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Wafer-thin and without solvents

Polyethylene is used for the production of many everyday objects. Since the mid-1950s, polyethylene has been used in large quantities to manufacture tubes for gas and water supply systems, for cable insulation as well as for packaging, for example shrink film packaging used for wood panels sold in hardware stores, and for covering fruit and vegetables. Polyethylene is also used for the manufacture of implants.

As a product, polyethylene stands out for its combination of superior gliding characteristics and abrasion resistance. Its production requires few resources, is environmentally friendly and physiologically harmless. In view of the decreasing availability of fossil raw materials, it will also be possible to efficiently produce polyethylene from renewable resources.

Layers of plastic, much thinner than a strand of hair, are of enormous interest to scientists and engineers. Applications include protective coatings. A team of researchers led by Prof. Stefan Mecking of the Department of Chemistry at Constance University has now developed a new method to produce wafer-thin layers. “Such research results are also an excellent start for the “Molecular Materials Sciences” course,” said Mecking explaining that the new 6-semester course with 25 student places will be offered from the winter semester of 2008/2009.

“We are developing films from individual, prefabricated nanocrystal building blocks. The conventional method used to produce ultrathin polymer films begins with a diluted solution of the polymer in an organic solvent, which is applied to a surface. High temperatures are usually required to break up the crystalline structure of the solid polymer and turn it into a solution. The ordered crystalline layer develops once the solvent is removed or cooled,” said the chemist explaining the conventional procedure.

Tiny crystalline lamella

Mecking and his team are using a completely new procedure. “We do not need such high temperatures, and more importantly, we do not need organic solvents. In the first step, the catalytic polymerisation of ethylene with nickel complexes leads to an aqueous dispersion of crystalline polymer particles. These are individual, separate single crystals consisting of crystalline lamella of about 25 x 6 nm surrounded by an amorphous layer 1 nm thick. Amorphous domains on the surface are typical for polymer crystals. “Amorphous” is virtually the opposite of crystalline and means that the polymer molecules are arranged in an irregular manner. In comparison to the crystalline domains, the amorphous areas are soft.”

Once this chemical process is finished, droplets of the aqueous dispersion are applied to a glass carrier. “Then the droplets are spun at 2,000 revolutions per minute. Excess liquid is spun away and the water evaporates, leaving behind a wafer-thin, uniform film 50 nm thick,” said Mecking (editor’s note: one nanometre is a billionth of a metre). The success of this production technique is based on the amorphous domains around the single crystals in combination with the tiny size of the crystals. Although the amorphous domains only make up a tiny portion of the volume of the particles, they interact very strongly with each other, holding the individual particles solidly in the film.

The research is being carried out by Qiaong Tong (M.Sc.) in her dissertation supervised by Professor Stefan Mecking at the Department of Chemical Materials Science at the University of Constance.

Source: University of Constance - 6th June 2008 (P)
Website address: https://www.biooekonomie-bw.de/en/articles/news/wafer-thin-and-without-solvents