MATSE 81
Materials in Today's World

Plastics

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Plastics are polymers derived from petroleum products. They are a perfect example of designing better, cheaper, and completely human-made materials. Plastics are inspired by nature, i.e., natural polymers, but are completely synthetic.

Most polymers are made up of carbon and hydrogen atoms, and many plastics are as well. Polymers that contain hydrogen and carbon atoms are called hydrocarbons. Carbon atoms can form single bonds to four other atoms. If the carbon atoms in a polymer are bound to four other atoms the polymer is referred to as a saturated hydrocarbon. If on the other hand, the carbon atom is not bound to four other atoms it will typically form double or triple bonds, as needed, with another carbon atom. In this case, the polymer is referred to as an unsaturated hydrocarbon. This distinction is important as unsaturated polymers are generally unstable and more reactive than their saturated cousins.

Around 1850 billiards was becoming increasingly popular, but there was a problem. The balls were made of ivory, which is in very limited supply and is, thus, very expensive. Not to mention it requires the killing of elephants to obtain. In 1956, the first human-made plastic (Parkesine) was patented by Alexander Parkes from Birmingham, England. Often called synthetic ivory it was composed of nitrocellulose – cellulose treated with nitric acid and a solvent. It was the first thermoplastic, but it failed as a commercial product due to poor product quality control. The following 10-minute video discusses the development of polymers to replace ivory billiard balls, the science behind some of the most-used plastics, and some examples of thermoplastic and thermosetting polymers.

To Watch

Click for the transcript of Polymers - Crash Course Chemistry #45.
Charles Darwin was a big fan of billiards he loved his billiard table it was one of his prized
possessions and one of the most valuable things he owned but not actually the table itself
in fact it was the balls that were so valuable pure ivory carved from the tusks of elephants
only the wealthiest could afford a full set luckily he was married to an heiress a full set of
16 billiard balls would require at least one possibly two full elephants worth of ivory the
idea that any bar in the world might contain a billiard table available for anyone to play
and not like run out of the bar with your pockets full of valuable ivory would have sounded
insane in 1850 and the billiard industry was well aware of this problem billiard balls were getting
more expensive elephants were getting more rare it was thus not with an environmental motive
that in 1867 the felon and collander Pool Supply Company offered $10,000 to anyone who could
come up with a substitute material that worked as well as ivory but could be produced more quickly
and sustainably than dead elephants an inventor named John Wesley Hyatt took on that challenge 
he used nitrocellulose a flammable solid created by mixing cotton with nitric acid to create a hard
shiny white sphere the properties were extremely similar to ivory billiard balls the company never
gave him the prize but he did patent the technique using it to create billiard balls piano keys and
even teeth becoming pretty dang wealthy in the process so he pretty much created the industry that made all of the polymer materials that surround you right now and that we'll be discussing today in crash course chemistry also elephants didn't go extinct so that's a plus the polymer that John Hiatt worked on was somewhat unsurprisingly kind of crummy it worked well once it was created but the manufacturing process was dangerous because nitrated cellulose can explode in a warm breeze so luckily some replacements started creeping and replacements with names that you probably recognize like polyvinyl chloride or PVC bakelite polystyrene polyester and nylon these are all polymers huge chains are sometimes three-dimensional networks of repeating organic units called monomers each polymer has monomer but they're all relatively simple at that basic 1 unit level the trick is that they can bond to each other on each side potentially forever though in reality the chains are sometimes hundreds sometimes thousands sometimes hundreds of thousands of units long in order to make a polymer all you need is a molecule that can easily bond to another identical molecule at two points and the simplest of those is ethene also known as ethylene its polymer you'll be unsurprised to here is polyethylene which you've probably heard about we'll talk more about the specifics in a second but basically because pi bonds in the double bond are weaker than Sigma bonds they can be broken and new monomers can be added just a note to avoid confusion polyethylene has that een sound in it right but it's not an alkene because all of those double bonds get broken to form new sigma bonds it's a polymerized alkene but the molecule itself is an alkane it's confusing so I thought it was worth pointing out now chemists might want a bunch of different things out of their polymers maybe they want it to be stretchy maybe strong may be transparent may be recyclable polyethylene is transparent and thermoplastic meaning it can be melted and reformed making it recyclable some other polymers like polyurethane or bakelite are thermoset which means that they change chemically during some kind of curing process and cannot be melted down and reformed polyethylene can actually be converted into a thermoset polymer by introducing cross links basically molecular bridges between those polymer chains any plumbers out there probably have heard of cross-linked ethylene or PEX pipe which is what this is extra super-strong because of those cross links polyethylene is also nice because its strength can be varied by changing the size of the molecules they're allowed to polymerize until they're tens of thousands of monomers long the plastic they will form will be all knotted up in these ultra long chains and it will be extremely strong that's why this HDPE high-density polyethylene is a strong bottle whereas this is much squishier this is low-density polyethylene however those ultra long chains also make it much more viscous when heated and thus more difficult to process it also loses some of its opacity it becomes more of that milky white color a polyethylene is great it's really great so great that it's the most common plastic in the world we produce over 80 million tons of it per year but we want a lot out of our plastics strength color elasticity resilience recyclability we need everything from saran wrap to car tires all of these are traits that chemists work tirelessly to create in the early middle 20th century and continue to work on even today one of the earliest technique that they used to try and bring out new properties was to change the substituents on the ethylene monomer just see what would happen like what if we swapped out one of the hydrogens for chlorine well you get poly chloro ethene kinda that's not what we call it okay so remember how benzene when attached to a chain is a phenyl group and how those two words have nothing to do with each other well the same goes for the ethene functional group which is called a vinyl group it's an old word super old it comes from the Greek word for wine and that is why chloro ethane is more commonly called vinyl chloride and poly chloro a thane is more commonly called poly vinyl chloride or PVC which is what this little ducky is made out of and also what records are made out of which is why we call them vinyl now what happens when we change out a hydrogen for a methyl group well then suddenly this molecule is a propane or if you're using the old ways a propylene and yes if you polymerize it it becomes polypropylene if one of the hydrogen's is replaced with a phenyl group well that chemical was first derived from trees in the styrax family so it's called styrene to linearize it polystyrene make a foam out of it styrofoam now if you change all four of the hydrogen's on the base at alene with fluorine it becomes tetrafluoroethylene polymerized that and instead of hydrogens that polymer is bound entirely to fluorine fluorine as we could guess from its spot on the periodic table loves electrons it is extremely electronegative but because it holds onto its electrons so tightly and is so satisfied in this polymerized chain the electrons are unavailable for even minimal inter actions with any other molecules I'm not just saying this stuff is super difficult to react with or it's really stable it's more than that the electrons aren't even available for the sort of interactions that make things stick to each other cause friction which is why you have heard of polymerized tetrafluoroethylene because it's super useful either by its abbreviation PTFE or by its brand name teflon so how do we actually make these things well ethene based polymers form through a process called addition polymerization the monomers are simply added together and no byproducts are formed in order to get the process kicked off you have to introduce a free radical to me that always sounded like some kind of crazy freedom fighter diving into battle without much thought for what could come after he was consumed in the firefight that's kind of what they are free radicals are atoms or ions that have a single unpaired electron this is crazy unstable it's basically like having half a covalent bond dangling off the atom anywhere this can form a bond it's going to form a bond and in the case of addition polymerization it attacks the double bond and joins with one of the carbons while the other carbon is itself left with an unpaired electron the molecule itself is now a newly formed free radical it attacks another nearby PI bond joining with another molecule of ethene forming another radical this process continues until two radicals meet each other consuming both free radicals without producing any more thus ending polymerization there are of course other sorts of polymerization as well sometimes a hydroxyl group from one molecule is happy to join up with a hydrogen from another forming water the water will break away as a byproduct leaving the two molecules bound together this often occurs when an amine group with its loosely held hydrogen meets a carboxylic acid with its loosely held o H group this is just what happens when hexamethylenediamine meets adipic acid forming another branded polymer nylon by dissolving hexa methylene diamine and adipic acid into two different immiscible or unmixable solvents we can actually create nylon right here the nylon forms at the interface between the two immiscible liquids and we can literally grab it and pull it out of the vial twisting and spooling it until we get a nice blob of nylon this works because hexa methylene diamine has an amine group on each end and it acid as a carboxylic acid on both ends thus when the two monomer unit called a dimer is formed there's still a carboxylic acid on one end and an amine group on the other allowing for further polymerization these amine acid condensation polymerization zall so allow for the creation of possibly the most important polymers on the planet natural polymers being created inside of you right now out of monomers that we call amino acids did you see that one coming amino acids polymerize through condensation reactions guided by the code in your DNA and some very complicated enzymes to form basically you other important polymers in your body include polysaccharides which we use to store energy and yeah DNA and RNA which we use to encode information for the formation of proteins but that would be back to biology which is a whole other crash course which to be clear is available if you'd like to watch it and thank you for watching this episode of crash course chemistry and it you learn that the first commercial polymer ever saved the lives of a lot of elephants that ethene is sometimes called ethylene and that a huge variety of polymers is based on the addition reaction of ethene based monomers including Teflon which is so frictionless because of fluorines extreme electronegativity you also learn how addition polymerization reactions work and that other polymers are formed by condensation reactions including the polymerization of amino acid monomers which along with other polymers like DNA and RNA and glycogen make up a lot of the stuff that is you this episode of crash course was written by me Hank green edited by Blake de pastino and our chemistry consultant was dr. Heiko Langner it was filmed edited and directed by Nicholas Jenkins our script supervisor was stephan jin our sound designer is Michael Aranda and our graphics team is thought cafe you

After watching this video, please proceed to the first (of two) reading assignments for this lesson.