Swelling of polymer gels

The swelling equilibrium and kinetics of weakly crosslinked, non-ionic poly(N-isopropylacrylamide) gels were studied in water at a constant temperature of 298 K. The gel structure was varied by changing the monomer concentration, the proportion of crosslinker, and the temperature at gel formation. The relationship between the fractional approach to equilibrium (F) and the square root of time ($ surd t$) was sigmoidal. A model was developed to describe the kinetics of gel volume change for swelling or collapse. The model was based on Fick's first law and accounted for the movement of the boundary of the gel. Numerical computations for one-dimensional cases involving planar, cylindrical and spherical geometries were performed with a constant diffusion coefficient. Experimental data for disk-shaped and spherical poly(NIPA) gels in water and for spheres of rubber in toluene were well fitted by the model using a constant mutual diffusion coefficient. Composition dependent mutual diffusion coefficients including those suggested by the scaling theory, also gave good agreement with experiments. A new categorization of diffusion in polymers based on the F versus $ surd t$ plot is proposed.