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A smartphone is not designed to stop a bullet.
And to be fair, a Kevlar vest makes a pretty bad touch screen.
But both rely on the same type of material to do their jobs: liquid crystals.
And these revolutionary materials might never have seen the light of day without their discoverer,
Stephanie Kwolek, intervening on their behalf.
Kwolek was a chemist at DuPont Chemical Company, where she was tasked with finding long molecules called polymers
that the company could turn into strong, lightweight fibers.
The hope was to find a replacement for the steel wire in car tires, which would save
on weight and make the cars more fuel-efficient.
Kwolek’s breakthrough came in 1964, when she was working with polymers called aramids.
Most polymer molecules are long chains of repeating units, and are flexible like wet
noodles because they can spin and twist around their chemical bonds.
But aramid molecules are super rigid, more like steel-reinforced dry spaghetti.
Flat, hexagonal structures in the molecule called aromatic rings keep individual aramid
molecules relatively rigid and make them resistant to heat.
Oxygen and hydrogen atoms that stick out from the sides of each chain act like magnets that
lock chains with one another and hold them in place.
This means they have the potential to be incredibly strong and resilient.
But to actually turn these molecules into a useful fiber, Kwolek first had to find a way to dissolve them.
And that meant finding the right chemical to get in between the aramid molecules and separate them.
Eventually, she found just the right thing: a mix of tetramethylurea and lithium chloride.
But the result looked wrong.
She expected a sort of clear glop with the consistency of molasses.
But her solution was cloudy, as though it was full of solid particles.
Then when she stirred it, it split into two layers:
clear and yellow on the top and foggy and white on the bottom.
Kwolek wanted to try and spin this stuff into a fiber.
But her colleague was afraid it was gonna clog up the fiber-spinning equipment (which
admittedly was probably pretty expensive), and he wouldn’t let her use it.
She argued for days, and eventually wore him down.
Turns out it worked fine, and she got a fiber out of it.
Lab tests showed that it was super strong and super stiff, especially when heat-treated.
When Kwolek’s solution was spun, almost all of the molecules rearranged themselves
to point parallel with the length of the fiber.
Each molecule is so sturdy because of its aromatic rings, and all of the molecules hold
together tightly because of their hydrogens and oxygens.
What’s more, by pointing all in the same direction, the molecules reinforce each other in the fiber.
So when you weave that fiber into a fabric, you get something as strong as, well, Kevlar,
which was developed from the initial discovery within a few years.
Modern Kevlar has a specific tensile strength about eight times that of steel wire.
That means that if you have a Kevlar fiber and a steel wire of the same length and weight,
the Kevlar can hold about 8 times as much weight as the steel can before it snaps.
Bulletproof vests use this property by stacking layers of Kevlar fabric with the fibers running perpendicular to each other.
If the vest gets shot, the bullet basically is stretching the fibers like a weight pulling
on them at the impact point.
The fibers are super strong, though, so they don’t snap, but instead distribute the force across the rest of the vest.
Instead of piercing the vest, the bullet gets caught like a soccer ball in a net.
DuPont started producing commercial quantities of Kevlar in 1971, and the first bulletproof
vests hit the market in 1975.
But Kevlar isn’t just for bulletproof vests.
It shows up in anything that needs super strong, lightweight fibers, like car tires and drum heads and sailboat sails.
Now while most of us are never going to have to use a Kevlar vest, the chemists at DuPont quickly realized that
Kwolek had actually invented something else along the way.
The strange two-layered liquid she had made was a liquid crystal solution.
A liquid is a state of matter where molecules are free to move around, and the stuff fills the shape of its container.
Whereas in a crystal, molecules are organized in a repeating pattern.
It sounds to me like the two circles of that venn diagram do not overlap.
In Kwolek’s solution, there are patches of rigid molecules all lined up in parallel to each other.
Each patch points in a different direction.
So, liquid, but also a crystal.
Chemists had known since 1888 that liquid crystals can exist in nature, but Kwolek’s
was the first liquid crystal solution made with polymers.
But if that sounds like a mere curiosity, it’s probably how you’re watching this video right now.
Because the screen of your phone, computer, tablet or TV is probably a liquid crystal
display, or LCD, first invented in 1968.
Unlike the dry-spaghetti-rigid-straight molecules in Kevlar’s liquid crystal, LCDs use boomerang-shaped molecules.
These molecules are separated into pixels, and an electrical current can be used to change
the orientation of the molecules in each one.
Oriented one way, the molecules change the path of light, and in combination with a filter,
that makes the pixel looks black.
Turned another way, light can pass through, and the pixel looks bright.
Put a bunch of those together and you can see me! Hello!
So whether you’re using a smartphone, or driving a fuel-efficient car, or wearing a
bulletproof vest,
remember it’s chemists like Stephanie Kwolek who made it possible.
Thanks for watching this episode of SciShow, which was brought to you by LCDs, but also
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