Why long chains are strong

Section 4 of 4

Inside each chain, strong covalent bonds hold the atoms together. Between neighbouring chains the pull is far weaker. A plastic's strength comes from how its chains lie together.

Chains attract their neighbours wherever they lie side by side. Each attraction is weak, but along a chain of thousands of atoms they add up. Long chains also tangle, like cooked spaghetti in a bowl: pull one strand and it drags others with it. So the longer the chains, the stronger the material. Short chains make wax: candle wax is mostly alkanes of about 20 to 40 carbons, and it snaps and crumbles easily.

Short chains lie loose, long chains tangle Two boxes. Left: many short wiggly chains scattered loosely, with one short chain in amber. Right: four long chains wound through each other in a tangle, with one long chain in amber, so you can follow a single molecule as it loops past the others. short chains long chains
Short chains lie loose and slide apart easily. Long chains wind through each other: follow the amber chain to see how far one molecule reaches.

The chains can also slide a little and uncoil, so many plastics bend and stretch without snapping. Strong and bendy at the same time is what makes plastics so useful.

Same monomer, different plastic

How closely the chains pack matters too. High-density poly(ethene), HDPE, has straight chains that lie close together, so it is stiff and strong: milk bottles and crates. Low-density poly(ethene), LDPE, has chains with short side branches, like a branched alkane's, that hold them further apart, so it is soft and bendy: squeezy bottles and food bags. HDPE melts at about 130 °C and LDPE at about 110 °C.

Straight chains pack closely, branched chains pack loosely Two boxes. Left: nine straight zigzag chains lie side by side, packed tightly together; this is high-density poly(ethene), HDPE. Right: four zigzag chains with short side branches lie further apart, because the branches keep them from packing closely; this is low-density poly(ethene), LDPE. One chain in each box is amber. straight chains pack closely: HDPE branched chains pack loosely: LDPE
Straight HDPE chains pack closely and attract each other strongly. The branches on LDPE chains hold them further apart.

Heat, reshaping and recycling

Poly(ethene), poly(propene), PVC, nylon and PET are thermoplastics: their chains are separate. Heat loosens the pull between the chains so they slide past each other, the plastic softens, and it can be moulded into a new form that holds once it cools. That is how these plastics are recycled. The number inside the triangle of arrows on many plastic items names the polymer: 1 is PET, 2 HDPE, 3 PVC, 4 LDPE and 5 poly(propene).

Thermosetting plastics have cross-links: covalent bonds that join each chain to its neighbours, making one giant network. They hold their form when heated, and at high temperatures they char. Melamine picnic plates are thermosetting, and so is Bakelite, the first fully synthetic plastic, made in 1907.

Separate chains in a thermoplastic, cross-linked chains in a thermosetting plastic Two boxes, each with four wavy chains lying one above another. Left, a thermoplastic: the chains are separate, so when warmed they can slide past each other. Right, a thermosetting plastic: short amber bonds, the cross-links, join each chain to the chains above and below it. thermoplastic separate chains thermosetting chains cross-linked
In a thermoplastic the chains are separate and can slide when warm. In a thermosetting plastic, cross-links (amber) tie the chains into one network.

Quick check

A poly(ethene) bottle softens when it is heated and can be moulded into something new. Why?

That's the end of lesson 9

In this lesson you found out:

  • What monomers and polymers are
  • How ethene's double bond opens to make poly(ethene), and how to draw a repeat unit
  • How nylon and polyesters are made by condensation
  • Why long chains make strong, bendy materials
Start Lesson 10: Molecules of life All lessons