At this point, you may be wondering what I'm doing over here other than intermittently updating this blog. My primary project looks like it's going to be about tribology, which is the study of how things behave when under friction.
Our basic goal is to reinforce polymers with nanomaterials (as well as some materials that are somewhat larger, on the order of microns), and increase the wear resistance of those polymers by doing so. We are also hoping to be able to exercise some kind of control over the coefficient of friction, which in effect determines how slippery or sticky the final material will be. There are some applications where you want as sticky a surface as possible, such as brake pads. Other applications, like in bearings or pistons, you want as little friction as possible. Polymers are already outperformed in these applications by metals and ceramics, and it is also important to note that polymers are not as temperature-stable as either of the other two types of materials. However, if polymer composites can be made good enough to replace metals and ceramics, such composites have three major advantages, namely, they are lighter, they are (potentially) cheaper and could (potentially) be used without additional lubrication.
To make the composites we first have to make the filler. Right now I'm working on doing so using a method known as high-energy ball milling. Envision a coffee can filled with ball-bearings. Now, put some powders in that coffee can, and in some way shake the can at high speed, letting the ball-bearings rattle around in there for 10 hours. That's basically what this process is, more or less. This is a solid-state method, meaning that chemical powders are being reacted as solids rather than dissolved in some kind of solvent.
Not using a solvent can be advantageous for a few reasons. First and foremost, a lot of solvents are dangerous for the environment as well as human health. Please don't misunderstand, this doesn't mean all solvents are equally dangerous or that you should be afraid of chemists and chemistry. Just that this is a consideration when doing chemical work that can be bothersome. Second, solvents can often have an effect on a reaction, from either participating directly in said reaction or by making some types of reagents more or less reactive. This is not always a bad thing: indeed, choosing the right solvent can make a world of difference. But sometimes it's nice to not have to worry about solvent effects. Finally, solvents can be expensive. Combining this with environmental concerns means that there are some reactions that we can easily accomplish on a small scale in a laboratory that simply do not work on the industrial scale. Solid-state is a way around some of these disadvantages.
However, solid-state chemistry has a few issues. First and foremost, when something is dissolved, molecules can diffuse easily, bump into each other, and react. When powders are used, this doesn't happen. Furthermore, a dissolved molecule is separated from other molecules of the same type a lot better than molecules that are stuck in a crystalline structure. So, your effective surface area is lower in the solid state than it is in solution. In short, fewer molecules can react at one time when powders are mixed than when they are dispersed in a solvent. Last, if your reaction is exothermic, meaning that it gives off heat, a solvent will help disperse some of that heat whereas solid-state doesn't have this kind of advantage. Generally, solid-state reactions need to be run at very high temperatures if they are to work, and that creates other problems.
Ball milling is a way of avoiding some of the problems of conventional solid-state reactions. The balls crashing into the chemical powders transfer some of their energy to those powders. This causes a distortion in the crystal structure of those reactants that in essence "stores" that energy. That stored energy can be used to initiate reactions with other materials present in the reaction vessel. The reactant powders are also being mixed at a high rate of speed. Further, when balls collide with each other or with the walls, because energy has to be conserved a lot of heat is generated locally. Therefore, it is possible to have locally-high temperatures exactly where the reactions will be taking place rather than having to heat the whole darn thing all the time. These factors combined means that a milled reaction can be carried out at room temperature over a span of time comparable or less than conventional methods. Of course, there are some drawbacks to this method such as generally low-crystalline product and the need to cool the reaction vessel. But in general, it's a reasonably good method as far as I can tell.
Now that the reinforcing material has been made it has to be introduced to the polymer matrix in some way or another. We haven't gotten to this point quite yet, but when we do there are are number of ways of going about it. Most likely, since we will be using a thermoset, we will simply mix the polymer resin together with the reinforcing material, and then polymerize. The trick is going to be getting a good, even dispersion of the particles, particularly as those particles tend to stick together particularly if we are able to get particles on the nanoscale. The short explanation is that very small particles have a very high surface area relative to their volume, and that surfaces are energetic. So, high surface area means high surface energy. One of the most important concepts in chemistry is that whenever something is high-energy it tries to reduce that energy in some way or another. The easiest way to do that is to reduce the total surface area by forming larger agglomerations of particles, as larger particles have a lower surface-area to volume ratio. We don't want agglomerates! It defeats the whole purpose of having small particles in there to begin with, and empirical evidence indicates that composites with a lot of agglomerates have much worse properties than those where the reinforcing material is evenly dispersed.
I think I'll hold off on a theoretical discussion of tribological mechanisms for another day, as those are a little complicated. But, I hope that this little discussion has at least made it clear what I more-or-less aim to do here in Poland, and generally how I aim to do it.
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