## 3 Results and Discussion
### Influence of Phthalic Anhydride Solution Concentration on the Surface Properties of PPTA Fiber
#### 3.2.4 Fiber Crystal Structures before and after Catalytic Modification  
Figure 11 depicts the XRD patterns of PPTA fiber after modification. Compared to PPTA-2 h fiber, there is no significant change observed in the diffraction peaks of PPTA-0.5 h (A) fiber and PPTA-1 h (A) fiber after catalytic modification, indicating that the aggregate structure of fibers remains unchanged and no new crystalline structure is formed after catalytic modification, which mainly occurs on the surface of the fibers. The position and range of the diffraction peaks of PPTA-1.5 h (A) fiber also show no significant change, suggesting that the crystalline structure of PPTA-1.5 h (A) fiber remains unchanged. However, the intensity of the two major diffraction peaks of PPTA-1.5 h (A) fiber noticeably  
Figure 10: C1s spectra 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). decreases, indicating that the chemical composition and structure of the fiber surface are affected. Prolonged catalytic modification leads to increased surface etching of fiber, causing the detachment of highly crystalline outer layers, thereby reducing the overall crystallinity of fiber. The broadening of diffraction peaks for PPTA-2 h (A) fiber suggests that prolonged etching continuously leads to surface peeling, resulting in a continuous decrease in fiber crystallinity, which can compromise the intrinsic strength of the fiber. Therefore, it is necessary to control the duration of catalytic modification; excessive modification time (1.5 h, 2 h) may decrease the intrinsic strength of the fiber. Influence of Modification Treatment on Properties of PPTA/DGEBA Composite Materials 3.3.1. Interfacial Properties of PPTA/DGEBA Composites before and after Anhydride Modification  
The PPTA fiber modified by phthalic anhydride solution of different concentrations was hot pressed with DGEBA to prepare the PPTA/DGEBA composite material. The interlaminar shear strength is shown in Figure 12; the interlaminar shear strength of the modified PPTA composites is higher than that of the unmodified PPTA composites. Specifically, the interlaminar shear strengths of the unmodified PPTA, PPTA-0.15, PPTA-0.30, and PPTA-0.45 composites are 10.79 MPa, 11.65 MPa, 12.51 MPa, and 12.20 MPa, respectively. The interlaminar shear strength of the PPTA-0.30 composite is 15.94% higher than that of the unmodified PPTA composite, indicating that the PPTA fiber modified with a 0.30 mol/L phthalic anhydride solution and then combined with DGEBA exhibits the highest interlaminar shear strength. The optimal treatment concentration for modifying PPTA fiber with phthalic anhydride solution is 0.30 mol/L. The interaction mechanism studied herein between the fibers and matrix is interface modification [47]. The mechanism involves introducing chemical functional groups onto the fiber surface through chemical treatment to enhance the adhesion and compatibility between the fiber and matrix [48]. As the chemical treatment progresses, it creates chemical active sites on the fiber surface, forming strong bonds with chemical groups in the matrix, thereby increasing the adhesion strength of the interface. However, with further treatment, more severe fiber etching occurs, adversely affecting the properties of the fibers themselves, resulting in a decrease in interface performance. Figure 11. XRD diagram of catalytic modified PPTA fiber. 3.2 Interfacial Properties of PPTA/DGEBA Composite Materials before and after Catalytic Modification  
The interlaminar shear strength of PPTA/DGEBA composite materials before and after catalytic modification of phthalal anhydride by anhydrous AlCl3 is shown in Figure 13. The interlaminar shear strength of the unmodified PPTA samples is 10.79 MPa. After modification, the interlaminar shear strength of the PPTA-2 h samples increases to 12.51 MPa.