**1. Introduction**

The term "hydrogel" is ambiguous, and it must first be clarified. We will call gel, a polymer network swollen in a solvent—a set of a large number of polymer chains chemically

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

(or physically) sewn together. More strictly, the polymer gel is a system consisting of at least two components, one of which is a mesh polymer and the other is a liquid present in a palpable amount.

Polymer gels can almost completely (by 99% or more) consist of a liquid and be very soft materials. Despite this, they have the inherent ability of solid bodies to maintain their shape. This is due to the fact that the polymer mesh that forms part of the gel plays the role of a framework that provides rigidity (elasticity) of the entire system, i.e., it does not allow it to flow under the action of a deforming force (if the force is not too great and does not last too long).

#### **1.1. Composite polymer hydrogels**

The combination of the properties of hydrogels inherent in solids predetermines a wide range of applications from technical spheres (sorbents, gas separating, and ion exchange membranes [1–3]) to the food industry and medicine (food structure, drug carriers, artificial substitutes for biological tissues, materials for soft and intraocular lenses, etc. [4–6]). The emergence of new fields of application of polymer hydrogels puts forward new requirements to their properties.

These goals can be achieved by obtaining fundamentally new materials—composite hydrogels containing at least two components, each of which performs certain functions. It is obvious that the characteristics of the composite hydrogel are due not only to the physicochemical properties of the individual components, but also to the structure of the material. Given the limited thermodynamic compatibility of the polymers, a variety of hydrogel structures are possible, from complete stratification of the polymer phases to the formation of matrix-nanoscale structures or the formation of structures in which both polymer phases are continuous.

In most cases, composite hydrogels are biphasic systems. The interphase boundary in such materials is not always clearly expressed. It can be a transition layer in which a gradual change in properties occurs (transition from the properties of phase 1 to the properties of phase 2). At least one of phases must be a polymer hydrogel. The hydrogel can be either a synthetic or a natural polymer. The second phase can be a synthetic hydrogel of synthetic or natural origin, a hydrophobic polymer and an inorganic substance. In accordance with the foregoing, it is possible to propose a classification that divides composite hydrogels into three groups [7]:


The nature of the interactions between the components may be due to covalent bonds in the block and graft copolymers, the formation of interpolymer complexes due to the formation of hydrogen bonds, donor-acceptor, ionic and hydrophobic interactions of functional groups, and the engagement of macromolecular chains in interpenetrating and semi-interpenetrating polymer networks.

The compositional polymeric materials, consisting of hydrophilic polymers, have most application in last time. The presence of charged polar groups in hydrophilic polymers lids to communication with solution particles at help of the formation of intermolecular hydrogen and ionic bonds between molecules. Under conditions of corresponding implementation of functional groups, covalent bonds can exist in composite hydrogels. The hydrogels form by the type of interpenetrating and semi-interpenetrating polymer networks usually. In this case, the intermolecular bonds are caused by physical interactions between polar groups.

Inorganic components in composition hydrogels are introduced either to modify the properties of conventional polymer hydrogels (changing mechanical properties, increasing the sensitivity of hydrogels to thermal effects, changing pH, etc.) or to impart new properties not typical for hydrogels (magnetic characteristics and antibacterial properties). Oxides, various clays, carbon materials, water-insoluble inorganic salts, and metals are most often used in organo-inorganic composite hydrogels.

Methods for the preparation of organo-inorganic hydrogels can be divided into two groups:


#### **1.2. Swelling and collapse of polyelectrolyte gels**

(or physically) sewn together. More strictly, the polymer gel is a system consisting of at least two components, one of which is a mesh polymer and the other is a liquid present in

Polymer gels can almost completely (by 99% or more) consist of a liquid and be very soft materials. Despite this, they have the inherent ability of solid bodies to maintain their shape. This is due to the fact that the polymer mesh that forms part of the gel plays the role of a framework that provides rigidity (elasticity) of the entire system, i.e., it does not allow it to flow under the action of a deforming force (if the force is not too great and does not last too

The combination of the properties of hydrogels inherent in solids predetermines a wide range of applications from technical spheres (sorbents, gas separating, and ion exchange membranes [1–3]) to the food industry and medicine (food structure, drug carriers, artificial substitutes for biological tissues, materials for soft and intraocular lenses, etc. [4–6]). The emergence of new fields of application of polymer hydrogels puts forward new require-

These goals can be achieved by obtaining fundamentally new materials—composite hydrogels containing at least two components, each of which performs certain functions. It is obvious that the characteristics of the composite hydrogel are due not only to the physicochemical properties of the individual components, but also to the structure of the material. Given the limited thermodynamic compatibility of the polymers, a variety of hydrogel structures are possible, from complete stratification of the polymer phases to the formation of matrix-nanoscale structures or the formation of structures in which both polymer phases are

In most cases, composite hydrogels are biphasic systems. The interphase boundary in such materials is not always clearly expressed. It can be a transition layer in which a gradual change in properties occurs (transition from the properties of phase 1 to the properties of phase 2). At least one of phases must be a polymer hydrogel. The hydrogel can be either a synthetic or a natural polymer. The second phase can be a synthetic hydrogel of synthetic or natural origin, a hydrophobic polymer and an inorganic substance. In accordance with the foregoing, it is possible to propose a classification that divides composite hydrogels into three groups [7]:

**1.** Hydrogels consisting of two hydrophilic polymers, each of which is capable of forming an

The nature of the interactions between the components may be due to covalent bonds in the block and graft copolymers, the formation of interpolymer complexes due to the formation

a palpable amount.

96 Recent Research in Polymerization

**1.1. Composite polymer hydrogels**

individual polymer hydrogel.

**2.** Hydrogel, including hydrophilic and hydrophobic polymers.

**3.** A polymeric hydrogel containing an inorganic phase.

ments to their properties.

continuous.

long).

Polymer gel, placed in a solvent, changes its volume, i.е., swells or contracts, appropriately absorbing or releasing the solvent, until it reaches an equilibrium swelling. The equilibrium degree of swelling of the gel, determined by the amount of solvent in it, depends both on the properties of the gel and on the properties of the solvent. The nature of this relationship was established for the first time in theoretical studies carried out by Flory and Rener [8, 9] and Kachalsky [10, 11]. According to Flori-Rener's postulate, the equilibrium of free swelling of the polymer network is determined by the balance between the mesh expanding the osmotic pressure and the elastic stress that arises in it. In 1977, Peppas and Merrill modified the Flory-Rehner theory in application to the production of hydrogels from polymer solutions. Due to elastic forces, the presence of water affects the change in the chemical potential within the system [12].

Of all polymer gels, the most interesting are gels based on chains containing charged units, polyelectrolyte gels. Since the macroscopic sample of the gel must be electrically neutral, the charge of the polymer chains of the gel must be compensated by the opposite charge of the low molecular weight counterions floating in the solvent in which the gel swells. When the gel swells in a large volume of water, the counterions should be advantageous to leave the gel and go to the external solution, which would lead to a significant gain in their translational entropy. However, this does not occur, since this leads to a violation of the electroneutrality condition of the gel sample. Counterions are forced to stay inside the gel and create there a bursting osmotic pressure. This osmotic pressure is responsible for the two most important effects associated with polyelectrolyte gels swelling in water.

First, a simple theory shows [13, 14] that the effect of the expanding osmotic pressure is very strong, it leads to a significant swelling of the gel in the water. Therefore, polyelectrolyte gels are used as superabsorbents of water.

Second, the superstrong swelling of polyelectrolyte gels in water leads to the fact that their concentration decreases extremely sharply with a deterioration in the quality of the solvent. The volume of the gel may decrease by a factor of thousands. This phenomenon, called the collapse of gels, was first predicted theoretically in [15] and was experimentally found in [16]. It is associated with the transition of the tangle-globule in the chains constituting the polymer gel. As a result, the gel sample collapses as a whole. In this case, the higher the degree of gel charge, the more sharply the collapse occurs [13]. The theory of collapse of gels developed in [13, 17, 18] shows that this is due to the fact that the collapsed phase is stabilized by the forces of attraction of uncharged links and the volume of the gel in this case depends little on the degree of charge, whereas the volume of the swollen gel is substantially increases with an increase in the degree of charge due to the expanding osmotic pressure of the counterions.

As discussed earlier in the works [19, 20], the equilibrium swelling of ionic hydrogels depends on the network structure, degree of crosslinking, hydrophilicity, and ionization of the functional groups. The major factor contributing to the swelling of ionic networks is the ionization of the network.
