Molecular Dynamics Simulation of Solvent-Polymer Interdiffusion. I. Fickian diffusion

TL;DR

This study uses molecular dynamics and Monte Carlo simulations to analyze solvent-polymer interdiffusion, confirming Fickian behavior with constant diffusivity and providing detailed concentration-dependent diffusion insights.

Contribution

It demonstrates that solvent-polymer interdiffusion follows Fickian laws with a constant diffusivity, validated through large-scale simulations and theoretical calculations.

Findings

Weight gain scales as t^{1/2} consistent with Fickian diffusion.

Concentration profiles fit Fick's second law well.

Diffusivity remains constant across relevant concentration ranges.

Abstract

The interdiffusion of a solvent into a polymer melt has been studied using large scale molecular dynamics and Monte Carlo simulation techniques. The solvent concentration profile and weight gain by the polymer have been measured as a function of time. The weight gain is found to scale as t^{1/2}, which is expected for Fickian type of diffusion. The concentration profiles are fit very well assuming Fick's second law with a constant diffusivity. The diffusivity found from fitting Fick's second law is found to be independent of time and equal to the self diffusion constant in the dilute solvent limit. We separately calculated the diffusivity as a function of concentration using the Darken equation and found that the diffusivity is essentially constant for the concentration range relevant for interdiffusion.