.V. Tsarevsky, Proc. Nat. Acad. Sci., 103, pp. 15309-14 (2006). 20. S. Pratt, Massey News, Issue 12 (2006). 21. A. Sandoval, E.Arias-Barrau, M. Arcos, G. Naharro, E.R. Olivera, and J.M. Luengo, Environmental Microbiology, 9 (3), pp. 737-51 (2007). 22. K.M. Tobin, N.D. O’Leary, A.D.W. Dobson, and K.E. O’Connor, 23. A. Pohlmann, B. Friedrich, et al., Nature Biotechnology, 24, 1257 24. C. Frank, M.E. Harmon, D. Kucklung, W. Knoll, and D. Myung, 7th International Biorelated Polymers Symposium, ACS meeting, 25. J. Haldar, D. An, L.A. de Cienfuegos, J. Chen, and A.M. Klibanov, Proc. Natl. Acad. Sci., USA, 103, 17667 (2006). 26. M-W. Moon, S.H. Lee, J-Y. Sun, K.H. Oh, A. Vaziri, and J.W. Hutchinson, Proc. Natl. Acad. Sci., USA, 104 (4), pp. 1130-33 27. T.R. Hendricks and I. Lee, Nano Lett., 7 (2), pp. 372-79 (2007). 28. J.S. Wang and K. Matyjaszewski, J. Am. Chem. Soc., 117, p. 5614 29. K. Matyjaszewski, Y. Gnanou, and L. Leibler, eds., Macromolecular Engineering: From Precise Macromolecular Synthesis to Macroscopic Materials Properties and applications, Wiley-VCH, February 2007. [This article was originally published in Plastics Engineering in June 2007 – Editor]
