## 3 Results and Discussion  
### Structural Characterization of the Composites  
The cross-sectional SEM images of FGi/PTFE composites, which underwent brittle fracture in liquid nitrogen, are shown in Figure 2a-d. The liquid nitrogen cooling freezes the molecular chain movement during the fracture process and preserves the original orientation of the polymer matrix, which can be observed through the crack extension paths in SEM images [26; 27; 28]. The cracks in Figure 2a are randomly arranged, indicating the random distribution of molecular chains in pure PTFE. After the introduction of FGi, the cracks in Figure 2b-d gradually exhibit parallel alignment, attributed to the in-plane parallel arrangement of lamellar FGi fillers during rolling, which induces interfacial molecular orientation along the same direction and leads to ordered molecular chain alignment. Additionally, the roughness of the cross-section can reflect the fracture behavior of PTFE with different crystallinities, due to the fact that the fracture of PTFE occurs at the interfaces between crystalline and amorphous regions [27]. As shown in Figure 2a, the fracture surface of pure PTFE has large granularity and crystalline domain sizes. In contrast, as shown in Figure 2b-d, the size of the concave and convex regions on the cross-section of the composite decreases with FGi addition, indicating that the addition of the filler reduces the crystallinity and then the crystalline domain size of the matrix. By comparing Figure 2b-d, it can be found that as the filler content increases, the concavity and convexity undulation of the cross-section increases (the contrast in the SEM images becomes more significant). This suggests that with the increase in filler content, the entanglement between PTFE molecules intensifies, and the deformation of the matrix during fracture increases [26; 28]. The orientation and steric hindrance effect of FGi enable the in-plane directional growth of PTFE molecules and reduce the crystallinity of PTFE. In addition, as shown in Figure 2b-d, due to the similar elemental composition and C-F bond, the FGi particles in the composite are tightly connected to PTFE [29]. It also can be found in Figure 2d that, at a high filler content, the filler particles in the PTFE-0.75 agglomerate, resulting in the formation of defects. Figure 2e-h show the WAXS results used to evaluate the effect of FGi on the molecular chain orientation during the crystallization of PTFE. As shown in Figure 2e, the scattering ring of PTFE is a complete circle, indicating that the molecular chains in pure PTFE are randomly arranged in the matrix. In Figure 2f-h, the scattering rings are replaced by arcs, and the intensity increases with the increase in FGi content, which reflects the filler-induced orientation of PTFE molecules [30; 31]. To quantitatively confirm the orientation of PTFE, Herman's orientation function is used to calculate the orientation factor \(f\) through azimuthal scans in Figure S2. \[f=\frac{3<cos^{2}\theta>-1}{2} \tag{1}\]  
\[<cos^{2}\theta>=\frac{\int_{0}^{\pi/2}\mathrm{I}(\theta)cos^{2}\theta sin \theta\mathrm{d}\theta}{\int_{0}^{\pi/2}\mathrm{I}(\theta)sin\theta\mathrm{d}\theta} \tag{2}\]  
where the \(<cos^{2}\theta>\) is the average value of the squared azimuthal cosine for the (100) peaks of tested samples, and \(\mathrm{I}(\theta)\) indicates signal intensity at an azimuthal angle of \(\theta\). The results show that with the increase in FGi content, the Herman orientation factor \(f\) of the samples increases from 0 for PTFE-0 to 0.60 for PTFE-0.75. The increased Herman orientation factor \(f\) indicates that the (100) crystal plane and corresponding molecular chains of PTFE are partly ordered along the in-plane direction, which is consistent with the parallelization of cracks in the SEM images. To investigate the effect of FGi on the crystallization behavior of PTFE, XRD and DSC tests are conducted on FGi/PTFE composites with various FGi contents.