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Nonequilibrium Nature of Flow-Induced Nucleation in Isotactic Polypropylene
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As an extremely external driven and kinetics controlled process, flow-induced crystallization (FIC) of polymer is of vital importance not only in scientific research but also in industrial application. Over past decades, great efforts have been dedicated to understand the formations of precursor and shish-kebab, the roles of molecular and flow parameters on nucleation, and other aspects related to FIC. However, the nonequilibrium (NE) nature of FIC is relatively poorly understood because of the limitations in the existing experimental techniques, and the same holds for theoretical understanding of FIC. A team from University of Science and Technology of China (USTC) has gained insight into the NE nature of FIC in polymer material. Their research was published on January 28th, 2015 in Macromolecules.

With a combination of in-situ synchrotron radiation ultrafast X-ray scattering and extensional rheology measurements carried out at BSRF, the team revealed a constant critical strain for nucleation in isotactic polypropylene (iPP) melt in a wide temperature range from 130 to 170 °C. This discovery contradicts the straintemperature equivalence predicted by classical entropic reduction model but unveils the nonequilibrium nature of FIC. To account for the temperature independence of flow-induced nucleation, a tentative kinetic pathway of nucleation describing stretch-induced hierarchical structural transitions is proposed through which the capability of flow as driving force is justified.

FIG 1. The kinetic pathway of nucleation of polymer under strong flow: coil (melt) → stiff segment (helix) → precursor → crystal.

The research is an important step on understanding the NE nature of FIC in polymers, and underscores the potential for further improving the processing-related property in the iPP industry. In the work, the sample underwent high-speed extension with a period of only a few hundred milliseconds, where ultrafast x-ray scattering with a time-resolution of tens of milliseconds has irreplaceable advantages to extract the relevant structure information. More brilliant synchrotron beams and higher time-resolution detections would be bound to offer a shortcut for studies on NE phase transitions of materials driven by much extreme external field.


Kunpeng Cui, Dong Liu, Youxin Ji, Ningdong Huang, Zhe Ma, Zhen Wang, Fei Lv, HaoranYang, Liangbin Li*, Nonequilibrium Nature of Flow-Induced Nucleation in Isotactic Polypropylene, Macromolecules, 2015, 48, 694−699.

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