Science - September 10, 2009

Molecular trampoline replaces duster

A PhD students has developed a removable polymer coating that deflects dirt.

Dirt is an inevitable part of life but there is new hope for those who would prefer to leave their dusters in the cupboard. Daily life could become a good bit cleaner thanks to the discovery of Wiebe de Vos at the Laboratory of Physical Chemistry and Colloid Science. 'By attaching a molecular brush made up of polymers to the surface, you can keep loads of objects clean for a long time.' If this nano-brush still gets dirty after a while, you can remove it chemically, says De Vos. He expects these molecular brushes to be usable on industrial filters, contact lenses and medical apparatus.
De Vos uses a polyethylene oxide (PEO)-based polymer for his dirt-resistant coating. This is an obvious choice since PEO is safe and is already used in substances ranging from toothpaste to foodstuffs. 'You can attach this polymer to a surface at one end, and the other end takes on a rounded form so that you get a ball on a rod', explains De Vos. 'If you then attach more and more polymers to the surface, they are squeezed so close together that the open end is forced to expand.' The density of the polymers is then so high that dirt particles cannot reach the surface. These particles press down on the polymers, which react like a spring, pulling together and bouncing the dirt particle off. De Vos: 'So the brush works like a sort of trampoline that keeps dirt away.'
This principle has been known for decades. But De Vos found a solution to the problem that the brush does get dirty after a while and stops working. He came up with the idea of the removable brush. 'You allow a totally negatively charged polymer to attach itself to the surface that you want to keep clean. Then you add another polymer that is positively charged on one side, while the other side is neutral and water-soluble. The positively charged end will connect with the negative charge of the attached polymer.' What you then get is an electrically charged polymer brush. Once this brush is dirty, you can break the contact by raising the acidity level of the solution or by adding salt to it. 'Then you just rip the old brush off', says the researcher. It will take some time before we see applications. De Vos: 'Polymer brushes are already used on some contact lenses, but it will be a few years before my 'zipper brush' is in use.
What are polymers?
Polymers, long chains of linked molecules, have long been part of our daily lives. They are the basis of plastics, silicones and paint. They occur in nature too, in rubber and in DNA, for example. Polyethylene oxide (PEO), a long chain of ethylene oxide molecules is one of the main industrial polymers. Its physical characteristics, such as stickiness, depend on the length of the ethylene oxide chain and how many branches are formed in it. But chemically, the different PEO polymers are identical.