North Carolina State University chemical engineers have discovered a way to make flexible coating materials more durable and water-resistant, without the use of environmentally harmful solvents.
The research, conducted by Dr Jan Genzer, assistant professor of chemical engineering, and Dr Kirill Efimenko, a research associate in Genzer’s lab, is the first to show that long-lasting superhydrophobic polymer surfaces can be created through mechanically assembled monolayers (MAMs).
‘We discovered, quite by accident, that you can tailor control a flexible material’s physical and chemical surface properties, such as water resistance and durability, by increasing its surface area before you chemically attach the layer of molecules that form its final coating,’ said Genzer.
Stretching the substrate material allows the fitting of more of the desired coating molecules – up to a critical point – onto its surface, said Genzer. Then, when the tension is released and the material resumes its original size, the chemically grafted molecules are squeezed and locked together in a much greater density and with much greater chemical stability than would occur naturally.
‘The relative simplicity of it is stunning,’ said Genzer. ‘For years, scientists have relied on oxygen plasma treatments, which are quite harsh on the substrate, to attach the molecules without being able to control the density of the anchored molecules. Here’s a mechanical way that’s cheaper, easier, less harsh and more controllable.’
Genzer’s method is also better for the environment because it doesn’t require the use of solvents that produce harmful fumes or by-products.
He and Efimenko have used polydimethyl siloxane (PDMS) networks – an elastic polymer film widely used in industry and research – as a substrate material in their experiments. Because PDMS is a model material for other polymer films made of cross-linked molecules, Genzer believes the process will work on other elastic materials as well.
In their experiments, the NC State researchers fabricated their material’s surface using an array of rigid, semifluorinated units composed mainly of CF2, and one CF3 group
To test the durability and impermeability of the experimental coating, Genzer and Ekimenko submerged strips of the MAM material in water for controlled periods of time and subsequently stored them in normal ambient laboratory conditions, and then studied their stability. ‘To our surprise, the surfaces of MAMs stayed virtually unchanged, even after six months in a dusty and humid atmosphere.
The MAMs chemical properties remained the same, and there was very little physical deterioration,’ said Genzer. In contrast, strips of coating materials made the conventional way usually begin to decay and deteriorate after sitting in water a relatively short time.
Despite the initial results, Genzer stresses that more research is needed before the mechanically assembled monolayer process can be put into commercial use.