A team of scientists from the Swiss Federal Institute of Technology (ETH) in Zurich and Empa, the Swiss Federal Laboratories for Materials Science and Technology, has succeeded for the first time in producing regularly ordered planar polymers that form a kind of molecular carpet on a nanometer scale.
The research group, led by ETH Zurich scientists Dr. Dieter Schlüter and Dr. Junji Sakamoto, describes breakthrough results in the synthetic chemistry of polymers in a paper published in Nature Chemistry.
It is known that 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. The question remained as to whether polymers can only polymerize linearly, i.e. in one dimension.
According to ETH Zurich, in order to develop a synthetic chemistry that generates two-dimensional molecules the ETH chemists had to first and foremost 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”.
The researchers decided to do the synthesis in a single crystal, i.e. a crystal with a homogeneous layer lattice. PhD student Patrick Kissel successfully used this to crystallize special monomers in layered hexagonal single crystals. The monomers he generated are photochemically sensitive molecules, for which such an arrangement is energetically optimum. When irradiated with light with a wavelength of 470 nanometers, the monomers polymerized in all the layers of the crystal. To separate the individual layers from one another the researchers boiled the crystal in a suitable solvent. Each layer represents a two-dimensional polymer.
“These two-dimensional polymers are extremely sensitive towards irradiation,” said Dr. Rolf Erni, a co-author of the paper and a researcher at Empa. “It’s really tricky to not destroy their structure during the TEM measurements, which made the analyses a real tough nut to crack”
Diffraction experiments at minus 196 degrees Celsius – the condensation point of nitrogen – and high-resolution images at a low electron dose allowed the Empa scientists to eventually provide proof that the cross-linked molecules indeed exhibit a regular two-dimensional structure. The polymerization method that was developed is so gentle that all the monomer’s functional groups are also preserved at defined positions in the polymer.
“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,” concluded Dr. Says Sakamoto.