KINETICS OF ONE-DIMENSIONAL GEL SWELLING AND COLLAPSE FOR LARGE-VOLUME CHANGE

The kinetics of one-dimensional gel swelling and collapse for large vo lume changes were described by a Fickian model which accounts for the movement of the gel surface. For a constant mutual diffusion coefficie nt, D-m, the fractional approach to equilibrium, F, is a function only of dimensionless time, tau(0), and the equilibrium volume ratio, Phi. Gel collapse is faster than swelling when D-m is the same for both. S welling curves, the variation of F with root tau(0), were computed for planar, cylindrical and spherical geometries with constant D-m. For s labs the swelling curves are initially linear for all Phi values, whil e for cylinders and spheres the swelling curves are linear for small P hi values, but sigmoidal for Phi greater than or equal to 2.5. For 0.5 less than or equal to Phi less than or equal to 2, a simple method gi ves experimental values of D-m which account for the movement of the g el boundary. Experimental data for weakly ionic poly(N-isopropylacryla mide) gel spheres in water (Phi = 15) and for non-ionic poly(N-isoprop ylacrylamide) gel disks in water (Phi = 7.7) were well fitted by the m odel. Copyright (C) 1996 Elsevier Science Ltd