Topic 10 of 12
Chain packing and crosslinks
Explain a material property from its chain structure.
A-Level 8873, revised syllabus (2026-2027)
Packing and crosslinks control how a plastic moves
Explain a material property from its chain structure.
A polymer contains strong covalent bonds along its backbone. Between separate chains are weaker intermolecular attractions. A thermoplastic, exemplified by linear poly(ethene), softens when enough interchain attractions are overcome to allow chains to move past one another. Cooling can restore a solid shape. This remoulding normally occurs without breaking the backbone into monomers.
A thermosetting polymer, exemplified by cross-linked poly(diallyl phthalate), has covalent links between chains, forming a network. The crosslinks restrict chain movement. After curing, heating cannot simply separate the chains into a flowing melt; sufficiently severe heating instead breaks or decomposes the network. It cannot be recycled by straightforward remelting in the way 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 chain arrangements differ. LDPE has more branching, which makes close regular packing harder. Fewer close interchain contacts per volume give lower density and a softer, more flexible material, useful for plastic bags. HDPE has less branching and more closely packed regions. More effective interchain attractions give greater stiffness and hardness, useful for plastic bottles. The difference is not that HDPE has stronger C-C covalent bonds.
Worked example
Infer 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.
- 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.