Transport Diffusion of Light Gases in Polyethylene Using Atomistic Simulations

Area of Science:

• Materials Science
• Polymer Science
• Computational Chemistry

Background:

• Understanding gas transport in polymer membranes is crucial for separation technologies.
• Polyethylene (PE) is a common polymer with potential applications in gas separation.
• Temperature significantly influences polymer morphology and gas diffusion dynamics.

Purpose of the Study:

• To investigate the temperature dependence of CO2, CH4, and N2 diffusion in PE membranes.
• To analyze the impact of temperature on PE membrane morphology and gas adsorption.
• To elucidate the relationship between gas properties, polymer structure, and transport mechanisms.

Main Methods:

• Equilibrium molecular dynamics simulations were employed to study gas diffusion.
• Polymer morphology was analyzed using radial distribution functions, free volume, and pore size distribution.
• Gas adsorption isotherms were determined using a two-step methodology and fitted with a 'two-mode sorption' model.

Main Results:

• PE membranes exhibit pores of 1.5-3 Å diameter.
• Increased temperature causes polymer swelling, affecting gas solubility and diffusion.
• CO2 shows strong adsorption and high solubility, while N2 exhibits weak adsorption and low solubility; temperature reverses this trend for N2.
• Self-diffusivities align with experimental data, while transport-diffusivities are significantly higher.
• Self-diffusivity follows Arrhenius behavior, whereas transport-diffusivity exhibits non-Arrhenius behavior with varying activation energies.

Conclusions:

• Temperature plays a critical role in modulating gas transport properties in PE membranes.
• The observed reverse solubility behavior for N2 is attributed to temperature-dependent pore accessibility.
• Molecular dynamics simulations provide valuable insights into gas-polymer interactions and diffusion mechanisms.