My 7yo snapped the clip off of a transparent pink plastic pen. He noticed that at the place where it broke, the transparent pink plastic became opaque white. Why does that happen (instead of it remaining transparent throughout)?
This is best illustrated by the pic I took of the [broken pen](https://imgur.com/S8rasqb).
In: Physics
fun fact, architects and structural engineers back in the day would make plastic scale recreations of their designs and apply a load to them. the most concentrated points of stress would become apparent due to this sort of discoloration as well as changes in light refraction due to the stressed plastic becoming thinner. really cool stuff
Plastics are made up of chains of molecules, when you stretch them, the molecules align, allowing bonding between the chains, this is crystallization, and results in opaqueness. In addition, when stretching the plastic on a microscopic level, voids form (this is crazing) these voids also result in opaqueness, so what you are seeing is a combination of the two effects. Even if the plastic doesn’t seem deformed on a macro level, it has been severely traumatized on a micro level.
Polymers are made of of many “mers”. These are chains of atoms. Normally they are randomly oriented and amorphous. When you bend or stretch plastic you are aligning these polymer chains which causes the optical properties to change.
Fun experiment. Go find a plastic shopping grocery bag. stretch the bag and watch it turn white.
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