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Name: Glenn
Status: educator
Grade: K-3
Location: IN
Country: USA
Date: N/A


Question:
I am trying to explain to my class why Neoprene is waterproof, but I lack the in-depth knowledge to do so. Please assist me


Replies:
Glenn,

Neoprene is a trade name for a class of synthetic rubber. Most rubbers are polymers built up from butadiene (H2C=CH-CH=CH2). Isoprene replaces the H of the second carbon with CH3, chloroprene replaces that H with Cl. In any case, there are two main reasons why rubbers are waterproof. The first being that the polymer does not form intermolecular attractive interactions with water. Most rubbers are only capable of either London Forces (LF) or Dipole-dipole (DD) interaction whereas water forms Hydrogen Bonding (HB) intermolecular forces with itself. The HB forces in water are so strong and makes the interaction so stable that in order for anything to interact with water that something must be capable of replacing the HB forces within water with an interaction that is equally as strong. Since LF and some DD forces are weaker than HB, the replacement does not happen. The other reason is that in order for water molecules to penetrate a sheet of rubber, the water molecule must be able to migrate through the gaps in between the polymer strands. The molecular strands in a rubber sample do form gaps that are large enough for a single water molecule to pass through. However, since water forms very strong interactions with itself, one does not find a single molecule of water by itself. Rather it is droplets of water, an agglomeration of water molecules that is trying to migrate through a sheet of rubber. Since the water does not interact with the rubber molecules, it will tend to have a high surface tension, retain its droplet size and as such is too big to pass through the gaps between the rubber molecule strands in the rubber sample.

Since you are teaching K-3 students, it might be better to forego the above explanation and try to let the students formulate their own conceptions that may not be founded on the more involved chemistry just explained.

I would try a more exercise driven discussion. Here is what I think. You will need some nail polish remover (mostly acetone and water). Some foamed pressed polystyrene (those take-out boxes from fast food places), some polystyrene sheets (cut out the plastic windows from envelopes and mailers), a few glasses or beakers, and some water.

Let the students discover that the foamed boxes do not dissolve in water but readily dissolve in acetone. They can then come to the conclusion that a bead of water on the surface of the polystyrene windows will not be able to penetrate the window. However, a drop of acetone on this same window will eventually push through and actually punch a hole. This could bring home the idea that controlling solubility properties is one of the factors in water-proofing.

For the permeability lesson, you can inflate a rubber balloon so that the skin is very tight and come back the next day to find that the balloon is not quite as inflated as it was before. A balloon filled with water on the other hand should remain just as inflated and should indicate to the students that the ability of a molecule to migrate through a skin controls whether the skin is proof against that molecule.

Hope this helped.
Greg (Roberto Gregorius).

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