The super absorbent polymer is generally a polymer electrolyte containing a hydrophilic group and a cross-linked structure. Before water absorption, the polymer chains are tangled together and cross-linked with each other to form a network structure, so as to achieve the overall fastening. In contact with water, water molecules penetrate the resin through capillary action and diffusion, and the ionized groups on the chain ionize in water. The polymer chain stretches and swells due to the electrostatic repulsion between the same ions in the chain. Due to the neutral requirement of electricity, counter ions cannot migrate to the outside of the resin, and the concentration of ions between the internal and external solutions of the resin forms a reverse osmotic pressure. Water is further introduced into the resin under the action of reverse osmosis pressure to form a hydrogel.
At the same time, the cross-network structure and hydrogen bonding of the resin itself limit the infinite expansion of the gel.
When the water contains a small amount of salts, the reverse osmotic pressure decreases, and at the same time, due to the counter-ion shielding effect, the polymer chains shrink, resulting in a significant decrease in the water absorption capacity of the resin. In general, the superabsorbent resin absorbs only about 1/10 of the deionized water in a 0.9% NaCl solution.
Water absorption and water retention are two aspects of the problem. Lin Runxiong has conducted a thermodynamic discussion on this issue. At a certain temperature and pressure, the high water-absorbent resin can spontaneously absorb water, and the water enters the resin, so that the free enthalpy of the entire system is reduced until it is balanced. If the water escapes from the resin and raises the free enthalpy, it is not conducive to the stability of the system. Differential thermal analysis showed that the water absorbed by the high water-absorbent resin remained above 50% in the gel network above 150°C. Therefore, even if pressure is applied at room temperature, water does not escape from the superabsorbent resin, which is determined by the thermodynamic properties of the superabsorbent resin.