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15.3 The Properties of Polymers

GCSE Chemistry · AQA · Chapter 15: Using Our Resources

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how monomers and reaction conditions change a polymer, LD and HD poly(ethene), and thermosoftening versus thermosetting polymers. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain how the properties of a polymer depend on the monomers used and the conditions under which it is made
  • Compare low density (LD) and high density (HD) poly(ethene) and link their structures to their uses
  • Describe the difference between thermosoftening and thermosetting polymers
  • Explain the properties of thermosoftening and thermosetting polymers using their structures
  • Use data such as density and softening temperature to identify and choose polymers, and plan a simple polymer investigation

Key words

  • Polymer: A very large molecule made by joining many small molecules (monomers) together in a long chain.
  • Monomer: A small molecule that joins with many others to make a polymer, e.g. ethene.
  • LDPE / HDPE: Low density and high density poly(ethene). Both are made from ethene, but under different conditions.
  • Intermolecular forces: Weak forces of attraction between separate polymer chains.
  • Thermosoftening polymer: A polymer made of separate chains that melts or softens when heated and can be remoulded.
  • Thermosetting polymer: A polymer with covalent cross-links between chains. It does not melt when heated.
  • Cross-link: A covalent bond (or short chain of atoms) joining one polymer chain to another.

Monomers and conditions

The properties of a polymer depend on what it is made from (the monomers) and the conditions under which it is made (temperature, pressure and catalyst). For example, poly(ethene) and poly(propene) are made from different monomers, so they have different properties: poly(propene) is stiffer and softens at a higher temperature.

n CH₂=CH₂ → –(CH₂–CH₂)n– ethene → poly(ethene)

The same monomer, ethene, can make two quite different plastics depending on the conditions.

  • Conditions; LDPE very high pressure (over 1000 atm), about 200 °C, a trace of oxygen to start the reaction; HDPE low pressure (only a little above atmospheric), about 50–60 °C, a catalyst
  • Chains; LDPE branched, so loosely packed; HDPE mainly straight, so packed closely
  • Density; LDPE about 0.92 g/cm³; HDPE about 0.95 g/cm³
  • Softening temperature; LDPE lower (about 85–100 °C); HDPE higher (about 120–130 °C)
  • Properties; LDPE soft, flexible, stretchy; HDPE stiffer, stronger
  • Uses; LDPE carrier bags, cling film, squeezy bottles; HDPE milk bottles, water pipes, crates, buckets

Thermosoftening polymers

Most everyday plastics (poly(ethene), poly(propene), PVC, PET) are thermosoftening. They are made of separate polymer chains tangled together. The chains are held to each other only by weak intermolecular forces.

When heated, the intermolecular forces are overcome, the chains can slide past each other, and the polymer softens and melts. On cooling it sets again in a new shape. This means it can be moulded, and also recycled by melting.

Thermosetting polymers

Thermosetting polymers have cross-links: covalent bonds between neighbouring chains. The chains are locked in place, so when the polymer is heated it does not melt. If it is heated strongly enough, covalent bonds break and it chars (decomposes).

Thermosetting polymers are hard, rigid and heat resistant. They are used for electrical plugs and sockets, saucepan handles, kitchen worktops (melamine) and adhesives such as epoxy resin.

Exam tip: In explanations about melting, the bonds broken in a thermosoftening polymer are the weak intermolecular forces between chains, not the covalent bonds inside the chains.

Using data to identify a polymer

Density is a quick way to tell LDPE and HDPE apart. The formula is density = mass ÷ volume.

Worked example: which poly(ethene)?

A piece of plastic from a crate has a mass of 7.60 g and a volume of 8.0 cm³.
density = 7.60 ÷ 8.0 = 0.95 g/cm³
This matches HDPE (about 0.95 g/cm³), not LDPE (about 0.92 g/cm³).

Answer: 0.95 g/cm³, so the crate is HDPE

  • softens and can be bent into a new shape at a fairly low temperature; Type of polymer thermosoftening
  • stays hard; eventually chars and gives off smoke; Type of polymer thermosetting

Worked example: investigating cross-linking with “slime”

PVA glue is a solution of long polymer chains. Adding borax solution makes weak links between the chains, so the runny glue turns into a thick, stretchy slime.
Plan: add 1, 2, 3, 4 and 5 cm³ of 4% borax solution to separate 20 cm³ portions of warm PVA solution and stir each for 2 minutes.
Independent variable: volume of borax. Dependent variable: how runny the slime is, e.g. time for 10 cm³ to flow out of a funnel. Control: volume and temperature of PVA, stirring time.
Wear eye protection and gloves; borax is harmful.

Answer: more cross-linking makes the polymer stiffer (less runny)

Common mistakes

  • Saying LDPE and HDPE are made from different monomers. Both are made from ethene; the conditions are different.
  • Calling cross-links “intermolecular forces”. Cross-links are strong covalent bonds between chains.
  • Saying a thermosetting polymer melts at a high temperature. It does not melt at all; it chars or decomposes.
  • Saying covalent bonds break when a thermosoftening polymer melts. Only the weak forces between the chains are overcome.

Revision questions

Try these, then check your answers below each one.

Question 1. LDPE and HDPE are both poly(ethene). Why do they have different properties? (Choose from: They are made under different conditions, They are made from different monomers, HDPE has cross-links between its chains, LDPE contains chlorine atoms.)

Answer: They are made under different conditions

Working: Both are made from ethene. Different conditions (pressure, temperature, catalyst) give branched chains (LDPE) or straight chains (HDPE).

Question 2. True or false? Thermosetting polymers can be recycled by melting them and remoulding.

Answer: False

Working: False. Cross-links stop the chains moving, so thermosets do not melt; they char instead.

Question 3. What type of bond forms the cross-links in a thermosetting polymer?

Answer: covalent

Working: Cross-links are covalent bonds between neighbouring chains.

Related lessons

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This lesson is section 15.3 (AGCCH1503) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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