**2. Experimental**

In this chapter, the sorption properties of two different types of hydrogel composites will be compared. The first type is a clay-containing hydrogel composite—a copolymer of acrylic acid and acrylamide, filled with bentonite. The second type is a hydrogel filled with polysaccharide—pectin. Pectin is a naturally occurring biopolymer that is finding increasing applications in the pharmaceutical and biotechnology industry. The combination of the hydrophilic acrylic polymer properties with the biodegradable character of pectin-based blends, can lead to interesting hydrogels with potential applications as biomaterials exhibiting different properties depending on the composition and on the type of interactions within the network, attending to chemical crosslinking and hydrogen bonding interactions. In addition, a hydrogel composite based on a copolymer of acrylic acid and acrylamide, filled with such a polysaccharide as pectin, is a hydrogel with a semi-interpenetrating network.

#### **2.1. Material**

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

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

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

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

In this chapter, the sorption properties of two different types of hydrogel composites will be compared. The first type is a clay-containing hydrogel composite—a copolymer of acrylic acid and acrylamide, filled with bentonite. The second type is a hydrogel filled with polysaccharide—pectin. Pectin is a naturally occurring biopolymer that is finding increasing applications in the pharmaceutical and biotechnology industry. The combination of the hydrophilic acrylic polymer properties with the biodegradable character of pectin-based blends, can lead to interesting hydrogels with potential applications as biomaterials exhibiting different properties depending on the composition and on the type of interactions within the network, attending to chemical crosslinking and hydrogen bonding interactions. In addition, a

with polyelectrolyte gels swelling in water.

are used as superabsorbents of water.

98 Recent Research in Polymerization

the counterions.

of the network.

**2. Experimental**

Pectin (chemical grade, MW 50,000) was purchased from Merck Chemical Co. (Germany). N,N′-methylene-bis-acrylamide (MBA), sodium hydroxide, ammonium persulfate, and bentonite were supplied by Sigma-Aldrich and were used without any further purification. Acrylic acid was distilled under reduced pressure before use. Acrylic acid (AAc) and acrylamide (AAm) were from Merck and were used without any further purification. All agents were of analytical grade quality.

#### **2.2. Preparation of superabsorbent composites**

Crosslinked acrylamide-sodium acrylate hydrogel and its composites were synthesized by free radical solution polymerization of AAm and AAc monomers in aqueous solution. A series of hydrogels were prepared by the following procedure.

Polymerization is carried out with constant stirring with a magnetic stirrer at a speed of 500 rev/min. According to the procedure, 10 ml of AAc monomer were dissolved in 3.5 ml distilled water. Acrylic acid was neutralized with 14 N aqueous solution of potassium hydroxide. The degree of neutralization is 0.8. Then 3 g of acrylamide monomer was added. To increase the crosslink density was added 0.001 g N,N-methylenebisacrylamide crosslinking agent. To initiate radical polymerization process a redox system consisting of 4 ml of a 2% ammonium persulfate aqueous solution and 4 ml of a 0.5% solution of TEMED are used. The polymerization was carried out at 35°C (**Figure 1**).

For the preparation of poly(AAm-co-AAc)/bentonite composite hydrogels, 1, 2, 3, or 4 mas.% bentonite was added into solution of sodium acrylate (AAcNa), AAm, and MBA, and was stirred for 10 min. Then, an oxidation–reduction system was added.

**Figure 1.** Schematic representation of a hydrogel with semi-interpenetrating networks—poly(AAm-co-AAc)/pectin and a dispersion-filled hydrogel composite—poly(AAm-co-AAc)/bentonite.

Poly(AAm-co-AAc)/pectin composite semi-IPNs were prepared using the same preparation method. However, the pectin powder was dissolved in a solution of sodium acrylate, stirred with a magnetic stirrer for 10 min, then acrylamide, a crosslinking agent and a redox system were added. To prepare highly swollen poly(AAm-co-AAc)/pectin (containing different contents of pectin) semi-IPNs, same method was used as mentioned above with the addition of 1, 2, 3, or 4 mas.% of pectin to solution of sodium acrylate.
