Small Molecules Transport Across Grafted Polymer Layers and Its Role for Contamination and Decontamination of Surfaces

The transport of small molecules across grated polymer layers presents interesting fundamental challenges and is of great practical importance. Such layers can be designed to protect surfaces against various chemical and biological contaminants. The diffusion of low molecular species towards the surface is identified as contamination, while the diffusion away from the surface corresponds to decontamination.<br/><br/>We offer a theoretical analysis of the contamination and decontamination process using classical Density Functional Theory (cDFT) in combination with a Smoluchowski type of equation describing the diffusion in an external field. The cDFT approach is based on defining a grand thermodynamic potential for an open system, which is then minimized by solving the resulting Euler-Lagrange equation. The output of the computation are the density profiles of all species (polymer segments, solvent, and solute). These profiles are used to calculate the potential of mean force (PMF) that a contaminant molecule is subjected to in a grafted polymer layer. The PMF is then introduced into the transport Smoluchowski equation as an external field, which is then solved to obtain the density profiles in time and space. Hence, we can assess both the structure and the transport dynamics in the grafted layers.<br/><br/>Grafted polymer layers in contact with solution present a rather complex system because the overall structure, and consequently the transport, depend on the sizes and specific interactions between all participants. Even the simplest example includes a substrate, polymer chains, solvent and a contaminant species. Hence, the resulting parameter field is 18-dimensional, which is presents an overwhelming explorative task from an experimental perspective. We offer reduction of the space by designing suitable variables that combine the various interaction energies, similarly to the Flory energy parameter. Next, we explore the contaminant transport in the two limiting cases of dry (solvent free) and swollen polymer layers. The analysis provides useful guidelines for the experimentalists and offer prediction for the protective quality of polymer layers that follow from the molecular interactions.