Coarse-Grained Simulations on Polyethylene Crystal Network Formation and Microstructure Analysis

Area of Science:

• Materials Science
• Computational Chemistry
• Polymer Physics

Background:

• Semi-crystalline polymers, like high-density polyethylene (HDPE), possess complex microstructures with crystalline and amorphous regions.
• Understanding these microstructures is vital for predicting and optimizing material properties for industrial applications.
• Current characterization methods require advanced simulation techniques to accurately model polymer behavior.

Purpose of the Study:

• To investigate crystal network formation in HDPE using coarse-grained molecular dynamics (CGMD) simulations.
• To analyze the resulting microstructure, including lamellar crystal stems and amorphous regions.
• To determine the mechanical properties, such as tensile strength, of the simulated semi-crystalline model.

Main Methods:

• Utilized modified Paul-Yoon-Smith (PYS/R) forcefield parameters for CGMD simulations of HDPE melt.
• Performed isothermal crystallization at 300 K and 1 atm to observe crystal growth and orientation.
• Analyzed one-dimensional density distributions to confirm lamellar-stack ordering and calculated densities for crystalline (ρcr) and amorphous (ρam) regions.

Main Results:

• Simulations predicted multi-nucleus crystal growth with alternating lamellar crystal stems and amorphous regions.
• Obtained semi-crystalline model density (ρcr ≈ 0.913 g·cm⁻³) and amorphous model density (ρam ≈ 0.856 g·cm⁻³), with a ratio (ρcr/ρam ≈ 1.06) consistent with experimental data.
• Achieved a degree of crystallinity (χc) of approximately 52% at 300 K, with observed increases indicating lamellar stem alignment.
• Stress-strain curves mimicked tensile tests, showing significant tensile strength at yield (≈100 MPa) and break (350%), with cavitation linked to lamellar stem alignment.

Conclusions:

• The PYS/R forcefield effectively models crystal network formation in HDPE, producing a reliable semi-crystalline microstructure.
• The simulated microstructure exhibits ordered crystalline and amorphous segments with robust mechanical properties.
• This simulation approach aids in predicting microstructure-mechanical property relationships in plastics under stress, valuable for material design and application.