Thermoplastic
Summary: Thermoplastic polymers soften when heated and harden when cooled — a reversible physical change. The individual polymer chains are held together by weak intermolecular forces (no cross-links). They can be melted and remoulded repeatedly, making them recyclable. Tags: igcse chemistry organic-chem polymers Created: 2026-07-14 Last Updated: 2026-07-23
Definition
A thermoplastic polymer is one that softens on heating and hardens on cooling — a process that can be repeated many times. The change is physical and reversible; the polymer can be melted, remoulded, and recycled.
Structure
In thermoplastics, the polymer chains are long individual molecules held together by:
- Weak intermolecular forces (van der Waals forces) between chains
- No covalent cross-links between chains
When heated:
- The weak intermolecular forces are overcome
- Chains can slide past each other
- The polymer softens and eventually melts
- On cooling, the intermolecular forces re-form in the new shape
Because there are NO cross-links, the chains remain as individual molecules that can move independently when heated.
Analogy
Think of thermoplastics like spaghetti in a bowl:
- Individual strands can slide past each other when hot and wet
- When cold and dry, they stick together but can be re-heated to move again
- There are no “welds” (cross-links) holding specific strands together
Properties
| Property | Description |
|---|---|
| Heat behaviour | Softens/melts when heated; hardens when cooled |
| Recyclability | Can be melted and remoulded — recyclable |
| Solubility | Some dissolve in organic solvents |
| Structure | Linear or branched chains; NO cross-links |
| Melting point | Defined melting range |
| Process | Physical change — reversible |
Examples of Thermoplastics
| Polymer | Common Name | Typical Uses |
|---|---|---|
| Poly(ethene) | Polythene | Plastic bags, bottles, cling film |
| Poly(propene) | Polypropylene | Food containers, rope, car bumpers |
| Poly(chloroethene) | PVC | Window frames, drainpipes, cable insulation |
| Poly(tetrafluoroethene) | PTFE / Teflon | Non-stick pan coating, plumber’s tape |
| Poly(phenylethene) | Polystyrene | Packaging, disposable cups, insulation |
| PET | Polyester / Terylene | Drinks bottles, clothing fibres |
| Nylon | Polyamide | Rope, clothing, parachutes |
Thermoplastic vs Thermosetting
| Feature | Thermoplastic | Thermosetting |
|---|---|---|
| Cross-links between chains? | No | Yes (covalent cross-links) |
| Effect of heating | Softens → melts (reversible) | Does NOT melt; chars/decomposes |
| Can be remoulded? | Yes | No |
| Recyclable? | Yes — can be melted and reformed | No — cannot be melted |
| Structure | Linear or branched chains | 3D network with cross-links |
| Heat resistance | Low — softens on heating | High — does not soften |
| Examples | Poly(ethene), PVC, nylon, PET | Bakelite, melamine, epoxy resins |
| Analogy | Spaghetti strands | A 3D net/lattice |
Why Thermoplastics Soften — Structure Explanation
The key structural feature is the absence of covalent cross-links:
- Polymer chains are separate molecules
- Only weak van der Waals forces hold them together
- Heat provides enough energy to overcome these weak forces
- Chains become mobile → polymer softens
- On cooling, weak forces re-form → polymer hardens in the new shape
This is in contrast to thermosetting polymers where strong covalent cross-links permanently lock the chains together → heating cannot separate them without breaking covalent bonds (decomposition).
Key Facts
- Thermoplastics soften when heated and harden when cooled
- The change is physical and reversible
- Chains held by weak intermolecular forces only — no covalent cross-links
- Can be remoulded and recycled many times
- Examples: poly(ethene), poly(propene), PVC, nylon, PET
- When heated: weak forces break → chains slide → polymer softens
- When cooled: weak forces re-form → polymer hardens in new shape