In one line
Atoms within a polymer chain are joined by strong primary covalent bonds, while separate chains are held together by weaker secondary forces (van der Waals, dipole–dipole and hydrogen bonds) or, in network polymers, by covalent cross-links.
Explained simply
Picture a bowl of cooked noodles. Each noodle is strong along its length (covalent bonds inside the chain). The noodles stick to each other only weakly (secondary forces), so you can pull them apart. If you glue the noodles together at many points (cross-links), the whole bowl becomes one rigid lump. That is the difference between a thermoplastic and a thermoset.
How it works
- Primary (intra-chain) bonds: covalent bonds such as C–C, C–H, C–O, C–N, Si–O along the backbone. These are strong (C–C about 348 kJ/mol) and decide chemical stability and the thermal degradation temperature.
- Secondary (inter-chain) forces:
- Dispersion (London/van der Waals) forces: weakest; present in all polymers; dominate in non-polar polymers such as polyethylene and polypropylene.
- Dipole–dipole forces: in polar polymers like PVC (C–Cl) and polyesters (C=O).
- Hydrogen bonds: strongest secondary force; in nylons (N–H···O=C), cellulose and proteins (O–H, N–H groups).
- Cross-links: covalent bonds between chains (sulphur bridges in vulcanised rubber; network bonds in Bakelite). They prevent chains from sliding, making the polymer rigid, insoluble and infusible.
Key formula
No formula. Order of strength of forces between chains: covalent cross-link ≫ hydrogen bond > dipole–dipole > dispersion.
Quick example
| Polymer | Main inter-chain force | Result |
|---|---|---|
| Polyethylene | Dispersion | Soft, flexible, low melting |
| PVC | Dipole–dipole | Harder and more rigid than PE |
| Nylon 6,6 | Hydrogen bonding | Strong fibres, high melting (about 265 °C) |
| Vulcanised rubber, Bakelite | Covalent cross-links | Elastic (light cross-links) or rigid (dense network) |
These forces explain why heating softens thermoplastics (secondary forces are overcome and chains slide) but does not soften thermosets (covalent cross-links do not break until the polymer decomposes).
Common mistakes
- Saying polymer chains are held together by ionic bonds; ionic bonding is rare (only in ionomers).
- Thinking melting breaks the covalent backbone; melting overcomes only secondary forces.
- Forgetting cross-links as a bonding type when explaining thermosets.
How MU asks it
"Explain chemical bonding in polymers" (3–5 marks) or as part of thermoplastic vs thermoset comparison. For full marks: distinguish primary and secondary bonding, list the three secondary forces with polymer examples, explain cross-links, and link the forces to properties (strength, softening).