## 3 Results and Discussion
### Mechanical Properties  
The results of the mechanical properties of the GF/mPPO composites show a dependence on the GF content, as presented in Figures 3-5. The GF/mPPO composites exhibit enhanced tensile and flexural properties compared to those of the neat mPPO. The tensile strength and modulus increased with increasing GF content. The tensile strength and tensile modulus of the 50 wt% GF composites increased by 186% and 500%, respectively. Moreover, the flexural strength and flexural modulus increased by 130% and 452%, respectively. However, the GF negatively influenced the impact strength, and the impact strength decreased by 58% for the 50 wt% GF composites. Figure 4: Effect of GF content on flexural strength and modulus of GF/mPPO composites. Figure 3: Effect of GF content on tensile strength and modulus of GF/mPPO composites. The advantage of the direct fiber feeding method is more pronounced in the manufacturing of composites with higher GF content than in that of composites with lower GF content with a comparison to the chopped GF composites prepared by compression molding and injection molding after the compounding process. The GF/PPO composite prepared by compression molding with a 20 vol% of the chopped GF has approximately 100 MPa of the tensile strength [3]. The tensile strength of the PPO-g-MA/PA6/GF composite prepared by injection molding with 30 wt% of GF content is 97.1 MPa [6]. The tensile strength of the PPO/high-impact polystyrene (HIPS)/GF composite with 30 wt% of GF content, prepared by injection molding, exhibits 132 MPa [7]. The tensile strengths decrease in the GF/PPO composite and in the PPO/HIPS/GF composite when the fiber content further increases above 20 vol% and 23.1 wt% in the respective studies, whereas, in this study, the tensile strength continuously increased up to 147.9 MPa as the fiber content increased up to 50 wt% of GF in the composite. This phenomenon may be explained by the decreased void fraction and the increased number of GFs oriented to the tensile direction. Therefore, it is demonstrated that the direct fiber feeding method may be more effective than the other methods for better tensile properties of the composites with a high GF content. For discontinuous short-fiber-reinforced composites, the reinforcing effects of the GF content can be estimated using the fiber efficiency factor, which can be expressed by the modified rule of mixtures equation [16; 18]. The fiber efficiency factors for the tensile strength and modulus were calculated using the following equations:  
\[\lambda_{\sigma}=\frac{\sigma_{c}-\sigma_{m}\Big{(}1-V_{f}\Big{)}}{\sigma_{f }V_{f}}\]  
\[\lambda_{E}=\frac{E_{c}-E_{m}\Big{(}1-V_{f}\Big{)}}{E_{f}V_{f}}\]  
where \(\lambda_{\sigma}\) is the fiber efficiency factor for the tensile strength of the composite; \(\sigma_{c}\), \(\sigma_{m}\), and \(\sigma_{f}\) are the tensile strengths of the composites, matrices, and fibers, respectively; \(\lambda_{E}\) is the fiber efficiency factor of the tensile modulus of the composite; and \(E_{c}\), \(E_{m}\), and \(E_{f}\) are the tensile moduli of the composites, matrices, and fibers, respectively. The calculation results are listed in Table 3. Although the GF/mPPO 50 exhibited the maximum tensile strength, its \(\lambda_{\sigma}\) decreased. This result is attributed to the fiber orientation distribution in the GF/mPPO  
Figure 5: Effect of GF content on impact strength of GF/mPPO composites. composites. Figure 6 shows the fracture morphology of the GF/mPPO composites after the tensile test, indicating the matrix fracture, GF pullout and fracture, and debonding between the GF and the mPPO. At 50 wt% of GF in the GF/mPPO composite, as shown in Figure 2, the increased core area results in decreased \(\lambda_{\sigma}\), although the tensile strength was enhanced. In contrast, \(\lambda_{E}\) increases with increasing GF content, and \(\lambda_{E}\) is higher than \(\lambda_{\sigma}\). This result implies that the tensile modulus was more affected by the fiber content than by the fiber length and orientation.