First of all, I would like to introduce what is 'Inverse vulcanization' and why we are interested in investigating on inverse vulcanization. Inverse vulcanization was coined by Pyun and co-workers in 2013, and it is a process of using small molecular to stabilize polymeric sulfur leading to chemically stable high sulfur content polymers. The polymers generated by this polymerization route show many unique properties thanks to their unique sulfur-sulfur backbone structures, such as high refractive index, recycling ability despite crosslinked structures, excellent heavy metal capture ability, and antibacterial activity. We are aiming to improve the performance and explore more potential applications of this kind of polymer. Ideally we want to see some common plastics in our real life can be replaced by inverse vulcanized polymers, as the raw materials of inverse vulcanized polymers are very cheap.
However, inverse vulcanized polymers are produced through bulk polymerization at high temperature (≥159 °C) normally. There are many accompanying problems in this polymerization process including but not limited to: inhomogeneous polymers obtained caused by less miscible monomers or different reactivity of monomers at high temperature, uncontrollable auto-acceleration, side reactions accompanied by hydrogen abstraction and H2S generation, and the limitation of co-monomer choice by boiling point. That is unsafe and hazardous in the operation. Therefore, at present stage, it is hard to apply inverse vulcanized polymers industrially. If we want to apply this new material industrially, alternative easier and safer synthesis methods should be explored.
Hence, we have tried to find an alternative synthesis method for making inverse vulcanized polymers. In this work, we have demonstrated that the mechanochemical synthesis, a green method, can be used into the production of inverse vulcanized polymers by using ball milling. Mechanochemical synthesis is sourced from mechanical energy and is an environmentally-friendly method which shows advantageous properties including shorter reaction time, homogeneous reaction, high atom economy and so on. Compared with traditional thermal synthesis route, no requirement for heating, fast reaction, solvent-free, reduced hydrogen abstraction, no auto-acceleration, broader monomer options, and more homogeneous reaction regardless of miscibility of sulfur with monomers can all be observed in this polymerization method.
In addition to the process advantages the method possesses, the polymer materials obtained by the mechanochemical synthesis method show many unexpected and interesting properties compared with the normal thermally synthesized products, including totally different thermal properties, and enhanced mercury capture efficiency. A notable and surprising finding was that the fallen iron filings from the steel milling balls are able to chemically react with the polymers to form thermally stable inorganic substances rather than being only physically dispersed in the polymers.
For more details, please read our paper: https://rdcu.be/cTOd7