## 3 Results and Discussion
### Influence of Phthalic Anhydride Solution Concentration on the Surface Properties of PPTA Fiber
#### 3.1.2 Study on Surface Element Compositions of Aramid Fiber before and after Modification  
To investigate the reaction mechanism between phthalalic anhydride and PPTA fiber, the surface elemental composition of PPTA fiber before and after modification were analyzed and compared. X-ray photoelectron spectroscopy (XPS) can determine the atomic bonding state and electron distribution state by measuring the chemical shift of a material's electronic spectrum, as well as identify atomic types and chemical states (excluding H, He) through the characteristic binding energies of the elements and the spectral lines in the XPS spectra. XPS is widely used for analyzing the surface modification of polymers. Figure 4 shows the survey scan spectra of the surface chemical composition of PPTA fiber after modification with different concentrations of phthalal anhydride solutions. Figure 4a is the XPS survey scan spectrum of unmodified PPTA fiber, while Figure 4b, Figure 4c, and Figure 4d, respectively, represent the XPS survey scan spectrum of PPTA fiber after modification with 0.15 mol/L, 0.3 mol/L, and 0.45 mol/L phthalalic anhydride solutions. In each survey scan spectrum, three strong peaks can be observed, corresponding to the presence of three elements on the surface of the PPTA fiber: carbon (C), nitrogen (N), and oxygen (O) elements. Furthermore, the relative content of the three elements can be quantitatively determined by the intensity of the peak. Comparing Figure 4a-c, with the increase in the concentration of phthalal anhydride, the C1s peak on the surface of PPTA \begin{table}
\begin{tabular}{c c} \hline
**Fiber Name** & **Fiber Modification Conditions** \\ \hline unmodified PPTA & unmodified \\ PPTA-0.15 & 0.15 mol/L phthalalic anhydride + 80 \({}^{\circ}\)C + 2 h \\ PPTA-0.30 & 0.30 mol/L phthalalic anhydride + 80 \({}^{\circ}\)C + 2 h \\ PPTA-0.15 & 0.45 mol/L phthalalic anhydride + 80 \({}^{\circ}\)C + 2 h \\ \hline \end{tabular}
\end{table} 
Table 3: Phthalic anhydride modified fiber conditions. Figure 3: SEM images of PPTA fiber surface before and after modification: (**a**) PPTA; (**b**) PPTA-0.15; (**c**) PPTA-0.3; (**d**) PPTA-0.45. fiber shows a slight decrease, while the N1s and O1s peaks significantly increase. Especially after modification with a 0.3 mol/L solution of phthalic anhydride, the enhancement of the O1s peak is notably pronounced (Figure 4c). These changes indicate that after modification with phthalic anhydride solution, the oxygen content on the surface of PPTA fiber significantly increases; when the concentration of phthalic anhydride is in the range of 0-0.3 mol/L, the oxygen content on the fiber surface notably increases with the concentration, reaching its highest value at 0.3 mol/L. In Figure 4d, when the concentration of phthalic anhydride solution reaches 0.45 mol/L, the O1s peak decreases significantly and the O content drops sharply. This change indicates that the etching of the fiber surface is dominant at this time, leading to delamination of the fiber surface layer, causing the shedding of oxygen-containing functional groups and the decrease in oxygen content. The relative percentage contents of the three elements carbon (C), nitrogen (N), and oxygen (O) on the surface of PPTA fiber are shown in Table 4. The O/C ratios on the surface of unmodified PPTA fiber and PPTA fiber modified by 0.15 mol/L, 0.3 mol/L, and 0.45 mol/L phthalic anhydride solutions are 0.149, 0.156, 0.176, and 0.148, respectively. The data indicate that the optimal modification concentration for phthalic anhydride modified PPTA fiber is 0.3 mol/L. At this concentration, the most oxygen-containing functional groups are introduced into the fiber surface, the surface oxygen content is the highest, and the surface etching is relatively light. Below this concentration, the oxygen content is not ideal, while above this concentration, severe surface etching and delamination of the fiber occur.