Two-molecule theory of polyethylene liquids

Area of Science:

• Computational chemistry
• Polymer physics
• Statistical mechanics

Background:

• Microscopic theories for radial distribution functions in molecular liquids are crucial for understanding material properties.
• The Reference Interaction Site Model (RISM) equation is a key theoretical framework, but requires accurate closures.
• Existing closures, like Percus-Yevick (PY), have limitations in predicting polymer liquid structures.

Purpose of the Study:

• To evaluate a specific two-molecule theory as a closure for the RISM equation.
• To compute equilibrium structural and thermodynamic properties of polyethylene liquids.
• To compare the accuracy of the two-molecule theory against molecular dynamics (MD) and polymer-RISM (PRISM) theories.

Main Methods:

• Applied the two-molecule theory to polyethylene liquids at melt densities.
• Utilized Monte Carlo simulation with an improved direct sampling algorithm to compute the radial distribution function and intramolecular structure function.
• Studied polyethylene chains of 24 and 66 united atom CH2 units.

Main Results:

• The two-molecule theory yields results comparable to full many-chain molecular dynamics (MD) simulations.
• It demonstrates superior accuracy in predicting liquid structure at both short and long ranges compared to PRISM-PY theory.
• The theory successfully models equilibrium structural and thermodynamic properties of polyethylene liquids.

Conclusions:

• The two-molecule theory provides a formally derived and accurate closure for the RISM equation.
• It overcomes limitations of previous polymer-RISM theories, offering improved predictions for polymer liquid structure.
• This method is a valuable tool for understanding and predicting the behavior of molecular liquids.