Step-wise surface oxidation mechani

Step-wise surface oxidation mechanism of UHMCFs is shown inFig. 9. In the process of electrochemical surface oxidation, chemicaletching and oxidation firstly happened to the surfaces of UHMCFs,which could destroy surface ordered crystallites layer. As a result, anobvious increase happened to the ID/IG value (as shown in Table 5)when the current density increased from 0 to 1.0A/m2, which provedthat the disorder structures emerged. The chemical etching and oxidation also resulted in an initial decrease in the value of the tensilestrength of UHMCFs (Fig. 8). As the electrochemical surface oxidationprogressed, a strong oxidative reaction occurred with the current density more than 1.0 A/m2. The graphite crystallite layers of UHMCFsurfaces were broken or even peeled off in the in-depth oxidation. According to the research by Liu et al. [54], the in-depth oxidation of CFscould peel off the surface crystallites layer, as a result, the inside disorder structure exposed. Therefore, the ID/IG value of electrochemicallyoxidized CFs was higher than that of untreated CFs. However, the insideordered graphite structure rather than disorder structure showed up asthe outmost layer of UHMCF surfaces was peeled off. Therefore, the ID/IG value of oxidized UHMCFs decreased due to the ordered structureinside the fiber as the current density increased more than 1.0 A/m2(Table 5). Meantime, a larger number of oxygen-containing functionalgroups were introduced onto UHMCF surfaces. The in-depth oxidationcould possibly result in chemical crosslinks between graphite layers ofUHMCF surfaces, as shown in Fig. 9(3).