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

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 structure in the process of hydrogel synthesis.

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

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 (HEMA) was substantiated [33, 46].

Such mechanism allows to explain the formation of grafted and few structured PVP copoly-

**1.** Adsorption of an initiator and solvation of a monomer on PVP macromolecules and forma-

The degree of grafting of PVP depends on the nature of a complex-forming solvent and the

Matrix effect increases with the increase of hydroxyl group number in the solution and with the increase of the molecular weight of a proton donor. As a result, CTC with polyvinyl alco-

This method allowed to obtain hydrogels with higher mechanical resistance based on the combined matrix PVP:PVA [49, 50]. Complex based on the PVA and PVP shows the highest

Efficiency of grafting (f) (inclusion of PVP macromolecules into the copolymer structure or its chemical cross-linking), and also cross-linking degree of macrochains in a polymer network (Mn), first of all depends on the value of stability constant of CTC (**Figure 3**) [47]. During polymerization of HEMA:PVP composition, Кst was changed by the replacement of certain

**(kg/mole)**

Comments: *M*n, molecular weight mass of the fragment of macrochain between two neighboring cross-linking nodes;

**at rupture (ε, %) PVP PVA HEMA H2**

**Tensile strength (σ, MPa)**

•

27

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

**Relative elongation** 

**4.** The chain transfer to the PVP as from initial radical R• and from macroradical Rm

Obtaining of grafted copolymers as a result of macroradical combination.

hol as a proton donor was found to have significant activating ability [49].

amount of water for the acceptor of protons (dimethyl sulfoxide (DMSO)).

− 30 30 200 15 10.5 500 5 30 200 175 0.29 281 10 30 200 180 0.45 390 15 15 200 185 0.21 260 10 30 200 DMSO 195 0.49 295 10 15 + 15 GMA 100 + 100 DMSO 146 0.87 510

**Table 2.** Physico-mechanical properties of membranes based on the hydrogels in hydrated state.

nature of an initiator of polymerization of HEMA-PVP compositions (**Table 1**).

mer [62]:

**2.** Initiation

**3.** Chain growth

tion of change transfer complex

**5.** Graft copolymerization on PVP

efficiency at the ratio of 2:1 (**Table 2**).

**Composition of forming solution (mass parts) Mn**

GMA, glycidyl methacrylate.

**O**

**Figure 2.** Quantum mechanical model of interaction by PVP with HEMA.

The constant of stability of CTC represents fraction of quantity of the molecules of a reaction mixture (molecules of monomer and elementary links of PVP), which form CTC, to their general quantity in the volume. The change of optical density of diluted solutions of the monomer and PVP in a chosen solvent is determined.

As a result of the monomer molecule solvation on the PVP macrochains through CTC, the rate of HEMA [43] polymerization increases significantly. The rate constant of the polymerization significantly depends on Кst of CTC. The polymerization rate increases with the increase of Кst of CTC with the maximum at the equimolar ratio of a proton donor (H<sup>2</sup> O) and segments of PVP (**Table 1**) [47].

An activation effect of PVP can be observed in proton donor solvents and allows the polymerization without initiators of radical type [48].

The results prove the matrix mechanism of polymerization—local concentration of monomer molecules activated with CTC on the chains of PVP.


<sup>1</sup> Comment: HEMA-PVP: solvent = 9:1:10 mass parts (without initiator), initiator-benzoperoxide.

**Table 1.** Influence of the nature of solvents on the stability constant of the complex and on the polymerization rate (V) of HEMA-PVP composition.

Such mechanism allows to explain the formation of grafted and few structured PVP copolymer [62]:


The constant of stability of CTC represents fraction of quantity of the molecules of a reaction mixture (molecules of monomer and elementary links of PVP), which form CTC, to their general quantity in the volume. The change of optical density of diluted solutions of the mono-

As a result of the monomer molecule solvation on the PVP macrochains through CTC, the rate of HEMA [43] polymerization increases significantly. The rate constant of the polymerization significantly depends on Кst of CTC. The polymerization rate increases with the increase of

An activation effect of PVP can be observed in proton donor solvents and allows the polym-

The results prove the matrix mechanism of polymerization—local concentration of monomer

**Viscosity1 ,** **V1** ⋅**104 (mole/**

**(dm3** ⋅**sec) (at 60°С))**

**(η**⋅**103 , Pa**⋅**sec)** O) and segments

**Degree of PVP graft, P, %**

Кst of CTC with the maximum at the equimolar ratio of a proton donor (H<sup>2</sup>

**Extinction coefficient (dm3**

Dimethyl sulfoxide 0 — 2.4 0 – Cyclohexanol 0.06 20.8 17.6 0.6 11 Butanol 0.12 10.0 2.1 0.8 – Ethylene glycol 0.17 5.6 14.4 1.1 14 Diethylene glycol 0.21 2.1 22.3 1.5 15 Water 0.28 5.3 5.3 3.8 18

<sup>1</sup> Comment: HEMA-PVP: solvent = 9:1:10 mass parts (without initiator), initiator-benzoperoxide.

**Table 1.** Influence of the nature of solvents on the stability constant of the complex and on the polymerization rate (V)

**/(mole**⋅**cm))**

mer and PVP in a chosen solvent is determined.

**Figure 2.** Quantum mechanical model of interaction by PVP with HEMA.

erization without initiators of radical type [48].

molecules activated with CTC on the chains of PVP.

**(dm3 /mole)**

of PVP (**Table 1**) [47].

26 Hydrogels

**Solvent Кst**

of HEMA-PVP composition.


Obtaining of grafted copolymers as a result of macroradical combination.

The degree of grafting of PVP depends on the nature of a complex-forming solvent and the nature of an initiator of polymerization of HEMA-PVP compositions (**Table 1**).

Matrix effect increases with the increase of hydroxyl group number in the solution and with the increase of the molecular weight of a proton donor. As a result, CTC with polyvinyl alcohol as a proton donor was found to have significant activating ability [49].

This method allowed to obtain hydrogels with higher mechanical resistance based on the combined matrix PVP:PVA [49, 50]. Complex based on the PVA and PVP shows the highest efficiency at the ratio of 2:1 (**Table 2**).

Efficiency of grafting (f) (inclusion of PVP macromolecules into the copolymer structure or its chemical cross-linking), and also cross-linking degree of macrochains in a polymer network (Mn), first of all depends on the value of stability constant of CTC (**Figure 3**) [47]. During polymerization of HEMA:PVP composition, Кst was changed by the replacement of certain amount of water for the acceptor of protons (dimethyl sulfoxide (DMSO)).


Comments: *M*n, molecular weight mass of the fragment of macrochain between two neighboring cross-linking nodes; GMA, glycidyl methacrylate.

**Table 2.** Physico-mechanical properties of membranes based on the hydrogels in hydrated state.

**Figure 3.** Dependences of the internodal molecular mass Mn (1) and grafting efficiency f (2) on the constant K of complex formation between 2-hydroxyethyl methacrylate and polyvinylpyrrolidone.

The molecular weight of the fragment of macrochain between two neighboring cross-linking nodes has been calculated according to the following formula:

$$M\_{\mu} = \frac{L^{s} \rho\_{p} \mathbf{v}\_{s}}{0.5 - \mu} \tag{1}$$

The dependencies which are appropriate for analytical forecast of the copolymer structure have been proposed. The experimental results of synthesis of hydrogels based on HEMA/ PVP at the various amounts of DMSO in the initial composition have been obtained (**Table 3**).

Comments: In examples 1–7, we used 2-hydroxyethyl methacrylate as HAMA; in example 8, we used 2-hydroxypropyl methacrylate; and in example 9, we used 2-hydroxypropyl acrylate. DMSO, dimethyl sulfoxide; Klt is the luminous

**(%)**

**σ (MPa) ε**

**(%)**

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

> **k·104 (m3 /(m2 ·h))**

29

Hydrogels based on the structured hydrophilic copolymers can be obtained due to water sorption. Water sorption by this (co)polymers occurs up to equilibrium-limited swelling of polymeric matrix due to the presence of hydrophilic groups –ОН, −С = О, −NH–, and –NH<sup>2</sup> in their structure. This process is going with different rates depending on the hydrophilic

> Volume Mass Linear *KV* <sup>=</sup> *<sup>V</sup>*\_\_\_*<sup>К</sup>*

*КМ* <sup>=</sup> \_\_\_\_ *m*max *m*0

The coefficient of linear swelling is within 1.13…1.20 [51], and the amount of water content is

<sup>O</sup> <sup>=</sup> *<sup>m</sup>*max \_\_\_\_\_\_\_ <sup>−</sup> *<sup>m</sup>*<sup>0</sup> *m*max

*KL* <sup>=</sup> *<sup>L</sup>*\_\_\_\_ max *L*0

⋅ 100% (7)

is the initial mass of the sample

(6)

**3. Effect of the amount of grafted PVP on the sorption parameters of** 

properties of polymer network and volume (bulk) of block sample. Equilibrium swelling is characterized by the coefficient of swelling:

**Table 3.** Influence of the amount and nature of solvents on the properties of hydrogels.

**Contents of the components, mass parts Mn (kg/mole) Klt**

**О DMSO**

transmission factor, and k is the permeability coefficient of water.

 20 100 0 24 95 0.40 235 52 20 99 1 25 95 0.40 235 53 20 90 10 31 95 0.41 240 57 20 80 20 41 95 0.41 245 63 20 70 30 52 95 0.42 250 70 20 60 40 65 95 0.42 255 76 20 40 60 67 — — — 77 20 80 20 38 96 0.43 230 57 20 80 20 44 94 0.40 240 70

**HAMA1 PVP Н<sup>2</sup>**

within 20…90%, which can be calculated with the equation:

where mmax is the mass of the sample after swelling and m0

W<sup>H</sup><sup>2</sup>

*V*0

**copolymers**

before swelling.

where L is the linear swelling coefficient, ρp is the polymer density (kg/m3 ), ν<sup>s</sup> is the molar volume of the solvent (m3 /(kg⋅mole)), and μ is the parameter of polymer-liquid interaction:

$$
\mu = 0.5 - \frac{\nu\_\circ \sigma\_\circ L^4}{RTU\lambda^2 - \lambda^{-4}\!\!/\!/} \tag{2}
$$

where σ∞ is the equilibrium voltage (kgf/m2 ):

$$
\lambda = 1 + \varepsilon, \ 0 < \varepsilon < 0.3 \tag{3}
$$

where ε is the equilibrium voltage strain.

Profitability for practical realization under the circumstances of predicted synthesis dependence of the Мn on the amount of DMSO as proton acceptor to water (A)

$$A = \ln \frac{6.25}{K\_d} - 6.875\tag{4}$$

Using this dependence, the exponential dependence of Мn on Кst is offered:

$$M\_n = M\_n^0 \cdot \exp\left(\text{--} \text{2.9 K}\_{\text{sl}}\right) \tag{5}$$

where *Mn* 0 is the molecular weight mass of the fragment of macrochain between two neighboring cross-linking nodes by Кst = 0 (in DMSO).


Comments: In examples 1–7, we used 2-hydroxyethyl methacrylate as HAMA; in example 8, we used 2-hydroxypropyl methacrylate; and in example 9, we used 2-hydroxypropyl acrylate. DMSO, dimethyl sulfoxide; Klt is the luminous transmission factor, and k is the permeability coefficient of water.

**Table 3.** Influence of the amount and nature of solvents on the properties of hydrogels.

The molecular weight of the fragment of macrochain between two neighboring cross-linking

**Figure 3.** Dependences of the internodal molecular mass Mn (1) and grafting efficiency f (2) on the constant K of complex

0.5 <sup>−</sup> *<sup>μ</sup>* (1)

/(kg⋅mole)), and μ is the parameter of polymer-liquid interaction:

λ = 1 + ε, 0 < ε < 0.3 (3)

− 6.875 (4)

<sup>0</sup> ⋅ exp(−2.9 *Kst*) (5)

\_\_\_\_\_\_\_\_\_ *RT*(*λ*<sup>2</sup> − *λ*<sup>−</sup>1)

):

Profitability for practical realization under the circumstances of predicted synthesis depen-

*Kst*

is the molecular weight mass of the fragment of macrochain between two neighbor-

), ν<sup>s</sup>

, (2)

is the molar

nodes has been calculated according to the following formula:

formation between 2-hydroxyethyl methacrylate and polyvinylpyrrolidone.

where L is the linear swelling coefficient, ρp is the polymer density (kg/m3

dence of the Мn on the amount of DMSO as proton acceptor to water (A)

Using this dependence, the exponential dependence of Мn on Кst is offered:

*<sup>М</sup><sup>n</sup>* <sup>=</sup> *<sup>L</sup>*<sup>5</sup> *<sup>ρ</sup><sup>p</sup> <sup>ν</sup>* \_\_\_\_\_*<sup>s</sup>*

*<sup>μ</sup>* <sup>=</sup> 0.5 <sup>−</sup> *<sup>ν</sup><sup>s</sup> <sup>σ</sup>*<sup>∞</sup> *<sup>L</sup>*<sup>4</sup>

where σ∞ is the equilibrium voltage (kgf/m2

where ε is the equilibrium voltage strain.

*A* = ln\_\_\_\_ 6.25

*Mn* = *М<sup>n</sup>*

ing cross-linking nodes by Кst = 0 (in DMSO).

where *Mn* 0

volume of the solvent (m3

28 Hydrogels

The dependencies which are appropriate for analytical forecast of the copolymer structure have been proposed. The experimental results of synthesis of hydrogels based on HEMA/ PVP at the various amounts of DMSO in the initial composition have been obtained (**Table 3**).
