## 3 Results and Discussion  
### Physical Properties  
The physical properties of the masterbatch and GF/mPPO composites are listed in Table 1. The constituent contents were calculated using the flowing equations:  
\[W_{f}=\frac{m_{f}}{m_{i}}\times 100\]  
\[V_{f}=\frac{m_{f}}{m_{i}}\times\frac{\rho_{c}}{\rho_{f}}100\]  
\[V_{m}=\frac{m_{i}-m_{f}}{m_{i}}\times\frac{\rho_{c}}{\rho_{m}}100\]  
\[V_{v}=100-\left(V_{f}+V_{m}\right)\]  
where \(W_{f}\) is the fiber weight fraction, \(V_{f}\) is the fiber volume fraction, and \(V_{m}\) is the matrix volume fraction of the specimen; \(m_{i}\) and \(m_{f}\) are the mass of the initial specimen and residual fiber, respectively; \(\rho_{c}\), \(\rho_{t}\), and \(\rho_{m}\) are the density of the specimen, fiber, and matrix, respectively. The masterbatch had a GF content of 52 wt%. The volume fractions of the fiber and void were 26.3 and 14.6 vol%, respectively. The high void fraction in the masterbatch was caused by the die swell of molten mPPO [13]. Although there were concerns about the high void fraction of the masterbatch, the void fraction in the GF/mPPO composites decreased after injection molding. The fiber length in discontinuous fiber-reinforced thermoplastics is a major factor that influences the mechanical properties. The average residual fiber length and fiber length distribution (FLD) are listed in Table 2. The average length by number (\(L_{n}\)) and average length by weight (\(L_{w}\)) were calculated using the following equations:  
\[L_{n}=\frac{\sum n_{i}L_{i}}{\sum n_{i}}\] \begin{table}
\begin{tabular}{c c c c c} \hline  & **Density** & **Fiber Weight** & **Fiber Volume** & **Void Volume** \\  & **(g/cm\({}^{3}\))** & **Fraction** & **Fraction** & **Fraction** \\  & **(wt\%)** & **(vol\%)** & **(vol\%)** & **(vol\%)** \\ \hline Masterbatch & 1.319 & 52.0 & 26.3 & 14.6 \\ GF/mPPO 30 & 1.275 & 29.1 & 14.5 & 1.8 \\ GF/mPPO 40 & 1.361 & 37.3 & 19.9 & 1.2 \\ GF/mPPO 50 & 1.495 & 48.9 & 28.6 & 0.7 \\ \hline \end{tabular}
\end{table} 
Table 1: The physical properties of the GF/mPPO masterbatch and composites. For the composite with a GF content of 30 wt%, the average fiber length was longer than the previously reported lengths of the short GF/mPPO composites [7; 14]. [PERSON] et al. [7] reported an average fiber length of 106 \(\upmu\)m in their PPO-g-MA/PA6/GF composite, and [PERSON] et al. [13] reported an average fiber length of 190 \(\upmu\)m for a short commercial GF/mPPO composite. However, [PERSON] et al. [11] reported an average fiber length of 2.28 mm for the direct GF roving yarn feeding to prepare the GF/mPPO composites. Although their processing is similar to the one employed in this study, the difference in the average fiber lengths would result from the different GF content of the pellets for the following injection molding. It is well known that fibers become damaged by shear stress and fiber-fiber interactions during extrusion and injection molding processes [15]. The higher fiber content leads to more fiber-fiber interaction; thus, the fibers become broken and shortened. The relatively long fibers of the GF/mPPO masterbatch degraded to shorter fibers in the composites because of fiber breakages during injection molding. The damaged and shortened fibers account for the decrease in \(L_{w}\) and FLD in Table 2, which is less compared to the masterbatch and with increasing fiber length. The fiber orientation distribution also influences the mechanical properties of discontinuous-fiber-reinforced thermoplastics [16]. The fibers in the injection-molded composites are oriented along the flow direction. Figure 2 shows the cross-section images of the tensile specimens of the GF/mPPO composites. The GFs in the GF/mPPO 30 were oriented along the specimen length direction. The core region (red dash ellipse in Figure 2), where the GFs were oriented perpendicularly to the specimen length direction, can be observed in the GF/mPPO 40. Moreover, the area of the core region in the GF/mPPO 50 was increased.