Coiled chains

Section 2 of 5

A rubber band stretches to several times its length and snaps straight back. The reason is in the way its long chains lie.

Every cis double bond bends a rubber chain back on itself. Between the double bonds, the links are joined by single bonds, and the two ends of a single bond turn freely, as the two ends of ethane do. So a rubber chain can bend into countless arrangements, and it moves from one to the next all the time.

Gutta-⁠percha, the dried latex of the Palaquium trees of Southeast Asia, is poly(isoprene) too, with every double bond trans. Each of its chains runs on straight, like the chain of elaidic acid, and straight chains pack tightly side by side into crystals. Gutta-⁠percha is hard and tough at room temperature, and it softens in hot water, ready to be moulded.

Cis chains in natural rubber wander, trans chains in gutta-percha lie straight Two boxes of skeletal chains, with each link's double bond in amber and a short line for its methyl group. Top, natural rubber: two chains whose links are all cis. Each chain bends back on itself at every double bond and wanders across the box, and the two chains lie apart. Bottom, gutta-percha: three chains whose links are all trans. Each runs straight across the box as a regular zigzag, and the three lie close together, side by side. natural rubber: every link cis gutta-percha: every link trans
Poly(isoprene) chains as skeletal formulas, with each double bond in amber and each methyl group a short line. The cis links of natural rubber bend the chain back at every double bond, so the chains wander. The trans links of gutta-percha leave each chain straight, so the chains line up side by side.

Stretch and let go

The bent chains of natural rubber pack together badly. They are non-⁠polar, so only the weak, fleeting pulls that hold alkane molecules together act between them. At room temperature each chain is a loose coil, wriggling and tumbling against its neighbours.

Pull on the rubber and the coils are drawn out along the pull. Let go, and the wriggling quickly tangles each chain back into a coil, and the rubber returns to its old length. Cold slows the wriggling: below about −70 °C the chains freeze in place, and natural rubber turns hard and glassy.

Coiled chains in a relaxed band straighten when the band is stretched Top: a short, deep piece of rubber holding six chains, each curled into a loose coil, one of them in amber. An arrow points down to the same piece stretched to twice its length and half its depth, with arrows pulling at both ends. The same six chains, the amber one among them, are now drawn out into long, gently waving lines that run along the pull. relaxed: chains coiled stretched: chains drawn out
The same six chains in a relaxed piece of rubber, and in the piece stretched to twice its length. Stretching draws each coil out along the pull, and letting go lets the chains coil up again.

Quick check

Gutta-percha has the same links as natural rubber, with every double bond trans. Why is it hard at room temperature?