Tuesday, December 11, 2012

Getting something good from Mechanochemistry

Mechanochemical reactions, especially thermomechanical chemical reactions are the bane of my career. Regardless of what I am working with, I know that every process step that a polymer is exposed to will do something to break it down. Even in reactive steps, such as polymerization, crosslinking or chain extending, there is some sort of shear used to ensure good mixing [*]. And that means degradation. The higher the temperature, the greater the degradation. The more the shear, the greater the degradation. The longer the process, the more the degradation.

But few aspects of science are ever pure evil, and that is even the case with mechanochemical reactions. While the value of these reactions have been known for some time (the Wikipedia article suggests that starting a fire with two sticks is the oldest such reaction), studying the fundamentals has always been challenging. Without knowing the fundamentals, monitoring something as simple as the degree of the reaction becomes a trial-and-error project.

That is all changing due to recent work that uses high energy X-rays to monitor the real-time, in situ kinetics of mechanochemical reactions in a ball mill. High energy x-rays from a synchrotron means that you take your reaction to the x-ray source and not the other way around, which further means that this isn't too practical as of yet for just any old lab to run (or even afford). But as technology advances, these techniques or similar ones will become more available.

The obvious appeal of these type of reactions is that they are solvent-free. By not having to add and then later remove solvent, the efficiency of the process is greatly improved.

[*] The only exception that I can think of is photochemistry of thin films and coatings. But even that isn't pristine and pure, as there is always some photodegradation occurring. Granted, it is overwhelmed by the polymerization and/or crosslinking, but photochemistry always has a degradation element to it. Always.