An experimental study of the swelling properties of spherical hydro-gel immersed in water and the formulation of a simple theoretical model for its explanation

The mechanism of variation of mass and volume, through the absorption of water, by the most commonly used and abundantly available spherical hydro-gel beads of various colors has been studied. Immersing the beads in distilled water, both mass and volume were measured at regular intervals. By an analysis of mass-versus-time data, using standard theoretical methods, the mechanism of diffusion of water has been studied. Relaxation controlled transport was found to be the predominant process of diffusion of water into the materials under study. The diffusion coefficient has been determined for hydro-gels of different colors. Various swelling related parameters such as equilibrium water content, equilibrium swelling ratio, characteristic time, initial rate of water absorption etc. were determined for these hydro-gels. The maximum attainable mass and the maximum number of water molecules that can be attached to active polymer sites in a single bead have been calculated. In this regard, the number of such active sites per unit volume and mass of dry hydro-gel was determined. A theoretical model has been developed, based on the consideration of water absorption through the occupation of vacant polymer sites in a highly porous hydro-gel. Considerations of cylindrical and spherical pores in this model predicted almost the same swelling behavior. Expressions relating mass and volume with time have been formulated and they agreed well with experimental observations. The dependence of characteristic time () upon structural parameters have been established. Values of swelling related parameters have been determined by a numerical fitting of theoretical model to experimental data. The agreement of experimental findings with theoretical predictions has been depicted graphically