**3. Swelling properties of superabsorbent hydrogels**

#### **3.1. Natural polymer**

The natural polymers copolymerized with synthetic polymers provided these natural polymers to have a suitable functional group and gain mechanical strength more than individual [7]. A variety of natural polymers such as chitosan, heparin, pectin, chitin, hyaluronic acid, agarose, dextran, and alginate are excellent to covalently cross-link and polymerize to form hydrogel. They have been explored as biocompatible, biodegradable hydrogels for biomedical applications [8–13]. Polysaccharide hydrogel is a biopolymer with high permeability for nutrients, oxygen, and other water-soluble metabolites, making it attractive scaffolds for use in cell encapsulation [14–16]. In addition, polysaccharides have been copolymerized with proteins such as laminin, gelatin, collagen, and fibrin to form an interpenetrating network polymer (IPN) or composite hydrogels [17–21]. This insoluble cross-linked biopolymer hydrogel allows immobilization of biomolecules and active agents. Owing to their high water retender, hydrogels resemble natural soft tissue more than any other type of polymeric biomaterials [22]. The water retention in SAH-based natural polymer promotes cell migration, growth, proliferation, differentiation, and adhesion, leading to tissue regeneration scaffolds [23].

#### **3.2. Gum and wax**

Hydrocolloid materials such as wax and gum and a few others such as surfactant or acidic oils have been copolymerized with hydrogel and novel systems of hydrocolloid hydrogel matrices with attractive properties. Emulsion polymerization is an efficient method for the production of new wax-hydrogel such as cetyl alcohol: stearic acid-based acrylamide hydrogel by using triethylamine (TEA) as an emulsifier [24]. A cross-linking reaction is performed at a dose of 20 kGy. This wax-hydrogel matrix bearing of acid and amide groups shows high swelling value behavior at different pH values.

**Figure 3** shows the SEM images of wax-hydrogel matrices of CtOH-StA/PAAm. It was observed that the perfect miscibility was between wax and hydrogel networks as one phase with the absence of a separation zone. **Figure 3a** shows the equilibrium swelling at pH 6

**Figure 3.** Scanning electron microscopy (SEM) images show the porous structure of CtOH-StA/PAAm (a) swelled at pH 6 and (b) swelled at pH 10.

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

and the interconnected porous structure. At pH 10 (**Figure 6b**), an alveolate morphology and highly uniform pores are observed.

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> in acrylamide groups and hydrolysis to COO− affected by NaOH. The electrostatic repulsion of the ionized groups COO− leads to increase the pore size.

#### **3.3. Rubber**

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)

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

The natural polymers copolymerized with synthetic polymers provided these natural polymers to have a suitable functional group and gain mechanical strength more than individual [7]. A variety of natural polymers such as chitosan, heparin, pectin, chitin, hyaluronic acid, agarose, dextran, and alginate are excellent to covalently cross-link and polymerize to form hydrogel. They have been explored as biocompatible, biodegradable hydrogels for biomedical applications [8–13]. Polysaccharide hydrogel is a biopolymer with high permeability for nutrients, oxygen, and other water-soluble metabolites, making it attractive scaffolds for use in cell encapsulation [14–16]. In addition, polysaccharides have been copolymerized with proteins such as laminin, gelatin, collagen, and fibrin to form an interpenetrating network polymer (IPN) or composite hydrogels [17–21]. This insoluble cross-linked biopolymer hydrogel allows immobilization of biomolecules and active agents. Owing to their high water retender, hydrogels resemble natural soft tissue more than any other type of polymeric biomaterials [22]. The water retention in SAH-based natural polymer promotes cell migration, growth, proliferation, differentiation, and adhesion, leading to tissue regen-

Hydrocolloid materials such as wax and gum and a few others such as surfactant or acidic oils have been copolymerized with hydrogel and novel systems of hydrocolloid hydrogel matrices with attractive properties. Emulsion polymerization is an efficient method for the production of new wax-hydrogel such as cetyl alcohol: stearic acid-based acrylamide hydrogel by using triethylamine (TEA) as an emulsifier [24]. A cross-linking reaction is performed at a dose of 20 kGy. This wax-hydrogel matrix bearing of acid and amide groups

**Figure 3** shows the SEM images of wax-hydrogel matrices of CtOH-StA/PAAm. It was observed that the perfect miscibility was between wax and hydrogel networks as one phase with the absence of a separation zone. **Figure 3a** shows the equilibrium swelling at pH 6

*<sup>M</sup>*<sup>∞</sup> ) <sup>=</sup> *logK* <sup>+</sup> *<sup>n</sup>*log*<sup>t</sup>* (2)

\_\_\_\_ *Mt*

**3. Swelling properties of superabsorbent hydrogels**

shows high swelling value behavior at different pH values.

which becomes:

48 Hydrogels

the intercept.

**3.1. Natural polymer**

eration scaffolds [23].

**3.2. Gum and wax**

log(

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

[26, 27]. Water-swollen composite rubber-hydrogel materials are highly permeable to various applications. For example, rubber-hydrogel of SBR/PVP/MAA composite was prepared by mixing styrene-butadiene rubber (SBR) and copolymer hydrogel (polyvinylpyrrolidoneco-methacrylic acid) cross-linked by gamma irradiation. A high miscibility was observed between the MAA/PVP hydrogel and the matrix of SBR with swelling degree of 25 (g/g) after 4 h. **Figure 5** shows a SEM image of SBR/PVP/MAA revealed to a uniform of surface morphol-

Superabsorbent

51

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

Superabsorbent hydrogels are prepared from either natural or synthetic polymers. The synthetic polymers are mechanically robust and stiffer compared to natural polymers. Their mechanically strength results in a slow degradation rate providing the durability as well, but mechanical strength of natural polymers is highly poor. These two inverse properties should be adjusted through ideal design [29]. The following section describes the physical

Freezing–thawing process is a physical method for creating a strong and highly elastic hydrogel [30]. The general advantage of physically forming hydrogel is the need for the addition of cross-linking and initiator entities. PVA is a common polymer that suitable cross-links by freezing–thawing techniques. Cross-linked PVA hydrogel is obtained when an aqueous solution of PVA highly crosslinked by the "repeated freezing-and-thawing cycles." The steps in this method were as follows. First, an aqueous solution of PVA was frozen at a low temperature 0°C, and then the frozen PVA solution stands to thaw at room temperature or at a temperature of 60°C. These cycles were repeated, with increasing the cycle of freezing the resultant PVA hydrogel with much cross-linked density, and the water resistance has also been increased [31]. **Figure 6** reveals why the "freezing–thawing" process is well involved in the preparation of hydrogel for the PVA aqueous solution [32]. **Figure 6a** shows the H bond formation of PVA chains dissolved in water. Once the PVA solution was frozen (**Figure 6b**), ice crystals were formed within all PVA molecules, and the chains were freezing (low motion). During the "thawing" process, the ice will melt gradually from free chains before bonding chains (H bonds). A free space with water allowing free chains to cling together (tangle) and form node is seen on the left of **Figure 6c**. The "repeated freezing-and-thawing" method

cross-linked PVA hydrogel with superporous structure that is formed (**Figure 6d**).

Moreover, physically cross-linked hydrogels have limitations in which a few kinds of polymeric materials could be cross-linked by this method. Chemically, cross-linkage could be carried out in the wide kinds of polymeric matrials in thr presence of initiators and

ogy indicating a high compatibility between the hydrogel and rubber matrices [28].

**4. Preparation methods of SAH (freezing/thawing, ionizing** 

**radiation, and chemical initiation)**

**4.1. Freezing: thawing process**

**4.2. Chemical cross-linkage**

and chemical methods reflecting the synthesis of hydrogels.

**Figure 5.** SEM image of SBR/PVP/MAA. The porous structure affected by elasticity of SBR chains.

**Figure 6.** The mechanisms for PVA hydrogel formation from PVA solution by a freezing–thawing method: (a) PVAwater system. (b) PVA-ice system and frozen of chains. (c) PVA-ice-water system obtained by the gradual thawing of the PVA solution causing coiling of PVA chains together to form node (helix hydrogel). (d) Cross-linked network of PVA hydrogel was obtained.

[26, 27]. Water-swollen composite rubber-hydrogel materials are highly permeable to various applications. For example, rubber-hydrogel of SBR/PVP/MAA composite was prepared by mixing styrene-butadiene rubber (SBR) and copolymer hydrogel (polyvinylpyrrolidoneco-methacrylic acid) cross-linked by gamma irradiation. A high miscibility was observed between the MAA/PVP hydrogel and the matrix of SBR with swelling degree of 25 (g/g) after 4 h. **Figure 5** shows a SEM image of SBR/PVP/MAA revealed to a uniform of surface morphology indicating a high compatibility between the hydrogel and rubber matrices [28].
