Operando tribo-Raman spectroscopic observation for wear processes of superlow frictional concentrated polymer brushes at frictional interface

Concentrated polymer brushes (CPBs) synthesized by advanced surface-initiated controlled radical polymerization have received increasing attention in the tribological field because CPBs in good solvents exhibit superlow friction phenomena under macroscopic severe sliding conditions. To realize tribological applications of CPBs, an understanding of the wear mechanisms of CPBs is required to develop a method to enhance their durability. In this study, an operando tribo-Raman spectroscopic experiment, which is an effective approach for revealing the wear mechanisms of tribo-materials, was conducted on the frictional interface between a polymethyl methacrylate (PMMA)-CPB and a bearing steel ball immersed in an ionic liquid (IL), which is a good solvent for PMMA-CPBs. From the operando tribo-Raman results, the concentrations and molecular structures of the PMMA-CPBs and IL at the frictional interface were found to be constantly changing with the sliding cycles during operation. Moreover, their time-dependent changes strongly affected the wear process and superlow friction phenomena of the PMMA-CPB lubricated with the IL. Consequently, our results suggest that controlling the degree of swelling of CPBs at frictional interface is an effective approach for realizing a durable-superlow frictional tribo-system of CPBs.