Metals and ceramics will go a long way in this world, but they're not always the best choice.

Every time you bounce a ball or drive your car on a set of tires, you're seeing a different kind of material in action –

one that's able to do things metals and ceramics just can't.

They're called polymers, and the world as we know it depends on them.

[Theme Music]

Imagine there's a 12 story building, and up on the top floor is a fire.

Naturally, your first response would be to call the fire department.

They'd come in their big trucks, hook up their hoses, and put out the fire. Right?

Well, unfortunately, it's not always that simple.

Many fire hoses, especially older ones, only have enough capacity and water pressure to reach about 30 meters straight up.

And in practice, firefighters on the ground rarely attempt to reach even half that distance with a hose – only around the 4th floor.

Even with a ladder truck, firefighters can only reach up to about the 10th floor until it becomes difficult and unsafe.

But that's still 2 stories short of where our fire is.

If we assume that we can't just use a different water source or another hose, then we're going to need to send the water up higher than it already is.

How could we do that?

Well, one answer is by using polymers!

They're already everywhere in this situation, like in the trucks the firefighters drive and the gear they wear.

And they can cause a special phenomenon that might help with the water height problem, too.

Polymers are materials made up of long, repeating chains of tiny molecules known as monomers

– a class of molecules that can react with others to form much larger molecules.

In a polymer, monomers are repeated over and over again until there are hundreds, thousands, or even millions of them in a long line.

Typically, polymers have lower densities compared to metals and ceramics, and they aren't usually as stiff or strong.

So you probably shouldn't try to build a house out of them.

Many polymers are also extremely ductile and pliable, so they're easily formed into more complex shapes.

They also don't generally react easily with other chemicals, making them a good material for volatile environments like a chemistry lab.

But since polymers depend on the monomers that they're made from, you can get a better sense of a polymer's characteristics by looking at its composition.

Ones that have longer chains tend to show more elastic properties.

These are often called elastomers, and are mainly found in rubber materials.

Take a rubber band, for example.

The molecules in the polymer can be all clumped together, or stretched out, like when you pull on the rubber band.

On the other hand, polymers with larger and more complex monomers tend to be stiffer.

You can also stiffen polymers through a process called cross-linking.

That's when polymers are bonded with each other to form a 3D, net-like shape called a polymer network.

Have you ever heard of vulcanized rubber?

It's the stuff used to make things like tires, like the ones you might find on a fire truck.

Well, vulcanization is the process of using sulfur to cross-link and strengthen rubber polymers!

Tires probably aren't the only polymers that you're familiar with.

Chances are good that you use plenty of them every day, like the ones you'll find in plastic bags and seat covers.

And while it's easy to take polymers for granted, they've only been widely used since about the 1940s.

But that doesn't mean they don't have a special place in history.

Research suggests that the first time people really used polymers was back in pre-Columbian Mexico and Central America, around 3,000 years ago.

Back then, they took latex, a milky, sap-like fluid found in some plants, and mixed it with juice from the morning glory plant.

This juice had sulfur in it, which effectively vulcanized the latex and made it less brittle.

And from there, they were able to make rubber goods like sandals, rubber bands, and even bouncing balls.

You can actually do a pretty similar and simple experiment if you want.

All you have to do is mix some common glue you'd find at a school supply store, which contains a polymer chain known as polyvinyl acetate,

with a borax solution, which will act as the cross-linker for our mixture.

And then you just combine that with some cornstarch.

Stir them together, roll it around in your hand, and poof!

You got yourself a rubber ball!

You can also do similar experiments to make a cool slime if you change up the ingredients and measurements a bit.

Now, even though we have evidence that polymers were used thousands of years ago, it still took a while for the modern world to find the right applications.

That's because we don't always get things right the first time.

For example, if you've ever played a game of pool or billiards, you know that the balls don't usually explode.

But that wasn't always the case.

Go back to the 1870s and celluloid, a hardened material made from the natural polymer cellulose,

was used to make billiard balls instead of ivory, which was what they were made from before that.

Problem was, celluloid was highly flammable, and when the billiard balls hit each other just right, they would explode on the pool table, which happened quite often.

Today billiard balls are still made of polymers, just ones that are far less explosive!

We also tried using cheaper, more flammable, substitutes for silk back in the day.

Of course, having more flammable fabrics, especially for clothing, is obviously a bad thing.

This all happened because it wasn't until about the early 20th century that we had a decent understanding of what a polymer was.

That's when Hermann Staudinger, a German chemist, entered the picture.

At the time, most chemists thought polymers were made up of small molecules bundled together by unknown forces.

But in 1922, Staudinger suggested that these materials were made up of larger molecules arranged in the long molecular chains that we know them to be, all bonded in a line.

His work earned him the Nobel Prize for Chemistry and laid the foundation for the explosion of the plastics industry in the 20th century.

Scientists then began using polymers as synthetic substitutes, like synthetic rubber and nylon.

World War II also had a big impact on the polymer world.

Wartime restrictions on natural materials helped drive the polymer industry into existence, especially the creation of synthetic polymers.

During the war, the Allies used polymer to create large inflatable tanks that served as decoys on the battlefield, part of what was known as the “Ghost Army.”

These fake tanks tricked enemies into thinking Allied forces had far more strength than they actually did,

or that there was a giant army in an area where there weren't really any soldiers at all.

The Ghost Army reportedly saved thousands of lives – I'd call that a win for polymers!

And a win for us engineers, because they set us up for the world of polymers that we have today.

It's this world that's helped us keep firefighters safe, using polymers to design better gas masks and other parts of their gear.

Just like with other materials, some kinds of polymers get used more than others.

The ones that you'll most likely come across are polyethylene, or PE, polyvinyl chloride, or PVC, and polyethylene terephthalate, or PET.

Polyethylene is the most common plastic in use today.

It's primarily found in packaging materials, like plastic bags and bottles.

Polyvinyl chloride comes in two main forms: rigid and flexible.

Rigid forms of PVC are often used in buildings and construction for things like plastic pipes and the frames for windows and doors.

They need to be strong, not just for daily use, but to hold up for as long as possible in a disastrous event like a fire.

PVC can also be made softer and more bendable by adding in a plasticizer, which can help it be more flexible or make it less thick and viscous.

You'll find flexible PVC in imitation leather, flooring, and inflatable products.

Polyethylene terephthalate is a bit different.

It's spun into fibers and used for clothing, as well as other things like photographic film and magnetic recording tape.

These are some of the more common ones, but there are a bunch of other unique polymers too!

There's nylon, which is a tough, lightweight, and elastic polymer, and kevlar, a strong fiber with bullet-stopping power.

We also have teflon, which is highly water resistant and one of the slipperiest solids.

And interestingly enough, if you add a very small concentration of a polymer to a moving fluid, the friction within the fluid can decrease by up to 80%.

That's called polymeric drag reduction.

The end result is a fluid that can flow even faster with the same pressure behind it.

Which is really important if your fire hose can only safely reach the 10th floor and you have a fire on the 12th.

So now we have a way to solve our problem!

Add in a small concentration of polymeric material and voila, the water can now reach the 12th floor and put out the fire!

Now, in reality, this probably wouldn't be the way to go.

It can be dangerous for firefighters to try and reach tall fires from the outside.

And with one as high up as this, they'd likely try to find a different solution, like trying to attach a hose to an adjacent building.

But as a thought exercise, it's a great way to see some of the more innovative uses of polymers and what they can do.

Today we learned all about the third main type of material that you'll encounter as an engineer: polymers.

We found out that they're made of long, repeating chains of smaller molecules known as monomers.

Then we saw the strange history of polymers and what led to how we use them today.

Finally, we went over some of the more common polymers and how the special properties of polymers could help us in a dangerous situation.

I'll see you next time, when we'll talk about electrical engineering materials.

Crash Course Engineering is produced in association with PBS Digital Studios.

And, if you're interested what's going on with our climate, check out Hot Mess,

which explores climate science, the effects of climate change, and how we can create a better future for our planet and ourselves.

Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C. Kinney Studio with the help of these wonderful people.

And our amazing graphics team is Thought Cafe.