The structural difference between chain-like polymers with linear repeat units (blue boxes) and their two-dimensional counterparts with areal repeat units (orange triangles). Image: Schlüter Research Group / ETH Zurich |
Scientists
under the direction of ETH Zurich have created a minor sensation in
synthetic chemistry. They succeeded for the first time in producing
regularly ordered planar polymers that form a kind of “molecular carpet”
on a nanometre scale.
At
ETH Zurich in 1920, the chemist Hermann Staudinger postulated the
existence of macromolecules consisting of many identical modules strung
together like a chain. For this he was initially rewarded with mockery
and incomprehension in professional circles. But Staudinger was to be
proved right: today the macromolecules described as polymers are known
as plastics, and by 1950 one kilogram of them was already being produced
per capita worldwide.
Today,
more than ninety years after Staudinger’s discovery—for which the
chemist was honoured with the Nobel Prize in 1953—about 150 million tons
of plastics are manufactured every year. A gigantic industry developed,
without whose products our daily life is no longer imaginable.
A
research group led by Professor A. Dieter Schlüter and Senior Lecturer
Junji Sakamoto at the Polymers Institute of ETH Zurich has now succeeded
in making a decisive breakthrough in the synthetic chemistry of
polymers: they have created two-dimensional polymers for the first time.
Intensive discussions led to success
Polymers
are formed when small single molecules known as monomers join together
by chemical reactions like the links of a chain to form high molecular
weight substances. Since qualifying as a lecturer, Schlüter was already
occupied by the question of whether polymers can only polymerise
linearly. Although graphene counts as a natural representative of a
two-dimensional polymer—the carbon atoms in graphene form a
honeycomb-like pattern through triple bonds—it cannot be synthesised in a
controlled way.
Nevertheless,
he said, if it is possible to produce giant molecules
“one-dimensionally” from monomers, or for example molecules in
pharmacology that are so small that they are practically
“zero-dimensional”, why then should it not be possible to develop a
synthetic chemistry that generates two-dimensional molecules? When
Schlüter and Sakamoto met at ETH Zurich a few years ago, they discussed
this topic intensively and together they looked for answers.
The
crux of the matter was to create oligofunctional monomers in such a way
that they join together purely two-dimensionally instead of linearly or
even three-dimensionally. Polymers of this kind must have three or more
covalent bonds between the regularly repeating units. The scientists
had to find out which bonding chemistry and environment was most
suitable for producing this kind of “molecular carpet”.
After
intensive analyses of previous studies and the possible ways of
generating two-dimensional polymers synthetically, they considered the
synthesis at a water-air interface or in a single crystal, i.e. a
crystal with a homogeneous layer lattice.
The
researchers decided in favour of the second alternative: the doctoral
student Patrick Kissel successfully used this to crystallise special
monomers which he had prepared into layered hexagonal single crystals.
For this he generated photochemically sensitive molecules for which such
an arrangement is energetically optimum. When irradiated with light
with a wavelength of 470 nanometres, the monomers polymerised in all the
layers.
Sheet-like polymers with regular structures
After
this the researchers boiled the crystal in a suitable solvent to
separate the individual layers from one another. Each layer represents a
two-dimensional polymer. The fact that the team really had succeeded in
producing sheet-like polymers with regular structures was shown by
special studies in an electron microscope carried out by Empa researcher
Rolf Erni and Marta Rossell from ETH Zurich at the Empa (Swiss Federal
Laboratories for Materials Science and Technology).
The
polymerisation method that was developed is so gentle that all the
monomer’s functional groups are also preserved at defined positions in
the polymer. The researchers have complete structural control over the
monomers in a way that would never be possible with graphene, for
example, because that process would need to be carried out at enormously
high temperatures. Sakamoto says, “Our synthetically manufactured
polymers are not conductive like graphene, but on the other hand we
would be able to use them for example to filter the tiniest molecules.”
In
fact there are small defined holes with a diameter in the sub-nanometre
range in the regularly arranged polymers. Moreover, tiny hexagons in
the polymers, formed by benzene rings with three ester groups, can be
removed by a simple hydrolytic process. This would form a “sieve” with
an ordered structure suitable for the selective filtration of molecules.
Unresearched physics
However,
before the researchers can think about practical applications, the task
now is to characterise the material’s properties. According to
Schlüter, this is mainly a job for the physicists. One of the exciting
questions in this respect will be how a two-dimensional polymer behaves
compared to a linear polymer, for which a good physical and
technological understanding is available. Schlüter assumes that
two-dimensional polymers could have a different physics and will
therefore also find different applications.
He
mentions the property of “elasticity” as an example: intertwined linear
polymers enable a stretched rubber band to snap back as soon as it is
released. But because flat sheets can hardly entangle together, this
would probably not work with planar polymers. However, the researchers
must first of all find a way to produce larger amounts and even larger
sheet sizes.
The
size of the crystals is currently only 50 ?m. Sakamoto stresses that
“those, however, are already enormous degrees of polymerisation at a
molecular level.”
