The proposed study aims to investigate the relationship between the crystallinity and chain orientation of injection-molded poly(butylene terephthalate) (PBT) as a function of depth profile using thermal analysis, X-ray diffraction, and polarized infrared (IR) spectroscopy. The crystallinity determined by differential scanning calorimetry (DSC) decreased from the skin to core layers, whereas that determined by X-ray diffraction increased from the skin to core layers. The difference in crystallinity determined by DSC and X-ray diffraction is due to the formation of a mesophase, which is formed in the skin layer due to the presence of the shear stress. The chain orientation was observed by polarized attenuated total reflection (ATR)-IR spectroscopy. At the gate of PBT plate, the dichroic ratio at 874 cm-1 was about 0.8 and 0.5 near the skin layer, whereas it reached around 1.0 and 0.75 at the core layer for PBT formed at injection molding rates of 40 and 80 mm/s, respectively, corresponding to the increased chain orientation in the skin layer. This indicated that more mesophase was formed due to the shear stress at the skin layer.
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