**4. Preparation methods of SAH (freezing/thawing, ionizing radiation, and chemical initiation)**

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 and chemical methods reflecting the synthesis of hydrogels.

#### **4.1. Freezing: thawing process**

**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.

50 Hydrogels

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**).

#### **4.2. Chemical cross-linkage**

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

cross-linking agents. In another known difference, the physically cross-linked hydrogels are not homogeneous networks, the chain entanglements forming irregular porous structure, while chemically cross-linked hydrogels are covalently cross-linked networks forming regular porous structure [33]. **Figure 7** shows the morphologies of synthetic PVA hydrogels by two different processes (physically and chemically). The change in the parameters during cross-linking leads to the spontaneous formation of various porous structures. **Figure 7c** shows SEM image of PVA hydrogel prepared which underwent eight freezing– thawing cycles [34] given irregular porous structure. While **Figure 7d** shows a SEM image

BO<sup>3</sup>

structure is more regular compared with the one in **Figure 7c**. **Figure 7e** and **f** shows an image of tubular chitosan prepared using the freezing–thawing method [36, 37]. **Figure 7g** and **h** shows SEM images of the chitosan cross -inked chemically by adding succinic acid [38] and by adding glutaric acid [39], respectively. Open-pore structure with a high degree of interconnectivity can be observed. It is indicated that the mechanism of cross-linking accompanied by reaction-induced phase separation leads to diverse morphologies of the resulting

Cross-linking by irradiation occurs using a high-ionizing energy, such as gamma rays (Co-60), x-ray, or electron beam (*e*-beam). Gamma irradiation is more economically rather than the rest of the other irradiation techniques [40, 41]. Gamma irradiation is a promising technique to fabricate a wide scale of different materials especially polymeric materials [42–44]. Particularly, at the first step, a polymer radical is formed with regard to water radiolysis by ionizing radiation follow-up polymerization reactions. Water radiolysis

beside *ehy* produced upon radiolysis of water are transferred by the polymeric solute to form carbon chain macroradicals. The average number of macroradical centers formed in a pulse would then be determined simply from the radiation chemical yield of hydroxyl radicals and the dose per pulse. For example, radiation cross-linkage of acrylic acid (AAc) monomer and polymer radicals preferably undergoes intra-cross-linking and inter-cross-linking reactions producing a porous structure. Macroporous structures are produced when inter-cross-linking predominates and intra-cross-linking would cause nano-porous structures [45]. Radicals are generated from the radiolysis of AAc aqueous solutions; the predominance of the inter-cross-linking reactions is achieved due to the large number of carbon–carbon double bonds. The beginning of intra-cross-linking under these conditions is confirmed by the macroporous formation. Also, higher yield of C-centered free radicals along the PAAc chain enhances the intra-cross-linking reactions. Dimmers are formed by combination of two macroradical molecules. In the same manner, a 3D cross-linked hydrogel will be obtained of PAAc polymer. **Figure 8** shows covalently cross-linked hydrogels. First, macroradicals are combined together, and then cellular structure consisted of small compartments or pores as rooms are filled with water. The network expansion probably takes place by means of an own pores are filled with water. At a certain

, H2 O2

[35], as seen in **Figure 7d**, the porous

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

Superabsorbent

53

, and O•). All •H and •OH radicals

and PVA cross-linked chemically by adding H<sup>3</sup>

porous structure of cross-linked hydrogel.

generates six reactive species (•H, •OH, *ehy*, H2

water pressure bonds of the networks, walls contract and relax.

**4.3. Ionizing radiation**

**Figure 7.** SEM images of morphologies of synthetic hydrogels. The changes in cross-linking method lead to the spontaneous formation of various porous structures. Physical hydrogels are not homogeneous, since clusters of molecular entanglements, or hydrophobically or ionically associated domains, can create homogeneities. Free chain ends or chain loops also represent transient network defects in physical gels. Physical hydrogel (a) chemical hydrogel (b) PVA hydrogel crosslinked (c) physically and (d) chemically (e) and (f) shows the chitosan hydrogel crosslinked physically as tubular porous structure and chitosan hydrogel crosslinked chemically using (g) succinic acid and (h) glutaric acid.

cross-linking agents. In another known difference, the physically cross-linked hydrogels are not homogeneous networks, the chain entanglements forming irregular porous structure, while chemically cross-linked hydrogels are covalently cross-linked networks forming regular porous structure [33]. **Figure 7** shows the morphologies of synthetic PVA hydrogels by two different processes (physically and chemically). The change in the parameters during cross-linking leads to the spontaneous formation of various porous structures. **Figure 7c** shows SEM image of PVA hydrogel prepared which underwent eight freezing– thawing cycles [34] given irregular porous structure. While **Figure 7d** shows a SEM image and PVA cross-linked chemically by adding H<sup>3</sup> BO<sup>3</sup> [35], as seen in **Figure 7d**, the porous structure is more regular compared with the one in **Figure 7c**. **Figure 7e** and **f** shows an image of tubular chitosan prepared using the freezing–thawing method [36, 37]. **Figure 7g** and **h** shows SEM images of the chitosan cross -inked chemically by adding succinic acid [38] and by adding glutaric acid [39], respectively. Open-pore structure with a high degree of interconnectivity can be observed. It is indicated that the mechanism of cross-linking accompanied by reaction-induced phase separation leads to diverse morphologies of the resulting porous structure of cross-linked hydrogel.

#### **4.3. Ionizing radiation**

**Figure 7.** SEM images of morphologies of synthetic hydrogels. The changes in cross-linking method lead to the spontaneous formation of various porous structures. Physical hydrogels are not homogeneous, since clusters of molecular entanglements, or hydrophobically or ionically associated domains, can create homogeneities. Free chain ends or chain loops also represent transient network defects in physical gels. Physical hydrogel (a) chemical hydrogel (b) PVA hydrogel crosslinked (c) physically and (d) chemically (e) and (f) shows the chitosan hydrogel crosslinked physically as tubular

porous structure and chitosan hydrogel crosslinked chemically using (g) succinic acid and (h) glutaric acid.

52 Hydrogels

Cross-linking by irradiation occurs using a high-ionizing energy, such as gamma rays (Co-60), x-ray, or electron beam (*e*-beam). Gamma irradiation is more economically rather than the rest of the other irradiation techniques [40, 41]. Gamma irradiation is a promising technique to fabricate a wide scale of different materials especially polymeric materials [42–44]. Particularly, at the first step, a polymer radical is formed with regard to water radiolysis by ionizing radiation follow-up polymerization reactions. Water radiolysis generates six reactive species (•H, •OH, *ehy*, H2 , H2 O2 , and O•). All •H and •OH radicals beside *ehy* produced upon radiolysis of water are transferred by the polymeric solute to form carbon chain macroradicals. The average number of macroradical centers formed in a pulse would then be determined simply from the radiation chemical yield of hydroxyl radicals and the dose per pulse. For example, radiation cross-linkage of acrylic acid (AAc) monomer and polymer radicals preferably undergoes intra-cross-linking and inter-cross-linking reactions producing a porous structure. Macroporous structures are produced when inter-cross-linking predominates and intra-cross-linking would cause nano-porous structures [45]. Radicals are generated from the radiolysis of AAc aqueous solutions; the predominance of the inter-cross-linking reactions is achieved due to the large number of carbon–carbon double bonds. The beginning of intra-cross-linking under these conditions is confirmed by the macroporous formation. Also, higher yield of C-centered free radicals along the PAAc chain enhances the intra-cross-linking reactions. Dimmers are formed by combination of two macroradical molecules. In the same manner, a 3D cross-linked hydrogel will be obtained of PAAc polymer. **Figure 8** shows covalently cross-linked hydrogels. First, macroradicals are combined together, and then cellular structure consisted of small compartments or pores as rooms are filled with water. The network expansion probably takes place by means of an own pores are filled with water. At a certain water pressure bonds of the networks, walls contract and relax.

**Figure 8.** (Left) The proposed radical polymerization mechanism induced by gamma irradiation on the preparation of polyacrylic acid. (Right) The framework of SAH network structure has a preferential spatial orientation.

**Figure 9.** Predominated shape design of hydrogels.

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

**Figure 10.** The two routs of polymerization reactions.
