Modelling of water transport in ionic hydrophilic polymers

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Abstract
Water and ion transport in thin sheets of initially dry, ionic, hydrophilic crosslinked polymers was modelled throughout the dynamic swelling process. The water transport was expressed in terms of a non‐Fickian equation with a diffusion term containing a Fujitatype concentration‐dependent diffusion coefficient coupled with a pseudoconvective term arising from the reasonable assumption that the stress in ionic polymers is proportional to the total number of ionized pendant groups in the polymer. Ion transport was expressed in terms of generalized Fickian equations with water concentration‐dependent diffusion coefficients. These equations were solved with appropriate boundary conditions to establish the water uptake as a function of time, pH and ionic strength in a citrate‐phosphate‐borate buffer solution. A new dimensionless number, the Stress Swelling number, A, was defined to quantify the relative importance of stress in the overall swelling process. Water uptake was a strong function of A. © 1994 John Wiley & Sons, Inc.