**5. Design of the polymerization reaction according to appropriate monomer**

A construction of 3D frameworks of superabsorbent hydrogel is random and amorphous of nature. But this does not prevent inspiration for the molecular architecture of hydrogels. The fabrication of various architectures for SAH is demonstrated in **Figure 9a** and **b** According to the IUPAC definitions [46], semi-IPN networks are composited of one linear polymer entrapped within the network of another polymer, while IPN networks comprise more than one polymer network structures interlaced on a molecular scale. **Figure 9c** is an image of a multilayer hydrogel (MLH) [47] in which single networks are stacked one onto the other, interpenetrating network films with the mixture of two networks in the same layer. **Figure 9f** is an image of hydrogel fibers [48–50]. The synthesis of hydrogel depends on the concentration of monomer, initiator, and cross-linked agents.

The polymerization reaction may occur by two growth polymerizations such as step and chain growth polymerizations individually or of both [51]. In case of chain growth, the polymerization reaction is very fast, whereas in step growth, the reaction is gradual and monomer concentration decreases gradually. In case of chain growth, the polymerized hydrogel conversion is high, while pore size and swelling are low. Whereas in step growth the hydrogel conversion is poor, pore size and swelling are high [52]. **Figure 10** shows the framework

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

**5. Design of the polymerization reaction according to appropriate** 

polyacrylic acid. (Right) The framework of SAH network structure has a preferential spatial orientation.

tion of monomer, initiator, and cross-linked agents.

A construction of 3D frameworks of superabsorbent hydrogel is random and amorphous of nature. But this does not prevent inspiration for the molecular architecture of hydrogels. The fabrication of various architectures for SAH is demonstrated in **Figure 9a** and **b** According to the IUPAC definitions [46], semi-IPN networks are composited of one linear polymer entrapped within the network of another polymer, while IPN networks comprise more than one polymer network structures interlaced on a molecular scale. **Figure 9c** is an image of a multilayer hydrogel (MLH) [47] in which single networks are stacked one onto the other, interpenetrating network films with the mixture of two networks in the same layer. **Figure 9f** is an image of hydrogel fibers [48–50]. The synthesis of hydrogel depends on the concentra-

**Figure 8.** (Left) The proposed radical polymerization mechanism induced by gamma irradiation on the preparation of

The polymerization reaction may occur by two growth polymerizations such as step and chain growth polymerizations individually or of both [51]. In case of chain growth, the polymerization reaction is very fast, whereas in step growth, the reaction is gradual and monomer concentration decreases gradually. In case of chain growth, the polymerized hydrogel conversion is high, while pore size and swelling are low. Whereas in step growth the hydrogel conversion is poor, pore size and swelling are high [52]. **Figure 10** shows the framework

**monomer**

54 Hydrogels

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

of both growths of polymerization reaction. As shown, the polymerization occurs which by chain growth would proceed by random dimerization of the neighboring monomers and then oligomerization formation until the cross-linked network is obtained. Step growth polymerization reaction has formed dimers, trimers, tetramers, etc., until short chains are formed within the combination of them to form cross-linked hydrogel.
