## 3 Results and Discussion
### Influence of Phthalic Anhydride Solution Concentration on the Surface Properties of PPTA Fiber
#### 3.2.3 Study of Surface Elemental Composition before and after Catalytic Modification of Fiber  
To investigate the catalytic mechanism of anhydrous AlCl\({}_{3}\), surface elemental analysis was conducted on unmodified PPTA fiber, uncatalytic modified PPTA-2 h fiber, and fiber modified with different durations under catalytic conditions. Figure 9 shows the XPS full spectra of the fibers before and after modification. Figure 9a depicts unmodified PPTA fiber, while Figure 9b shows PPTA-2h fiber modified with 0.3 mol/L phthalal anhydride for 2 h. Figures 9c, 9d, 9e and 9f, respectively, illustrate PPTA-0.5 h (A), PPTA-1 h (A), PPTA-1.5 h (A), and PPTA-2 h (A) fibers modified with 0.3 mol/L phthalal anhydride for 0.5, 1, 1.5, and 2 h under catalytic conditions with anhydrous AlCl\({}_{3}\). From Figure 9a, it can be observed that the surface of unmodified PPTA fiber mainly consists of three elements, C, N, and O, which are consistent with its molecular composition. In Figure 9, the C1s peak and N1s peak of the catalytically modified fibers are lower compared to those of unmodified PPTA fiber and PPTA-2 h fiber, while the O1s peak is significantly enhanced. Relative percentages of the three elements were calculated using Avantage 5.9931 software, and the results are presented in Table 7. The surface element O/C ratios of PPTA-0.5 h (A) fiber, PPTA-1 h (A) fiber, PPTA-1.5 h (A) fiber, and PPTA-2 h (A) fiber are 0.307, 0.389, 0.355, and 0.318, respectively, nearly twice as high as those of unmodified PPTA fiber and PPTA-2 h fiber. This indicates that the addition of the catalyst anhydrous AlCl\({}_{3}\) in the modification process accelerates the reaction rate, increases the reaction extent, introduces more polar oxygen-containing functional groups on the fiber surface, and enhances the oxygen content on the fiber surface. However, if the catalytic modification time is too long (1.5 h, 2 h), the excessive etching effect will exacerbate the degree of fiber surface delamination and the loss of polar oxygen-containing functional groups, leading to a decrease in the relative oxygen content. Figure 8: AFM diagram of PPTA fiber before and after catalytic modification: (**a**) PPTA; (**b**) PPTA-2 h; (**c**) PPTA-0.5 h (A). To further analyze the changes in the surface functional groups of the PPTA fiber during catalytic modification and investigate the chemical reaction mechanism occurring on the fiber surface during this process, peak fitting of the C1s peak was performed using Avantage software, and the results are shown in Figure 10. In Figure 10a, the surface functional groups of unmodified PPTA fiber mainly include C-C (284.5 eV), C-N (285.5 eV), and C=O (287.7 eV). In Figure 10b, C-O (286.3 eV) is introduced on the \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Fiber Name**} & \multicolumn{2}{c}{**Surface Element Content**} & \multirow{2}{*}{**Element Ratio**} \\ \cline{2-2} \cline{4-4}  & **C** & **N** & **O** & **O/C** \\ \hline unmodified PPTA & 79.02 & 9.21 & 11.77 & 0.149 \\ PPTA-2 h & 76.58 & 9.93 & 13.49 & 0.176 \\ PPTA-0.5 h (A) & 73.01 & 4.61 & 22.38 & 0.307 \\ PPTA-1 h (A) & 70.01 & 2.75 & 27.24 & 0.389 \\ PPTA-1.5 h (A) & 70.83 & 4.05 & 25.12 & 0.355 \\ PPTA-2 h (A) & 70.53 & 7.02 & 22.45 & 0.318 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Surface element content of PPTA fiber before and after modification. Figure 9: XPS full spectrum of PPTA fiber before and after catalytic modification: (**a**) PPTA; (**b**) PPTA-2 h; (**c**) PPTA-0.5 h (A); (**d**) PPTA-1 h (A); (**e**) PPTA-1.5 h (A); (**f**) PPTA-2 h (A). surface of PPTA-2 h fiber. From Figure 10c-f, it can be observed that the types of polar oxygen-containing functional groups introduced on the fiber surface remain unchanged after catalytic modification, but the number of polar oxygen-containing functional groups increases.