When I was writing about PTFE on the page
about polymerisation of alkenes, I spent ages trying to find out why PTFE was
non-stick - and failed completely.
Part of the information I found on the web
I know to be untrue or illogical, but there is a mass of stuff which, to be
frank, I simply don't understand. Quite a lot of what is out there is written
by physicists or other non-chemists who speak a quite different language from
me! It also seems to me that there is a reluctance to start right back at the
level of the molecules and explain what is happening in terms of molecular
interactions.
Following a number of discussions with
various knowledgeable people over recent years, what follows is, I hope,
logical. Whether it is also the best explanation that can be given is another
matter.
The structure of PTFE molecules
PTFE, poly(tetrafluoroethene), is made by
polymerising lots of tetrafluoroethene molecules.
This simple diagram for PTFE doesn't show
the 3-dimensional structure of the molecule. In the simpler molecule
poly(ethene) the carbon backbone of the molecule just has hydrogen atoms
attached to it, and the chain is very flexible - it definitely isn't a straight
molecule.
However, in PTFE, the fluorine atoms in one
CF2 group are big enough to interfere with those on the neighbouring groups.
You need to remember that each fluorine atom will have 3 lone pairs sticking
out from it.
The effect of this is to inhibit rotation
about the carbon-carbon single bonds. The fluorine atoms will tend to line up
so that they are as far apart as possible from neighbouring fluorines. Rotation
will tend to involve a clash of lone pairs between fluorines on adjacent carbon
atoms - and this makes rotation energetically unfavourable.
The repulsions lock the molecules into a
rod-like shape with the fluorines arranged into very gentle spirals - a helical
arrangement of the fluorines around the carbon backbone. The rods will then
tend to pack together a bit like long thin pencils in a box.
This closely touching arrangement has an
important effect on the intermolecular forces as you will see.
Note:
Actually, this is a simplification. You will get some kinking in the
chains especially as temperature is increased.
Intermolecular forces and the melting point
of PTFE
The melting point of PTFE is quoted as
327°C. That's quite high for a polymer of this sort - so there must be sizeable
van der Waals forces between the molecules.
But . . . several web sites talk about PTFE
having very weak van der Waals forces. If it had very weak van der Waals
forces, it would be a gas - not a fairly high melting point solid. So we have a
problem here!
Why do people claim the van der Waals
forces in PTFE are weak?
van der Waals dispersion forces are caused
by temporary fluctuating dipoles set up as electrons in the molecules move
around. Since PTFE molecules are large, you would expect the dispersion forces
to be large as well, because there are a lot of electrons which can move.
It is generally the case that the bigger
the molecule, the greater the dispersion forces.
However, there is a problem with PTFE.
Fluorine is so electronegative that it tends to hold the electrons in the
carbon-fluorine bonds closely to itself - so closely that the electrons are
prevented from moving as much as you would expect. We describe the
carbon-fluorine bonds as not being very polarisable.
van der Waals forces also include
dipole-dipole interactions. But in PTFE each molecule is sheathed in a layer of
slightly negative fluorine atoms. The only interactions possible between
molecules in this case are repulsions!
So the dispersion forces are weaker than
you might expect, and dipole-dipole interactions are going to tend to cause
repulsion. It is no wonder that people claim that van der Waals forces are weak
in PTFE. You don't actually get repulsion because the effect of the dispersion
forces outweighs that of the dipole-dipole interactions, but the net effect is
that the van der Waals forces will tend to be weak.
And yet PTFE has a high melting point, and
so the forces holding the molecules together must be strong.
How can PTFE have a high melting point?
PTFE is very crystalline in the sense that
there are large areas where the molecules are lying in a very regular
arrangement. Remember that PTFE molecules can be thought of as long thin rods.
These rods will pack very closely together.
That means that although PTFE molecules
can't generate really big temporary dipoles, the dipoles that are produced can
be used extremely effectively.
So are the van der Waals forces in PTFE
weak or strong?
I think you could argue it both ways! If
you had PTFE chains arranged in such a way that the chains didn't have much
close contact, then the forces between them would be weak, and the melting
point would be low.
But in the real world, the molecules are
closely touching. The van der Waals forces may not be as strong as they could
be, but the structure of the PTFE means that they are felt to the maximum
effect, producing overall strong intermolecular bonding and a high melting point.
Non-stick properties and friction
Virtually every site that I have looked at
treats the relative lack of friction of PTFE and its non-stick properties as if
they were the same effect. I don't think that's true.
The non-stick properties
This is about why things like water and oil
don't stick to the surface of PTFE, and why you can fry an egg in a PTFE-coated
pan without lots of it ending up stuck to the pan.
You need to consider what forces might hold
other molecules to the PTFE surface. Possibilities might include some sort of
chemical bonding, van der Waals forces or hydrogen bonds.
Chemical bonding
Carbon-fluorine bonds are very strong, and
there is no way that any other molecule could get at the carbon chain to enable
any sort of substitution reaction to take place. No sort of chemical bonding
could take place.
van der Waals forces
We've seen that the van der Waals forces in
PTFE aren't very strong, and only work to give PTFE a high melting point
because the molecules lie so close together and there is very effective contact
between them.
But it is different for other molecules
approaching the surface of the PTFE. A relatively small molecule (like a water
or an oil molecule) will only have a small amount of contact with the surface,
and will only produce a small amount of van der Waals attraction.
A large molecule (like a protein, for
example) isn't going to be rod-like and so, again, there isn't going to be
enough effective contact between it and the surface to overcome the low
tendency of the PTFE to polarise.
Either way, van der Waals forces between
the PTFE surface and whatever is around it are going to be small and
ineffective.
________________________________________
Note:
As a similar example, it has been pointed out to me (February 2015) that
long-chain perfluoroalkanes (big alkanes in which all the hydrogens have been
replaced by fluorine atoms) can form a third phase if they are mixed with water
and a hydrocarbon solvent. Because they are so weakly attracted by both, they
form a third layer instead of dissolving in one or the other.
________________________________________
Hydrogen bonds
The PTFE molecules on its surface are
completely encased in fluorine atoms. Those fluorine atoms are very
electronegative and so will all carry some degree of negative charge. Each
fluorine also has three lone pairs of electrons sticking out.
Those are exactly the conditions needed for
hydrogen bonding to be possible between lone pairs on the fluorines and
hydrogen atoms in water for example. But it clearly doesn't happen - otherwise
there would be strong attractions between PTFE molecules and water molecules
and water would stick to the PTFE.
In November 2013, an Iranian PhD student
pointed out to me a 1997 paper by Dunitz and Taylor with a title "Organic
Fluorine Hardly Ever Accepts Hydrogen Bonds". If you are interested, you
can find it from this site if you have the right access.
They found that only a tiny number of
compounds containing C-F bonds would form hydrogen bonds, whereas compounds
like HF or the F- ion formed strong hydrogen bonds.
What they didn't come up with, however, was
any definite explanation for this, although they suggested that a possible
explanation could lie in the fact that the fluorine atom holds its electrons
very tightly in towards the nucleus, and as a result the C-F bond isn't very
polarisable. The electrons won't move sufficiently towards a hydrogen from
water (or anything similar) in order for a hydrogen bond to form.
Personally, I have a problem seeing why
that is different from the situation in H-F or a fluoride ion, both of which
can form hydrogen bonds with water.
And their final sentence said:
"At the same time, it has to be
admitted that, in spite of the vast amount of work on hydrogen bonding over the
years, the chemical factors influencing the strength of hydrogen bonds
(especially factors influencing H-bonding acceptor ability) are still not
completely understood."
Summary
There are no available methods for other
molecules to attach themselves successfully to the surface of the PTFE, and so
it is has a non-stick surface.
The low friction
PTFE has a very low coefficient of
friction. What this means is that if you have a surface coated with PTFE, other
things will slide on it very easily.
What follows is just a quick summary of
what is happening. This comes from a 1992 paper called Friction and wear of
PTFE - a review which is available free from this link.
• At the start of sliding,
the surface of the PTFE fractures, and lumps are transfered to whatever it is
sliding against. That means that the PTFE surface tends to wear away.
• As sliding continues,
the lumps are spread out to a thin film.
• At the same time the
surface of the PTFE is dragged out into an organised layer.
• The two surfaces in
contact now both have well organised PTFE molecules which can slide over each
other.
What holds the PTFE layer onto the
substance it is sliding against is quite complicated, and thoughts on this may
have changed since the paper was written. If you are interested, you will find
it discussed on the page numbered as 203 of the paper (page 11 of 19 on my pdf
reader).
