Double melting behavior of poly(trimethylene terephthalate) (PTT) was studied in detail by means of differential scanning calorimetry (DSC) and optical microscopy. The results indicate that the low-temperature melting peak of PTT at ca. 218℃ for the samples crystallized isothermally at 203℃ is associated with the melting of crystals produced by secondary crystallization, while the high-temperature melting peak of it at about 227℃ is related to the melting of the crystals produced by primary crystallization. The results further demonstrate that the PTT crystals growing non-isothermally during cooling process are thermodynamically unstable and can undergo structure reorganization during the DSC heating scan. The reorganized crystals melt at temperature higher than the crystals produced by secondary crystallization at 203 ℃. Consequently, for the non-fully crystallized samples, the crystals grown during cooling also exhibit contribution to the high-temperature melting peak.
Time-resolved FTIR,WAXD/SAXS and DSC have been used to investigate the structural variation of non-isothermally crystallized poly(trimethylene terephthalate)(PTT) during the heating process.The three-phase model:the lamellar phase,the mobile amorphous phase(MAP) and the rigid amorphous phase(RAP) between lamellae is suggested to describe the structure of meltcrystallized PTT.According to FTIR results,the conformation of RAP in the constrained state is different from that of MAP.The increased content of amorphous phase in the temperature range from 120 to 192℃ is ascribed to the relaxation of RAP,rather than the melting of defective crystals.When the PTT is heated to a temperature above 192℃,the recrystallization/crystal perfection of original defective lamellae occurs without a pre-melting process,which leads to an increase in lamellar thickness and probably connects two adjacent lamellar stacks.This is responsible for an increase in crystallinity as well as a higher major melting temperature.
