Molecular dynamics simulation of three plastic additives' diffusion in polyethylene terephthalate

Area of Science:

• Polymer Science
• Materials Science
• Computational Chemistry

Background:

• Accurate diffusion coefficient data for polymer additives is crucial for predicting migration.
• Understanding additive diffusion in polymers like PET is essential for material safety and performance.

Purpose of the Study:

• To estimate the diffusion coefficient of three plastic additives (UV-P, BHT, DEHP) in PET using molecular dynamics (MD) simulations.
• To compare MD-simulated diffusion coefficients with experimental data and the Piringer model.
• To elucidate the microscopic diffusion mechanism of additives in PET.

Main Methods:

• Molecular dynamics (MD) simulations were conducted for UV-P, BHT, and DEHP in PET at temperatures ranging from 293 K to 433 K.
• Diffusion coefficients were calculated using the Einstein relationship based on mean-square displacement data.
• Analysis of additive movement trajectories within PET cell models.

Main Results:

• MD-simulated diffusion coefficients for the additives in PET generally agreed with experimental values within one order of magnitude.
• The study identified four key factors influencing additive diffusion: interaction energy, fractional free volume, molecular size/shape, and polymer self-diffusion.
• Additive movement in PET was characterized by slow oscillations and occasional hops into free-volume holes.

Conclusions:

• MD simulation is a valuable tool for predicting the microstructure and diffusion coefficients of plastic additives in polymers.
• The findings support the use of MD simulations to estimate the migration levels of additives from PET packaging.
• A detailed understanding of the microscopic diffusion mechanism was achieved, aiding in material design and safety assessments.