Different from small molecules, polymer would fold into lamellar crystals during crystallization and further assemble into lamellar stacks. Synchrotron Small-Angle X-Ray Scattering (SAXS) is an important tool to characterize such nanoscale structure and understand polymer crystallization. However, its scattering mechanism in semi-crystalline polymers is not elucidated completely yet, which prevents severely using this powerful tool to obtain these parameters.

Recently, a group led by Professor TIAN Xingyou from Institute of solid state physics, Hefei institutes of physical science (HFIPS), proposed a complete set of new methods to characterize polymer lamellar crystals using SAXS.

"We’ve been working on this novel models for 4 years,” said LI Xiangyang, a scientist of physics. “It's been a long process to put the theory into practice."

It was in 2019 when they first found that the existence of evanescent wave-induced scattering in transmission SAXS was possible. Moreover, the evanescent wave-induced scattering was much stronger than the scattering induced directly by incident X-ray, and the scattering of interfacial electrons within was the main origin of SAXS signal. They started to try the approaches to obtain lamellar thickness, long period and lateral size. 

"The first breakthrough was made one year later." recalled LI. Basing on new SAXS image, they successfully isolated the evanescent wave-indued scattering from the real and reduced the influence of the scattering of bulk electrons, form factor and Porod scattering, obtaining the information on lamellar thickness and long period.

This time, another big step was made forward. Memory effect is an important phenomenon in polymer crystallization. It was assumed that some residual crystals existed in the structured melt, which lateral size is larger than critical nucleation size. However, it is hard to be demonstrated because of the difficulty in characterization. In this research, they succecssfully acquired the lateral size with the same approach. With the new methodologies, they characterized residual crystals, demonstrating previous conjecture.

This work is financially supported by the National Natural Science Foundation of China (21774133).

Link to the paper: Detection and characterization of folded-chain clusters in the structured melt of isotactic polypropyl-ene

Melting (a) and recrystallization processes (b-d) after melting at different temperatures (Image by LI Xiangyang)

Contact:

ZHAO Weiwei
Hefei Institutes of Physical Science (http://english.hf.cas.cn/)
Email: annyzhao@ipp.ac.cn