Plastics are substances made from polymers such as poly(ethene), polyvinylchloride and nylon. They have weak intermolecular forces of attraction between the polymer molecules so they have low melting points and can be stretched easily as the polymer molecules can slide over one another.
2.9
Although the intermolecular forces between polymer molecules are weaker than the strong bonds along the chains, there are many of these forces so together they are relatively strong and so these substances are solids at room temperature. The weaker the force, the lower the melting point. Type of intermolecular force or bond
Property 1
Property 2
weak intermolecular forces of attraction
stretches easily
lower melting point
strong cross-links – chemical bonds between the chains
rigid
higher melting points
A polymer is a long chain and is represented by a ‘repeating unit’. In Figure 2.28, the repeating unit has two carbon atoms joined, with four hydrogen atoms; outside the bracket is an ‘n’ – that means this unit is joined to itself ‘n’ times into a very long chain. H
H
C
C
H
H n
Figure 2.29 Polymer chains with weak intermolecular forces (as in Figure 2.27) can slide over one another, chains with cross-links cannot slide.
poly(ethene)
Figure 2.28 Show the repeating unit in brackets. ‘n’ is a very large number. 2
Figure 2.28 shows the ‘repeating unit’ of a polymer chain. Draw three repeating units of this chain.
3
Suggest why the polymer melamine cannot stretch and is rigid.
KEY INFORMATION
Bonding and structure Polymers can be recognised from their structure as they form long chains of repeating units.
Remember the two brackets have a bond through each one. This signifies that the bonds are continuing to join other repeat units.
DID YOU KNOW? Figure 2.30 Model of polymers such as nylon
Each repeat unit is made up from a group of atoms bonded together with strong covalent bonds. The group is shown inside a pair of brackets with two bonds through the brackets, showing each bond that joins to the next repeating units. As there are many repeating units, the value ‘n’ is written outside to represent a large number. 4
Gore-Tex is a registered product made by expanding PTFE.
Draw three repeating units of the polymer made from the unit: CH3 H C
C
H
H
Figure 2.31 Gore-Tex is a popular polymer used by walkers.
Google search: 'polymers and polyester'
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