Chemistry • Year 11 • Module 1 • Lesson 11
Polymers: Structure and Properties
Apply your understanding of polymerisation mechanisms, structural features, and IMFs to real data, comparative scenarios and reasoning tasks.
1. Interpret polymer property data
The table below gives selected properties for five common polymers. Use the data to answer the questions below. 9 marks
| Polymer | Polymerisation type | Melting point (°C) | Solubility in organic solvents | Tensile strength (MPa) |
|---|---|---|---|---|
| LDPE | Addition | 105–115 | Slightly soluble (hot) | 8–20 |
| HDPE | Addition | 120–140 | Slightly soluble (hot) | 20–37 |
| PVC | Addition | ~100 (with plasticiser) | Soluble in THF, DMF | 40–60 |
| Nylon-6,6 | Condensation | 265 | Insoluble (most solvents) | 75–85 |
| Cross-linked epoxy resin | Condensation | Does not melt (>300°C chars) | Insoluble (all solvents) | 55–130 |
Data from Callister & Rethwisch, Materials Science and Engineering 10th ed. (2018). Illustrative ranges.
(a) Identify the polymer with the highest melting point. Using IMF theory, explain what structural feature causes this polymer to have a higher melting point than HDPE. (3 marks)
(b) Cross-linked epoxy resin does not melt and is insoluble in all solvents. HDPE can be dissolved in hot organic solvents. Explain this difference at the molecular level. (3 marks)
(c) PVC has higher tensile strength than LDPE and HDPE despite being an addition polymer like them. Use the substituent group on PVC’s repeat unit to explain this difference. (3 marks)
2. Compare addition and condensation polymerisation
Complete the comparison table. Some cells have been filled as prompts. 10 marks (1 per blank cell)
| Feature | Addition polymerisation | Condensation polymerisation |
|---|---|---|
| Monomer requirement | Must contain a C=C double bond | |
| Byproduct formed? | Yes — small molecule (e.g. H2O) | |
| Atom economy | Less than 100% | |
| Linkage type in backbone | C–C single bonds only | |
| Named example (polymer) | ||
| Named example (monomer) | ||
| Recyclability of typical product | Often not recyclable if cross-linked | |
| Identify from repeat unit: key clue | Look for —COO— or —CONH— in backbone |
3. Cause-and-effect chain — why HDPE is stronger than LDPE
Each cause box below is filled in. Write the effect that follows logically in the empty boxes on the right. The final box should state the overall outcome for the material’s properties. 5 marks (1 per correct effect)
Cause 1
HDPE chains are linear (very little branching).
Effect 1: the chains can…
Cause 2
The chains pack closely in an ordered arrangement.
Effect 2: the contact surface area…
Cause 3
Increased contact area between linear chains.
Effect 3: total dispersion force strength…
Cause 4
Stronger total IMF between chains.
Effect 4: chains require…
Overall outcome (so…)
4. Predict and justify
Read the scenario, then answer the question in 3–5 sentences. 4 marks
Scenario. A materials engineer adds a plasticiser (a small organic molecule) to rigid PVC powder and heats the mixture. The resulting material is significantly more flexible than unplasticised PVC and can be bent without cracking. The engineer notes that the plasticiser does not react chemically with PVC chains.
Q. Using your knowledge of intermolecular forces and polymer structure, explain why adding a plasticiser makes PVC more flexible. Predict what would happen to flexibility if the plasticiser molecules were gradually removed (e.g. by evaporation over many years).
5. Graph interpretation — polymer chain length and tensile strength
The graph below shows how tensile strength (MPa) varies with average chain length (expressed as degree of polymerisation, n) for a series of polyethylene samples with negligible branching. 6 marks
(a) Describe the overall trend shown in the graph. Use values from the graph to support your description. (2 marks)
(b) Explain, using intermolecular force theory, why tensile strength increases as chain length increases. (2 marks)
(c) The curve appears to plateau at very high values of n. Suggest one reason why further increasing chain length beyond ~150,000 would have diminishing returns for tensile strength, and predict one practical processing disadvantage of using extremely long chains. (2 marks)
Q1(a) — Highest MP polymer
Nylon-6,6 has the highest melting point (265°C). Its amide linkages (—CONH—) contain N–H bonds; nitrogen is electronegative, making the N–H partially positive. These N–H groups form hydrogen bonds (N–H···O=C) with the carbonyl oxygen on adjacent nylon chains. Hydrogen bonds are significantly stronger than the dispersion-only forces acting between HDPE chains, so far more energy is required to separate nylon chains during melting.
Q1(b) — Epoxy vs HDPE solubility
HDPE chains are held together only by dispersion (London) forces — intermolecular forces. Hot organic solvent molecules can compete for these interactions and displace the chain–chain contacts, dissolving the polymer. Cross-linked epoxy chains are joined by covalent bonds (the cross-links). Solvent molecules cannot break covalent bonds under normal conditions, so the epoxy does not dissolve. Similarly, no amount of heating can slide cross-linked chains past each other, so the material does not melt.
Q1(c) — PVC vs PE tensile strength
PVC’s repeat unit contains a —Cl substituent. Chlorine is highly electronegative (χ = 3.0), making each C–Cl bond polar (δ+ on C, δ− on Cl). The permanent dipoles along PVC chains attract each other through dipole–dipole forces between adjacent chains. These are stronger than the dispersion-only forces between polyethylene chains (which have only non-polar C–H bonds). Stronger interchain forces resist stretching more effectively, resulting in higher tensile strength.
Q2 — Comparison table (blank cells only)
Monomer requirement (condensation): Must be bifunctional (contain two reactive functional groups, e.g. —OH + —COOH, or —NH2 + —COOH).
Byproduct (addition): No byproduct released (100% atom economy).
Atom economy (addition): 100%.
Linkage type (condensation): Ester (—COO—) or amide (—CONH—) bonds in backbone.
Named example polymer (addition): Any of PE, PP, PVC, polystyrene, PTFE.
Named example polymer (condensation): Nylon-6,6 or PET (polyester).
Named example monomer (addition): e.g. ethene (CH2=CH2).
Named example monomer (condensation): e.g. hexamethylenediamine + adipic acid (for nylon-6,6).
Recyclability (addition): Usually recyclable (thermoplastic — IMFs only).
Repeat-unit clue (addition): No ester or amide groups in backbone; all C–C single bonds in chain; can reconstruct C=C in monomer.
Q3 — Cause-and-effect chain
Effect 1: …pack closely together in an ordered, regular arrangement (high crystallinity).
Effect 2: …between adjacent chains increases significantly, maximising IMF contact.
Effect 3: …between chains increases (stronger total dispersion forces acting over more contact points).
Effect 4: …more energy to be separated, meaning more force to slide chains past each other.
Overall outcome: HDPE has higher tensile strength, higher density, higher melting point, and greater rigidity than LDPE — making it suitable for rigid pipes and containers rather than flexible bags.
Q4 — Plasticiser predict and justify
PVC chains have polar C–Cl bonds that create dipole–dipole forces between adjacent chains, holding them together and making the polymer rigid. Plasticiser molecules (small, polar organic molecules) insert themselves between PVC chains, physically separating them and disrupting chain–chain dipole–dipole interactions. With the chains further apart, they can slide past each other more easily when a force is applied, so the material becomes flexible. If the plasticiser evaporates over time, the chains move back closer together, their dipole–dipole interactions are restored, and the PVC becomes brittle and rigid again. (This is observed in old PVC products such as aged electrical insulation and early vinyl records.)
Q5(a) — Trend description
Tensile strength increases as degree of polymerisation increases, but the rate of increase slows at higher values of n (the curve levels off / plateaus). At low chain lengths (n ≈ 1000–10,000) strength rises steeply from approximately 2 MPa to 30 MPa. At very high chain lengths (n > 100,000) the curve flattens, approaching approximately 150 MPa.
Q5(b) — IMF explanation
Longer chains have more atoms and therefore greater surface area available for contact with neighbouring chains. Dispersion (London) forces act between all adjacent atoms, so more surface area = more dispersion force contact points = greater total intermolecular attraction between chains. Greater attraction means chains resist being pulled apart, giving higher tensile strength.
Q5(c) — Plateau and processing
Plateau: Once chains are sufficiently long, additional length adds entanglement rather than new discrete contact surfaces; the incremental gain in total IMF per extra monomer unit diminishes because chains are already highly entangled and the increase in contact area per unit of extra length becomes small.
Processing disadvantage: Extremely long chains create very high melt viscosity (the polymer barely flows when heated), making injection moulding, extrusion, or other melt-processing techniques very difficult or impractical (as seen with UHMWPE).