Stretching Temperature Dependency of Fibrillation Process in Isotactic Polypropylene

Area of Science:

• Polymer Science
• Materials Science
• Crystallography

Background:

• Isotactic polypropylene (IPP) is a semicrystalline polymer with a spherulitic morphology.
• Fibrillation is a key process in the mechanical deformation of semicrystalline polymers, transforming isotropic structures into oriented ones.
• Understanding fibrillation is crucial for tailoring polymer properties through controlled processing.

Purpose of the Study:

• To investigate the fibrillation behavior of isotactic polypropylene during tensile deformation at elevated temperatures.
• To elucidate the role of annealing temperature and deformation conditions on the fibrillation process.
• To map the resulting morphologies (mesophase, α crystallites) as a function of processing parameters.

Main Methods:

• In situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques were employed.
• Tensile deformation was performed at elevated temperatures on polypropylene samples annealed at different temperatures.
• Analysis focused on structural transitions from spherulitic to fibrillar morphology.

Main Results:

• Fibrillation occurs via a stress-induced melting and recrystallization mechanism.
• Three distinct fibrillation regions were identified based on the formation of mesophase and/or α crystallites.
• Lower annealing temperatures and higher deformation temperatures favored the formation of the thermodynamically stable α crystallites.

Conclusions:

• Molecular mobility, influenced by annealing and deformation temperatures, dictates the fibrillation pathway.
• Optimized processing conditions (lower annealing, higher deformation temperature) promote the formation of the desired α crystalline modification.
• Limited chain mobility at lower deformation temperatures results in a significant amount of oriented mesophase structure.