A polymer chain can be linear, branched or cross-linked, and the arrangement decides what the plastic does: linear and branched chains slide past each other when heated and so melt, while cross-links lock chains into a network that cannot. Few cross-links give an elastic solid such as vulcanised rubber and many give a rigid one such as Bakelite, so the number of links is the dial between soft and hard.
Part of Plastics and Polymers
Chain structure describes how the monomer units of a polymer are arranged into linear, branched or cross-linked molecules and how that arrangement sets the properties.
Monomers Become Chains
A monomer is the repeating unit, a polymer is the finished chain, and crosslinking is the extra bonding between chains. Polymerization describes the two ways chains are built, and Polymer Chemistry applies them to plastics.
Three Arrangements, Three Behaviours
| Arrangement | What it is | Behaviour |
|---|---|---|
| Linear | Long chains, no ties between them | Soften and melt on heating; can dissolve in the right solvent |
| Branched | Chains with side branches | Pack less tightly, so softer and lower in density (general knowledge) |
| Cross-linked | Chemical bridges between chains | Do not melt; swell in solvent but do not dissolve |
The casein, celluloid and Bakelite examples fit the table:
- Celluloid is built from cellulose, a natural chain, modified and softened with camphor. It is linear in behaviour, so it softens when warm.
- Casein is a protein chain. Hardening it with formaldehyde forms bridges between chains: see Formaldehyde Hardening.
- Bakelite is a dense network. Each phenol molecule can bond to formaldehyde at three positions, so the network grows in three dimensions: see Phenolic Resins.
- Vulcanised rubber has sulfur bridges between rubber chains: Sulfur Vulcanization.
Cross-Link Density Sets Stiffness
The more bridges per chain, the less the chains can move. The corpus’s thermoset article states it as: the more links per chain, the harder and more brittle the result. Thermosets shows the rubber case, where a high-sulfur version is hard ebonite. For plastics, light cross-linking toughens, heavy cross-linking makes the part rigid and brittle, which is why Bakelite is filled with wood flour or fabric to take impact.
Practical Consequences
- You can recycle linear and branched plastics, because chains slide when heated: Recycling and Reprocessing.
- You cannot recycle cross-linked plastics; scrap is ground to filler.
- Hardness and heat resistance rise with cross-linking, at the cost of toughness.
- Solvents behave differently: a linear plastic dissolves (celluloid in acetone for lacquer), while a cross-linked one only swells.
Common Mistakes
- Confusing chain length with cross-linking — long chains tangle; cross-links bond.
- Expecting a branch to behave like a cross-link — branches stay attached to one chain.
- Cross-linking beyond what is needed — the part becomes brittle.
Cross-link reactions release heat
Curing a thermoset is an exothermic reaction, and phenolic cures release steam and fumes of phenol and formaldehyde. Never cure in a sealed vessel and keep the work ventilated. See Synthesis and Molding.
What’s Next
- Learn next: Thermoplastic vs Thermoset — choosing between the two families
- Combine with: Polymer Chemistry — how the chains are built