Chapter 15 Heterochain thermoplastics
A. Polyamides and polypeptides:
Nylons are one of the most common polymers used as a fiber.
Nylon is found in clothing all the time, but also in other places, in the form
of a thermoplastic. Nylon's first real success came with it's use in women's
stockings, in about 1940. They were a big hit, but they became hard to get,
because the next year the United States entered World War II, and nylon was
needed to make war materials, like parachutes and ropes. But before stockings or
parachutes, the very first nylon product was a toothbrush with nylon bristles.
Nylons are also called polyamides, because of the characteristic amide groups
in the backbone chain. These amide groups are very polar, and can form hydrogen
bond with each other. Because of this, and because the nylon backbone is so
regular and symmetrical, nylons are often crystalline, and make very good
fibers.
The nylon in the pictures on this page is called nylon 66, because each repeat unit of the polymer chain has two stretches of carbon atoms, each being six carbon atoms long. Other nylons can have different numbers of carbon atoms in these stretches.
Nylons can be made from diacid chlorides and diamines. Nylon 66 is made from the monomers adipoyl chloride and hexamethylene diamine.
This is one way of making nylon 66 in the laboratory. But in a nylon plant, it's usually made by reacting adipic acid with hexamethylene diamine:
Another kind of nylon is nylon 6. It's a lot like nylon 66 except that it only has one kind of carbon chain, which is six atoms long.
It's made by a ring opening polymerization form the monomer caprolactam. Nylon 6 doesn't behave much differently from nylon 66.
Other polyamides
Aramids are a family of nylons, it is applied to aromatic polyamides, including Nomex and Kevlar. Kevlar is used to make things like bullet proof vests and puncture resistant bicycle tires.
Blends of Nomex and Kevlar are used to make fireproof clothing. Nomex-Kevlar blends could be used to protect fire fighters.
Kevlar is a polyamide, in which all the amide groups are separated by para-phenylene groups, that is, the amide groups attach to the phenyl rings opposite to each other, at carbons 1 and 4. Kevlar is shown in the big picture at the top of the page.
Nomex, on the other hand, has meta-phenylene groups, that is, the amide groups are attached to the phenyl ring at the 1 an 3 positions.
Kevlar is a crystalline polymer. It took a long time to figure out how to make anything useful out of Kevlar because it wouldn't dissolve in anything. So processing it as a solution was out. It wouldn't melt below a right toasty 500 oC, so melting it down was out, too.
Aramids are used in the form of fibers. They form into even better fibers than non-aromatic polyamides, like nylon66
Polypeptides (proteins)
Proteins are one of many types of natural polymers, and they are the most versatile by far. Some proteins, called enzymes, make certain chemical reactions in your body take place up to a million times faster than they would without enzymes. One protein called hemoglobin is found in your blood, and carries oxygen from your lungs to your cells. Another protein called collagen is a strong and tough material that makes up your skin, hair, and fingernails.
A protein is a naturally occurring polyamide.
In your body, these proteins are made from monomers called amino acids:
There are a twenty different amino acids. Each one has a different R' group. Also, each protein has a specific sequence of the different amino acids in it. So in each protein, there is a different sequence of R' groups hanging off of the backbone chain. This sequence determines the properties of the protein.
B. Polyesters, polyethers, and related polymers
Polyesters are used for shatterproof plastic bottles that hold your favorite refreshing beverages and balloons. The balloons are made of a sandwich, composed of polyester and aluminum foil. Materials like this, made of two kinds of material, are called composites.
Polyesters have hydrocarbon backbones which contain ester linkages, hence the name.
The structure in the picture is called poly(ethylene terephthalate), or PET for short, because it is made up of ethylene groups and terephthalate groups.
The ester groups in the polyester chain are polar, with the carbonyl oxygen atom having a somewhat negative charge and the carbonyl carbon atom having a somewhat positive charge. The positive and negative charges of different ester groups are attracted to each other. This allows the ester groups of nearby chains to line up with each other in crystal form, which is why they can form strong fibers.
Polyurethanes are the most well known polymers used to make foams.
Polyurethanes are the single most versatile family of polymers there is. Polyurethanes can be elastomers, and they can be paints. They can be fibers, and they can be adhesives.
Polyurethanes are called polyurethanes because in their backbones they have a urethane linkage.
One unusual polyurethane thermoplastic elastomer is spandex, which DuPont sells under the trade name Lycra. It has both urea and urethane linkages in its backbone. What gives spandex its special properties is the fact that it has hard and soft blocks in its repeat structure. The short polymeric chain of a polyglycol, usually about forty or so repeats units long, is soft and rubbery. The rest of the repeat unit, you know, the stretch with the urethane linkages, the urea linkages, and the aromatic groups, is extremely rigid. This section is stiff enough that the rigid sections from different chains clump together and align to form fibers. Of course, they are unusual fibers, as the fibrous domains formed by the stiff blocks are linked together by the rubbery soft sections. The result is a fiber that acts like an elastomer! This allows us to make fabric that stretches for exercise clothing and the like.
Polycarbonate, or specifically polycarbonate of bisphenol A, is a clear plastic used to make shatterproof windows. General Electric makes this stuff and sells it as Lexan.
Another polyercarbonate can be used to make eyeglass lenses.
This molecules can be used to form a crosslinked material that looks like this:
The carbonate-containing groups (shown in blue) for the crosslinks between the polymer chains (shown in red). This crosslinking is makes the material very strong, so it won't break nearly as easily as glass will.
Poly(phenylene sulfide), or PPS, is one of those really high-performance plastics that is very strong and can resist very high temperatures. PPS doesn't melt until around 300 oC. It's also flame resistant. People in the plastics business call high performance plastics like PPS engineering thermoplastics.
PPS is expensive, so it's used only when good heat resistance is needed.
This gives you a low molecular weight PPS, which is a good thing if you want to use it as a coating. But if you want to use it as a material, you need to heat it in oxygen, and this will give you higher molecular weight. This usually crosslinks it, too.
C. Cellulosic polymers
Starch is found in potatoes, and in grains such as corn and wheat. Starch is made up of glucose repeat units.
In your body, enzymes (which are also polymers, by the way) break starch down into glucose, so your body can burn it for energy. If you're eating a healthy diet, you get most of your energy from starch in this way.
Starch has a few other uses other than food. It's used in pressing clothes to keep them from wrinkling. It's also used to make a foam packing. Starch is biodegradable, so starch foam packing is an environmentally-friendly alternative to styrofoam packing.
Cellulose is one of many polymers found in nature. Wood, paper, and cotton all contain cellulose. Cellulose is an excellent fiber. Wood, cotton, and hemp rope are all made of fibrous cellulose. Cellulose is made of repeat units of the monomer glucose. This is the same glucose which your body metabolizes in order to live, but you can't digest it in the form of cellulose.
Starch is also called a polysaccharide. It is very similar to cellulose.
D. High-temperature and inorganic polymers
Polyimides are a group of incredibly strong and astoundingly heat and chemical resistant polymers. Their strength and heat and chemical resistance are so great that these materials often replace glass and metals, such as steel, in many demanding industrial applications. Polyimides are even used in many everyday applications. They can also be used in circuit boards, insulation, fibers for protective clothing, composites, and adhesives.
Polyimides usually take one of two forms. The first of these is a linear structure where the atoms of the imide group are part of a linear chain. The second of these structures is a heterocyclic structure where the imide group is part of a cyclic unit in the polymer chain.
Aromatic heterocyclic polyimides, like the one on the left, are typical of most commercial polyimides, such as Ultem from G.E. and DuPont's Kapton. These polymers have such incredible mechanical and thermal properties that they are used in place of metals and glass in many high performance applications in the electronics, automotive, and even the aerospace industries. These properties come from strong intermolecular forces between the polymer chains.
Another interesting property of polyimides which makes them excellent for use in construction and transportation industries is they burn. When an aromatic polyimide catches on fire, which by the way is difficult to begin with, a surface char develops which smothers the flame, blocking it from the fuel to burn. Then you just wipe it off, and it's just like the fire never happened.
Poly(phenylene oxide), or PPO, is one of those high-performance polymers we like to call engineering thermoplastics. It's biggest strength is it's resistance to high temperatures. It has a very high glass transition temperature, 210 oC. But there is a price for being resistant to heat. Most polymers are processed at high temperature in a liquid-like state. But if your polymer won't become liquid-like at reasonable temperatures, you can't process it! For this reason, PPO is often made into blends with high-impact polystyrene (HIPS for short). Blending PPO with HIPS makes the PPO easier to process, plus it gives PPO some resilience. PPO needs this toughening because by itself PPO can be brittle in some situations. General Electric makes PPO/HIPS blends and sells them under the name NorylTM.
Structurally, PPO is made of phenylene rings linked together by ether linkages in the 1,4 or para- positions, with a methyl group attached to carbon atoms in the 2 and 6 positions.
This polymer should really be called poly(2,6-dimethylphenylene oxide), but we call it poly(phenylene oxide).
PPO is made by what we call oxidative coupling polymerization of the monomer 2,6-dimethylphenol. Water is a by-product, and thus this is a condensation polymerization.
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