Possibility of surfaces that can be manipulated

Using new combinations of well-known polymers is one way of producing new materials. For this work, the use of polymers that respond to external stimuli are of particular interest, and is the line followed by the chemist Leire Ruiz-Rubio in the research for her PhD thesis. She has studied the blending of certain polymers that can be controlled when the conditions of their medium (the pH or temperature, for example) have been altered.Once the behaviour of these polymers is known, it is possible to design new materials to use in many different applications."We do basic chemistry; in other words, we study the behaviour of these polymers in more and more advanced conditions, and then the applications that can be developed thanks to these materials will be sought." Ruiz has in fact studied the polymer blends first of all in solution and then when they are on a solid surface.

The aim was to find out the conditions in which polymer chains disperse and when they end up joined together.Having examined that, Ruiz went on to study how the conditions can be manipulated, so that the material formed by the polymer blend can be altered from a dense layer to one with holes.

The blends in this research were produced by adding one of each of the two polymer groups.In one group there were polyacids, in other words, proton donors (acrylic polyacid and methacrylicpolyacid and phenolic polyvinyl). In the second group there are several poly(N-vinylamides), in other words, polymers that can create hydrogen bridges with the protons of the ones before them.

"We have studied the relationship between polymers.In other words, to see whether polymers of one blend move away from each other, or whether, by contrast they become adhered to each other."And they studied that relationship first in solution, because the solvent can greatly influence that.Polymer chains will move away from each other if they have a greater tendency to attach themselves to the molecules of the solvent (dispersive tendency).By contrast, if the hydrogen bridges between the polymers are more stable than the relationship towards the solvent, the polymer chains will tend to stay together (associative behaviour).

The balance is a tricky one, because there are all kinds of interactions."That's the problem," says Ruiz."Polyacids, for example, not only interact with the solvent and other polymers, they also interact with themselves, but some polyamides, for example, prefer to join with other polymers.The same thing happens depending on the quality of the solvent.If the solvent is very good, the polymers tend to join with it."

Most of the solvents used in this work are alcohols, (methanol, ethanol, propanol, etc.) and the effect of their molecule size has been studied.In fact, the size has a lot to do with whether the solvent is good or bad:small alcohols are better solvents that large ones.

Ruiz and her colleagues repeated their solution-based research in the conditions of a surface.They wanted to know what kind of behaviour was displayed by these polymer blends on a surface.They used silicon surfaces covered with a co-polymer layer and added a number of functional groups to that layer."They end up as if they were strands of hair," says Ruiz.The polymer blends were placed on top and they formed a second layer.These kinds of systems can be manipulated, for example by changing the pH or the temperature, and the chemical and physical properties of the surface change.And in the end, the polymer blend will behave in one way or another according to that.

"We do basic chemistry, but you always have applications in mind.For example, drugs can be inserted between these layers and attached to the hairs of the surface.With the drugas the filling, it would end up like a kind of sandwich."In that case, by manipulating the pH or temperature, the polymer blend can be dismantled and the drug released at the desired moment.So, one potential application is a capsule that opens at the desired moment."But that's still a long way off.We have studied what happens in the solution, and then in a more complex medium, on a surface.When you have studied that, you'll be able to go one step further."But the capsule application is still a long way away.The aim of this thesis was to study the behaviour of certain polymers.