Kinetics of one-dimensional gel swelling and collapse for large volume change

Abstract

The kinetics of one-dimensional gel swelling and collapse for large volume changes were described by a Fickian model which accounts for the movement of the gel surface. For a constant mutual diffusion coefficient, D m , the fractional approach to equilibrium, F, is a function only of dimensionless time, τ 0, and the equilibrium volume ratio, Φ. Gel collapse is faster than swelling when D m is the same for both. Swelling curves, the variation of F with ✓τ 0 , were computed for planar, cylindrical and spherical geometries with constant D m . For slabs the swelling curves are initially linear for all Φ values, while for cylinders and spheres the swelling curves are linear for small Φ values, but sigmoidal for Φ ≥ 2.5. For 0.5 ≤ Φ ≤ 2, a simple method gives experimental values of D m which account for the movement of the gel boundary. Experimental data for weakly ionic poly( N-isopropylacrylamide) gel spheres in water ( Φ = 55) and for non-ionic poly( N-isopropylacrylamide) gel disks in water ( Φ = 7.7) were well fitted by the model.