11A. More on POLYMERS - synthetic macromolecules

Polymers are long chain molecule formed from lots of repeating units joined together by strong covalent bonds.

Modifying polymers, thermoplastics and thermosets

• First some reminders from section 7. about addition polymers which were discussed in some detail.
• As an example the formation of PVC is shown below.

  • the long chain PVC molecules

  • and what the molecules look like in the structure of PVC or any other thermoplastic.

  • Although the PVC molecules look straight, in reality, the long molecules will be all twisted-jumbled up as in the thermoplastic diagram above (a bit spaghetti like!).

• This is a typical addition polymer (formed by simple addition of monomer molecules), just like polythene and polystyrene etc. AND they are examples of thermoplastics, that is they can be heated and softened, reshaped and cooled to keep their new moulded shape.

• In addition polymerisation there is only one monomer molecule with a double bond and one product, the polymer, and the linking occurs via a reactive double bond. Detailed addition polymer notes

• In condensation polymerisation there are monomer molecules with a reactive functional groups.

  • There are two products, the condensation polymer itself, and, the small molecule that is eliminated between the two monomer molecules when the linking bond is formed from the two functional groups.

    • Notes on condensation polymers

    • Condensation polymers are usually thermoplastics too.

• Thermoplastic polymers (thermoplastics)

• A molecular model for a thermoplastic

• In thermoplastics the intermolecular forces between the polymer molecules are quite weak compared to the strong covalent bonds (C-C) holding the chain of atoms together.

• Because the 'intermolecular bonding' is weak, this explains that when heated, these 'plastic' materials will soften quite easily, which is why they are called 'thermoplastic' and have relatively low softening points and melting points.

• Even at room temperature the plastic is easily distorted because the polymer chains can slide over each other i.e. the external physical force applied on bending overcomes the intermolecular forces between the polymer molecules.

• Despite their relative weakness, on controlled heating until they are quite soft (but NOT molten), they are readily extrusion moulded or drawn out into useful shapes which retain their new formation on cooling.

• So, overall, thermoplastics are not that heat resistant or exceptionally rigid/strong - but their properties do vary quite widely e.g. poly(propene) and nylon can be drawn into strong fibres and both can be manufactured into quite strong and rigid forms.

• See nylon and Terylene

• COMPARISON OF THERMOPLASTICS, THERMOSETS and FIBRES

  • First a reminder that the use of a polymer mainly depends on its physical properties (which are derived from the polymers structure).

    • You can chemically modify polymers to change their physical properties to suit a particular use or application.

      • If you increase the chain length of the polymer molecules you increase the intermolecular forces between the chains so it is stronger and less flexible and has a higher softening/melting point.

      • If you shorten the average chain length the polymer has lower softening/melting and is easier to shape, and the plastic is more flexible.

      • Cross-linking, i.e. adding a cross-linking agent to the monomer/polymer mixture which forms strong chemical bonds between polymer chains is discussed below and this modification is most important when comparing thermosoftening plastics and thermosetting plastics. Cross

• In thermosoftening plastics like poly(ethene), poly(propene) or poly(chloroethene) PVC, because the inter-molecular attractive forces between the chains are weak, the plastic softens when heated and hardens again when cooled. See also addition polymers page.
  • It also means the polymer molecules can slide over each other especially when heated to their relatively low softening/melting points.
  • This means they can be easily stretched or moulded into any desired shape.
  • They are examples of thermoplastics (thermosoftening plastics), because they can be heated to make them softer - more plastic, reshape it e.g. in an injection mould system, and on cooling the plastic object retains its new shape - bottle, bowl, toy etc.
  • However it is possible to manufacture and process plastics in which the polymer chains are made to line up. This greatly increases the intermolecular forces between the 'aligned' polymer molecules and strong fibre strands of the plastic can be made.
  • Examples: The addition polymer poly(propene) and the condensation polymers nylon and Terylene.
• Thermosoftening polymers like poly(ethene) and poly(propene) consist of individual, tangled polymer chains and melt relatively easily when they are heated. This contrasts with thermosetting polymers consist of polymer chains with strong cross-links between them and so they do not melt when they are heated.
• When a thermosetting plastic is formed you not only get polymerisation to form long molecules, you also get chemical bonds formed between various points in one polymer chain molecule across to another polymer molecule.
  • These extra bonds are called cross-links and hold the linear polymer chains together in a much more rigid structure.
    • These cross links do not usually occur in the simpler addition polymerisations when thermoplastics like poly(ethene) and PVC are made.
  • Commercially, many thermosets consist of a partially polymerised (but not cross-linked) resin, which contains a cross-linking agent and a catalyst, so that when the mixture is exposed to air or a the mixture warmed, cross-linking polymerisation occurs and the hard thermoset is formed. This type of mixture is used to make fibre-glass reinforced structures e.g. light car bodies or the hulls of sailing boats and canoes.
• These extra cross-linking covalent bonds formed between adjacent chains of the polymers change the physical properties considerably and thermoset polymers have much higher high melting points (giving greater heat resistance and thermal stability) as well as greatly increased strength and rigidity.
  • Compared to thermosoftening plastics, thermoset polymers do not soften or melt and only break down and degrade at much higher temperatures compared to the softening/melting points of thermoplastics described above.
    • Thermosets are harder, more rigid/stiffer and not as easily bent or stretched, in fact they can be quite brittle and almost impossible to stretch (not very elastic!).
  • Note that thermosets type polymers can be formed at room temperature, heating may not be required.
    • Many super glues form this kind of structure.
  • However, you have to get it right first time because thermosetting polymers cannot be softened with heat and therefore cannot be stretched or re-shaped, but the advantage is that thermosets are much more heat resistant than thermoplastics.
  • But these cross-linked thermoset polymers are much more rigid (e.g. can't be stretched) and stronger material (though they can be brittle) and not as flammable as most thermoplastics.
  • On heating them strongly they do NOT melt, but tend to char, gradually giving off gases.

  • 

  • A simple diagram of the polymer molecules in the three different situation.

    • Thermoplastic: The polymer molecules tend to be randomly jumbled up, but no cross-linking bonds.

    • Fibres: Fibre molecules are thermoplastic molecules but manufactured in such a way to get the 'molecules more lined up' to increase intermolecular forces between the long molecules, and this increases the strength of the fibre, but no cross-linking bonds are formed.

    • Thermosets: Their great strength and very rigid structure derives from the strong cross-links between the polymer strands. These cross-links are full chemical covalent bonds, NOT the much weaker intermolecular forces/bonding in thermoplastics.

      • In thermoplastics you have intermolecular bonding (weak attractive forces) between polymer molecules.

      • In thermosets you have intramolecular chemical bonding (very strong attractive forces) between the adjacent polymer molecule chains.

  • Heat resistant polymers are usually thermosets e.g. like melamine resin (plastic plates), but even thermoplastics like poly(propene) can be used in hot situations e.g. plastic electric kettles.

• Examples of Thermosets:
  • Melamine (used in furniture), Bakelite (was used for electrical fittings, a horrible brown colour but a good insulator, not used now?), Formica (table tops) and some super glues are examples of thermosetting polymers.
• See also ....
  • Problems with using plastics are in section 7. Polymers-plastics page.
  • Some basic notes on polymers and plastics in the Section 7 Oil Products Polymer Notes e.g. the formation of poly(ethene), polypropene and poly(chloroethene) PVC.
  • Some important structure, strength and 1D, 2D 3D dimension structural concepts) in the Chemical Bonding notes.
  • See also A survey of the properties and uses of a wide range of materials

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Key revision points

Thermosoftening polymers have tangled chains that soften when heated, thermosetting polymers have rigid cross-linked structures that do not melt, and synthetic fibres are condensation polymers with strong intermolecular forces giving them durability and textile applications.

Students must compare structure → properties → uses across exam boards, while avoiding common misconceptions such as confusing thermoplastics with thermosets.

Comparative Revision Notes

1. Thermosoftening Polymers (Thermoplastics)

• Structure: Long chains with no cross-links, tangled together.
• Properties:
  • Soften/melt when heated (reversible).
  • Flexible, can be reshaped.
• Examples: Poly(ethene), Poly(propene), PVC.
• Uses: Plastic bags, bottles, insulation.
• Exam Tip: Link weak intermolecular forces to low melting point and recyclability.

2. Thermosetting Polymers (Thermosets)

• Structure: Chains with strong covalent cross-links forming a rigid 3D network.
• Properties:
  • Hard, brittle, heat-resistant.
  • Do not soften on heating (irreversible).
• Examples: Melamine (kitchenware), Bakelite (electrical plugs), Epoxy resins.
• Uses: Electrical fittings, adhesives, cookware.
• Exam Tip: Stress that cross-links prevent melting. Misconception: students often think they “melt at higher temperatures” — they actually char/decompose.

3. Synthetic Fibres

• Structure: Often condensation polymers (e.g. Nylon, Terylene).
  • Chains held by hydrogen bonding or dipole interactions.
• Properties:
  • Strong, durable, resistant to wear.
  • Can be woven into fabrics.
• Examples: Nylon (ropes, parachutes), Terylene/polyester (clothing).
• Uses: Textiles, industrial fibres.
• Exam Tip: Distinguish between addition polymers (polyethene) and condensation polymers (nylon).

Exam Board Alignment

All boards (WJEC, CCEA, CIE, AQA, Edexcel, OCR Gateway, OCR 21st Century) require:

• Comparison of thermosoftening versus thermosetting polymers (structure, properties, uses).
• Recognition of synthetic fibres as condensation polymers.
• Application questions: e.g. Why is Bakelite used for plugs? (heat resistance).
• Diagrammatic understanding: polymer chains with/without cross-links.

Common Misconceptions

• Thinking thermosets “melt at higher temperatures” → they do not melt.
• Confusing synthetic fibres with natural fibres (cotton, wool).
• Forgetting that polyethene is an addition polymer, while nylon is a condensation polymer.
• Assuming all plastics are recyclable → thermosets are not.

Student Tips

• Always link structure → property → use in exam answers.
• Use comparative language: “Unlike thermosets, thermoplastics soften when heated.”
• Draw simple chain diagrams: tangled versus cross-linked.
• Practise past-paper questions that ask for examples (exam boards often test recall here).
• Remember: synthetic fibres are polymers designed for strength and flexibility in textiles.

Comparative Table

Final Tip for Students:
When revising, compare by example (polyethene versus Bakelite versus Nylon) and always connect structure → property → use. This is the most exam-aligned way to secure marks across all boards.