Zeming Qi

Influence of the memory effect of a mesomorphic isotactic polypropylene melt on crystallization behavior

In this study, the memory effect of a polymer is investigated with a mesomorphic isotactic polypropylene (iPP) melt as the initial state. It is found that the nucleation density and crystallization rate at low temperatures are strongly dependent on the initial melting temperature and melting time, indicating a strong memory effect. High melting temperatures decrease the nucleation density, while a short melting time causes a faster crystallization rate than in the melt with its thermal history removed. At 180 °C, it takes about 1 hour for the crystallization rate to be restored to the normal value found in the melt with erasure of its thermal history at 220 °C. Further experiments indicate that the increase in spherulite growth rate is mainly responsible for acceleration of the overall crystallization kinetics. Through comparison between meso crystallization and self-seeding crystallization, it is suggested that some ordered structures with higher thermal stability exist in the mesomorphic iPP melt. The high thermal stability of the ordered structure may be due to the random arrangement of helices of different tacticity. We propose that the ordered structure accelerates spherulite growth, however, long melting times at 180 °C can break down the ordered structure, leading to the formation of an ideal melt and restoration of the spherulite growth rate. This study indicates that a thermodynamically unstable ordered structure can survive in a supercooled melt for a long time and is involved in the crystallization process.

Resonant absorption induced fast melting studied with mid-IR QCLs

We demonstrate the use of a pump-probe setup based on two mid-infrared quantum cascade lasers (QCLs) to investigate the melting and crystallization of materials through resonant absorption. A combination of pump and probe beams fulfills the two-color synchronous detection. Furthermore, narrow linewidth advances the accuracy of measurements and the character of broad tuning range of QCLs enables wide applications in various sample and multiple structures. 1-Eicosene was selected as a simple model system to verify the feasibility of this method. A pulsed QCL was tuned to the absorption peak of CH2 bending vibration at 1467 cm-1 to resonantly heat the sample. The other QCL in continuous mode was tuned to 1643 cm-1 corresponding the C=C stretching vibration to follow the fast melting dynamics. By monitoring the transmission intensity variation of pump and probe beams during pump-probe experiments, the resonant absorption induced fast melting and re-crystallization of 1-Eicosene can be studied. Results show that the thermal effect and melting behaviors strongly depend on the pump wavelength (resonant or non-resonant) and energy, as well as the pump time. The realization and detection of melting and recrystallization can be performed in tens of milliseconds, which improves the time resolution of melting process study based on general mid-infrared spectrum by orders of magnitude. The availability of resonant heating and detections based on mid-infrared QCLs is expected to enable new applications in melting study.