Time-resolved Fourier transform infrared spectroscopy was used to investigate the structural differences between cold- and melt-crystallized poly(trimethylene terephthalate) (PTT) samples. To elucidate the arrangement of different chain segments in the crystalline phase obtained through different crystallization processes, characteristic bands associated with the phenylene rings, the CH<sub>2</sub> segments, and the C-O-C segments were followed during the cold and the melt crystallizations. The results show that the alignment of the phenylene rings and the C=O groups in the PTT crystals produced via cold- and melt-crystallization processes are essentially the same, while the alignment of the CH<sub>2</sub> segments depends on the crystallization condition. It was found that the CH<sub>2</sub> segments in the melt-crystallized sample were more stable than those in the cold-crystallized sample. This was ascribed to the different crystallization environment. Melt-crystallized PTT chains have higher mobility, and the variation of flexible segments is quicker than that of the rigid segments. However, in cold crystallization, the CH<sub>2</sub> segments are restricted by the adjacent environment in the glass state, which weakens their capability of transforming their conformation; this leads to a cooperative change of CH<sub>2</sub> segments, phenylene rings, and C=O groups. Therefore, the stability of CH<sub>2</sub> segments in crystalline structure is lowered accordingly.

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