When you think of electricity, what's the first thing that comes to mind?
Probably a wire. We're used to thinking of metals as the best way to conduct
electric current.
And it's true--metals are found in pretty much all the electronics that we use:
in light bulbs, phones, computers, TVs,
and countless other appliances. But believe it or not, there are certain non-metals,
such as conductive plastics that can act as conductors, too.
Their special properties challenges our assumptions about non-metals,
and allow them to harness the power of electricity. Although electricity can
seem like magic,
the science behind it is pretty simple. An electric current is just the flow of
electrons from
atom-to-atom. Different atoms are better at connecting electrons than others
which is why materials like metals are generally more conductive than others.
This is because the electrons metals are delocalized,
meaning that the outermost electrons in a metal atom are held loosely,
allowing them to flow more freely. In contrast,
the outer electrons in the atoms of a non- metal are held more closely to the
nucleus,
preventing movement. Think about an electrical wire.
Inside, the charges flow easily through the metal filaments.
On the outside, the rubber coating keeps these charges from flowing into our
bodies
and electrocuting us when we touch the wire. But electrically-conductive plastics
turn this idea on its head.
These plastics are polymers, long chains made of smaller,
repeating units known as monomers. By changing the structure of
the atoms within the monomers, these plastics can actually become
just as conductive as certain metals. Sounds pretty cool,
right? You may think that some brilliant chemist made these plastic specifically
for this purpose.
But, like so many other important discoveries in science,
this one was an accident. It happened in the Lab of Hideki Shirakawa,
when the chemical compound polyacetylene was mixed incorrectly.
Normally black and powdery, the compound instead formed a silvery
film that looked distinctly metallic. Wondering if this strange substance would
exhibit metallic properties,
Shirakawa shared the discovery with chemist Alan MacDiarmid
and physicist Alan Heeger. After experimenting with many different
processes,
they discovered that they could actually increase the conductivity
of polyactylene to rival that of certain metals. With the addition of bromine gas,
for instance,
the conductivity of polyacetylene increased 10 million--fold--
to a level that approached that copper. Although a different conductive polymer,
polyaniline, had existed for almost two centuries,
the accidental discovery of polyacetylene brought the field of conductive polymers
to the forefront and opened up a revolutionary new possibilities for
experimentation.
While metals are generally the best conductors we have available today,
there are many downsides to their use. Mining,
shipping, and processing metals is expensive. Conductive polymers
are attractive alternatives because they are lightweight, cheap
and functionally versatile. So how do these conductive polymers work?
Like in metals, the flow of electric charge in conducting polymers is
generated by voltage--
a difference in the electrical potential the negative and positive poles of a
battery
or an outlet. The negatively charged electrons are drawn towards the positive
pole,
and they create a current as they pass from atom-to-atom. But like we mentioned,
the electron of polymers usually aren't delocalized, and cannot easily move around
to conduct current.
But let's take another look at polyacetylene.
if you look closely, you'll notice that there's an alternating pattern of single
and double bonds holding the atoms of the polymer together.
This is known as a conjugated backbone. Along this backbone,
the sigma bonds hold each atom together by concentrating electrons directly between them.
The pi bonds, which constitute the second bond in each double bond,
strengthen the connection between their atoms by attracting electrons above and
below the plane and the molecule.
This forms delocalized orbitals, where the mobility of electrons can be
enhanced
through a process called "doping." This process essentially changes the number of
electrons in the polymer,
by either removing or adding electrons to the atoms. Removing electrons
create empty spaces in the outermost orbital the atom,
allowing the remaining electrons to move around more freely.
Adding electrons forces an atom to create another orbital--
and so long as this orbital isn't full, the electrons have more space to
move around
and hop from atom-to-atom. You can think of it like a pool table full of billiard balls.
The balls would be able to move around, if they weren't getting in each other's way.
But if you take out a few balls, the rest of them can move
around. Or alternately, if you add more balls,
you need to add a new pool table. In this new pool table,
they have lots of room to move around. The more easily they can move around,
the more easily they can transport charges and conduct electrical currents.
Although initially no one believed that polymers could conduct electricity,
the scientific community has since cut up to Shirakawa's research.
In 2000, Shirakawa, MacDiarmid
and Heeger were awarded the Nobel Prize in Chemistry--
and the research around conductive polymers has grow exponentially
While the full potential of conductive polymers has yet to be discovered,
there are countless possibilities for developments in technology and medical since,
such as high-capacity batteries, artificial muscles, and biosensors.
So while we probably won't see plastics entirely replacing metals in our
lifetime,
the field of conductive polymers has huge potential,
especially for something that began as a mistake.