**2. Kinetics of water absorbed (swelling) in hydrogel**

The swelling process of a hydrogel is a transition from solid state to fluid state without dissolution or dissociation. The two interfaces become one interface called "gel." Eq. (1) was used to determinate the nature of diffusion of water molecules into the hydrogels:

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

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

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 con-

The swelling process of a hydrogel is a transition from solid state to fluid state without dissolution or dissociation. The two interfaces become one interface called "gel." Eq. (1) was used

representation of the 3D structure of a swollen and dried hydrogel.

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

46 Hydrogels

ditions, expansion and shrinkage of hydrogel are controlled [4].

**2. Kinetics of water absorbed (swelling) in hydrogel**

to determinate the nature of diffusion of water molecules into the hydrogels:

$$\frac{M\_r}{M\_{\omega}} = K \, t^u \tag{1}$$

where *Mt* 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 *Tg* 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 compared to the relaxation processes (0.50 < n < 1), which appears when the *Tg* of hydrogel is well above the experimental temperature. In this situation, the hydrogel chains are not adequately mobile to permit urgent penetration of water into the hydrogel (Rdiff >> Rrelax). Case III is the anomalous diffusion. It is observed when the diffusion and relaxation rates are comparable (Rdiff ≈ Rrelax) [6]. To detect the diffusion mechanisms, the swelling curves are fitted to Eq. (1) which becomes:

$$
\log\left(\frac{M\_i}{M\_{\text{-}}}\right) = \log K + n \log t \tag{2}
$$

and the interconnected porous structure. At pH 10 (**Figure 6b**), an alveolate morphology and

**Figure 4.** Scanning electron microscopy (SEM) images of (PAAm-g-XG) (A) swelled at pH 1.2 and (B) swelled at pH 7.4.

**Figure 3.** Scanning electron microscopy (SEM) images show the porous structure of CtOH-StA/PAAm (a) swelled at

Another example of pH-sensitive gum (xanthan) based on acrylamide hydrogel is given (PAAm-g-XG) by radical polymerization [25]. **Figure 4** shows SEM images of the pH-sensitive swelling behavior reflecting the surface morphology of (PAAm-g-XG) exposed to acidic and alkaline pH. **Figure 4A** at pH 1.2 surface of (PAAm-g-XG) shows no pores as there was minimum swelling. In **Figure 4B**, at pH 7.4, the surface morphology of PAAm-g-XG reveals highly porous structure compared with the surface of PAAm-g-XG incubated in pH 1.2. As shown above, the swelling in alkaline medium is higher than in the acidic one due to the CONH<sup>2</sup>

Rubber could be used for improving the elasticity of many materials due to their flexibility and softness that has glassy transition temperatures bringing down the ambient temperature

leads to increase the pore size.

in

Superabsorbent

49

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

affected by NaOH. The electrostatic repulsion of

highly uniform pores are observed.

pH 6 and (b) swelled at pH 10.

acrylamide groups and hydrolysis to COO−

the ionized groups COO−

**3.3. Rubber**

The diffusional exponent n is calculated from the slopes and K (kinetic rate of swelling) from the intercept.
