Properties, Intermolecular Forces and Uses of Condensation Polymers Revision Notes | A Level AQA Chemistry

1. Why Condensation Polymers Behave the Way They Do

You have already met condensation polymers — polyesters like PET and polyamides like nylon. Now the question shifts from how they form to why they behave as they do. The answer lies entirely in the intermolecular forces between polymer chains.

Unlike addition polymers such as poly(ethene), condensation polymers contain polar functional groups along every chain. All polymers experience London dispersion forces, but condensation polymers also have stronger interactions such as permanent dipole-dipole forces or hydrogen bonding. These additional stronger forces help explain their higher softening or decomposition temperatures, greater tensile strength, and different solubility behaviour.

2. Intermolecular Forces in Polyesters

Polyesters contain the ester linkage -CO-O- repeated along the backbone. The C=O bond is strongly polar because oxygen is considerably more electronegative than carbon, creating a permanent dipole. The C-O single bond is also polar for the same reason.

These permanent dipoles allow neighbouring polyester chains to attract one another through permanent dipole-dipole interactions. The carbonyl oxygen carries a partial negative charge ($\delta-$) and the carbonyl carbon carries a partial positive charge ($\delta+$). Adjacent chains align so that opposite partial charges are close together, lowering the overall energy of the system.

Polyesters do not form hydrogen bonds between chains in the way polyamides do, because the ester oxygen has lone pairs but there is no O-H or N-H group present to act as the hydrogen-bond donor. This is a critical distinction that examiners test regularly.

3. Intermolecular Forces in Polyamides

Polyamides contain the amide linkage -CO-NH-. This group is polar because the electronegative oxygen and nitrogen atoms withdraw electron density, giving a polar C=O bond and a polar N-H bond. Polyamides then go one step further: the N-H group acts as a hydrogen-bond donor, and the C=O oxygen acts as a hydrogen-bond acceptor.

This means neighbouring polyamide chains can form hydrogen bonds directly between the N-H of one chain and the C=O of the next. Hydrogen bonds are stronger than dipole-dipole interactions. This difference is most dramatically illustrated by Kevlar, which decomposes above 450 °C, far higher than any comparable polyester. For Nylon-6,6 versus PET, the melting points are actually similar (~265 °C and ~260 °C respectively), but the tensile strength difference is more pronounced — reflecting that hydrogen bonding has a greater impact on resistance to chain slippage under load than on the energy needed to melt the bulk material.

In Kevlar, the polyamide chains are arranged in a highly ordered, parallel fashion. The extensive network of hydrogen bonds between chains creates an exceptionally strong and rigid material — strong enough to be used in body armour. The regularity of the chain alignment maximises the number of N-H...O=C hydrogen bonds that can form simultaneously.

4. Comparing Polyesters and Polyamides: A Structured Analysis

Now that both polymer types have been covered, it is worth placing them side by side. The table below summarises the key differences in intermolecular forces and the properties that follow from them.

Notice that the difference in intermolecular force type — dipole-dipole versus hydrogen bonding — cascades directly into differences in physical properties, which in turn determine the applications. This chain of reasoning (structure → forces → properties → uses) is exactly what AQA examiners reward in extended-answer questions.

5. Typical Uses of Condensation Polymers

The uses of condensation polymers follow logically from their properties, which in turn follow from their intermolecular forces. Knowing why a polymer is used somewhere is far more valuable in an exam than memorising a list.

Polyesters (e.g. PET — poly(ethylene terephthalate)): PET is used to make clothing fibres (often sold as 'polyester fabric'), plastic drinks bottles, and packaging films. The moderate dipole-dipole interactions give PET enough strength and rigidity for these applications, while its relatively low density and ease of processing make it commercially attractive. PET bottles can also be recycled by hydrolysis of the ester linkages back to the original monomers.

Polyamides (e.g. Nylon-6,6): The strong hydrogen-bonding network in nylon gives it high tensile strength and good resistance to abrasion. This makes it suitable for ropes, toothbrush bristles, clothing (particularly hosiery and sportswear), and engineering components such as gears and bearings where mechanical durability matters.

Kevlar: Kevlar is an aromatic polyamide (aramid) in which the benzene rings in the backbone allow the chains to pack in a highly ordered, parallel arrangement. The combination of extensive hydrogen bonding and the rigidity introduced by the aromatic rings produces a material with an exceptionally high strength-to-weight ratio. Kevlar is used in bullet-resistant vests, helmets, and high-performance tyres. Its strength per unit mass exceeds that of steel.

6. Exam Technique: Answering 'Explain' and 'Evaluate' Questions on Condensation Polymers

AQA questions on this topic typically use the command words explain, compare, or evaluate. Each demands a different response structure. An 'explain' answer must state the observation and then give the scientific reason using 'because'. A 'compare' answer must address both similarities and differences explicitly. An 'evaluate' answer must weigh evidence and reach a conclusion.

Worked example: Constructing an 'Explain' Answer: Polyamide vs Polyester Melting Points

Explain why nylon-6,6 has a higher melting point than a polyester of similar molar mass. [4 marks]

1. Nylon-6,6 contains amide linkages (-CO-NH-). The polyester contains ester linkages (-CO-O-). — [B1] Naming the correct linkages earns the first mark and sets up the rest of the answer.
2. Nylon-6,6 forms hydrogen bonds between the N-H group of one chain and the C=O group of an adjacent chain (N-H...O=C). — [B1] Specifying both the donor (N-H) and acceptor (C=O) is required for full credit. Vague references to 'hydrogen bonding' without identifying the groups involved typically score zero.
3. The polyester only forms permanent dipole-dipole interactions between the polar C=O and C-O bonds. There is no N-H group, so no hydrogen bonds form between chains. — [B1] Explicitly stating the absence of hydrogen bonds in the polyester, and the reason why, is what distinguishes a full-mark answer from a partial one.
4. Hydrogen bonds are stronger than dipole-dipole interactions, so more energy is required to overcome the intermolecular forces in nylon-6,6. Therefore nylon-6,6 has a higher melting point. — [B1] The conclusion must explicitly connect force strength to the energy needed to separate chains. This is the 'because' that earns the final mark.

Answer: Nylon-6,6 has a higher melting point because it forms hydrogen bonds (N-H...O=C) between chains, which are stronger than the permanent dipole-dipole interactions in the polyester. More energy is needed to overcome these stronger forces, so a higher temperature is required to melt nylon-6,6.

Notice the structure: linkage identified → force named with specific groups → comparison made → conclusion drawn. Every step earns a mark. Skipping straight to the conclusion without the mechanistic reasoning is the most common way students lose marks on this type of question.

7. Common Mistakes and Exam Insider Guidance

This topic has a small number of high-frequency errors that appear in mark-scheme reports year after year. Knowing them in advance is a genuine advantage.

To consolidate the full picture: the type of intermolecular force present in a condensation polymer is determined by the functional groups in its linkage. Polyesters have dipole-dipole interactions; polyamides have hydrogen bonds. Stronger forces mean higher melting points, greater tensile strength, and more demanding applications — which is precisely why Kevlar stops bullets and nylon makes ropes, while PET makes bottles and clothing. Structure drives forces, forces drive properties, and properties drive uses.