At the time when I was still post-doc in the group, I was helping out Prof. Picchioni in the supervision of students for their MSc thesis in Chemical Engineering. I had the idea of putting together two topics we were working on: synthesis of polymers via ATRP, and reversible Diels-Alder chemistry, for the preparation of self-healing materials. My idea was based on introducing furfuryl methacrylate, a relatively inexpensive and possibly even bioderived molecule, in the polymer structure. With ATRP we can add (at least in theory...) how many units we want, and where we want them. Then, we can reversibly cross-link the polymers with a bis-maleimide (see figure below) to obtain thermally reversible materials. The latter is something that has been done in the group for many years, but now, with the aid of a controlled radical polymerization method, we can introduce a lot more variability and we can study the influence of the polymer different structure (diblock, triblock, star-block, random) and composition (relative amount of furan groups in the structure) on the final properties of the material. As partner for our monomer, we choose styrene, simply because its polymerization with ATRP is very well studied, and I had experience with it.
The idea was not entirely new at the time (of course, as a researcher, the most common experience is finding out that somebody else already published your idea...), but a systematic study was never performed, on such a large variety of structures. Francesco liked it, and put two of our best Master students to work on it for their thesis, under my supervision.
One of the results I am most proud of, is represented by this graph below. It shows nicely how the cross-linking density (how many cross-links are present per unit volume of material), measured by swelling experiments correlates with the relative amount of furan groups (which makes sense, because these are responsible of cross-linking), but also (and this is more surprising) with the polymer architecture: 6-arm star polymers have a significantly higher cross-linking density than linear polymers, at the same relative amount of furan groups. In this one graph there is a lot of work condensed. The paper contains other interesting observations (if I may say so myself...) that may apply in general to self-healing materials based on Diels-Alder chemistry.
As I mentioned, this work was done by two very nice students we had in our group, Amanda and Julian. We had a great time back then in the lab, and they produced a very large amount of data, that took me very long time to put together in this work, on and off during more than 5 years. They are long gone now, continuing with their career away from the university, but they were both happy that we finally managed to publish their effort. It was worth it in the end. You can read the paper open access here, if you are interested. The journal offered me to publish an inside cover for the issue where this paper will appear. We'll see how that goes... Stay tuned!
As a follow up of this work, we repeated a similar approach to make reversible rubber materials, by replacing styrene with butyl acylate. We got interesting results, but it will probably take 5 more years to get those published too...
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