**1. Introduction**

A cross-linked polyelectrolyte polymer that has the capability to absorb water up to 150– 1500 times of their own size in a dry state and keep water in a three-dimensional (3D) structure is called superabsorbent hydrogel (SAH) (**Figure 1**). Several descriptions are used to express superabsorbent hydrogel (SAH): (I) SAH has synthetic three-dimensional swollen networked structures. The product from covalently cross-linked to certain synthetic

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 1.** Hydrogel is a hydrophilic monomer that would be cross-linked.

monomer or nature polymer [1–3]. (II) SAH is a fine white powder-like sand or tiny granule-like sugar and has a high ability of water absorption. (III) SAH is the diffusion of the solvent (typically water) into the hydrogel network when the hydrogel is placed in contact with water; the water molecules begin to diffuse inside the solid hydrogel network. (IV) By another way, at equilibrium swelling hydrogel contains small fractions of solid and large fractions of water so it can be described as a hydrogel that begins to diffuse into the water. The diffuser of the water outlet of the hydrogel is a deswelling process. Over time, when the maximum swelling of hydrogel soaked in water is achieved, the bonds of the network will relax by evolving apportion of the water molecules (deswelling) and absorb of water molecules again (swelling) with the time interval. This deswelling/swelling process affecting by the pressure of water molecules on the bonds of hydrogel networks that given the swelling curve line saw shape (zigzag), this phenomenon so-called hydrogel breath. **Figure 2** is a representation of the 3D structure of a swollen and dried hydrogel.

From **Figure 2** it can be said that the hydrogel chains are in close and function groups tightly interacting with each other due to H bonds. As water diffuses inside the hydrogel network, the function groups begin to hydrate, and the interactions such as H bonds will terminate. With further water molecules absorbed, the chains will gain pressure with a gain swell. At appropriate conditions, the hydrogel will reach a state where the pores are fully filled with water and chains reach the maximum expanded. Responding to many external stimulus conditions, expansion and shrinkage of hydrogel are controlled [4].

*Mtr* \_\_\_\_

where *Mt*

*Tg*

*M*∞*<sup>r</sup>*

**Figure 2.** Illustration in the left hydrogel in a dry state or deswelling state (shrinking).

pared to the relaxation processes (0.50 < n < 1), which appears when the *Tg*

and *M∞* are the amount of water diffusion into the hydrogel at the time (t); at the

infinite time, respectively, K is a constant related to the structure of the network; and n is a characteristic exponent of the transport mode of the water solvent [5]. Depending on the relative rates of water diffusion and hydrogel network relaxation, three cases of diffusion mechanisms are distinguished. The Fickian diffusion may be described by Case I which appears when the

 of hydrogel is below the water medium temperature. In this case, the hydrogel chains have a high mobility and relaxation, and the water penetrates more easily into the relaxed network. Therefore, the water diffusion rate, Rdiff, is much less than the hydrogel chain relaxation rate Rrelax (n = 0.50) (Rdiff << Rrelax). Case II is the non-Fickian diffusion, in which diffusion is very rapid com-

above the experimental temperature. In this situation, the hydrogel chains are not adequately

= *K tn* (1)

of hydrogel is well

Superabsorbent

47

http://dx.doi.org/10.5772/intechopen.74698
