- [Steve] This is a strip of self-healing polymer.
That means I can slice it into two pieces,
and then with the application of gentle pressure,
push the two pieces together,
and it'll become,
once again, one single piece.
At first, you might think,
"Well, that's not so unusual,
"like, if I took a strip of Blu Tack
"and slice that into two pieces,
"and then press those two pieces together,
"I would, once again,
"have a single piece of Blu Tack."
You might also say,
"Look, if I have two droplets of water,
"and I push them together,
"they become a single droplet of water."
The difference between Blu Tack
and this self-healing polymer
is that, well,
Blu Tack is sticky,
it sticks to my hand,
it sticks to a wall,
it sticks to the back of this picture
of Patrick Stewart.
So, it's not surprising
that Blu Tack sticks to itself,
but this polymer isn't sticky.
The same goes for droplets of water,
they're sticky and they flow,
because they're a liquid,
so, we should naturally expect different behavior
from droplets of water,
as we do from a strip of polymer.
We can actually flip this question on its head,
instead of saying,
"Why is this polymer self-healing?"
We can say,
"Why aren't other things self-healing?
"Why isn't metal self-healing?"
Like, if I cut this piece of metal in two,
why is it that I can't just push the two ends together,
and it fuses back into a single piece of metal?
Like, if you think about what's happening
at the molecular level,
a metal is a lattice of ions
in a cloud of mobile electrons.
So, if you slice this lattice in two,
and bring those two ends back together,
well, (chuckles) to me,
that looks like a single metal lattice,
so, why doesn't it become one?
In the case of metal,
the answer's actually quite straightforward,
it's oxidation.
I've got some sodium metal here,
which is soft enough to cut with a knife,
and when I do,
you can see it's actually a really shiny metal,
but that doesn't last forever,
it doesn't take long
for that shiny surface to become dull in appearance.
That's because the metal is reacting
with oxygen in the air.
Actually, the same thing happens
when you cut any metal,
it's just that that oxidation layer isn't as visible
in the case of, say, steel,
as compared to sodium.
So, it's understandable then
that if you took two pieces of metal
and pushed them together,
they wouldn't form one single piece of metal,
because there is a layer of oxidized metal
in between.
In fact, if you were to cut a piece of metal
in the vacuum of space,
and then push those cut surfaces together,
they would fuse together.
We know this,
because it's happened in situations
when we don't want it to happen,
at least, once.
In April, 1991,
the Galileo spacecraft
was supposed to open its high-gain antenna,
looks a bit like an umbrella,
but three ribs of that umbrella
had cold welded to the body of the spacecraft,
and so, it couldn't open.
This stuff is a bit different
to a metal, though,
because it's a polymer,
so, it would be more like if I cut a piece of plastic
into two pieces,
and then try to push those two pieces together.
In this case, clearly,
they don't fuse back together.
And I suppose that makes sense,
like, if you're cutting a piece of plastic in two
then you're cutting through individual polymer chains,
you're breaking bonds between carbon atoms.
When you break a covalent bond like this,
you're left with radicals,
that's atoms that have unpaired valence electrons,
they're incredibly reactive,
so, we should expect the exposed surface atoms
to react with molecules in the atmosphere,
or if it was in a vacuum,
I imagine, they would react with the atoms around them
in the substance itself,
and so, that exposed surface becomes unreactive.
It's not sticky anymore.
If you press those two surfaces together,
those bonds aren't going to reform,
unless you can introduce quite a high activation energy.
So, why is it then that I can cut this material
into two pieces,
push the two pieces together,
and it reforms into a single piece?
Well, the full explanation it seems
is hidden behind trade secrets,
but there is some information out there.
I want to announce this Patreon exclusive live Q and A
I'm doing on Monday,
the 20th of June,
but I don't know where to put it in the video.
Put it anywhere.
Okay, thanks, Patrick.
Not all self-healing polymers are the same,
and it seems that there are,
at least, two possible explanations
for what's happening with this one.
The first is a polymerization reaction,
and the second is a crosslinking reaction.
A polymerization reaction is just a reaction
that joins lots of small molecules together,
into long chain molecules,
into polymers.
It can also be when you take two existing polymer chains
and join them together into one longer polymer chain,
and that's potentially what's happening
with this self-healing polymer.
When you bring two surfaces together,
some of the ends of the polymers on one surface
are joining to some of the ends of the polymers
on the other surface,
creating longer polymer chains
that span the break.
Unlike with the plastic,
this polymerization reaction can happen
at room temperature,
perhaps because when you break the polymers,
you're not left with highly reactive radicals,
or perhaps because of the presence of a catalyst.
Actually, that might explain
why the polymer isn't sticky.
If the catalyst has a high specificity
to that particular polymerization reaction,
and very few others,
that would explain why it's only sticky to itself.
The second possibility,
crosslinking, is what happens
when you have lots of individual chains,
and you link them together
at different positions along the chain.
Like, for example, I've got loads of little chains here,
and maybe I'm gonna join these two chains together
at this point,
these two chains together at this point,
and so on.
Crosslinking strengthens a polymer.
For example, I've got this liquid latex here,
it's liquid, because the individual polymer chains
are able to slide over each other,
but I can cause crosslinks to form
between these separate chains
by changing the pH.
So, by adding vinegar,
for example, I turn this liquid latex
into a bouncy ball.
It's interesting to think that before,
I had lots of individual molecules,
and now, I have one giant molecule,
the size of a bouncy ball.
And this is true for things like car tires.
This car tire is one giant molecule.
How does this relate to self-healing polymers?
Well, when two surfaces of the polymer
are brought together,
chains on one surface,
and chains on the other surface,
come into contact,
and a crosslinking reaction occurs.
It's interesting that that reaction can happen
at room temperature.
And it's interesting that the polymer
is able to create bonds with itself,
but not other things.
It's interesting that it's not sticky.
Whether it's a crosslinking reaction
or a polymerization reaction,
the explanation for why this substance
is only sticky to itself,
and not other things,
seems to be in the molecular detail,
and that detail seems to be a trade secret.
But I'll keep digging,
and if I find out any more,
I'll tell you.
Actually, there is a third possibility,
but I don't think it's what's going on here,
and that is micro encapsulation.
You have these tiny microcapsules
throughout the substance,
and inside those capsules is some kind of sticky substance,
some kind of glue type of thing,
or maybe a catalyst,
or something like that.
When the material is damaged,
it tears open those microcapsules,
and the content spills out and gets to work,
healing the damage.
You can buy something quite similar
from a standard hardware store,
usually, called self amalgamating tape,
but the fusing isn't quite so good,
but it's a similar idea.
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