Evidence for pressure-induced diffusion of solvent in dense polymer membranes

Knowledge of the nature of solvent transport through polymers is crucial for developing future membranes for novel separations. It has been debated whether pressure-driven transport of solvent through dense polymer membranes occurs via convective flow through nano-scale pores or diffusive flow induced by the mechanical conditions imposed on the polymer. A key distinction between these two models is the role of membrane pressure. In the diffusive model, membrane pressure is approximately constant while a concentration gradient of penetrant exists in the film. In the pore explanation, solvent flow is driven by pressure drop. Historical evidence has favored diffusion models through the direct observation of pressure-induced concentration gradients of water and organic solvents in polymer films. Recently, the diffusion-based explanation for membrane transport has been challenged. However, our work presents evidence for pressure-induced diffusion in polymers, including cellulose acetate, xl-poly(ethylene glycol diacrylate), xl-poly(styrene sodium sulfonate), and Nafion 117. By observing the pressure-induced concentration gradient across a stack of polymer films, we evidence to support the constant pressure assumption. Additionally, we verify the proposed relationship between pressure, water activity, and diffusivity in polymers by measuring water flux at pressures up to 250 bar. Finally, we discuss the validity of the constant pressure assumption in the context of contact mechanics.