Topic 11 of 12
Choosing a polymer
Functional groups explain creasing, water response and chemical resistance.
A-Level 8873, revised syllabus (2026-2027)
Choose the material for the job
Functional groups explain creasing, water response and chemical resistance.
A useful material comparison needs a chain of reasoning: identify the structural difference, explain the different interactions or reactions, then connect that difference to the required use. "It is strong" or "it is polar" alone does not explain why one named polymer is the better choice.
| Polymer | Condensed repeat-unit representation | Feature to inspect |
|---|---|---|
| PET, poly(ethylene terephthalate) | [-O-CH2-CH2-O-C(=O)-C6H4-C(=O)-]n | Ester links in the backbone; the two substituted positions of the aromatic ring are opposite one another. |
| Nylon 6,6 | [-NH-(CH2)6-NH-C(=O)-(CH2)4-C(=O)-]n | Amide links provide N-H donors and carbonyl O acceptors for interchain hydrogen bonding. |
| PVA, poly(vinyl alcohol) | [-CH2-CH(OH)-]n | Many OH side groups can hydrogen-bond with water. |
| PVC, poly(vinyl chloride) | [-CH2-CHCl-]n | C-Cl bonds are polar, but the chain lacks the repeated OH groups that provide strong hydration in PVA. |
| PP, poly(propene) | [-CH2-CH(CH3)-]n | A hydrocarbon backbone and methyl side groups; no ester link to hydrolyse in aqueous alkali. |
PET fabric is slightly less prone to creasing than nylon 6,6 in the syllabus comparison. Nylon amide groups can form many interchain hydrogen bonds. During deformation and exposure to moisture, some original contacts can be disrupted and new contacts formed in displaced positions, helping retain a crease. PET has polar ester groups but no repeated N-H or O-H hydrogen-bond donors along the ideal backbone, so this particular interchain hydrogen-bonding effect is smaller. PET still has intermolecular attractions; neither polymer is completely crease-proof, and fabric construction and treatment also matter.
PVA can be water-soluble and is used in eye drops. Its many OH groups form favourable hydrogen bonds with water, allowing suitable chains to become hydrated and dispersed. PVC is water-resistant and is used in raincoats. Its C-Cl bonds are polar, but polarity by itself does not ensure dissolution: it lacks PVA's extensive OH-water hydrogen bonding, and interactions with water do not sufficiently compensate for separating the chains. Covalently bound chlorine here is not equivalent to a hydrated chloride ion.
Choose PP rather than PET for strongly alkaline cleaning solutions. The alkali can hydrolyse PET's ester links, cutting its backbone into shorter fragments and weakening the container. PP has no hydrolysable ester links and its hydrocarbon chain is relatively resistant to this chemical attack. This is a chemical-resistance decision; stiffness alone does not determine suitability.
Worked example
Use supplied structure to recommend a material
Polymer X contains many neutral -C(=O)-O- links in its backbone. Polymer Y has a -CH2-CH(CH3)- backbone. Which is the better candidate for a reusable container holding concentrated aqueous alkali, and what additional evidence would help?
- X contains ester links, which can undergo alkaline hydrolysis and cleave the chain.
- Y is a polyalkene-type hydrocarbon structure with no corresponding hydrolysable link, so it is the stronger chemical-resistance candidate.
- For a real selection, still compare supplied data on temperature resistance, mechanical strength and long-term compatibility. The functional-group argument identifies a major risk but does not establish every performance property.
Y is preferred on the stated chemical criterion. A defensible answer names the ester link and the consequence of backbone cleavage.