Relations Between Dynamic Localization and Solute Diffusion in Polymers.

Area of Science:

• Materials Science
• Polymer Physics
• Computational Chemistry

Background:

• Unintended leaching of additives and impurities from polymeric materials (medical devices, food packaging) poses public health risks.
• Solute diffusivity (D) is a key parameter, but experimental and simulation methods are often impractical for glassy polymers at physiological temperatures due to slow diffusion.

Purpose of the Study:

• To investigate a simulation approach for rapidly calculating solute diffusivity (D) in polymers.
• To explore the relationship between dynamic localization (Debye-Waller factor, ⟨u^2⟩) and D.
• To develop a predictive model for D in glassy polymers.

Main Methods:

• Atomistic molecular dynamics (MD) simulations were employed.
• Short, high-temperature MD simulations were used to estimate D at physiological temperatures.
• The Debye-Waller factor (⟨u^2⟩) was analyzed in relation to D.

Main Results:

• A quantitative relationship (ln D ∝ 1/⟨u^2⟩) was found to predict D for small solutes and provide upper-bound estimates for larger solutes.
• Comparison with the Piringer model highlighted the utility of upper-bound estimates.
• Rescaling the Debye-Waller factor by mode coupling temperature (Tc) improved D estimates when Tc was carefully chosen.

Conclusions:

• The proposed simulation approach offers a faster alternative for estimating solute diffusivity in polymers.
• The method provides quantitative predictions for small solutes and useful upper-bound estimates for larger ones.
• Further research is needed to predict adjustable parameters a priori for truly predictive models in glassy polymers.