Chemistry • Year 12 • Module 7 • Lesson 22
Condensation Polymers: Polyesters & Polyamides
Build Band 5–6 extended-response technique: evaluate a real data set, critique a source claim, and reach evidence-based judgements about condensation polymer chemistry.
1. Data + scenario — evaluate polymer choice for outdoor Australian sportswear (Band 5–6)
8 marks Band 5–6
Stimulus. An Australian outdoor sportswear manufacturer (similar to those supplying Rebel Sport) is selecting a polymer fibre for a new performance fabric line designed for high-UV, high-sweat Australian summer conditions. Two candidates are under consideration: Polyester (PET fibre) and Nylon 6,6 (polyamide fibre). The R&D team has collected the following comparative data.
| Property | PET polyester fibre | Nylon 6,6 polyamide fibre |
|---|---|---|
| Melting point (°C) | ~260 | ~265 |
| Tensile strength (GPa) | 0.45–0.80 | 0.50–0.85 |
| Moisture absorption (% by mass at 65% RH) | 0.4 | 4.0 |
| UV resistance (relative scale 1–10) | 8 | 4 |
| Hydrolysis rate in dilute acid (relative) | Slow | Slow–moderate |
| Cost index (AUD per kg) | 2.4 | 4.8 |
| Recyclability (Australian facilities) | Yes (RIC code 1; kerbside in many states) | Limited (no kerbside; specialist only) |
Data adapted from: CSIRO Textile Technology, Fibre Properties Database (2021); Pacific Brands sustainability report (2022).
Q1. Evaluate which polymer — PET polyester or Nylon 6,6 — is the more appropriate choice for this Australian outdoor sportswear application. In your response you must:
- Define condensation polymerisation and explain what linkage type each polymer contains (ester vs amide).
- Compare the two polymers on at least three criteria drawn from the data table, using chemical reasoning to explain each difference.
- Refer to at least one Australian context (manufacturing, recycling, or environmental impact).
- Reach an evidence-based judgement that identifies which polymer is preferred and under what specific conditions that preference could change.
2. Source critique — evaluate a claim about condensation polymers (Band 5–6)
7 marks Band 5–6
“Condensation polymers like PET and Nylon 6,6 are just as environmentally persistent as addition polymers like polyethylene. Because all three are synthetic plastics, none of them can be broken down by water or living organisms. Once they enter the ocean they will remain intact indefinitely. The only structural difference between these polymers is that PET and Nylon contain more carbon atoms per repeat unit than polyethylene does.”
— From a fictional Year 12 student’s study notes posted on an online forum.
Q2. Evaluate this claim. Identify the specific chemical errors and misconceptions it contains, explain the correct chemistry for each, and reformulate the claim into a scientifically defensible statement. Your response should address at least three distinct errors.
Q1 — Sample Band 6 response (8 marks), annotated
Condensation polymerisation is the reaction of bifunctional monomers with loss of a small molecule (water or HCl) at each bond formed, building a long polymer chain. PET forms when ethylene glycol (-OH ends) reacts with terephthalic acid (-COOH ends) to produce ester linkages (-COO-) with 2 mol H&sub2;O released per repeat unit. Nylon 6,6 forms when hexane-1,6-diamine (-NH&sub2; ends) reacts with hexanedioic acid (-COOH ends) to produce amide linkages (-CO-NH-) with 2 mol H&sub2;O per repeat unit. [1 mark — defines condensation polymerisation + identifies ester vs amide linkage]
Criterion 1 — Moisture absorption: PET absorbs 0.4% moisture vs Nylon 6,6 at 4.0% — a 10-fold difference. The amide -CO-NH- groups in Nylon form strong hydrogen bonds with water molecules (N-H···O and C=O···H-O), making Nylon hygroscopic. PET’s ester -COO- groups are weaker hydrogen-bond acceptors and the rigid benzene ring reduces chain mobility, limiting water uptake. In a high-sweat Australian summer context, lower moisture absorption means PET fabric stays lighter and dries faster. [1 mark — criterion compared with chemical reasoning]
Criterion 2 — UV resistance: PET scores 8/10 vs Nylon’s 4/10. The benzene (aromatic) ring in PET’s terephthalic acid component absorbs UV radiation without photochemical chain scission, acting as a UV stabiliser within the polymer backbone. Nylon’s aliphatic chain has no aromatic chromophore, so UV radiation causes photodegradation (chain scission and yellowing) more rapidly. For outdoor Australian summer use at high UV index, PET’s superior UV resistance is a significant advantage. [1 mark — criterion compared with chemical reasoning]
Criterion 3 — Recyclability: PET has Australian kerbside recycling in most states (RIC code 1) and is collected through programs like Clean Up Australia’s single-use plastic campaigns. Nylon 6,6 currently has no kerbside recycling stream in Australia and requires specialist facilities. For a sportswear brand targeting sustainability-conscious Australian consumers, PET’s recycling infrastructure is a clear advantage. [1 mark — Australian context linked to criterion]
Additional criterion (optional) — Cost: PET costs $2.40/kg vs Nylon’s $4.80/kg, halving raw-material cost for a high-volume manufacturer. [1 mark if discussed with chemical or economic reasoning]
Judgement: For the specified application — high-UV, high-sweat Australian summer outdoor sportswear — PET polyester is the more appropriate choice. It outperforms Nylon 6,6 on UV resistance (critical for Australian conditions), moisture management (stays lighter when sweating), cost, and recyclability. However, the preference would change for applications requiring higher tensile strength with comparable resistance (e.g. climbing harnesses or parachute cords), where Nylon’s amide H-bonding provides superior strength-to-weight ratio under repeated mechanical stress; or for applications requiring the anti-microbial properties associated with nylon’s nitrogen-containing backbone. [1 mark — evidence-based judgement with condition under which preference reverses]
Marking criteria:
- 1 mark — Defines condensation polymerisation correctly and identifies ester linkage in PET and amide linkage in Nylon.
- 1 mark × 3 — Three criteria compared with chemical reasoning (e.g. moisture absorption: H-bonding in amide vs ester; UV resistance: benzene ring in PET; cost/recyclability; tensile strength).
- 1 mark — Australian context explicitly linked (recycling, Clean Up Australia, Pacific Brands / Rebel Sport, outdoor conditions).
- 1 mark — Evidence-based judgement that names PET as preferred for this application and states at least one condition under which Nylon would be preferred instead (not a one-winner ranking).
- 1 mark — Overall quality: precise use of chemical terminology (ester, amide, hydrogen bond, bifunctional, hydrolysable) throughout.
Q2 — Source critique: errors identified and corrected (7 marks)
Overall judgement: The claim is largely incorrect. It conflates the hydrolysis resistance of addition polymers with all synthetic polymers and contains at least four distinct chemical errors. [1 mark — overall evaluative judgement]
Error 1 — “None can be broken down by water”: PET and Nylon 6,6 can both be hydrolysed, because their ester (-COO-) and amide (-CO-NH-) linkages are susceptible to nucleophilic attack by water. Under acidic or alkaline conditions, the C-O bond (ester) or C-N bond (amide) is cleaved, regenerating the original monomers. This is actually exploited in chemical recycling of PET bottles in Australia. Polyethylene has only C-C bonds which water cannot attack. The claim is correct only for addition polymers like polyethylene. [1 mark]
Error 2 — “Living organisms cannot break them down”: PETase-producing bacteria (Ideonella sakaiensis, discovered 2016) can hydrolyse PET’s ester bonds enzymatically. Esterases and proteases in soil microorganisms can hydrolyse polyesters and polyamides respectively. Natural polyamides (proteins) are hydrolysed efficiently by proteases in all living systems. While the rate is slow for synthetic PET under ambient conditions, the claim that NO living organism can degrade them is incorrect. [1 mark]
Error 3 — “The only structural difference is carbon atom count”: The fundamental structural differences between PET/Nylon and polyethylene are the presence of ester (-COO-) or amide (-CO-NH-) functional groups in the backbone — not merely the number of carbon atoms. These heteroatom-containing linkages are the entire chemical basis for hydrolysability. Polyethylene has no heteroatoms in the backbone. Describing the difference as “more carbon atoms” is chemically meaningless and misses the structural argument entirely. [1 mark]
Error 4 — “Remain intact indefinitely” for condensation polymers in the ocean: While PET and Nylon persist for decades in ocean water at ambient conditions (hydrolysis is very slow at neutral pH and low temperature), they do not remain intact indefinitely. Ocean hydrolysis, UV degradation, mechanical fragmentation into microplastics, and emerging enzymatic degradation (PETase) all contribute to their breakdown over time. “Indefinitely” is only defensible for addition polymers like polyethylene (which has no hydrolysable linkage), and even then UV fragmentation occurs. [1 mark]
Defensible reformulation: “Condensation polymers like PET and Nylon 6,6 are highly persistent in ocean environments over human-relevant timescales, but unlike addition polymers such as polyethylene, they contain ester (-COO-) or amide (-CO-NH-) linkages in their backbone that can be slowly hydrolysed by water under acidic or basic conditions and cleaved by specialised microbial enzymes. Addition polymers with only C-C bonds lack any hydrolysable functional group and are more persistent. The key structural distinction is not carbon content but the presence or absence of heteroatom-containing linkages in the polymer backbone.” [1 mark — defensible reformulation using correct chemical terminology]
Marking criteria:
- 1 mark — Overall evaluative judgement (e.g. “the claim is largely incorrect”).
- 1 mark — Error 1 identified and corrected: PET and Nylon are hydrolysable; their ester/amide bonds can be cleaved by water (acid/base); only C-C addition polymers are resistant.
- 1 mark — Error 2 identified and corrected: living organisms can break down condensation polymers; PETase and proteases hydrolyse ester/amide bonds.
- 1 mark — Error 3 identified and corrected: the structural difference is the presence of heteroatom-containing linkages (-COO-, -CO-NH-), not carbon atom count.
- 1 mark — Error 4 identified and corrected: “indefinitely” is not defensible for condensation polymers; hydrolysis pathways exist even if slow.
- 1 mark — Scientifically defensible reformulation that acknowledges high persistence while correctly identifying the structural basis for degradability vs non-degradability.
- 1 mark — Quality: uses precise chemical terminology (ester, amide, hydrolysis, C-C, heteroatom, enzymatic) throughout; no unsupported generalisations.