A three-dimensional (3D) porous network can be prepared on the PEEK surface by sulfonation with enhanced osseointegration and antibacterial properties. However, few studies have been conducted on the formation mechanism of a 3D porous network. In this work, the surface and cross-sectional morphologies, chemical compositions, functional groups, surface wettability, and crystalline states of sulfonated PEEK were investigated at different sulfonation times and coagulant concentrations. The results show that the number of nodular structures and broken fibers on the sulfonated PEEK surface as well as the size of macrovoids in the cross sections increase with increasing sulfonation times when water is used as a coagulant. In contrast, dilute sulfuric acid as a coagulant can inhibit the formation of surface porous structures and macrovoids in the cross sections. Moreover, all of the sulfonated PEEK samples have the same chemical compositions but exhibit better hydrophilicity as the number of microsized pores decreases. It is proposed that non-solvent-induced phase separation (NIPS) occurs during the sulfonation process, and the formation mechanism of surface and cross-sectional morphologies is discussed. Furthermore, it is assumed that the air is trapped in the microsized pores, leaving the surface of the 3D porous network in the Cassie-wetting state. All of these preliminary results throw light on the nature of the sulfonation process and may guide further modification of the structures of sulfonated PEEK.
Keywords: Cassie model; PEEK; formation mechanism; non-solvent-induced phase separation; sulfonation.