Crystallization and melting behaviors of poly(trimethylene terephthalate)

What Thermal Analysis Can Tell Us About Melting of Semicrystalline Polymers: Exploring the General Validity of the Technique

Thermal characterization of semicrystalline polymers can constitute a difficult task due to the metastable nature of polymer crystals. It is well documented that polymer structure can reorganize during the thermoanalytical experiment. It has become also clear that thermal analysis alone cannot discriminate the reorganization processes from multiple melting events. Therefore, instead of studying the initial sample state the measurements may simply reflect the structural evolution uncontrollably occurring during the experiment. Here an original setup combining in situ ultrafast chip calorimetry with millisecond time-resolved X-ray scattering is used to find the structural signature of the reorganization processes. The information is further used to construct the heating-rate versus crystallization-temperature reorganization (HR-CT-R) diagram. The diagram allows rationally designing thermoanalytical experiments in which one can completely exclude uncontrolled evolution of the semicrystalline structure. For a typical aromatic polyester, poly(trimethylene terephthalate), the critical heating rate above which all reorganization processes cease to exist can reach 1000 K/s and more.

Structural differences between cold- and melt-crystallized poly(trimethylene terephthalate) samples

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 CH2 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 CH2 segments depends on the crystallization condition. It was found that the CH2 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 CH2 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 CH2 segments, phenylene rings, and C=O groups. Therefore, the stability of CH2 segments in crystalline structure is lowered accordingly.

Study on Morphology, Rheology, and Mechanical Properties of Poly(trimethylene terephthalate)/CaCO3Nanocomposites

For preparing good performance polymer materials, poly(trimethylene terephthalate)/CaCO3nanocomposites were prepared and their morphology, rheological behavior, mechanical properties, heat distortion, and crystallization behaviors were investigated by transmission electron microscopy, capillary rheometer, universal testing machine, impact tester, heat distortion temperature tester, and differential scanning calorimetry (DSC), respectively. The results suggest that the nano-CaCO3particles are dispersed uniformly in the polymer matrix. PTT/CaCO3nanocomposites are pseudoplastic fluids, and the CaCO3nanoparticles serve as a lubricant by decreasing the apparent viscosity of the nanocomposites; however, both the apparent viscosity and the pseudoplasticity of the nanocomposites increase with increasing CaCO3contents. The nanoparticles also have nucleation effects on PTT’s crystallization by increasing the crystallization rate and temperature; however, excessive nanoparticles will depress this effect because of the agglomeration of the particles. The mechanical properties suggest that the CaCO3nanoparticles have good effects on improving the impact strength and tensile strength with proper content of fillers. The nanofillers can greatly increase the heat distortion property of the nanocomposites.