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CLASSES YOU MAY HAVE MISSED




TARA MEYER
ON THE FUTURE OF PLASTIC



There are a few polymer products that many of us would rather forget--after all, only Elvis looked good in polyester leisure suits. But plastic technology overall has engineered inexpensive building materials for everything from compact discs to latex paint. Your tennis shoes are a little nation of plastics. Even if we're not wearing them, rarely can we sit in a room without being surrounded by them. Polymers, better known as plastics, are everywhere.

Considering how much polymer chemists have learned about plastics since they first came into production in the 1930s, in some areas little innovation can be expected. We've become about as good at making polyethylene milk jugs as we'll ever be. So what does the future hold?

Exploring the frontiers of polymer research first calls for understanding the basic chemistry. Polymers consist of long interwoven chains of molecules. Most of their useful properties, such as flexibility, elasticity, and strength, arise from the tendency of these long chains to get tangled with one another. They slide around each other, so it's a struggle to unlink any one chain. The whole mass can be bent, but the tangling makes it virtually unbreakable.

Researchers at the cutting edge of polymer technology are trying to create plastics that can conduct electricity or act like magnets as if they were made of metal, without sacrificing the flexibility with which metals are never blessed. This may put some metals out of work, as there's always been a clear division of labor between the two. Metal wires in lamp cords, for example, are coated with plastic so we, thankfully, don't get electrocuted. Plastics typically make great protective coatings because they're excellent insulators and lousy conductors. Metals are just the opposite. But by teaching plastics some of the skills that metals have always monopolized, polymer researchers are now getting quite a charge from creating exceptions to this rule.

Turning an insulator into a conductor requires altering the way its atoms are bound--a bond being the sharing of electrons between two or more atoms. Metal atoms link up in such a way that they form a conduction band, an electronic highway, on which electrons are free to travel. Electrons in plastics, on the other hand, are normally very localized. They stick to one atom and sit still. But we've discovered that when polymer-forming elements like carbon and nitrogen are connected in a pattern of alternating single- and double-bonds, they become capable of supporting a conduction band that's worthy of a metal. Once the alternating pattern is established, then some electrons must be removed so that others can circulate and traffic jams won't tie up the highway. Treating the polymer with iodine, a process called "doping," alleviates this congestion.

Compared to conducting polymers, which are a well-known phenomenon at this point, magnetic polymers have been very elusive. Yet the two are closely related, distinguished by only a slight rearrangement of electrons. Making magnetic polymers also requires increasing their energetic state through doping. But rather than dispatching and receiving free-flowing electrons, some atoms along the many interwoven chains of a magnetic polymer need to end up with an extra electron stuck to them. Like all electrons, these come in two flavors. They spin with an either clockwise or counterclockwise motion, and this determines the direction of the tiny magnetic fields that they emit. These extra electrons can see each other, they're aware of each other, and they can communicate inasmuch as they induce each other to spin in unison. Whereas in a nonmagnetic material, tiny up and down fields all over the place cancel each other out, these synchronously spinning, unpaired extras create a considerable net magnetic field for the whole chain. That much we've been able to accomplish. But we have yet to overcome the second-order problem of making the magnetic field of each chain correspond. So for now, we have to be satisfied with polymeric magnetism in only one dimension.

We may never produce a polymer that rivals a metal in its ability to conduct electricity or behave like a magnet, but even the current level of performance is generating some fascinating results. The process by which researchers develop products using this technology involves thinking about what polymers are good at and metals aren't. Take an overhead transparency. It's transparent, lightweight, flexible, and pretty strong--remarkably strong considering it's so thin. So think of a transparency, then ask, "What if I could make it conduct electricity? What could I do with that?"

Visual displays, an application derived from transparencies and diodes, are, in fact, one of the most exciting applications of conducting polymers. Most people know of diodes as the tiny lights that indicate the volume of their stereo. Made with conventional semiconductor technology, they can't exceed one-quarter inch in diameter. But with polymers, you can make lightweight, flat, and flexible lights the size of an office wall or bigger. A special class of double-duty polymers, electrochromic polymers, holds the key to these giant, flat lights. They conduct electricity, but instead of just operating as closed circuits, they convert some of the juice that passes through them into light. This is nothing new--light bulbs have been doing it for almost a century. But instead of encasing metal leads in a glass bulb, this technology involves coating both sides of a flat electrochromic polymer with a conducting polymer. And the result could be a thin, transparent, billboard-sized sandwich of light.

Thinking futuristically, electrochromic polymers could be the basis for "smart windows." Flip a switch and your window turns tan to block the sun--instant lightshade. An electrochromic polymer could propel the production of lightweight, durable, roll-up computer monitors. Conventional computer screens can't be bent. In fact, they're very prone to damage. But a roll-up screen would be much more amenable to being transported. If they can be made cheaply enough, they may even become our daily newspaper. Instead of tiptoeing outside in our pajamas every morning to search for the paper in the hedges, we'd unroll a screen, plug it in, download the news into a little memory chip, and read it as it plays.

Another frontier in conducting polymer research, a little less sexy but probably far more useful, is electromagnetic shielding. All electronic devices, as well as the bundles of cables that connect them, pose a threat to each other. Signals from one can leak out and sabotage the flow of information in another. So all the cables that carry signals within a network, as well as the computers that they connect, require shielding. This has been managed by dispersing graphite particles throughout a wire's plastic casing. By conducting electricity within the casing, the graphite prevents signals from straying. Coaxial cables use braided copper filament instead of particles. But these methods pose problems. They weigh down the cable. Also, weaves and dispersions are never perfect. Signals leak from the tiny holes like water from a hose.

So why not just use a polymer that can conduct electricity and, in turn, act as a uniform shield? Conducting polymers will not only protect equipment from spurious signals, but they'll also help dissipate the threat of static charge. Anyone who uses computers has seen the warning against taking them apart and messing around with what's inside. A single static shock from your finger can fry electronic components and completely erase their memory. But with a painted-on conducting polymer, there would be no need to worry.

Academic and industrial labs all around the world are exploring these frontiers of polymer chemistry. In the next century, plastics will not only surround us, but they'll be full of life like never before. They'll be pulsing with electricity, exerting a magnetic attraction, and maybe even making static shocks in winter a thing of the past.

So stay tuned. We may have seen the last of the leisure suits. But plastics, well, they're more like Elvis. They'll be around forever.


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