What Happens When You Bend a Plastic Cup?

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SUMMARY

Bending a plastic cup, specifically made from polyethylene (PE) or polyethylene terephthalate (PET), causes a white patch to form due to alterations at the molecular level. This phenomenon occurs as a result of stress patterns that stretch molecular bonds, leading to changes in the refractive index of the material. The discussion highlights that the white appearance is linked to polarization effects, similar to those observed when stretching Saran wrap. Understanding these effects is crucial for comprehending the physical properties of polymers under mechanical stress.

PREREQUISITES
  • Basic knowledge of polymer chemistry, particularly polyethylene and polyethylene terephthalate.
  • Understanding of molecular bonding and stress patterns in materials.
  • Familiarity with the concept of refractive index and its implications in optics.
  • Experience with physical properties of materials under mechanical stress.
NEXT STEPS
  • Research the molecular structure of polyethylene and polyethylene terephthalate.
  • Explore the effects of mechanical stress on polymer materials.
  • Learn about the principles of polarization in optics and its applications.
  • Investigate the relationship between temperature, friction, and material properties in polymers.
USEFUL FOR

This discussion is beneficial for materials scientists, polymer chemists, and physics students interested in the mechanical properties and behavior of polymers under stress.

Mephisto
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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?
 
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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.
 
Last edited:
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.
 

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