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Properties of Some Polymers

 
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Students learn to describe the uses of the polymers made from the above monomers in terms of their properties

Poly(vinyl chloride), polystyrene, and polyacrylonitrile are all polymers that are rigid at room temperatures - we polymer scientists would say that they have a high glass transition temperature (more on glass transition temperatures later).

Although there are no strong forces between molecules of these polymers, the individual molecules are large enough that the weak forces between them make them solids at room temperature.

Since the "side groups" of these polymers are all small, it is easy for the chains to pack closely together - this forms tiny crystalline regions within the polymer.

Poly(vinyl chloride) is used to make PVC piping, among many other applications. Although many chlorinated hydrocarbons are toxic, the size of the molecules in poly(vinyl chloride) means that it is impossible for it to interact with living cells and harm them; you can eat as much of it as you like.

Some chewable toys are made from PVC; you will have noticed that these so not have the same rigidity as PVC piping - they are more rubbery and less glassy. This is achieved by adding a plasticiser which helps the chains to move past one another more easily, disrupting the crystalline packing. A few years ago, some evidence emerged that some of the plasticisers used might be toxic and quite dangerous. This is still an on-going argument... Have a look at some pages on the cases for and against the use of these plasticisers, and some of the other ongoing arguments about PVC.

One of the plasticisers in question is diethyl phthalate, shown below


diethyl phthalate, a nifty plasticiser once found in chewable toys. Lucky no-one has yet spotted the spurious resemblance to terephthalic acid, main constituent of PET plastic bottles!

Poly(styrene) is another glassy polymer, though we think of it is a soft substance - this is because most common applications are of poly(styrene) as a blown foam. If you add a little acetone (nail polish remover) to a piece of polystyrene, you may be surprised to see how little solid material is actually there...

(Blown polystyrene appears white for the same reason that a bowl of sugar is white; while a single crystal of sugar and a single air-filled bubble are both transparent, the multiple scattering of light as it enters and leaves many crystals or bubbles gives a diffuse white appearance overall.)

Hard polystyrene can also be made into useful products other than by foaming; the lid of the pen you may well be using now is likely to be made of a rubbery polystyrene polymer - that is, polystyrene plus a plasticiser.

You will almost never see anything made from poly(acrylonitrile). Acrylonitrile (one of the nastier and more carcinogenic substances around) is an invaluable comonomer in making synthetic rubber (styrene, acrylonitrile, and a third monomer, buta-1,3-diene, combined in one polymer chain forming a copolymer). Poly(acrylonitrile) is chemically modified to make acrylic fibres and the carbon fibres found in tennis rackets and high-tech bicycles.

A good source for information on commercial polymers and their applications is the Macrogalleria. As you will see when you get there, they have won all kinds of awards.

Coming soon: a number of different kinds of spectra of a simple polymer; IR, UV, NMR of poly(styrene), for example, with marks to show what is what.