Quantitative Theory for Polyelectrolyte and Liquid-Crystalline Brushes

Polymeric brushes refer to surfaces densely coated with polymer molecules, each attached to the surface by one of their ends. Brushes offer an easy and versatile way of modifying surface properties, such as friction, adhesion and wetting behaviour. The rich selection of polymeric materials with which this can be done provides a diverse range of surface coatings. With new synthetic techniques it is now becoming possible to create brushes of impressive precision and ever increasing complexity, and likewise state-of-the-art experiments are able to measure their properties with unprecedented detail. On the other hand, brush theories have advanced very little over the last couple decades. To keep pace with experiments, the present proposal aims to develop advanced theories with the capability of making accurate quantitative predictions.Much of the proposed research will focus on polyelectrolyte brushes in which the polymer chains become electrically charged. One of their most intriguing properties is the ultra-small (almost unmeasurable) friction that occurs when two opposing brushes slide past each other, even when they are pushed against each other at a pressure of several atmospheres. It seems that nature itself makes use of this property by coating the outer cartilage surface of mammalian joints with biological polyelectrolyte polymers. Thus one could imagine doing the same with artificial hip or knee joints. Polyelectrolyte brushes also have another useful property that they can spontaneously switch from an extended state to a collapsed state; this has a number of potential applications in the emerging field of nano-technology, which involves the building of ultra-small devices. For instance, this transition can be used to create small motors (i.e., actuators) or to open and close (i.e., gate) small porous membranes. The transition can be induced by changes in temperature, the pH (for brushes immersed in water), or by the application of an external electric field.This research will also investigate the behaviour of liquid-crystalline brushes, where the polymer chains have small mesogen units attached along their length. Potential uses include smart responsive surfaces with electro-optic, mechano-optic and electro-mechanic behaviours. There is also a theoretical calculation predicting that the alignment of the mesogens units can cause the brush to collapse, causing an analogous transition to that of the polyelectrolyte brushes with similar potential applications. Again, we should be able to induce the transition by changing temperature or by applying an electric field. There are also good reasons to believe that liquid-crystalline brushes will have superior properties for liquid-crystal displays (LCD's), and they may also provide important components for organic semi-conductors.