When Flow Creates Order: Nematic Alignment and Form Birefringence in Shear for High-Density Polyethylene Melts

Area of Science:

• Polymer physics
• Rheology
• Materials science

Background:

• High-density polyethylene (HDPE) exhibits complex behavior under shear flow.
• Understanding chain dynamics is crucial for predicting material properties.

Purpose of the Study:

• To quantify chain stretching in HDPE under shear flow.
• To investigate the relationship between chain stretching, nematic alignment, and rheological properties.
• To explore the validity of the stress-optical rule in the nematic regime.

Main Methods:

• Utilizing the exponential character of the high molecular weight tail of HDPE.
• Measuring birefringence and Raman intensity to track chain stretching.
• Monitoring apparent viscosity and primary normal stress difference.
• Employing optical techniques to confirm form birefringence.

Main Results:

• Quantified the fraction of stretched chains in shear flow, correlating with birefringence and Raman intensity.
• Observed shear rate dependence of viscosity and birefringence indicating nematic alignment at high shear rates.
• Demonstrated a failure of the linear stress-optical rule in the nematic regime due to increased birefringence and decreased normal stress.

Conclusions:

• Chain stretching in HDPE is quantifiable and linked to nematic alignment.
• Nematic alignment significantly influences rheological and optical properties.
• Form birefringence, arising from nematic alignment, explains the deviation from the linear stress-optical rule.