## 3 Results  
### The Characterization of the COF-TPB-DMTP and COF-TPB-DMTP Coating  
Considering the gully-like size of the GS-SSF, it was essential to control the diameter of the coating material. The parameters that influenced the sizes of the COFs included the proton number, reaction concentration and temperature. When the amount of acetic acid was varied from 0.1 mL to 0.5 mL at 90 \({}^{\circ}\)C, the spherical diameter of the COF-TPB-DMTP decreased gradually (Figure 2a-d). This could be attributed to the promotion of nucleophilic attack behavior with the increase in the amount of acid. However, an excessive acetic acid concentration could lead to N-protonation, which could reduce or eliminate the nucleophilicity and hinder the nucleophilic attack behavior. In addition, the influence of the reaction temperature on the morphology structure of the COF-TPB-DMTP was also investigated. As shown in Figure 2c,e,f, when the temperature varied from 90 \({}^{\circ}\)C to 150 \({}^{\circ}\)C, the diameter of the spherical COF remained almost unchanged.  
The appearance of C=N stretching bands at 1612 cm\({}^{-1}\) illustrated the successful synthesis of COF-TPB-DMTP since the C=N was nonexistent in the TPB and DMTP (Figure 3a). The disappearance of the N-H stretching bands (3433, 3355 and 3217 cm\({}^{-1}\)) and the weakening of the -CHO stretching bands (1679 and 2869 cm\({}^{-1}\)) of COF-TPB-DMTP in comparison with those of the TPB and DMTP also suggest the Schiff base condensation reaction had finished, which agreed with previously reported results [57]. The TGA revealed that the weight loss (<3%) of the prepared COFs was not obvious when the temperature was below 400 \({}^{\circ}\)C (Figure 3b). This result indicates that the resultant COF-TPB-DMTP possessed high thermal stability. The porosity and surface area of the prepared COF-TPB-DMTP were evaluated by nitrogen adsorption-desorption measurements at 77 K (Figure 3c). The type-III adsorption isotherm indicated the existence of a mesoporous structure in the COF-TPB-DMTP. The specific surface area, pore volume and pore diameter of the resultant COF-TPB-DMTP were calculated to be 81.01 m\({}^{2}\).g\({}^{-1}\), 0.243 cm\({}^{3}\).g\({}^{-1}\) and 3.429 nm, respectively. The abundant mesoporous structure is seen in Figure 3d.  
The SEM image (Figure 4a-c) of the GS-SSF exhibited a distinct gully-like surface structure. The nano-sized spherical COF-TPB-DMTP particles coated the gullies (Figure 4d-f). In comparison with the XRD pattern of the COF-TPB-DMTP (Figure 4g), one of the COF-TPB  
Figure 2: The SEM images of COF-TPB-DMTP synthesized under different conditions with (**a–d**) acetic acid amounts of 0.1, 0.3, 0.5 and 0.7 mL at 90 \({}^{\circ}\)C and (**e,f**) 0.5 mL of acetic acid at 120 \({}^{\circ}\)C and 150 \({}^{\circ}\)C.  
DMTP coatings scraped from the prepared fiber was not altered remarkably. The results revealed that the crystallinity of the COF-TPB-DMTP fabricated with two approaches was identical. In fact, the FT-IR spectra of the COF-TPB-DMTP and COF-TPB-DMTP coatings could give the same result (see Figure 4h). The TPB-DMTP coating material possessed a superior thermal stability and could be combined with GC (Figure 4i).  
Figure 4: (**a**) The SEM images of the 304 SSF at 200\(\times\); (**b**,**c**) the SEM images of the 304 stainless steel after etching (**b**) at 200\(\times\) and (**c**) at 3000\(\times\); (**d**–**f**) the SEM images of the TPB-DMTP-GS-SSF (**d**) at 200\(\times\); (**e**,**f**) the SEM images of the TPB-DMTP-GS-SSF  
200\(\times\), (**e**) at 2000\(\times\) and (**f**) at 5000\(\times\); (**g**) the XRD images of the COF-TPB-DMTP versus the COF-TPB-DMTP-GS-SSF; (**h**) the FT-IR image of the COF-TPB-DMTP and TPB-DMTP-GS-SSF; (i) the TGA image of the COF-TPB-DMTP and TPB-DMTP-GS-SSF.