Shear-induced smectic ordering and crystallisation of isotactic polypropylene

Area of Science:

• Polymer Science and Engineering
• Materials Science
• Crystallography

Background:

• Understanding polymer crystallization under external fields is crucial for material property control.
• Isotactic polypropylene (iPP) is a widely used semicrystalline polymer whose crystallization behavior is complex.
• Previous studies have suggested mesophase formation in quenched iPP, but its role in crystallization was unclear.

Purpose of the Study:

• To investigate shear-induced smectic ordering and crystallization in isotactic polypropylene (iPP).
• To elucidate the role of shear-induced smectic structures in the subsequent crystallization process.
• To develop a model explaining the observed smectic ordering and its relationship with crystalline structures.

Main Methods:

• In-situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) were employed.
• Experiments were conducted on iPP samples of varying molecular weights and sources.
• An anisotropic drop model was developed to interpret the structural observations.

Main Results:

• Shear-induced smectic bundles with ~4 nm periodicity were observed in iPP, both above and below the melting point.
• Smectic bundles formed fibrillar morphologies and exhibited higher melting temperatures than crystalline regions.
• Crystals grew epitaxially on smectic bundles, suggesting these bundles act as 'shish' in a modified shish-kebab structure.

Conclusions:

• Shear-induced smectic bundles are primary nuclei for iPP crystallization, challenging previous models.
• The high-temperature smectic phase serves as a model for studying shear-induced ordering and coupling.
• Smectic domains represent a metastable state during the transition from smectic phase to crystal.