## 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
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. The flexural performance of the PPTA/DGEBA composite material is significantly improved after catalytic modification. In Figure 16b, it can be observed that the peaks of the stress-strain curves for all specimens exhibit an arc shape, and the increase in load after reaching the peak gradually decreases with increasing displacement. During the bending failure process, the failure modes of the PPTA/DGEBA composite material include compression failure in the upper layer, shear failure in the middle layer, tensile failure in the lower layer, and delamination. The compression strain of the PPTA/DGEBA composite material is four times that of the tensile strain, but its compression strength is only one-fourth of the tensile strength. Therefore, in bending tests, the PPTA fiber on the compression side will exhibit a \"yielding\" phenomenon at low strains under load, manifested as a turning point appearing before the peak on the stress-strain curve. At this point, the entire PPTA/DGEBA specimen still has a significant load-carrying capacity, eventually forming an arc-shaped peak in the stress-strain curve. A slight fluctuation can be observed on the stress-strain curve of the unmodified PPTA specimen, which is due to the occurrence of fracture in the DGEBA matrix at that time. Through data comparison, it was found that the order of bending stress magnitude for the specimens is as follows: PPTA-0.5 h (A) > PPTA-1 h (A) > PPTA-2 h > PPTA-1.5 h (A) > PPTA-2 h (A) > PPTA. Combining the analysis of the microstructure and element composition of the fiber surfaces before and after modification, the catalytic action of anhydrous AlCl\({}_{3}\) increases the number of polar functional groups on the fiber surface and increases surface roughness. This enhancement strengthens the chemical bonding and mechanical interlocking between the fiber and the DGEBA matrix, thereby improving the interfacial adhesion and tensile strength of the PPTA/DGEBA composite material. However, prolonged modification can lead to fiber damage and surface layer detachment, resulting in a decrease in intrinsic strength. Consequently, the excellent properties of the fibers as reinforcing agents are not fully utilized, thereby negatively affecting the tensile performance of the composite material. Figure 17 depicts the side-view micrographs of the PPTA/DGEBA composite materials after bending failure at different catalytic modification times. In Figure 17a,b, at the interface delamination after bending failure, both the PPTA-0.5 h (A) fibers and the PPTA-1 h (A) fibers are encapsulated by resin, with multiple fibers bonded together through resin, collectively sharing the bending load. Additionally, good interfacial adhesion effectively transfers stress, significantly enhancing the bending strength of the composite material. Figure 16: (**a**) Flexural strength of PPTA/DGEBA composites; (**b**) PPTA/DGEBA stress–strain curve during flexural process. However, in Figure 17c, fiber fracture is evident due to prolonged catalytic modification of 1.5 h, causing surface delamination and detachment of fibers, resulting in structural damage to the fibers and leading to fracture failure under load, consequently reducing the bending strength of the composite material. In Figure 17d, fiber fracture is similarly observed, with less resin attached to the fiber surface, indicating exacerbated surface etching of fibers due to prolonged 2 h catalytic modification, resulting in severe fiber damage and another decrease in the bending strength of the composite material.