what we can see is, we start out with a mer in which I've indicated
there are double bonds along the polymer back bone.
So we have a couple of double bonds, and if we can break some of those bonds
periodically, what we can do is develop a covalent bond between the two chains.
So as illustrated on the right, we break those one of those bonds and
we do this by putting sulfur into the system.
So in the case of natural rubber, we refer to this process as vulcanization.
So now what we've done is to periodically go through, replace those double bonds
and, as we do that, we begin to develop cross-linking between them.
And so consequently what that does is help develop a rigid
structure as a result of those covalent bonds between the chains.
Another example, a polyester,
as indicated by the molecular diagram on the left and the right.
In the case on the left we have a saturated chain in which we have only
single bonds, and the one on the right, we have a double bond so it's unsaturated.
Now, if we take the polymer to the left, and we connect the mers together,
then what we would do is to produce a thermoplastic polymer.
On the other hand, if we maintain that unsaturated bond, and then we go through
a chemical reaction, we can by virtue of the fact that we break the double bond.
We're able to then create a covalent bond that exists between those two chains,
making the material then, what we refer to as again a thermoset.
Now another type of material Bakelite, and
this a common material with respect to electronic devices.
It's used as a resistor in an electronic device.
And, when you start out with this very complicated structure that's given here
for Bakelite, what can readily happen is, we can develop cross-linking.
With respect to adding together various groupings of this
functional group that we have on the left.
And then eventually, what we wind up doing is to develop
a three-dimensional network of the structure that we refer to as Bakelite.
And one of the things that happens here is that in a structure
where we have the strong cross linkings these types of
materials tend not to be easily recyclable.
In the case of materials that are thermoset what we can
do is to remelt the materials and start over and fabricate them in a new way.
On the other hand when we have these cross link, because of
the strength of that cross linking and the fact that it is a covalent bond
prohibits the process of recycling.
And what will happen is rather than having the polymer melt there'll be
a chemical reaction and the structure will wind up reacting and burning.
And that's associated with the unpleasant smell that you have in
some of your electronic devices when there happens to be a short circuit.
So these are the things that will ultimately control the characteristics of
the polymer.
That is, what's happening between the individual chains.
Thank you.