The super absorbent polymer is generally a polymer electrolyte containing a hydrophilic group and a crosslinked structure. Before absorbing water, the polymer chains are entangled with each other and crosslinked with each other to form a network structure, thereby achieving overall fastening. When in contact with water, water molecules penetrate into the SAP through capillary action and diffusion, and the ionizing groups on the chain are ionized in water. Due to the electrostatic repulsion between the same ions on the chain, the polymer chain stretches and swells. Due to the requirement of electrical neutrality, the counterions cannot migrate to the outside of the resin, and the difference in ion concentration between the solution inside and outside the resin forms the reverse osmotic pressure. Under the action of reverse osmotic pressure, water further enters the resin to form a hydrogel.
At the same time, the cross-linked network structure and hydrogen bonding of the resin itself limit the infinite expansion of the gel.
When a small amount of salt is contained in the water, the reverse osmotic pressure is reduced, and at the same time, due to the shielding effect of the reverse ions, the polymer chain shrinks, which causes the water absorption capacity of the polymer to be greatly reduced. Generally, the super absorbent polymer's water absorption capacity in 0.9% NaCl solution is only about 1/10 of that in deionized water.
Water absorption and water retention are two aspects of a problem. At a certain temperature and pressure, a superabsorbent polymers can absorb water spontaneously, and water enters the resin, reducing the free enthalpy of the entire system until equilibrium. If water escapes from the resin and increases the free enthalpy, it is not conducive to the stability of the system. Differential thermal analysis shows that 50% of the water absorbed by the SAP(super absorbent polymer) is still enclosed in the gel network above 150 ° C. Therefore, even if pressure is applied at normal temperature, water will not escape from the super absorbent polymer, which is determined by the thermodynamic properties of the super absorbent polymer.
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