Lesson 10 of 12
Chain packing and crosslinks
Explain a material property from its chain structure.
A-Level 8873, revised syllabus (2026-2027)
I can compare thermoplastic and thermosetting polymers.Syllabus
Syllabus 8873, 9(o). Compare thermoplastic and thermosetting polymers.
I can explain all named polymer structure-property-use comparisons.Syllabus
Syllabus 8873, 9(p). Explain all named polymer structure-property-use comparisons.
I can predict polymer physical properties from structure.Syllabus
Syllabus 8873, 9(q). Predict polymer physical properties from structure.
Packing and crosslinks control how a plastic moves
Explain a material property from its chain structure.
Make a guess. You are not marked.
Poly(ethene) can be made with straight chains or with many branched chains. Which form is denser and stronger?
Show the answer
The straight-chain form, as its chains pack closely
Straight chains pack closely and line up, so there are more attractions between chains and less empty space.
A polymer has strong covalent bonds along its backbone. Between separate chains there are weaker intermolecular attractions. A thermoplastic, such as straight-chain poly(ethene), softens when heating overcomes enough of these attractions for the chains to slide past each other. Cooling makes it solid again. This reshaping does not break the backbone into monomers.
A thermosetting polymer, such as cross-linked poly(diallyl phthalate), has covalent bonds between its chains, which form a network. These cross-links stop the chains moving. Once it has set, heating cannot separate the chains into a liquid. Strong heating breaks down the network instead. So it cannot be recycled by simply melting it again, as a suitable thermoplastic can.
Separate chains can slide; a covalent network cannot
The thermoplastic schematic has separate chains connected only by dashed intermolecular attractions. The thermoset schematic has solid covalent links joining chains into a network. It illustrates the network principle rather than the atomic structural formula of poly(diallyl phthalate).
| Property | Linear thermoplastic: poly(ethene) | Cross-linked thermoset: poly(diallyl phthalate) |
|---|---|---|
| Softening and remoulding | Can soften and be reshaped within a suitable temperature range; potentially recyclable by remelting. | Does not soften into a reusable melt after curing; decomposition occurs on sufficiently severe heating. |
| Rigidity | Chains can move relative to one another more readily. Flexibility depends on packing, crystallinity and conditions. | Crosslinks restrict movement and usually give greater rigidity for comparable structures. |
| Strength | Resistance to deformation depends on how well chains pack and interact. | Covalent connections resist chains being pulled apart or sliding; a rigid network can nevertheless be brittle. Do not equate strength with ability to bend. |
LDPE and HDPE are both poly(ethene), but their chains are arranged differently. LDPE has more branches, which stop the chains packing closely. Fewer close contacts between chains give a lower density and a softer, more flexible material, useful for plastic bags. HDPE has fewer branches, so its chains pack more closely. Stronger attractions between chains make it stiffer and harder, useful for plastic bottles. HDPE does not have stronger C-C bonds.
Worked exampleInfer a property from a changed structure
Two samples have the same backbone chemistry and similar average chain length. Sample A has more side branches; sample B has longer unbranched stretches. Predict which is likely to pack more closely.
Try it first. Then show the worked stepsHide the worked steps
- Branches hinder chains from approaching in a regular arrangement. Sample A therefore tends to have less efficient packing.
- Sample B can form more closely packed regions and more effective interchain contacts.
- For these otherwise comparable samples, B is expected to be denser and stiffer; A is expected to be softer and more flexible. The stated comparison controls other factors such as molecular mass.
Use chain shape → packing → attractions → property. Do not infer every property from the polymer name alone.
Check your understanding
Why does being a thermoplastic not guarantee that an item will be recycled?
Show the answer
It can be remoulded, but collection, sorting, contamination and cost decide if it is.
Thermoplastic describes what the material can do when heated. Whether recycling happens also depends on collection, sorting, additives, damage and demand.