## 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
Since the outstanding performance of the fibers as reinforcing agents cannot be fully realized, the mechanical properties of the composite material exhibit a decreasing trend within the concentration range of 0.3 to 0.45 mol/L of phthalic anhydride. Figure 14: Tensile strength and flexural strength of PPTA/DGEBA composites before and after modification. .3.4 Mechanical Properties of PPTA/DGEBA Composite Materials before and after Catalytic Modification  
The tensile strength of PPTA/DGEBA composite materials before and after catalytic modification of PPTA fiber is shown in Figure 15a. The stress-strain curves during the tensile process of PPTA/DGEBA composite materials before and after catalytic modification are shown in Figure 15b. In Figure 15a, the tensile strength of the unmodified PPTA samples is 196.64 MPa, while that of the PPTA-2 h samples is 231.91 MPa. The tensile strength of the PPTA-0.5 h (A) samples is 306.77 MPa, which is 32.28% higher than that of the PPTA-2 h samples and 56.01% higher than that of the unmodified PPTA samples. With increase in the catalytic modification time, the tensile strength of the composite materials decreases. The tensile strengths of samples modified for 1 h, 1.5 h, and 2 h are 241.40 MPa, 153.97 MPa, and 138.57 MPa, respectively. The flexural strength of the unmodified PPTA samples is 138.27 MPa, while that of the PPTA-2 h samples is 199.31 MPa. The flexural strengths of the PPTA-0.5 h (A), PPTA-1 h (A), PPTA-1.5 h (A), and PPTA-2 h (A) samples are 248.95 MPa, 232.12 MPa, 160.66 MPa, and 147.12 MPa, respectively. Among them, the flexural strength of the PPTA-0.5 h(A) samples is 24.91% higher than that of the PPTA-2 h samples. Figure 15b depicts the stress-strain curves of the PPTA/DGEBA composite materials before and after catalytic modification during tensile testing. Initially, when subjected to axial tensile loading, the composite material mainly bears the load through the DGEBA matrix. As the tensile displacement increases, both the fiber and matrix, as well as their interfacial adhesion, collectively bear the load. Due to the lower toughness of the resin matrix compared to the fiber reinforcement, matrix fracture begins to occur, corresponding to the inflection point on the curve where the slope gradually decreases. After catalytic modification, the increased presence of polar oxygen-containing functional groups on the surface of PPTA-0.5 h (A) fibers significantly enhances the adhesion between the fiber and matrix, resulting in an increase in stress at the inflection point of the stress-strain curve. However, excessively prolonged catalytic modification time (PPTA-1 h (A), PPTA-1.5 h (A), PPTA-2 h (A)) exacerbates surface erosion of the fiber, leading to potential detachment of the fiber's surface layer and a decrease in the intrinsic strength of the fiber. Consequently, the interfacial adhesion between the fiber and resin cannot be improved, resulting in a decrease in stress at the inflection point and a reduction in tensile performance of the composite material. As the tensile displacement continues to increase, DGEBA matrix failure occurs, and the tensile load is primarily borne by the fiber reinforcement until the sample is damaged, reaching its maximum tensile strength. The flexural strength of the PPTA/DGEBA composite materials before and after catalytic modification of PPTA fiber is shown in Figure 16a. The stress-strain curves during the bending process of the PPTA/DGEBA composite materials before and after catalytic modification are shown in Figure 16b. In Figure 16a, the flexural strength of the unmodified  
Figure 15: (**a**) Tensile strength of PPTA/DGEBA composites; (**b**) PPTA/DGEBA stress–strain curve during tensile process. PPTA samples is 138.27 MPa, while that of the PPTA-2 h samples is 199.31 MPa. The flexural strengths of the PPTA-0.5 h (A), PPTA-1 h (A), PPTA-1.5 h (A), and PPTA-2 h (A) samples are 248.95 MPa, 232.12 MPa, 160.66 MPa, and 147.12 MPa, respectively.