**Hydrogels Based on Polyvinylpyrrolidone Copolymers**

**Hydrogels Based on Polyvinylpyrrolidone Copolymers**

DOI: 10.5772/intechopen.72082

#### Oleh Suberlyak and Volodymyr Skorokhoda Additional information is available at the end of the chapter

Oleh Suberlyak and Volodymyr Skorokhoda

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72082

#### **Abstract**

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22 Hydrogels

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The role of polyvinylpyrrolidone (PVP) complex formation with water-soluble 2-hydroxyalkyl methacrylates is described. The impact of the complexation on both the polymerization kinetics and the formation of a copolymer structure initiated by radical initiators has been studied. The activating effect of iron(II) and iron(III) sulfates has been revealed for the initiator-free polymerization of the formulation. An analytical approach to determining the molecular weight of the chain fragments located between two neighboring crosslinking nodes in the polymer network (Mn) has been developed depending on the values of the stability constant (Кst) for the charge-transfer complexes. The basic regularities of hydrogels obtaining based on PVP copolymers with high sorption capacity and diffusion characteristics are presented. The main directions of practical application of synthesized hydrogels are considered.

**Keywords:** hydrogels, polyvinylpyrrolidone, complex with charge transfer, cross-linked copolymers, permeability membranes, capsulation particles, drugs, soft contact lenses, biomedical, properties

#### **1. Introduction**

The concentration of colloid polymer solutions is accompanied by increasing viscosity up to a critical value when a gel is formed. A gel (a jellylike material) is a system which exhibits no flow and is based on a fluctuation polymer network swollen in a solvent. The formation of gels is accompanied by the appearance of physical nodes between macromolecular chains. The stability of fluctuation nodes and, therefore, the gel stability increase with increasing energy of the interaction between solvent molecules and polymer chains. In the case of using aqueous solutions of natural or synthetic polymers, a "hydrogel" is formed. This is a hydrogel of the "second type." Such a product consists of two phases and is unstable. During significant change of temperature or dynamic load it divides into two phases that hydrogel—formed a

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 IntechOpen. 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.

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syneresis process occurs [1]. That is why a gel of the "second type" cannot be recommended for long-lasting exploitation under variable conditions.

It has been stated [25] that oxygen permeability depends just on the water content and does

Hydrogels Based on Polyvinylpyrrolidone Copolymers http://dx.doi.org/10.5772/intechopen.72082 25

A highly hydrophilic matrix of a hydrogel can also be obtained due to chemical cross-linking of water-soluble polymers. For example, polyvinyl alcohol (PVA) cross-linked by heating in the presence of sodium tetraborate [26] or by initiated graft polymerization, in particular, PVA with glycidyl methacrylate [27]. To this end, polyvinylpyridine, poly(ethylene glycol), and hydroxypropyl cellulose are also applied besides PVA. [13, 15, 28–31]. Such polymers are mainly used for the reduction of internal tensions due to them washing out during hydration process and increase of matrix-free volume that decreases spatial obstacles for the conforma-

Method of grafted copolymerization of water-soluble monomers on polyvinylpyrrolidone (PVP) appears to be particularly promising with significant possibilities of hydrogel polymeric matrix formation [32, 33]. PVP is used by itself as a sorbent, a thickener of cosmetic ointments and for encapsulation of medical drugs [34]. Due to its high surface energy, PVP is also an attractive (a promising) substance in the formation of metal nanopowders [35, 36] as well as silicate nanopowders from corresponding solutions [37]. PVP keeps adsorbed drugs

Macromolecule of PVP in a free state has a helicoidal structure with pyrrolidone rings outside, which promotes the interaction of the peptide groups with substances by complex formation. PVP is characterized by high complexation ability. It forms complexes with organic and inorganic electron donor as well as electron acceptor compounds. Complexes form highly polarized peptide groups of the pyrrolidone rings due to mesomeric effect. Specific role is played by complexes of PVP with vinyl monomers which polymerize in the presence of PVP.

Important and notable property of PVP is the ability to form complexes [40–42]. This ability significantly influences the kinetic of polymerization and the formation of a polymeric matrix

As it has been shown in the research papers [43, 44], PVP can form charge-transfer complexes not only with medical drugs but also with water-soluble vinyl monomers. The results of spectral analysis and quantum mechanical calculations with the application of package Chem3D [45] shows that –C = C– bond of a monomer molecule, negative charge of which significantly changes, and nitrogen atom of the pyrrolidone cycle (−N–), the charge of which increases

The complex was characterized by the constant of complex stability (Кst), and its value increases with the presence of water or primary alcohol groups [43, 46]. Based on this information, the structure of a charge-transfer complex (CTC) with, for example, 2-hydroxyethyl methacrylate

**2. Role of polyvinylpyrrolidone in the kinetics and formation of** 

from +0.35 to +0.42, both participate in the formation of a complex (**Figure 2**).

not depend on the chemical structure of a hydrogel matrix.

tional changes of structured polymer chains.

**structure**

on the pyrrolidone rings of the macromolecule [38, 39].

structure in the process of hydrogel synthesis.

(HEMA) was substantiated [33, 46].

Polymeric gels of the "first type" are formed upon swelling of chemically cross-linked hydrogels, and their matrix consists of macromolecule segments located between chemical crosslinking nodes. It leads to the formation of chemically cross-linked network that swells due to the sorption of a solvent. Chemical bonds between macromolecules provide non-fluidity of the system. Network swells partially as a result of change of the segment conformation under the effect of a solvent.

Conformational static macromolecular coil (**Figure 1**) of chain segments between cross-linking nodes causes significant reversible deformation which corresponds to highly elastic deformation under the influence of external force field. Hydrogels are formed during the swelling of chemically cross-linked highly hydrophilic polymers in water. A large number of hydrogels, obtained through the polymerization in water or in bulk with the following swelling of the synthesized polymer in water, are known [2].

Water-soluble monomers such as vinylpyrrolidone [3], hydroxyalkyl (met)acrylates (HAMA) [4–6] and their homologs (C3 –C13) [7], propylene glycol methacrylates [8], etc. are used for the synthesis of a polymer matrix.

In the method [9], a chemically cross-linked structure is formed due to the usage of a bifunctional monomer of similar nature in reaction mass. Cross-linking agents (CA), which are used for the polymerization of monofunctional monomers, are bis-(met)acrylates of glycols [10–14], bis-allylic esters [15, 16], triallyl cyanurate [17], dialdehydes [18], and polyethylene glycol dimethacrylates [19].

The number of CA affects the degree of polymer matrix cross-linking and molecular weight of intermolecular crosslinks [20].

Content of water can vary from 5 to 90% depending on the quantity of cross-linking agent and its molecular weight [20, 21]. The quantity of CA with low molecular weight, such as dimethacrylates, can be 0.25–2%, which would provide a sufficient amount of water in a hydrogel [22–24]. It has been mentioned that hydrogels based on hydroxyalkyl (met)acrylates, used for production of contact lenses, have quite low oxygen permeability. Oxygen permeability of hydrogel with 28% of water is 35∙10−10 сm<sup>2</sup> ∙mL О<sup>2</sup> /mL∙s∙mm.

**Figure 1.** Schematic diagram of swelling of hydrogel [(●) cross-linking node and (○) water molecule].

It has been stated [25] that oxygen permeability depends just on the water content and does not depend on the chemical structure of a hydrogel matrix.

syneresis process occurs [1]. That is why a gel of the "second type" cannot be recommended

Polymeric gels of the "first type" are formed upon swelling of chemically cross-linked hydrogels, and their matrix consists of macromolecule segments located between chemical crosslinking nodes. It leads to the formation of chemically cross-linked network that swells due to the sorption of a solvent. Chemical bonds between macromolecules provide non-fluidity of the system. Network swells partially as a result of change of the segment conformation

Conformational static macromolecular coil (**Figure 1**) of chain segments between cross-linking nodes causes significant reversible deformation which corresponds to highly elastic deformation under the influence of external force field. Hydrogels are formed during the swelling of chemically cross-linked highly hydrophilic polymers in water. A large number of hydrogels, obtained through the polymerization in water or in bulk with the following swelling of the

Water-soluble monomers such as vinylpyrrolidone [3], hydroxyalkyl (met)acrylates (HAMA)

In the method [9], a chemically cross-linked structure is formed due to the usage of a bifunctional monomer of similar nature in reaction mass. Cross-linking agents (CA), which are used for the polymerization of monofunctional monomers, are bis-(met)acrylates of glycols [10–14], bis-allylic esters [15, 16], triallyl cyanurate [17], dialdehydes [18], and polyethylene glycol

The number of CA affects the degree of polymer matrix cross-linking and molecular weight

Content of water can vary from 5 to 90% depending on the quantity of cross-linking agent and its molecular weight [20, 21]. The quantity of CA with low molecular weight, such as dimethacrylates, can be 0.25–2%, which would provide a sufficient amount of water in a hydrogel [22–24]. It has been mentioned that hydrogels based on hydroxyalkyl (met)acrylates, used for production of contact lenses, have quite low oxygen permeability. Oxygen permeability of

∙mL О<sup>2</sup>

**Figure 1.** Schematic diagram of swelling of hydrogel [(●) cross-linking node and (○) water molecule].

/mL∙s∙mm.

–C13) [7], propylene glycol methacrylates [8], etc. are used for the

for long-lasting exploitation under variable conditions.

under the effect of a solvent.

24 Hydrogels

[4–6] and their homologs (C3

dimethacrylates [19].

synthesis of a polymer matrix.

of intermolecular crosslinks [20].

hydrogel with 28% of water is 35∙10−10 сm<sup>2</sup>

synthesized polymer in water, are known [2].

A highly hydrophilic matrix of a hydrogel can also be obtained due to chemical cross-linking of water-soluble polymers. For example, polyvinyl alcohol (PVA) cross-linked by heating in the presence of sodium tetraborate [26] or by initiated graft polymerization, in particular, PVA with glycidyl methacrylate [27]. To this end, polyvinylpyridine, poly(ethylene glycol), and hydroxypropyl cellulose are also applied besides PVA. [13, 15, 28–31]. Such polymers are mainly used for the reduction of internal tensions due to them washing out during hydration process and increase of matrix-free volume that decreases spatial obstacles for the conformational changes of structured polymer chains.

Method of grafted copolymerization of water-soluble monomers on polyvinylpyrrolidone (PVP) appears to be particularly promising with significant possibilities of hydrogel polymeric matrix formation [32, 33]. PVP is used by itself as a sorbent, a thickener of cosmetic ointments and for encapsulation of medical drugs [34]. Due to its high surface energy, PVP is also an attractive (a promising) substance in the formation of metal nanopowders [35, 36] as well as silicate nanopowders from corresponding solutions [37]. PVP keeps adsorbed drugs on the pyrrolidone rings of the macromolecule [38, 39].

Macromolecule of PVP in a free state has a helicoidal structure with pyrrolidone rings outside, which promotes the interaction of the peptide groups with substances by complex formation. PVP is characterized by high complexation ability. It forms complexes with organic and inorganic electron donor as well as electron acceptor compounds. Complexes form highly polarized peptide groups of the pyrrolidone rings due to mesomeric effect. Specific role is played by complexes of PVP with vinyl monomers which polymerize in the presence of PVP.
