So let's put the models of polymer chemistry
that we have to work, in order to understand strengthening.
And we've got two models or sort of two levels to our model.
The first model is just this basic string model.
And the second level to the model
is really what the individual chemistry, or the mer
chemistry, is of those strings.
So starting with a string model though,
we can say, OK, what if we load it in tension.
And we've actually covered this already, right?
We covered already what happens to a polymer
when you stretch it.
Generalised stress-strain behaviour for a polymer looks
something like this.
OK.
And so we know, after necking, where
we get extensive plastic deformation,
you can get continued load bearing
because of some strengthening mechanism.
So what we saw was we had this chain orientation
along the loading axis.
We're loading up and down, and that's the direction
that these strings or these molecules become oriented.
So we'd call that chain orientation, and something
that happens during plastic deformation
of a lot of polymers.
But it's also a strengthening mechanism.
Chain orientation, we can withdraw fibres from the melt
at a careful rate.
And as the polymers are coming from the vat,
the molecules are becoming aligned.
And you draw these little fibres of oriented polymers,
and they're quite high strength along the fibre directional,
lightweight because they're polymers,
so we can use those to make strong fabrics.
I'll give you a couple of examples
here of oriented polymers.
Spectra is a trade name for an oriented polyethylene.
Kevlar is also another fibre that has a high degree
of orientation to it.
Chains become aligned like that.
This process is also called conditioning.
So Spectra fibre is, in fact, conditioned polyethylene.
I'm going to pause this and fix that.
Red star for spelling the word.
Sorry about that.
OK.
Let's continue.
There we go.
We've got one strengthening mechanism under our belts
already.
And we were able to explain it from these molecules
as these chains or strings.
Let's look at another way we can achieve this.
In fact, we can build on this model we've got up here.
What do we see here?
We see chains that are highly organised.
And so, in fact, that's what we call crystallinity.
It's a regular repeating arrangement.
And in polymers, it's a repeating arrangement
of molecules, not individual atoms but molecules.
So that's crystallinity.
So could we achieve crystallinity
without deformation?
Yeah, in fact, you can.
So we can strengthen by favouring,
or designing the molecule, to favour crystallisation.
So how do you do that?
Well, in a short statement-- and it'll
make a little more sense as we dive further
into the structure of polymers-- you
design with simple and small mer units.
And like I said, we'll explore what that really
means in a bit more detail.
But something like polyethylene, for example,
polyethylene is very simple chemistry,
just carbon, carbon, carbon, carbon with hydrogens
off on the side.
And we'll draw them.
And you've seen it before.
But it's quite simple.
Really, it's roughly linear, at least in that depiction.
And so it's quite simple.
Let's actually explore an extended analogy here
with polyethylene.
And you may have encountered these before in food packaging.
You look at the bottom, you're going to recycle it,
and you might see this code there.
It says LDPE.
You might have also seen HDPE.
So what's going on?
What's the difference between these things?
What does it mean?
So LD refers to Low Density polyethylene.
And HD refers to High Density.
So how can we achieve a different density?
Well, in fact, what we're doing is
we're achieving crystallinity.
We're achieving an organisation.
If you have organised molecules, they occupy less volume
for the same mass.
It's like if you go to the store and you
buy-- what are you going to buy-- you buy some noodles,
right, and you buy fusilli verses spaghetti.
If you buy the dry noodles there, spaghetti,
they're packed really tightly into a little box,
whereas fusilli, these sort of spirally noodles, they
occupy a lot more space.
You buy a big cardboard box of them,
but it's a larger box for the same mass,
is if it was spaghetti.
So the density of the polymer, similarly,
if the molecules are organised, is higher.
So we can achieve a higher density of polyethylene
by organising them somehow.
Well, how do you do that?
Well, it goes down to this mer chemistry.
So low density polyethylene has a molecule
that looks sort of like this.
What I'm doing?
I'm drawing on these branches.
So this is a new concept.
I'll explain it for you quickly.
This is called a branch.
It's actually quite difficult to make a molecule that's
completely linear.
We don't.
You always get some little branches off the side.
And I got to explain to you what that really is.
So if we look here at this little part
where the branch is coming off the molecule,
let's explore what that really looks
like close to the atomic level.
So we have the main chain of carbon atoms.
And if it's polyethylene, we've got hydrogen
coming off the side.
Hydrogen, you've got one mer unit, you've got another.
So here's all the hydrogens.
A branch is when coming off of a mer unit,
you have the same mer unit coming out.
So again, it's connected by these strong bond.
And this may continue.
These could be quite long.
So it's important to realise this branch here is the same
mer chemistry, in this particular example, at least.
And it's coming off, and it's strongly bonded.
That connection right there is a strong bond.
It's a primary bond to the main chain.
So you've got this.
And this would have hydrogen off the side, as well.
I won't draw them all in.
That's what a branch is.
You've got these great long branches coming off the side,
and quite a few of them in low density polyethylene.
High density polyethylene is processed so
that-- a little more careful.
There's branches still, but there's fewer of them.
And they're relatively short.
And so what happens is you can now pack these more closely.
Well, not you do it, but the molecules themselves
pack more closely.
And what's that mean?
Well, that leads to a higher density.
Or another way of saying, really,
that same thing is that when the molecules are organised side
by side like this and packed closely, what that is
is that is increased crystallinity.
OK.
So we increase the crystallinity.
Molecules become packed more closely together like this.
They're aligned.
Well, they don't have to be necessarily aligned
with the loading axis, but they're
packed closely together.
And so then what happens is you get more opportunities
for friction, if you will.
We're going to learn that they're
called secondary interactions between the chains.
You can imagine, it's just like they these chains,
these strings are all lined up, packed nicely organised.
And there's all of these places where
they come in contact with each other along the length
of the molecule, versus the case where it's amorphous,
and then they're just disorganised like.
Well, where are the regions where they touch each other?
Well, there's just a few where they kind of
come in the vicinity of one another
and they rub past each other.
Not all that many.
And so what you have to think about
when you think about the properties of the polymers,
well, how hard would it be to go and grab
one of these molecules-- say this one, for example.
That's a hand.
I better label that so you know.
That's a molecular-level hand.
And what's it doing?
Well, you're going to try to pull one of those molecules
out.
We're building on this model.
And it's sounds kind of childish,
but actually, it's quite accurate.
It was supposed to be shading.
It looks a hairy hand.
Anyway, there we go.
So the hand is pulling that molecule out.
But what does that explain?
That explains plastic deformation.
If you can move through the application of the load,
an extra load, you can move one molecule away from
the neighbour by overcoming these friction or secondary
interactions, these weak interaction.
That's plastic deformation.
In fact, it also explains melting.
So just from this little sort of mind exercise of thinking
of a molecular-level hand-- that's
the molecular hand, molecular-level hand-- you're
moving that molecule way, you can explain yielding.
You can explain what happens upon heating.
I'll call it just melting at this point.
You could even explain what happens when
you try to dissolve a polymer.
What happens if you get a short chain molecule
like acetone or something?
And it's got to work its way in there.
Well, how hard is that to do?
If it's amorphous, it's quite easy.
If it's crystalline, it's a lot more difficult
for that little molecule to work its way in between those.
Similarly, if you're going to apply heat to consider melting,
you have to put more heat into the system
if it's highly organised, highly crystalline,
because there's all these interactions
between these highly organised molecules.
So we're explaining a lot of stuff
with this simple little model.
So I think what I'll do is actually
wrap this video up here, and we'll continue on
in a separate video.