Polyvinyl alcohol

Polyvinyl alcohol, or PVA, is an unusual plastic: it dissolves in water. That's why it's used for the film around laundry pods. Its films are also clear, strong and stretchy, and they soak up iodine well, which puts it at the heart of nearly every LCD polarizer.

It can't be made directly from its own monomer, because vinyl alcohol doesn't last on its own: the molecule rearranges into acetaldehyde. So chemists take a detour. First, vinyl acetate is polymerized into long chains of polyvinyl acetate, the same material as white wood glue. Then the chains are treated with methanol and a little sodium hydroxide, which swaps each acetate side group for a simple -OH group. What's left is polyvinyl alcohol. How completely the acetate is swapped out changes how it behaves; for polarizers, it's almost complete.

For a polarizer, PVA is cast into an extremely even film, then soaked in iodine and stretched in one direction. The -OH groups help it hold water and iodine, and the stretching lines up the chains so the iodine lines up too. Optical-grade film is demanding to make, and a few companies, mostly in Japan, supply most of it.