"I’m guessing you haven’t thought much about the science of coolers, but it’s pretty astounding; these workhorses use a special molecular structure to literally trap cold air inside."
From It’s Elemental by Dr. Kate Biberdorf, Copyright © 2021 by Kate Biberdorf. Published by Park Row Books.
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In Austin, we can get to the ocean in just under four hours. All we have to do is throw the dogs in the back of the car, open the sunroof, and enjoy a sunny drive to Galveston or Corpus Christi. In the summer of 2019, being able to get to the water was almost a means of survival. In August of that year, we had temperatures of 100°F or higher for twenty days of the month. It was brutal.
My husband and I practically lived at the beach that summer, which was when I started to notice the chemistry in action by the ocean. From the avobenzone in my sunscreen to the polymers in my swimsuits, I could spot a real-world example of my favorite science in every direction.
Case in point: my cooler.
The one thing we absolutely relied on that summer was a quality ice chest to keep our food and drinks insulated and miraculously cold despite the sizzling heat. Our favorite one is made from polyethylene, but polystyrene can be used too.
I’m guessing you haven’t thought much about the science of coolers, but it’s pretty astounding; these workhorses use a special molecular structure to literally trap cold air inside. There are two different forms of polyethylene commonly found in most big, sturdy coolers so let’s look at that polymer first. Just like the name implies, polyethylene polymers are comprised of many ethylene molecules, and it is currently our most common form of plastic. Ethylene is a hydrocarbon with the formula H2C=CH2, and it is an extremely flammable gas. It’s a nonpolar species (the electrons are evenly distributed across the molecule), which means it can only form dispersion forces with its neighboring molecules.
However, under extremely high pressure, ethylene can react with itself chemically to form a huge ethylene chain. When this happens, the double bond breaks, leaving a single bond that holds the original molecule together. After this step, the carbon atoms can form new covalent bonds with different carbon atoms, resulting in a long hydrocarbon chain.
Let me break that down using the Ryan Reynolds example from the first part of this book. If you remember, we discussed how I could form a double bond with Ryan by holding both of his hands. But if I want to go through polymerization reaction, I need to release one of Ryan’s hands and reach out to form a new connection with another adorable celebrity, like Joe Manganiello. Ryan, of course, will do the same thing, therefore he can produce a new bond with Blake Lively.
This domino effect will continue until all carbon atoms are surrounded with four covalent bonds (because of its position on the periodic table). The product, polyethylene, is nonpolar, with dispersion forces between the polymer fibers. These interactions are very similar to the IMFs formed between individual ethylene molecules.
These molecules are gargantuan with molecular weights of 10,000–100,000 g/mol. Since polyethylene is a huge, nonpolar molecule, it is not soluble in water, which makes this polymer a perfect molecule to use in the production of an ice chest. It’s insolubility in water allows us to fill the chest with ice and bring it to the ocean.
Polyethylene is also used in sandwich bags, which protects our sandwich from getting wet and soggy from the ice! But what is the difference between the polymer used to make an ice chest and the one in our sandwich baggies?
For starters, the exterior plastic of most coolers is made from high-density polyethylene (HDPE), while the sandwich bags are made from low-density polyethylene (LDPE). Let’s talk about LDPE first, so I can explain the difference. LDPE was first used back in the 1930s, and it has a lower density than HDPE (obvious, I know). But even though the two plastics have the exact same atoms and covalent bonds within the polymers, the way in which the bonds have formed is very different.
The LDPE is a polymer made from covalent bonds between neighboring ethylene molecules. Like we discussed before, when the ethylene molecules start reacting with each other, the double bond breaks and a new single bond forms between nearby carbon atoms. This process forms what is referred to as a branched hydrocarbon chain. So, instead of forming one orderly, single-file line of carbon atoms, the bonds are made between random carbon atoms to form T’s all over the structure. A linear shape would be nice and orderly, like a chain of kindergarteners heading to lunch, while the branched shape is a chaotic mess, like kindergarteners at recess.
The branched shape of LDPE makes it much weaker than HDPE polymers (and a lot more stretchy). Because this shape makes it difficult for the molecules to pack closely together with the other LDPE polymers, they cannot form strong dispersion forces. This means that there is a lower-density of polymers in a given space, specifically, a density range of 0.917–0.930 g/cm3.
What that really means is actually quite significant, and it’s why we use these types of polymers so often in our daily lives. They are stretchy and resilient, which makes them perfect for sandwich bags (and plastic beach bags—before everybody realized they created a lot of excess waste, and converted to reusable bags). LDPE polymers are ideal for packing big sandwiches because they can be wrapped around the fragile bread, yet still keep your lunch safe from moisture, especially when compared to something natural, like paper, that’s completely rigid and does not stretch or bend or shield from water.
Technically, polyethylene is not considered a hard or rigid polymer, which means that, in theory, we should be able to mold the molecules into different arrangements. Our ability to change the shape of these plastics has to do with how the molecules are lined up within the polymer. For example, if we grab two sides of a plastic sandwich bag and pull, we will see the entire shape of the bag change. It will immediately adapt to the stress by increasing in length and decreasing its width, basically forming the shape of a dumbbell. If we pull too hard, the plastic will eventually snap.
This process, called necking, occurs whenever the molecules within a polymer try to adjust to stress. Before we started messing with the plastic bag, the molecules were arranged in a haphazard manner, like wet pasta in a pan. But as soon as we start to pull on the plastic bag, the molecules began to straighten out. Like if we were to zap our pan and change all of the wet spaghetti into dry spaghetti in an instant. With stress, the molecules change shape from bent to straight, and all line up in perfect order. The combination of the long, thin shape and the organized arrangement of molecules allows for the plastic to be stretched out, forming the middle part of the dumbbell shape I mentioned earlier.
But if you release the bag, it’ll mostly go back to its original, chaotic shape. (There may be some damage to the spots where the plastic connected with your fingers, but otherwise, the polymers on the inside should return to their initial arrangement.)
Excerpted from It’s Elemental by Kate Biberdorf, Copyright © 2021 by Kate Biberdorf. Published by Park Row Books.
Dr. Kate Biberdorf is a scientist and a chemistry professor at The University of Texas. She has a PhD in inorganic chemistry and has published her work in Catalysis, Science, and Technology. Her 6-book series for kids with Penguin breaks down the image of the stereotypical scientist, while reaching those who may be intimidated by science. She has appeared on The Today Show, Wendy Williams Show, and Late Night with Stephen Colbert. She lives in Austin TX with her family.
