## 1 Introduction  
Glass-fiber-reinforced thermoplastics (GFRTs) are applied in various industries owing to their excellent mechanical properties, low cost, and easy processability [1]. In particular, the reprocessability and recyclability of GFRs make them environmentally friendly [2]. Polypropylene (PP), polyamide (PA), and polybutyleneerephthalate are widely used as composite matrices. Furthermore, high-performance polymer composites, such as polyphenylene oxide (PPO), polyphenylene sulfide, and polyetherimide, have been developed and are emerging as promising materials. PPO is an amorphous polymer that exhibits good mechanical properties, thermal resistance, and dimensional stability. However, it has poor processability and a high melt viscosity because of its low chain mobility with a higher glass transition temperature (T\({}_{\text{g}}\)) of approximately 200 \({}^{\circ}\)C. To overcome this drawback, PPO is generally modified by blending with polystyrene (PS) or polyamide (PA) [3]. Because PS is a miscible polymer with PPO, phase separation does not occur in PS-modified PPO (PS-mPPO), and it exhibits good impact strength. In addition, PA-modified PPO (PA-mPPO) exhibits good mechanical properties, even though PA is an immiscible polymer with PPO. Some studies have reported that other polymers or additives contribute to the modification of PPO or to the improvement of the interfacial strength between GF and PPO. In a study of GF/PPO composites, the PPO composites were prepared by melt-blending of PPO with GF-reinforced syndiotactic PS (GFRsPS). The tensile and flexural moduli increased with increasing GFRsPS content, whereas the tensile and flexural strengths decreased [4]. The thermotropic liquid crystal polymer (TLCP) used in the GF/TLCP/PS-mPPO reduced the damage of GF during processing and moderately increased the tensile strength of the composite owing to the fiber orientation affected by TLCP in the composite [5]. [PERSON] and [PERSON] investigated PPO composites in which glass beads, GFs, and graphite fibers were used as reinforcements [6]. Their study reported that a silane coupling agent improved the tensile strength and fracture toughness of the PPO composites. [PERSON] et al. reported that maleic anhydride-grafted PPO (PPO-g-MA) was helpful in maintaining the fiber length during the melt processing, and the tensile strength of the GF/PA-mPPO composites was increased by PPO-g-MA owing to the improved interface adhesion [7]. The mechanical properties of GFRs could be improved not only by material modification but also by improving the manufacturing methods. In particular, the fiber length is believed to be an important factor for improving the mechanical properties of discontinuous-fiber-reinforced plastics [8; 9; 10]. A longer fiber length in GFRs would be obtained by direct feeding of the GF roving strand to an injection molding machine or extruder. The direct fiber feeding method has already been investigated and commercially applied to polymers, such as PP and PA, because it significantly improves their mechanical properties. In the majority of the studies, chopped GF was compounded to prepare GF/mPPO composites. To the authors' knowledge, the study reported by [PERSON] et al. was the only one that explored the direct feeding of GF roving yarns for GF/mPPO composites [11]. The effects of GF content, MA, and various processing methods on the thermal and mechanical properties of GF/mPPO composites were studied. The GF roving yarn was directly fed into a vent in the extruder to prepare long-fiber GF/mPPO pellets. However, the usefulness of the direct fiber feeding method has not been fully explained because of the difficulty in investigating the effects of various parameters. In this study, a GF/mPPO composite was prepared through injection molding using a masterbatch with a GF content of approximately 50%, which was prepared by the direct feeding extrusion of GF roving yarns.