On the Surface of Things


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From pens to pans, from hard disks to compact discs, an increasing number of products are made of polymers, giant complex molecules with repeating structural units.

Different polymers are often mixed together in liquid form and then allowed to harden in order to impart the best properties of each to the final product. During the mixing, certain types of polymers are more likely than others to move to the surface, forming a coating that thickens with time.

It is important to know which constituent is more likely to move to the outside: for example, a polymer that excels in dirt-resistance or lubrication will be of greatest benefit if it settles closer to the surface. It is also important to know whether the polymer layer at the surface will have time to thicken sufficiently before the molten mixture hardens.

A series of Weizmann Institute studies -- the latest of which appears in a recent issue of Physical Review Letters -- helps to answer these questions and may lead to better polymer materials.

Prof. Jacob Klein and his student Ullrich Steiner of the Institute's Materials and Interfaces Department have determined that the more flexible polymer chains in a mixture -- the ones easier to bend -- will be found at the surface.

They also discovered that the rate of thickening is primarily controlled by the weak forces of attraction between neutral molecules, known as van der Waals forces. Due to the weakness of these forces, the thickening is extremely slow -- it proceeds in accordance with a mathematical formula involving approximately the power of 4. If, for example, a wetting layer grows to a certain thickness after one hour, then it will double its thickness after 16 hours (2 to the 4th power), and double once again after 256 hours (4 to the 4th power), and so on -- until the mixture hardens and this process stops.

These studies were carried out at the Weizmann Institute's 3 MV Van de Graaff particle accelerator, where the nuclei of polymers were bombarded by atoms propelled to high speeds. An analysis of the reactions occurring within these nuclei provides detailed data about the formation of the polymer surface coating and its properties.

These findings advance the basic understanding of processes taking place in polymer mixtures. They may also help optimize the surface properties of materials made up of different polymer components.

Prof. Klein's research was partially supported by the German-Israeli Foundation for Scientific Research and Development (GIF), the Minerva Foundation, the Commission of the European Union and Israel's Ministry of Science. Prof. Klein holds the Hermann Mark Chair of Polymer Physics. Ullrich Steiner now conducts research at the University of Konstanz in Germany.

The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.