A model for the swelling of superabsorbent polymers

When dry superabsorbent polymers are immersed in water they swell by two or three orders of magnitude. Both the ultimate degree of absorption and rate of absorption are important in practical applications. Experimentally they depend on the type of polymerisation, e.g. suspension or solution; the monomer composition, e.g. proportions of acrylic acid, sodium acrylate and acrylamide; and on the type of cross-linker, e.g. water or oil-soluble. In this paper relationships between the various parameters are investigated in order to improve understanding and to identify the basic limiting factors. Plots of swelling against time have been found to fit mathematical relationships which are based on spring and dashpot models and are applied in the polymer field to viscoelastic systems. By associating the spring element with resistance to expansion by the absorbent and the dashpot element with resistance to permeation and by relating the behaviour of the elements to the polymerisation system, underlying molecular factors influencing absorption behaviour could be identified. Relationships were also established between the ultimate degree of swelling and the ratio of cross-linker to monomer for the different polymerisation systems.