Okay, welcome to today's lecture for cross-linked polymer systems.

Today we're going to talk about the last few groups of synthetic elastomers, basically

just silicones and polyurethane rubbers are left.

These are quite special elastomers, however they're quite important as they feature unique

properties.

So, as you can see, we'll start with the silicone rubbers, which have the short-term Q in it

because the cyloxane group.

And yeah, silicones, as we already talked about, were discovered by Kipping in the 1900s when

he was doing some small chemistry experiments, however he never thought that those pasted

substances could be of any use for the industrial world.

In the 1930s, James Franklin Hyde from the USA enabled commercial production of synthesis

of polysiloxanes and he is commonly seen as the father of silicones.

As he developed also the synthesis for the silicones and also for the silica, the inorganic

filler we will talk about later.

The real industrial hit came in the 1940s when the Müller-Rochow synthesis was developed.

Müller was from Germany, Rochow was from Russia, so he immigrated to USA, and they

invented the industrial production of methylchlorosilane, which is one of the precursor molecules to

get our monomer molecule.

And as you know, it's always a struggle if you talk about synthetic elastomers to achieve

the monomers in the first time because nature doesn't do it for us.

It's basically a polycondensate of organic silane compounds.

Here we see a dechlorosilane with two residues.

These two residues can be organic and of various shape as we will see later.

From this monomer, we have a polycondensation reaction, which is the elimination of a small

molecule which usually condenses at the beginning, condensed at the glass beakers of the chemical

synthesis.

We get two terminal hydroxy groups, which then can be used for, again, a polycondensation,

this time with water, to this polysiloxane polymer.

As you can see here in our polymer backbone, we don't have any carbon atoms.

It's just anorganic, it's silicon and oxygen, which forms the polymer backbone.

And this brings some important differences compared to aliphatic or organic elastomeric

substances.

The residues for sure can be organic, however, the polymer backbone is anorganic.

If you look at the properties of the bonds, here we have the silicon-oxygen bond.

We see that it has great stability.

The bond energy with around 450 kilojoule per mole is quite strong.

It's way stronger than the carbon-carbon bond with 340 kilojoules per mole.

It's even stronger than the carbon-hydrogen bond with 415 kilojoules per mole.

This already indicates that this bond and this polymer will be way stronger against

chemical attacks.

The silicon-carbon bond is a bit weaker, and the carbon-oxygen bond lies in between those

two extremes.

Not only the bond strength changes, also the length of the polymer bond is different.

Here we see that we have 10 to 15 percent increase in bond length here, 164 picometers

compared to the 153 picometers.

So the bond is stronger, longer, and if you look at the angle, 143 versus the tetra-EDA

angle of 109.5 is way flatter.

So it's longer, flatter, yet stronger.

Talking about the polymer backbone here.

For other ones you basically get the tetra-EDA angle.