Crystal structure and thermal behavior of cold-crystallized poly(trimethylene terephthalate)

The structure and thermal behavior of cold-crystallized poly(trimethylene terephthalate) (PTT) are revealed in detail by DSC, AFM, TEM, and WAXD as well as in situ FTIR and SAXS techniques. There is no effect of crystallization temperature and initial state on the crystal modification, yet the morphology is strongly affected by these two factors. First, the small rod-like lamellae for PTT are obtained during the cold crystallization instead of the spherulites formed in the melt crystallization. Second, the edge-on lamellar orientation in thin films is identified during the cold crystallization. The thickness and the lateral width of rod-like lamellae get larger and larger with increasing crystallization temperature. Thin lamellar crystals assemble randomly when the cold-crystallization temperature is lower, while lamellar stacks composed of thicker lamellae are observed when the PTT was annealed at elevated temperature. Moreover, for the cold-crystallized PTT, the final melting temperature does not vary with the crystallization temperature. This phenomenon is explained by the structural improvement during the heating process. For the cold-crystallized PTT sample at lower temperature, three transitions occur when it is heated again: the relaxation of the rigid amorphous phase, the reorganization of molecules in the intermediate phase, and then the melt–recrystallization behavior. Those transitions finally lead to thicker lamellae besides a higher crystallinity before the final fusion. Therefore, the final melting peak of these lamellae is at the same temperature.