The model above is an image of the pdb model you can view
by clicking here or you can just click on the image itself.
Either way, be sure to close the new window that opens up
with the 3D model in it when you are ready to come back here.
Polyesters are the polymers, in the form of fibers, that were used back in the seventies
to make all that wonderful disco clothing, the kind you see being modeled
on the right. But since then, the nations of
the world have striven to develop more tasteful uses for polyesters, like
those
nifty shatterproof plastic bottles that hold your favorite refreshing
beverages. Polyesters can be both plastics and fibers.
Another place you find polyester is in the fancy balloons you get when you're sick or having a birthday party. These are made of a polyester film made by DuPont called Mylar combined with aluminum foil. Materials made of two kinds of material like this are called composites.
Polyesters have backbones which contain ester linkages, as shown below, 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 (duh!). I realize that
terephthalate is not the kind of word an English-speaker (or anyone else in the world) is used to saying, but with practice you should be able to say it with
only a slight feeling of awkwardness as it rolls off your tongue.
The ester groups in the polyester chain are polar, with the carbonyl oxygen atom (the one with a double bond to it) 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.
Another molecular component that affects final properties is the flat aromatic ring. The rings on adjacent chains like to associate and come near each other. This makes films made with PET pretty dense and slow to pass gas (sorry for that but I couldn't help myself). That's why you can buy sodas in PET bottles: the carbon dioxide "bubbly" can't get out through the PET film walls.
Now I'm sure everyone out there is just dying to have two important questions
answered, namely:
Why isn't the most common carbonated "adult beverage" (adult? really? beer?) available in plastic bottles like sodas are?
And the second one, which I'm positive everyone is wondering about, is:
How come peanut butter comes in neato shatterproof jars but jelly doesn't?
The second of these riveting questions, as it turns out, is related not to the crystalline part of the polyester bottle wall, but to the amorphous part. That's the part that isn't held tightly in a crystalline lattice. The chain molecules are more loosely packed and can wiggle around a lot, especially if the temperature is elevated some. So, then, the answer is that PET has too low a glass transition temperature, or Tg, and that is the temperature at which the PET becomes soft. It's the temperature at which all those amorphous chains begin to move around a lot, letting gas molecules (and other kinds of molecules as well) get through. More important here, once the temperature goes above the Tg, the soft amorphous phase can't help hold the bottle's shape. That's also why you shouldn't put a PET bottle into the dishwasher, unless you're actually trying to find new forms of plastic art.
Now reusing a soft drink bottle would require that the bottle be sterilized before it is filled up again. This means washing it at really high temperatures, temperatures too high for PET because it softens. And to actually answer your very insightful question, filling a jar with jelly is also carried out at high temperatures. Down at your local jelly factory, the stuff is shot into the jars hot (so it flows like a liquid), at temperatures which would cause PET to become soft. So PET is no good for jelly jars because of a Tg of around 70 oC. What to do, what to do. Can we come up with a molecular answer to this important problem?
PEN Saves the Day!
Yes, we can, and it's in the form of a new kind of polyester that is just the thing needed for jelly jars, returnable bottles and even the beer bottles sold at athletic events. It is poly(ethylene naphthalate), or PEN for short. See the molecular structure below to help you understand why.
PEN has even bigger and flatter aromatic rings in the polymer backbone. The size increase means that it has a higher glass transition temperature than PET. Remember, that's the temperature at which a polymer gets soft. The glass transition temperature of PEN is high enough (at 120 oC) so that it can withstand the heat of both sterilizing bottle washing and hot strawberry jelly. PEN is so good at standing the heat that you don't even have to make the bottle entirely out of it. Just mixing some PEN in with the regular PET gives a bottle that can take the heat a lot better than plain old PET.
Now what about beer bottles? How can PEN work better for that then PET? Surely, the Tg difference isn't the reason, is it?
Great observation and hypothesis! Nope, it's not the Tg so much as what the Tg is related to. Remember how big and flat the naphthalene rings are? In the crystal domains, neither PET or PEN can pass gas of any kind, so that's not the answer. So it must be how that bigger aromatic ring affects the amorphous domains. In fact, there seem to be two affects. One is that the density of the PEN amorphous domains is higher than that of the PET amorphous domains. Well and good. But there's also all that wiggling going on at the molecular level. And there's less of it in PEN than in PET. More dense, less wiggling. Overall that means that even oxygen can't easily penetrate the PEN bottle walls.
Wait, how did oxygen get into this discussion? I thought we were talking about carbon dioxide.
Well, sure, CO2 needs to be kept inside for beer just like for sodas. But even more important is "skunkiness." What? you ask? Turns out that beer is much more affected by oxygen than sodas are, which actually aren't affected much at all. Beer is a much more complex mixture of sugars, enzymes, proteins plus a little bit of alcohol. And it's how oxygen reacts with some of those that generates a bad smell and even worse taste. Makes you think of how a skunk smells, and maybe, how one might taste.
So the key is this: both PET and PEN keep carbon dioxide IN, but only PEN keeps oxygen OUT. And that means PEN works for plastic beer bottles where PET won't. Case solved, Sherlock.
And now for your edification and delight, let's take a look at the 3D structure of PEN. The figure on the below shows the structure in 2D with fancy colors and all. Click on the image and up pops a true 3D version that you can rotate or zoom in to see the structure better. Try it! It's fun (not to mention educational).
All right, this polyester stuff is the best thing since
sliced bread.
Agreed. But how does one make it?
Ok here goes...
In the big plants where they make polyester, it's normal to start off
with a compound called dimethyl terephthalate. This is reacted with
ethylene glycol is a reaction called transesterification. The
result is bis-(2-hydroxyethyl)terephthalate and methanol. But if we heat
the reaction to around 210 oC the methanol will boil away and
we don't
have to worry about it anymore.
Then the bis-(2-hydroxyethyl)terephthalate is heated up to a balmy 270 oC, and it reacts to give the poly(ethylene terephthalate) and, oddly, ethylene glycol as a by product. Funny, we started off with ethylene glycol.
If you want to know how all these reactions go down, click here.
But in the laboratory, PET is made by other reactions. Terephthalic acid
and ethylene glycol can polymerize to make PET when you heat them with an
acid catalyst. It's possible to make PET from terephthoyl chloride and
ethylene glycol. This reaction is easier, but terephthoyl chloride is
more expensive than terephthalic acid, and it's a lot more dangerous.
There are two more polyesters on the market that are related to PET.
There is poly(butylene terephthalate) (PBT) and poly(trimethylene
terephthalate).
They are usually used for the same type of things as PET, but in some
cases these perform better.
Click on either of the two images above to pop up a 3D version of PBT (on the left) ad PTT (right). You can rotate, expand and convert to stick-and-ball structures, for example. Fun!
Other polymers used as plastics:| |
Other polymers used as fibers: | |
| Polypropylene | Polypropylene | |
| Polyethylene | Polyethylene | |
| Polystyrene | Nylon | |
| Polycarbonate | Kevlar and Nomex | |
| PVC | Polyacrylonitrile | |
| Nylon | Cellulose | |
| Poly(methyl methacrylate) | Polyurethanes |
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