Chemistry • Year 12 • Module 8 • Lesson 16
Polymers: Structure, Properties & Applications
Apply polymer structure–property reasoning to data tables, real scenarios and cause-and-effect chains about recycling and microplastics.
1. Interpret polymer property data
The table below compares five common polymers. Use it to answer the questions. 8 marks
| Polymer | Type | Backbone feature | Melting / softening point (°C) | Flexibility | Chemical resistance |
|---|---|---|---|---|---|
| LDPE (low-density PE) | Addition | Branched hydrocarbon | ~110 | High (flexible) | Good |
| HDPE (high-density PE) | Addition | Linear hydrocarbon | ~130 | Lower (stiffer) | Good |
| PVC (polyvinyl chloride) | Addition | Chloro-substituted chain | ~80 (softened) | Rigid or flexible (plasticiser-dependent) | Good |
| Nylon-6,6 | Condensation | Amide links (H-bond capable) | ~265 | Moderate | Moderate |
| PTFE (Teflon) | Addition | Fluorinated chain | ~327 | Low (rigid) | Exceptionally high |
1.1 Compare LDPE and HDPE. Use the data to explain why they have different flexibility, referring to chain structure. 3 marks
1.2 Explain why nylon-6,6 has a higher melting point than LDPE, referring to intermolecular forces. 2 marks
1.3 PTFE has an exceptionally high melting point and chemical resistance. Use the data and your knowledge of polymer structure to explain why. 3 marks
2. Cause-and-effect chain — microplastic formation in the ocean
The chain below traces how a plastic bag ends up as a widespread marine pollutant. Fill in the empty effect boxes. 5 marks
| A plastic bag made from LDPE enters the ocean. | → | Effect 1 (UV, wave action and mechanical stress): _______________ |
| The bag breaks into fragments smaller than 5 mm. | → | Effect 2 (name and define these particles): _______________ |
| Microplastic particles enter food webs. | → | Effect 3 (environmental persistence and spread): _______________ |
| The LDPE polymer chain resists biological breakdown. | → | Effect 4 (why the polymer does not biodegrade): _______________ |
| No practical biodegradation occurs even after decades. | → | Effect 5 (implication for ocean environment): _______________ |
3. Case study — Recycling codes and polymer chemistry in NSW
7 marks
NSW households are required to sort waste plastics using numerical recycling codes (1–7), printed inside the recycling triangle. Code 1 (PET) and Code 2 (HDPE) are widely accepted in kerbside recycling. Code 3 (PVC) is rarely accepted because it is difficult to separate and process cleanly. Code 7 (other/mixed) includes many thermosets that cannot be remoulded. Some councils are trialling chemical recycling programs, in which mixed plastic waste is broken down into monomer feedstocks rather than just melted and reformed.
3.1 Use the concept of thermoplastic vs thermosetting polymer to explain why Code 7 thermoset plastics cannot be processed by standard mechanical recycling (melting and reforming). 2 marks
3.2 PET (Code 1) is a condensation polyester. Explain how chemical recycling (breaking PET back into monomers) connects to the condensation chemistry used to make it. 2 marks
3.3 Identify one structural feature of HDPE that makes it suitable for mechanical recycling compared with a thermoset. 1 mark
3.4 Explain why mixing different polymer types in a recycling stream creates a chemical problem during mechanical recycling. 2 marks
Q1.1 — LDPE vs HDPE flexibility
LDPE has a branched chain structure. Branches prevent adjacent chains from packing closely together, reducing crystallinity and intermolecular contact, so it is more flexible. HDPE has a predominantly linear chain structure that allows closer packing of chains, higher crystallinity and stronger overall van der Waals forces between chains, making it stiffer and slightly denser than LDPE.
Q1.2 — Nylon-6,6 vs LDPE melting point
Nylon-6,6 has amide (–CO–NH–) links in its backbone, which are capable of hydrogen bonding between adjacent chains. These are much stronger intermolecular forces than the van der Waals dispersion forces between LDPE hydrocarbon chains. More energy (higher temperature) is needed to overcome the hydrogen bonds, so nylon-6,6 melts at a much higher temperature (~265°C vs ~110°C for LDPE).
Q1.3 — PTFE properties
PTFE has a fully fluorinated carbon backbone. The C–F bond is one of the strongest bonds in organic chemistry, and the fluorine atoms shield the carbon backbone from chemical attack, giving PTFE exceptional chemical resistance. The fluorinated chains also interact only weakly with solvents and reactive chemicals. Its high melting point (~327°C) reflects the need to overcome strong C–F bonds and the very efficient packing of linear fluorocarbon chains, which maximise van der Waals interactions.
Q2 — Cause-and-effect chain (sample answers)
Effect 1: UV radiation, wave action and physical stress cause the plastic bag to become brittle and begin breaking apart into smaller fragments.
Effect 2: The fragments become microplastics — plastic particles less than 5 mm in size that spread through the water column and accumulate in sediments.
Effect 3: Microplastics are ingested by marine organisms and enter food webs, spreading up through food chains and to distant parts of the ocean.
Effect 4: The hydrocarbon backbone of LDPE is stable under normal conditions; micro-organisms lack the enzymes to break its C–C bonds, so the material persists rather than biodegrading.
Effect 5: The ocean accumulates a growing burden of persistent microplastic particles that cannot be removed easily and continue to affect marine life.
Overall outcome: A plastic bag entering the ocean ultimately becomes persistent microplastic particles that accumulate in food webs and ecosystems, because synthetic addition polymers do not biodegrade on human timescales.
Q3.1 — Why thermosets cannot be mechanically recycled
Thermosetting polymers contain extensive covalent cross-links between chains that form permanently during curing. Heating a thermoset does not cause it to soften and flow; instead, it decomposes. Without the ability to remelt and reshape, the material cannot be processed through mechanical recycling.
Q3.2 — Chemical recycling of PET
PET is a condensation polymer formed when diol and diacid monomers react with loss of water, creating ester links. Chemical recycling reverses this process (hydrolysis of the ester bonds) to recover the original diol and diacid monomers, which can then be repolymerised into fresh PET. This exploits the same ester chemistry used in the synthesis.
Q3.3 — HDPE structural feature enabling mechanical recycling
HDPE is a thermoplastic with no permanent cross-links between chains. It can be remelted and reshaped because heating overcomes the intermolecular forces holding the chains together, allowing the material to flow into new forms.
Q3.4 — Why mixing polymer types causes recycling problems
Different polymers have different melting points, chemical compositions and mechanical properties. When mixed, they cannot all be melted at the same temperature to produce a uniform melt. The resulting material often has poor mechanical properties because incompatible polymer chains do not form a homogeneous blend, producing a weaker, lower-quality product.