Which material EVA or PS, would be expected to be a better ...

[BrownTown3]

Which material EVA or PS, would be expected to be a better barrier material against water? Why?
 
  What will be an ideal response?

Question 2

Low-density polyethylene is normally translucent. If a piece of this material is bent into a tight radius, it translates less light (becomes chalky white). What is this phenomena called? Explain why this happens. Explain why the material often breaks after repeated bends.
 
  What will be an ideal response?

[Jody Vaughn]

Answer to Question 1

Water is a polar molecule so it would be excluded best by a non-polar polymer. EVA has many polar atoms in the pendent group and would allow water to

permeate easily. Polystyrene is nonpolar and would exclude polar molecules like water. The size of the side chain for EVA is large, as is the pendent group for polystyrene. Both plastics would have about the same openness so size would not be a factor in determining the permeation differences between these two plastics. Therefore, polystyrene is a better barrier material for mater than is EVA.

Answer to Question 2

The change in light transmission due to stress like bending is called blushing. Blushing forms where areas of crystallinity are created caused by these stresses. As a rule, crystalline areas transmit less light than an amorphous region. The break will come as the induced crystallinity increases and those areas become brittle. Failure will occur when that crystallinity becomes significant.