Hydrogels are cross-linked, three-cross-linked hydrogels with the ability to swell without dissolving in water. are polymers with a dimensional network structure. The three-dimensional structure of hydrogels is formed by chemical bonds or ionic interaction, hydrogen bonding, physical interactions, Van der Waals forces, through cohesion forces such as hydrophobic interactions. Hydrophobic interactions directly affect the swelling behavior of the gel as they are physical crosslink-like interactions. In physical interactions, cross-linked gels are known to be highly dependent on external environmental conditions (Hoffman, 2012). Hydrogels are very similar to living tissues in terms of many physical properties they carry such as containing a lot of water in their structures, soft and flexible structures. show a great similarity when they are compared.

Hydrogen bonds in hydrogels are bridges between water molecules and hydrophilic groups of the hydrogel. They play an effective role in the water absorption capacity of the hydrogel. If the number of hydrophilic groups in a hydrogel increases, water absorption also increases. Hydrogen bonds weaken and break with increasing temperature. After a certain point, polymer-water interactions are replaced by polymer-polymer and water-water interactions. Another force is Van der Waals interactions. This interaction is a polymer-polymer interaction and is in the direction of shrinkage, short distance. It is analyzed under three headings: dipole-dipole, dipole-excited dipole and dispersion forces. These forces make it difficult for solvent molecules to diffuse between polymer chains in dry gels. In the swollen gel, it helps to expel water out of the hydrogel, i.e. shrinkage. The other force is the interaction between hydrophobic groups. Hydrophobic groups, as known, are groups that do not like water, that is, repel water. These non-polar groups attract each other with increasing temperature. Therefore, it acts in the direction of shrinkage. At a certain temperature, these interactions dominate the interactions between water-hydrophile groups and the gel starts to shrink. Another important force is ionic interactions. Ionic interactions have the strongest effect on the phase transition of gels.These interactions are caused by ionizable functional groups on the monomers forming the hydrogel. In the ionized state, it is explained as contributing to shrinkage or swelling as a result of the attraction or repulsion of oppositely or identically charged ions (Akdemir et al., 2005; Shahalom et al., 2006; Kurusic et al., 2006; Zhao et al., 2009; Wang et al., 2013b).
Writtter: Banu HASGÜL Thesis on “Synthesis, characterization and investigation of potential sorption capacities of new polymer/clay composite hydrogels doped with carrageenan
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