Reverse-selective diffusion in nanocomposite membranes

TL;DR

This paper presents a theoretical model explaining how nanoinclusions in polymer membranes can increase permeability through a polymer-segment depletion layer, leading to reverse selectivity and improved membrane performance.

Contribution

It introduces a simple classical diffusion model incorporating a depletion layer to explain permeability enhancement in nanocomposite membranes, linking microstructure to permeability.

Findings

Permeability increases with nanoinclusion volume fraction.

Depletion layer causes local diffusivity to rise, enhancing permeability.

Model aligns with experimental observations of size and volume effects.

Abstract

The permeability of certain polymer membranes with impenetrable nanoinclusions increases with the particle volume fraction (Merkel et al., Science, 296, 2002). This intriguing observation contradicts even qualitative expectations based on Maxwell's classical theory of conduction/diffusion in composites with homogeneous phases. This letter presents a simple theoretical interpretation based on classical models of diffusion and polymer physics. An essential feature of the theory is a polymer-segment depletion layer at the inclusion-polymer interface. The accompanying increase in free volume leads to a significant increase in the local penetrant diffusivity, which, in turn, increases the bulk permeability while exhibiting reverse selectivity. This model captures the observed dependence of the bulk permeability on the inclusion size and volume fraction, providing a straightforward connection between membrane microstructure and performance.