## 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
For the catalytically modified PPTA-0.5 h(A), PPTA-1 h(A), PPTA-1.5 h (A), and PPTA-2 h (A) samples, the interlaminar shear strengths are 16.15 MPa, 14.15 MPa, 13.71 MPa, and 13.03 MPa, respectively. Among them, the interlaminar shear strength of the PPTA-0.5 h (A) samples is increased by 49.68% compared to the unmodified PPTA samples and by 29.10% compared to the PPTA-2 h samples. Combining the previous analysis of the microstructure and elemental composition of the PPTA fiber surfaces before and after modification, it is evident that the addition of the catalyst anhydrous AlCl3 during modification introduces more polar oxygen-containing functional groups on the fiber surface. These polar functional groups form chemical bonds with the DGEBA matrix, thereby improving interfacial adhesion and bond strength. Additionally, the formation of \"protrusions,\" small grooves, and stripes on the fiber surface  
Figure 12: Interlaminar shear strength of PPTA/DGEBA composites before and after anhydride modification. Figure 13: Interlaminar shear strengths of PPTA/DGEBA composites before and after catalytic modification. enhances physical anchoring, strengthening the mechanical interlocking between the fiber and the matrix. Moreover, these \"protrusions,\" grooves, and stripes increase the specific surface area of fibers, thereby increasing the free energy of the fiber surface. This facilitates better spreading of the matrix on the fiber surface, improving wettability, and enhancing interfacial adhesion. Among these, the interlaminar shear strength of the PPTA-0.5 h (A) sample shows the most significant improvement, indicating optimal modification effects on its fibers and optimal interfacial bonding performance of the composite material. 3.3 Mechanical Properties of PPTA/DGEBA Composite Materials before and after Maleic Anhydride Modification  
The tensile strength and flexural strength of the PPTA/DGEBA composite materials after modification with different concentrations of phthalic anhydride are shown in Figure 14. The tensile strength of the unmodified PPTA specimens is 196.64 MPa. For the PPTA-0.15 specimens, the tensile strength is 208.91 MPa, while for the PPTA-0.3 specimens, it is 231.91 MPa, representing an increase of 17.94% compared to the unmodified PPTA specimens. The tensile strength of the PPTA-0.45 specimens is 177.55 MPa. The flexural strength of the modified composite materials is higher than that of the unmodified composite materials. The flexural strength of the unmodified PPTA specimens is 138.27 MPa, while for the PPTA-0.15 specimens, it is 154.01 MPa. The PPTA-0.3 specimens exhibit a flexural strength of 199.36 MPa, representing a 44.18% increase compared to the unmodified PPTA specimens. The flexural strength of the PPTA-0.45 specimens is 145.44 MPa. With an increase in the concentration of phthalic anhydride, the mechanical properties of the PPTA/DGEBA composite materials exhibit a trend of initially increasing and then decreasing. When the concentration of phthalic anhydride is 0.3 mol/L, the mechanical properties of the PPTA/DGEBA composite materials reach their optimum. This is attributed to the modification of PPTA fibers by phthalic anhydride, which can increase the surface roughness and polarity functional groups of the fibers, thereby enhancing the mechanical interlocking and chemical bonding between the fibers and the resin matrix. Consequently, within the concentration range of 0 to 0.3 mol/L of phthalic anhydride, the mechanical properties of the PPTA/DGEBA composite materials are strengthened with increasing concentration of phthalic anhydride. However, when the concentration of phthalic anhydride exceeds 0.3 mol/L, the surface etching of the fibers intensifies, and the fiber coating begins to peel off, resulting in a decrease in the intrinsic strength of the fibers.