Simulic
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Polymer structure and properties – thermoplastics, thermosets and elastomers

ChemistryBiomolecules & PolymersAges 17–18

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A chain model of a polymer: set the chain length, branching, cross-links and the forces between chains, then heat or stretch the sample. Linear thermoplastics soften, flow and can be remoulded, thermosets with covalent cross-links char instead, and lightly cross-linked elastomers stretch and spring back. Softening temperature, density, crystallinity and tensile strength update live (LDPE vs HDPE), and the Make polymers screen compares addition and condensation polymerisation (polyesters, polyamides, water lost) and hydrolyses chains to show which plastics are biodegradable.

Lesson: Polymers: structure and properties, thermosoftening and thermosetting plastics, addition and condensation polymers

What it shows

Polymer properties depend on how the long chains are held together. In a thermoplastic only intermolecular forces act between chains, so heating lets the chains slide: it softens, flows and can be remoulded. Stronger forces, longer chains and closer packing raise the softening temperature and strength; branching, as in LDPE, lowers density and strength compared with linear HDPE. A few covalent cross-links give an elastomer that springs back, while many give a rigid thermoset that chars instead of melting. Addition polymers form from alkenes with no by-product; condensation polymers lose water, and their ester or amide links can be hydrolysed.

How to use

On Heat, choose a preset such as LDPE or HDPE, or set Chain length, Branches per chain, Cross-links per chain and Forces between chains, then press Heat ▲ and Cool ▼ and watch the height graph. On Stretch, press Stretch and Release and compare the stress–extension curves. New sample keeps the last curve in gray. On Make polymers, choose a polymer, press Join next monomer or Auto, then Hydrolyse.

Parameters you can change

  • Test Heat, Stretch, Make polymers
  • Chain length (each bead is many repeat units) 10–60 beads
  • Branches per chain 0–10
  • Cross-links per chain 0–10
  • Forces between chains Weak (London forces), Medium (dipole–dipole), Strong (hydrogen bonds)
  • Polymer (Make polymers screen) Poly(ethene) – addition, PET – polyester (condensation), Nylon-6,6 – polyamide (condensation), PLA – biodegradable polyester (condensation)

Questions to explore

  1. Why does LDPE have a lower density and softening temperature than HDPE made from the same monomer?
  2. Why does a thermosetting polymer char when it is heated instead of melting like a thermoplastic?
  3. Why can condensation polymers such as PLA biodegrade, while poly(ethene) persists in the environment?

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