Rigidity Percolation Dictates Rheological Hysteresis Regime in Polypropylene during Crystallization and Melting

Area of Science:

• Polymer Science
• Materials Science
• Rheology

Background:

• Structure-property relationships in polymer crystallization and melting are poorly understood.
• Simultaneous measurement of crystallinity and rheological properties presents challenges.
• Existing rheological models do not account for crystallization-melting asymmetry.

Purpose of the Study:

• To directly measure rheological behavior as a function of crystallinity during polymer processing.
• To investigate the asymmetry between polypropylene crystallization and melting.
• To develop a model explaining the observed hysteresis.

Main Methods:

• Simultaneous rheology and Raman spectroscopy were employed.
• Polypropylene's rheological behavior was measured as a function of crystallinity.
• A generalized effective medium (GEM) model was utilized to analyze data.

Main Results:

• Polypropylene's rheological behavior differs significantly between crystallization and melting at identical crystallinity.
• Hysteresis onset aligns with the percolation threshold calculated by the GEM model.
• Two distinct hysteresis regimes were identified: pre-percolation and post-percolation.

Conclusions:

• The study quantifies hysteresis and identifies distinct rheological regimes based on structural differences.
• The findings provide a deeper understanding of polymer processing and structure-property relationships.
• The results challenge existing rheological models by highlighting the asymmetry in polymer phase transitions.