Chemguide: Support for CIE A level Chemistry


Learning outcome 21.3(b)

This statement is about the way that side-chains and intermolecular forces affect the properties of polymers.

Before you go on, you should find and read the statement in your copy of the syllabus.


The effect of side-chains in polythene

I want to start by considering the effect of simple side chains in addition polymers.

You should read about high and low density polythene on the page about the polymerisation of alkenes.

You don't need to worry about the way they are manufactured. Just concentrate on how the presence of side chains affects their physical properties.


PTFE (Teflon)

You will find that PTFE is discussed at the bottom of the page about the polymerisation of alkenes that you have just read.

You could also follow the link in the final green box on that page to a discussion page about PTFE which is gradually getting closer to the truth as people have fed me new ideas over the years.

I am not sure what CIE want for this, because this is new for the 2016 syllabus and there won't be a real question before 2016 at the earliest. My best guess is . . .

  • The fluorines arrange themselves around the carbon chain in a way that forces the molecules to be essentially straight. They can therefore lie closely together.

  • van der Waals forces between the molecules aren't as strong as you might expect because the carbon-fluorine bonds aren't very polarisable. The electrons are held quite tightly towards the fluorine end, and so you don't get as strong van der Waals dispersion forces as you would think given the number of electrons in the molecules.

  • Off-setting this, the molecules can lie tidily close together. Even though the van der Waals forces may not be as strong as they could be, they are still very effective at holding the molecules together in the solid. So the melting point of PTFE is quite high for a polymer.


Kevlar

Kevlar is a very tough polyamide. It is made by polymerising benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene.

If you line these up and remove water between the -COOH and -NH2 groups, you get the structure of Kevlar:

Kevlar is a very strong material - about five times as strong as steel, weight for weight. It is used in bulletproof vests, in composites for boat construction, in lightweight mountaineering ropes, and for lightweight skis and racquets - amongst many other things.

What is interesting about it from the point of view of this particular syllabus statement is the way that the molecules can line up to form very strong sheets. The bonding between the individual strands of Kevlar is hydrogen bonding.

The next diagram shows this for three strands of Kevlar. The red dotted lines represent hydrogen bonds.

Make sure you understand what is going on. A past question asked you to do just this - draw a second strand to show how it was attached to one drawn in the exam question.


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© Jim Clark 2011 (last modified July 2014)