Polymers are long chains made from small repeating units, and composites combine two materials so each covers the other’s weakness. Questions ask you to connect a property to a use, so the property is the point.
This lesson is part of SPM Chemistry manufactured materials. If you want the chemistry of the chains themselves, see the polymers topic.
How do polymer properties decide their uses?
A polymer is a long chain of repeating units. Its properties depend on how the chains are arranged and held together.
- Light and low density: bags, bottles and food containers.
- Does not conduct electricity: insulation around cables.
- Does not corrode or react easily: pipes and water tanks.
- Flexible and elastic (rubber): tyres, gloves and seals.
The same low reactivity that helps a pipe also makes plastic waste slow to decompose. A good answer names the benefit and, when asked, the environmental cost.
Thermoplastic or thermosetting?
| Feature | Thermoplastic | Thermosetting |
|---|---|---|
| Effect of heat | Softens, can be remoulded | Stays hard, will not soften |
| Chain link | Weak forces between chains | Cross-links between chains |
| Example use | Shopping bag, drink bottle | Kettle handle, electrical plug |
| Recyclable by melting? | Yes | No |
A shopping bag is cheap and is shaped once by melting, so a thermoplastic is fine. A kettle handle sits near steam and heat. It must not soften, so a thermosetting polymer is chosen.
What does a composite add?
Name the two parts first, then what each contributes. This habit keeps your answer specific.
| Composite | Parts | What each part gives | Use |
|---|---|---|---|
| Reinforced concrete | Concrete and steel bars | Concrete resists pressing, steel resists pulling | Buildings, bridges |
| Fibre glass | Glass fibres in plastic | Fibres give strength, plastic gives shape | Boat hulls, roofs |
| Photochromic glass | Glass with silver chloride | Darkens in bright light | Sunglasses |
Worked example: a kettle handle and a shopping bag
Question. One polymer makes a kettle handle and another makes a shopping bag. Explain why they must be different types, for 3 marks.
- The kettle handle is heated by hot water and steam, so it must stay hard and keep its shape.
- A thermosetting polymer has cross-links between chains, so it does not soften when heated.
- A shopping bag can be made by melting and shaping only once, so a thermoplastic with weak forces between chains is enough and is cheaper to mould.
Common error. Writing “the kettle handle is stronger”. Strength is not the deciding property here. Heat behaviour is, and the question was built around it.
Check yourself
Explain why a bridge is built from reinforced concrete instead of plain concrete, naming what each part contributes.
Answer
Concrete is strong when pressed but weak when pulled or bent, so plain concrete cracks under a bending load.
Steel bars are strong when pulled. Together they form a composite that resists both pressing and pulling, so the bridge carries traffic without cracking.
What to study next
Put the last skill together in evaluating a material choice using given requirements. Test yourself on the manufactured materials practice set.
If you want a teacher to go through polymer and composite questions with you, see online one-to-one Chemistry tuition.