Lesson focus: polymerA very long molecule made by joining many small molecules called monomers in a chain. properties arise from long molecular chains, intermolecular attractions, branching, cross-linkA chemical link joining one polymer chain to another, which makes the material stiffer. and processing.
Relate thermoplasticA plastic that goes soft when heated and hard again when cooled, so it can be reshaped., thermosetA plastic that stays hard when heated again and cannot be melted and reshaped. and elastomerA stretchy polymer, like rubber, that goes back to its shape after being stretched. structures to properties and evaluate polymer tests and uses.
Build on earlier learning
The EarthYear 7 compared plastics as engineered materials. This lesson explains their properties using chain structure and processing.
Practical
Comparing polymers
- Test strips of different polymers for stretch, flexibility and response to warm water.
- Use the same strip size for each.
Record: A table of properties linked to chain structure.
You will be successful when you can…
- Define monomer and polymer.
- Compare thermoplastics and thermosets.
- Explain cross-link effects.
- Evaluate polymer choices across a life cycle.
Words that carry the science
- monomer
- A small molecule capable of joining with others to form a polymer.
- polymer
- A substance containing very long molecules made from repeating units.
- thermoplastic
- A polymer that can soften when heated and harden again on cooling.
- thermoset
- A heavily cross-linked polymer that does not remelt after setting and eventually decomposes when strongly heated.
- cross-link
- A covalent connection between polymer chains.
- elastomer
- A polymer able to deform substantially and recover, often aided by sparse cross-links.
Connect polymer-chain structure to behaviour
01Long chains produce collective properties
A polymerA very long molecule made by joining many small molecules called monomers in a chain. contains long chain molecules formed from repeating units. Covalent bonds hold atoms within each chain; attractions and entanglements between chains help determine stiffness, softening behaviour and strength.
Chain length, branching, crystallinity, additives and processing all matter. The repeating-unit formula alone does not fix every property.
02Cross-links restrict chain movement
thermoplasticA plastic that goes soft when heated and hard again when cooled, so it can be reshaped. have chains able to slide more when heated, allowing reshaping and often mechanical recycling. thermosetA plastic that stays hard when heated again and cannot be melted and reshaped. have dense cross-linkA chemical link joining one polymer chain to another, which makes the material stiffer. that prevent flow; strong heating eventually causes decomposition rather than simple remelting.
elastomerA stretchy polymer, like rubber, that goes back to its shape after being stretched. often have flexible chains with enough cross-links to return toward their original arrangement after stretching. Too few links permit permanent flow; too many can make the material rigid.
03A fair polymer test defines the property
“Strong” is ambiguous. Tensile strength, stiffness, toughness, hardness and flexibility require different tests. Compare equal dimensions, loading rate, temperature and sample preparation.
Life-cycle decisions include feedstock, manufacturing energy, service lifetime, repair, reuse, collection and end-of-life. Durability can reduce replacement but create persistence if poorly managed.
Polymer chains and cross-links
Polymers are long chain molecules. In a thermosoftening plastic, the chains are held together only by weak forces, so heating lets them slide past each other: the plastic softens and can be remoulded. In a thermosetting plastic, strong cross-links join the chains, so it does not soften; strong heating chars it.
Blue lines: polymer chains. Red lines: cross-links (strong covalent bonds between chains).
Controls
Softening and charring temperatures are approximate and vary between plastics.
Explain a cable material choice
Why is a flexible thermoplastic useful around a copper wire?
- The coating must electrically insulate the conductor.
- Flexible chains and additives allow bending without cracking.
- Thermoplastic processing allows continuous extrusion around wire.
- Temperature rating and fire behaviour must still match the application.
How to make the reasoning visible: The choice combines function, manufacture and safety rather than relying only on “plastic is an insulator.”
Interpret polymer test evidence
Equal-size strips were tested at 20 °C; values are illustrative.
| Sample | Extension before break / mm | Force at break / N | Recovery | Likely behaviour |
|---|---|---|---|---|
| A | 4 | 220 | Little extension | Stiff, strong strip |
| B | 85 | 70 | Almost complete | Elastomeric |
| C | 32 | 145 | Partial | Tough, flexible thermoplastic |
| D after heating | — | — | Does not soften; chars | Thermoset-like |
Why can sample A be strong but not tough?
Why must strip dimensions be identical?
Check your understanding
Answer from memory, then check against the suggested answer.
What is a polymer?
What do cross-links do?
Can a thermoset simply be remelted?
Why can warming a thermoplastic make it easier to shape?
Explain why adding a plasticiser can increase flexibility but reduce stiffness.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Thermoplastics and thermosets are both polymers.
Compare the structure and heating behaviour of thermoplastics and thermosets.
(5)Total for this question: 5 marks
Command word: For compareA command word: say how two or more things are similar and how they are different., write paired structure → property statements.