Molecular models for creep in oriented polyethylene fibers

Area of Science:

• Materials Science
• Polymer Physics
• Computational Materials Science

Background:

• Highly oriented and crystalline polyethylene (PE) fibers exhibit high failure stress under rapid loading.
• Significant creep at lower stresses limits the practical applications of PE fibers.

Purpose of the Study:

• To investigate the mechanism of creep in highly oriented PE fibers.
• To model dislocation nucleation at chain ends using molecular dynamics and a Frenkel-Kontorova model.
• To determine the activation energy and volume for dislocation nucleation and estimate the limiting yield stress.

Main Methods:

• Molecular dynamics simulations were used to parameterize a Frenkel-Kontorova model.
• The model was used to derive analytic expressions for stress-dependent dislocation nucleation.
• Results from four hydrocarbon potentials were compared to assess robustness and quantify uncertainties.

Main Results:

• The study found that activated dislocation nucleation at chain ends is a key mechanism for creep in PE fibers.
• An Eyring model described the results with a zero-stress activation energy (Ea0) of approximately 1.5 eV and an activation volume (V*) of about 45 ų.
• The limiting yield stress was estimated to be around 8 GPa, consistent across different potentials.

Conclusions:

• Activated dislocation nucleation at chain ends is an important mechanism contributing to creep in highly oriented PE fibers.
• The findings provide quantitative insights into the mechanical behavior and limitations of PE fibers under stress.
• The results suggest potential strategies for improving the creep resistance of PE materials.