Water Proof Neoprene

name         Glenn
status       educator
grade        K-3
location     IN

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
---------------------------------------
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).
====================================================================