And by playing around with this molecular structure, polymer designers can vary its strength and behaviour enormously. Pack the chains close together so they crystallise and you might end up with a hard, rigid plastic bottle; space them further apart, with fewer microscopic plastic crystals joining them together, and you have a bottle you can squeeze ketchup out of. Vary the length and something similar applies: the bulletproof vest version of polyethylene (ultra-high molecular weight) involves strands that – if they were the width of spaghetti – would be about 250 metres long.
