What Happens When You Bend a Plastic Cup?

[Mephisto]

I was drinking from a plastic cup today with some physics friends, and we noticed that when we bent the cup, a white patch of plastic would form there, and the plastic would not be transparent at that point anymore. We were baffled by this observation, and none of us could really explain what happened at the molecular level. Does anyone know what happens there?
my best theory was that through friction, heat and pressure, cross-linkages formed in the polymer chains of the poly-ethelene, or whatever it is, basically altering the structure of the material... but I'm not sure how right that is... does that seem at all plausible?
Does anyone happen to know the answer?

 

[Danger]

That sounds reasonable, but I'm not sure about the friction and heat contributions. You can very slowly bend a piece of PET, to minimize those factors, and the result will be the same. It seems to me that it's a stress pattern from stretching some of the molecular bonds. In my experience, you'll also notice that you get a prismatic effect in the area, which indicates that the refractive index of the plastic is messed up.

 

[Loren Booda]

You can make a simple polarizer by stretching Saran wrap in one direction. That's the basic phenomenon I believe acted upon your cup - polarization along the directions of stress.