Explain why carbon fibres used in high-performance materials are strong and stiff, yet nylon (a polyamide) can be drawn into flexible fibres, despite both being used as structural fibres.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Carbon fibre composites are used in aircraft and racing cars because of their exceptional strength-to-mass ratio. Nylon fibres are used in clothing and ropes because of their flexibility and toughness.
Model answer (4 marks)
Carbon fibres are essentially bundles of graphene sheets; each carbon atom is covalently bonded to three others, forming a giant covalent network. The strong covalent bonds within each sheet and the van der Waals forces between the aligned sheets resist stretching, giving the material very high stiffness and strength.
Nylon (a polyamide) is a chain polymer whose chains are linked by hydrogen bonds between the amide groups (–NH–CO–). These hydrogen bonds are much weaker than covalent bonds and can be broken under stress, allowing the chains to slide past one another. This chain‑sliding gives nylon fibres flexibility and toughness.
Thus carbon fibres are strong and stiff because of their rigid covalent network, whereas nylon can be drawn into flexible fibres because its structure relies on weaker, reversible hydrogen bonds.
Nylon (a polyamide) is a chain polymer whose chains are linked by hydrogen bonds between the amide groups (–NH–CO–). These hydrogen bonds are much weaker than covalent bonds and can be broken under stress, allowing the chains to slide past one another. This chain‑sliding gives nylon fibres flexibility and toughness.
Thus carbon fibres are strong and stiff because of their rigid covalent network, whereas nylon can be drawn into flexible fibres because its structure relies on weaker, reversible hydrogen bonds.
Examiner tips
- Use the word "covalent" for carbon and "hydrogen" for nylon; mention the sheet structure and van der Waals forces for carbon.
- Explain that hydrogen bonds can break and allow chain sliding, giving flexibility; contrast with the rigid covalent network.
Common mistakes
- Confusing hydrogen bonds with covalent bonds in nylon; describing nylon as a covalent polymer.
- Failing to mention the sheet structure or van der Waals forces for carbon, or not linking bond strength to stiffness and strength.
Mark scheme (4 marks)
- Carbon fibres consist of layers of graphene-like sheets in which each carbon atom is covalently bonded to three others in a giant covalent/network covalent structure.
- The strong covalent bonds within the carbon layers and van der Waals forces between aligned sheets resist stretching, giving high stiffness and strength.
- Nylon is a polymer held together by hydrogen bonds between the amide (–NH–CO–) groups on adjacent chains.
- Hydrogen bonds are weaker than covalent bonds and can be disrupted under stress, allowing chains to slide past one another, so nylon fibres can flex and deform without fracturing.
Key terms in this question
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More From models to materials questions
- Explain why fullerene C₆₀ has a much lower melting point than diamond, despite b…
- Explain why silicon dioxide (SiO₂) has a much higher melting point than carbon d…
- Explain why graphite can conduct electricity whereas diamond cannot, despite bot…
- Explain why graphene is an electrical conductor, whilst poly(ethene) is an elect…