**3. Results and discussions**

#### **3.1. Characterization of hydrogels**

IR spectra confirm the formation of a copolymer of acrylamide and acrylic acid, as seen from the bands that appeared in the range of 3100–3500 cm−1 (O─H and N─H stretching) (**Figure 2**). Absorption bands located in the region 3350–3330 and 3200–3185 cm−1 corresponds to the asymmetrical and symmetrical stretching ─NH2 of acrylamide [21].

On the other hand, the broad absorption band of 3400–2950 cm−1 may be attributed to the ─OH of the carboxyl group. Absorption bands in the regions of 2950–2940 and 2915–2900 cm−1 correspond to asymmetric and symmetric stretching of the ─CH2 groups. In addition, the absorption band at 2790–2770 cm−1 is characteristic stretching ─CH group of the polymer chain. The stretching of the ─C═ group of acrylamide and acrylic acid in the frequency of 1653–1645 cm−1 appear in all spectra of the hydrogel composites. The typical band, which

**Figure 2.** ATR FTIR spectra of poly(AAm-co-AAc)/pectin (1), poly(AAm-co-AAc)/bentonite composites (2), and poly(AAm-co-AAc) hydrogel (3).

appeared in the range of 1620–1600 cm−1, results to deformation vibrations group ─NH2 acrylamide. The absorption band in the 1455–1445 cm−1 belongs to the stretching vibrations group ─CH2 . The absorption band, which is located in the region 1450–1410 cm−1, is a characteristic for the C─H stretching. The absorption bands at 1560 and 1406–1410 cm−1 results to the symmetric and asymmetric stretching of ─COO-acrylate (acrylic acid neutralized with NaOH). The presence of absorbed water can be seen through the band at 3300 cm−1 in all spectra of the samples.

The absorption band 987 cm−1 of Si─O─Si of bentonite in the spectra composites is shifted from the frequency of 1048 cm−1. This indicates that the composite components interact with each other by complexation to form homogeneous gels. From the FTIR it is clear that there is no significant shift in major peaks, which indicates that there is no chemical interaction between the polymer and the pectin used.

As can be seen from the SEM image (**Figure 3**), pectin is evenly distributed throughout the volume of the hydrogel. The structure of composite existing pectinate scaffold resulted in a double network architecture, where filamentous polyAA-co-AAm networks penetrated through pores of the pectin network.

#### **3.2. Study of water absorption properties**

#### *3.2.1. Measurement of swelling*

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

IR spectra confirm the formation of a copolymer of acrylamide and acrylic acid, as seen from the bands that appeared in the range of 3100–3500 cm−1 (O─H and N─H stretching) (**Figure 2**). Absorption bands located in the region 3350–3330 and 3200–3185 cm−1 corresponds to the asym-

On the other hand, the broad absorption band of 3400–2950 cm−1 may be attributed to the ─OH of the carboxyl group. Absorption bands in the regions of 2950–2940 and 2915–2900 cm−1

absorption band at 2790–2770 cm−1 is characteristic stretching ─CH group of the polymer chain. The stretching of the ─C═ group of acrylamide and acrylic acid in the frequency of 1653–1645 cm−1 appear in all spectra of the hydrogel composites. The typical band, which

**Figure 2.** ATR FTIR spectra of poly(AAm-co-AAc)/pectin (1), poly(AAm-co-AAc)/bentonite composites (2), and

of acrylamide [21].

groups. In addition, the

1, 2, 3, or 4 mas.% of pectin to solution of sodium acrylate.

correspond to asymmetric and symmetric stretching of the ─CH2

**3. Results and discussions**

100 Recent Research in Polymerization

**3.1. Characterization of hydrogels**

poly(AAm-co-AAc) hydrogel (3).

metrical and symmetrical stretching ─NH2

Swelling kinetic experiments were carried out by immersing a known amount of the dried hydrogels with 100 mL of distilled water in a constant temperature at 25°C. Gravimetrical measurement method was used to measure the swelling rate of the hydrogels. The water absorption amount Q (g/g) was calculated as follows:

$$Q = \frac{m - m\_o(1 - \gamma)}{m\_o(1 - \gamma)},\tag{1}$$

**Figure 3.** SEM image at 20.0 kV, showing surface structures of poly(AAm-co-AAc)/bentonite 3% (a) and poly(AAm-co-AAc)/pectin 3% (b).

where m0 and m (g) are the weights of the dry and swollen sample, respectively, and γ is the water content of the hydrogel. Q was calculated as grams of water per gram of dry sample.

The hydrophilic properties of poly(AAm-co-AAc) and its composites hydrogels filled with bentonite and pectin were investigated by measuring their water uptake. Water uptake values were obtained by the mass ratio of the swollen hydrogel to dried hydrogel. **Figure 4** shows the dependences of water uptake as a function of the immersion time of the hydrogels swelled in distilled water.

Polymer hydrogel swells when it is brought into contact with a distilled water and salt aqueous solution. Swelling of the polymer hydrogel continues until the forces due to swelling of the polymer balance the osmotic pressure, driving the solvent into the swollen polymer. The swelling process of hydrogels is a complicated phenomenon and involves three successive steps: (i) the diffusion of water molecules into the polymer network, (ii) the relaxation of hydrated polymer chains, and (iii) the expansion of the polymer network into the surrounding aqueous solution [22].

**Figure 4** presents the dynamic swelling data for poly(AAm-co-AAc)/pectin and poly(AAmco-AAc)/bentonite in distilled water.

For each samples, the swelling ratio increases with time until a certain point, when it becomes constant, i.e., the equilibrium state is reached. At the beginning, the swelling is very fast. Besides, all hydrogels exhibit a salt-responsive swelling. Values of *Q*eq decrease with increasing salt concentration. Moreover, the results clearly reveal that all hydrogels undergo drastic mass and/or volume change in the definite concentration range.

Poly(AAm-co-AAc)/pectin and poly(AAm-co-AAc)/bentonite hydrogel composites differ in their molecular structure (hydrogel containing dispersed filler particles and a mixture of linear and crosslinked polymers—semi-interpenetrating networks), however, the degree of crosslinking, the content of ionizable groups, and the supramolecular structure (degree of crystallinity)

**Figure 4.** Swelling curves of poly(AAm-co-AAc)/pectin semi-IPN hydrogels (a) with 0% (1), 1% (2), 2% (3), 3% (4), 4% (5) pectin and poly(AAm-co-AAc)/bentonite (b) with 0% (1), 1% (2), 2% (3), 3% (4), 4% (5), 5% (6) bentonite in water.

are the same. Bearing in mind these differences in molecular and supermolecular structure of dispersed composite and semi-IPN and all factors affecting the swelling of ionic hydrogels, we could look for some differences in poly(AAm-co-AAc)/pectin and poly(AAm-co-AAc)/bentonite swelling behavior.

It was shown that there was a decrease in the equilibrium swelling (*Q*eq) of the semi-IPN systems when pectin was added to the hydrogel systems. Incorporation of pectin into the copolymer network leads to lower degrees of swelling. The reason of this is the polymeric structure of pectin. Here, it could be said that chains of pectin were placed in the crosslinked polymeric systems, instead of crosslinked AAm and AAc molecules. So, it was seen that there was a decrease in the value of the *Q*eq, because of the decrease in the hydrophilic character at crosslinked polymeric systems. But there was generally an increase in the *Q*eq of the hydrogel systems when bentonite was added to the hydrogel systems. It was seen that there was an increase in the value of the *Q*eq, because of the increase in the hydrophilic character at crosslinked polymeric systems. However, an increase in the percentage of filling bentonite in the hydrogel composite leads to a deterioration in the swelling properties, since at high filler concentrations it acts as an additional crosslinking agent.

To characterize the effect of the swelling medium on the kinetics of water uptake of poly(AAmco-AAc)/bentonite and poly(AAm-co-AAc)/pectin hydrogel composites, kinetic modeling was conducted on the basis of the Fickian diffusion law.

#### *3.2.2. Diffusion water*

**Figure 4.** Swelling curves of poly(AAm-co-AAc)/pectin semi-IPN hydrogels (a) with 0% (1), 1% (2), 2% (3), 3% (4), 4% (5) pectin and poly(AAm-co-AAc)/bentonite (b) with 0% (1), 1% (2), 2% (3), 3% (4), 4% (5), 5% (6) bentonite in water.

and m (g) are the weights of the dry and swollen sample, respectively, and γ is the

water content of the hydrogel. Q was calculated as grams of water per gram of dry sample.

The hydrophilic properties of poly(AAm-co-AAc) and its composites hydrogels filled with bentonite and pectin were investigated by measuring their water uptake. Water uptake values were obtained by the mass ratio of the swollen hydrogel to dried hydrogel. **Figure 4** shows the dependences of water uptake as a function of the immersion time of the hydrogels swelled

Polymer hydrogel swells when it is brought into contact with a distilled water and salt aqueous solution. Swelling of the polymer hydrogel continues until the forces due to swelling of the polymer balance the osmotic pressure, driving the solvent into the swollen polymer. The swelling process of hydrogels is a complicated phenomenon and involves three successive steps: (i) the diffusion of water molecules into the polymer network, (ii) the relaxation of hydrated polymer chains, and (iii) the expansion of the polymer network into the surround-

**Figure 4** presents the dynamic swelling data for poly(AAm-co-AAc)/pectin and poly(AAm-

For each samples, the swelling ratio increases with time until a certain point, when it becomes constant, i.e., the equilibrium state is reached. At the beginning, the swelling is very fast. Besides, all hydrogels exhibit a salt-responsive swelling. Values of *Q*eq decrease with increasing salt concentration. Moreover, the results clearly reveal that all hydrogels undergo drastic

Poly(AAm-co-AAc)/pectin and poly(AAm-co-AAc)/bentonite hydrogel composites differ in their molecular structure (hydrogel containing dispersed filler particles and a mixture of linear and crosslinked polymers—semi-interpenetrating networks), however, the degree of crosslinking, the content of ionizable groups, and the supramolecular structure (degree of crystallinity)

where m0

in distilled water.

102 Recent Research in Polymerization

ing aqueous solution [22].

co-AAc)/bentonite in distilled water.

mass and/or volume change in the definite concentration range.

Many mathematical models have been proposed to describe the kinetics of hydrogel swelling. Most dynamic hydrogel swelling models are based in some way on Fick's laws of diffusion. To determine the type of diffusion of water into hydrogels the simple and commonly used method, based on the power-law expression (Eq. (3)) was applied [23]:

$$\frac{Q\_t}{Q\_\eta} = kt^u \tag{2}$$

where Qt and Qeq represent the amount of solvent diffused into the gel at time t and at infinite time (equilibrium state), respectively, k is a constant related to the structure of the network, and the exponent n is a number that determines the type of diffusion. This equation can be applied only to the initial stages of swelling, i.e., to a 60% increase in the mass of hydrogel (Qt / Qeq ≤ 0.6; log(Q<sup>t</sup> /Qeq) ≤ −0.22).

The phenomenon of water sorption by hydrogels depends on the diffusion of water molecules into the gel matrix and subsequent relaxation of macromolecular chains of the hydrogel. The mechanism of water transport in a swelling hydrogel is greatly affected by many factors, such as equilibrium water content, chemical composition of the hydrogel, swelling rate, etc. [24]. In general, three models are used to describe transport phenomena into polymer networks [25].

According to the relative rates of diffusion (Rdiff) and polymer relaxation (Rrelax), three classes of diffusion can be distinguished. For a planar geometry, the value of (i) n = 0.5 indicates a Fickian diffusion mechanism (Case I) in which the rate of diffusion is much smaller than the rate of relaxation (Rdiff ≪ Rrelax, system controlled by diffusion), (ii) n = 1.0 indicates Case II, where the diffusion process is much faster than the relaxation process (Rdiff ≫ Rrelax, system controlled by relaxation), (iii) 0.5 < n < 1.0 indicates non-Fickian (anomalous) diffusion mechanism, which describes those cases where the diffusion and relaxation rates are comparable (Rdiff ≈ Rrelax). Occasionally, values of n > 1 have been observed, which are regarded as Super Case II kinetics. When the water penetration rate is much below the polymer chain relaxation rate, it is possible to record the n values below 0.5. This situation, which is classified also as Fickian diffusion, is called as "Less Fickian" behavior.

Calculation of the exponent n and constant k was achieved by plotting the data in log-log plots, according to Eq. (3) and estimating the obtained curves by linear functions

$$
\log \frac{Q\_t}{Q\_\eta} = \log k + n \log t \tag{3}
$$

The study of diffusion phenomena of water in hydrogels is of value in that it clarifies polymer behavior. **Table 1** shows that the number determining the type of diffusion (n) is over 0.50. Hence, the diffusion of water into the hydrogel systems is generally found to have a non-Fickian character. When the diffusion type is anomalous behavior, the relaxation and diffusion time are of the same order of magnitude. Thus, both diffusion and polymer relaxation control the overall rate of water uptake. However, poly(AAm-co-AAc) has the values of n > 1, which means a Super Case II kinetics, where the diffusion process is much faster than the relaxation process (Rdiff ≫ Rrelax, system controlled by relaxation). Thus, the addition of bentonite and pectin to the hydrogel increases the rate of diffusion of water into the hydrogel composite, due to the increased hydrophilic properties of this composite. Moreover, pectin increases the rate of diffusion of water into the hydrogel to a greater extent.

#### *3.2.3. Swelling kinetics*

To describe the swelling kinetics of different hydrogel, three empirical models, namely, Peleg's, first-order, and Schott's second-order absorption kinetic model, are used.


**Table 1.** Some diffusion parameters of hydrogel systems.

Peleg proposed a two-parameter model to describe water absorption by hydrogels [26]:

$$Q\_t = Q\_0 \pm \frac{t}{k\_1 + k\_2 t'} \tag{4}$$

where *Q*<sup>0</sup> is the swelling content at *Q* = 0 (g/g d.b.), *Q*<sup>t</sup> is the swelling content at any time (g/g d.b.), *t* is the swelling time (s), *k*<sup>1</sup> is the kinetic constant of the model (h(g d.b.)/g), and *k*<sup>2</sup> is a characteristic constant of the model (g d.b.)/g). In Eq. (4), "±" becomes "+" if the process is absorption or adsorption and "−" if the process is drying or desorption.

This section examines the possibility that diffusion-controlled swelling follows fist-order kinetics, as is frequently assumed. According to first-order kinetics, the rate of swelling at any given time (*t*) is directly proportional to the uptake of swelling medium that has yet to occur before the maximum or equilibrium uptake (*Qeq*) has been reached. If Q is the uptake at time *t*, *Qeq* − *Q* is the unrealized uptake of swelling medium. If *k* is the proportionality constant between the rate of swelling and the unrealized swelling capacity, then [27]:

$$\frac{dQ}{dt} = k\_1 (Q\_{eq} - Q\_\epsilon) \,\tag{5}$$

which integrates to:

rate of relaxation (Rdiff ≪ Rrelax, system controlled by diffusion), (ii) n = 1.0 indicates Case II, where the diffusion process is much faster than the relaxation process (Rdiff ≫ Rrelax, system controlled by relaxation), (iii) 0.5 < n < 1.0 indicates non-Fickian (anomalous) diffusion mechanism, which describes those cases where the diffusion and relaxation rates are comparable (Rdiff ≈ Rrelax). Occasionally, values of n > 1 have been observed, which are regarded as Super Case II kinetics. When the water penetration rate is much below the polymer chain relaxation rate, it is possible to record the n values below 0.5. This situation, which is classified also as

Calculation of the exponent n and constant k was achieved by plotting the data in log-log

The study of diffusion phenomena of water in hydrogels is of value in that it clarifies polymer behavior. **Table 1** shows that the number determining the type of diffusion (n) is over 0.50. Hence, the diffusion of water into the hydrogel systems is generally found to have a non-Fickian character. When the diffusion type is anomalous behavior, the relaxation and diffusion time are of the same order of magnitude. Thus, both diffusion and polymer relaxation control the overall rate of water uptake. However, poly(AAm-co-AAc) has the values of n > 1, which means a Super Case II kinetics, where the diffusion process is much faster than the relaxation process (Rdiff ≫ Rrelax, system controlled by relaxation). Thus, the addition of bentonite and pectin to the hydrogel increases the rate of diffusion of water into the hydrogel composite, due to the increased hydrophilic properties of this composite. Moreover, pectin increases the

To describe the swelling kinetics of different hydrogel, three empirical models, namely,

Peleg's, first-order, and Schott's second-order absorption kinetic model, are used.

Poly(AAm-co-AAc) 0.4449 1.0507 Super Case II transport

Poly(AAm-co-AAc)/bentonite 2% 0.0029 0.8757 Non-Fickian diffusion Poly(AAm-co-AAc)/bentonite 3% 0.0031 0.9217 Non-Fickian diffusion Poly(AAm-co-AAc)/bentonite 4% 0.0025 0.9325 Non-Fickian diffusion Poly(AAm-co-AAc)/pectin 1% 0.0078 0.8277 Non-Fickian diffusion Poly(AAm-co-AAc)/pectin 2% 0.0088 0.8008 Non-Fickian diffusion Poly(AAm-co-AAc)/pectin 3% 0.0044 0.9211 Non-Fickian diffusion Poly(AAm-co-AAc)/pectin 4% 0.0057 0.8321 Non-Fickian diffusion

Poly(AAm-co-AAc)/bentonite 1% 0.0038 0.8480 Non-Fickian (anomalous) diffusion

**Hydrogel k (min−1) n Mechanism**

**Table 1.** Some diffusion parameters of hydrogel systems.

= *logk* + *n logt* (3)

plots, according to Eq. (3) and estimating the obtained curves by linear functions

*Qt Qeq*

Fickian diffusion, is called as "Less Fickian" behavior.

rate of diffusion of water into the hydrogel to a greater extent.

*log*\_\_\_

*3.2.3. Swelling kinetics*

104 Recent Research in Polymerization

$$\ln\left(\frac{Q\_{\eta}}{Q\_{\eta}-Q\_{\bullet}}\right) = k\_{\text{i}}t.\tag{6}$$

The Schott's second-order equation for swelling is [27]:

$$\frac{dQ}{dt} = k\_z (Q\_q - Q\_v)^2,\tag{7}$$

where *Qeq* is the equilibrium water swelling ratio, *Qt* is the water swelling ratio at time *t*, *k*<sup>2</sup> is the swelling rate constant, respectively. After definite integration between the limits *Q =* 0 at t = 0 and Q = Q at t = t and rearrangement, Eq. (7) can be rewritten as follows:

$$\frac{t}{Q\_{\epsilon}} = \frac{1}{Q\_{\epsilon\eta}^2} \mathbf{t}\_2 + \frac{1}{Q\_{\eta}} \mathbf{t}. \tag{8}$$

To test the all kinetics models described above, *ln*( *Qeq* \_\_\_\_\_ *Qeq* <sup>−</sup> *<sup>Q</sup>*) vs. t, t/Q vs. t, and 1/Q vs. 1/t graphs were plotted for analyzed hydrogel composites. Values of swelling rate constant and equilibrium swelling ratio, which were calculated from the slope and the intersection of the lines, respectively, are presented in **Table 2**.

**Figure 5** shows the comparison graphs of experimental data on the swelling kinetics in water with the three models described above. Many authors believe that the swelling kinetics behavior of ionic hydrogels (anionic and cationic hydrogels) is in good accordance with Schott's second-order diffusion kinetics [28, 29]. As can be seen from the **Table 2**, the most accurate process of swelling of hydrogel filled with bentonite describes the first-order kinetic model. Values of the calculated equilibrium degree of swelling *Qeq calc* are in good agreement with the experimental data. A second-order kinetic model also yields adequate results. For gels with pectin, all models give an acceptable result, although the most accurately nevertheless the kinetics of swelling describes a model of the first order.


**Table 2.** Swelling kinetic parameters for poly(AAm-co-AAc)/bentonite and poly(AAm-co-AAc)/pectin hydrogel composites in distilled water.

**Figure 5.** Comparison of the experimental swelling data with first-order, second-order kinetic models, and Peleg's model for a poly(AAm-co-AAc)/bentonite 4% (a) and poly(AAm-co-AAc)/pectin 4% (b).

**Figure 6.** Collapse curves of poly(AAm-co-AAc)/bentonite composites and poly(AAm-co-AAc)/pectin composites filled with 0% (1), 1% (2), 2% (3), 3% (4), and 4% (5) bentonite (a) and pectin (b) in a 1 M solution of calcium chloride.

#### *3.2.4. Collapse kinetics*

**Content of bentonite in hydrogel composites (%)**

**0**

*Peleg's model*

k1 k2 R2 Qexp Qcalc

276.749 *First-order absorption kinetic model*

kR1

R2 Qcalc

244.305 *Second-order absorption kinetic model*

kR2

R2 Qcalc **Table 2.**

261.096

280.899

305.810

278.551 Swelling kinetic parameters for poly(AAm-co-AAc)/bentonite and poly(AAm-co-AAc)/pectin hydrogel composites in distilled water.

265.251

189.753

207.039

166.389

166.666

0.9975

0.9960

0.9955

0.9961

0.9948

0.9995

0.9996

0.9992

0.9994

1.03E−5

0.93E−5

0.76E−5

1.23E−5

1.02E−5

3.89E−5

3.48E−5

3.50E−5

2.63E−5

259.474

277.954

254.178

237.243

185.447

200.764

161.373

158.642

0.9808

0.9968

0.9957

0.9814

0.9920

0.9972

0.9995

0.9967

0.9995

0.0008

0.0012

0.0011

0.0020

0.0017

0.0047

0.0040

0.0042

0.0027

140.4422

136.1533

259.1549

120.0033

216.941

226.848

210.621

170.177

247.0146

259.77287

278.517

254.182

237.26

185.447

200.764

161.373

158.642

0.9989

0.9977

0.9973

0.9995

0.9938

0.9987

0.9988

0.9984

0.9994

0.004

0.0069

0.0071

0.0036

0.0080

0.0044

0.0042

0.0044

0.0056

2.1810

1.3559

1.4929

1.4593

1.7098

1.1032

0.9791

1.6753

1.7822

106 Recent Research in Polymerization

**1**

**2**

**3**

**4**

**1**

**2**

**3**

**4**

**Content of pectin in hydrogel composites (%)**

Deswelling (or collapse) kinetic experiments were carried out by immersing a known amount of the fully swollen hydrogels at 25°C of the distilled water in various solutions with different ionic strength. At predetermined time intervals, the hydrogels were taken out and weighted. The percentage water retention was calculated using the following equation:

$$\mathcal{Q} = \frac{m\_2 - m\_o}{m\_o} \text{ } \tag{9}$$

where *m*<sup>2</sup> (g) and *m*<sup>0</sup> (g) are the weights of swollen hydrogel and of the original dry hydrogel in solution with different ionic strength.

**Figure 6** shows the kinetic curves of collapse in a 1 M solution of calcium chloride. It can be seen from the figure that the collapse of composite gels occurs with both to bentonite, and to pectin. However, it can be noted that the presence of bentonite and pectin in polyelectrolyte hydrogel composites to some extent prevents collapse, reducing the rate of desorption of water into the solution. This effect is most typical for polymer hydrogels with higher filler content. **Figure 6** shows that the collapse curves have a high collapse rate at the initial section for an unfilled hydrogel than for a polymer composite with 5 wt.% bentonite and 4% pectin. A plausible explanation is that the spatial interactions between the bentonite plates exclude the further collapse of the gels. As can be seen from **Figure 6b**, a hydrogel with a semi-interpenetrating network has a lower collapse rate, which indicates large spatial interactions between the polymer network and the polysaccharide chains, as compared to bentonite particles.

The first-order collapse rate constants calculated from Eq. (6) are shown in **Table 3**. It can be seen from the table that the rate constants of collapse in various salts decrease with increasing percentage of bentonite and pectin filling in the polymer composite, which indicates a decrease the rate of collapse depending on the content of fillers.


\*Kinetic constants are calculated at the initial stage of the collapse of hydrogels (0–50 min).

**Table 3.** The rate constants of the collapse, calculated from the Peleg's model and the first-order kinetic model.

**Figure 7** compares the experimentally obtained dependence of Q on the time of the collapse process with the theoretical dependences obtained in the approximation of the experimental data to the dependences (6) and (4) corresponding to the kinetic models of the pseudo-firstorder collapse and Peleg, respectively. As can be seen from the graph, the kinetics of the collapse of the poly(AAm-co-AAc)/bentonite hydrogel in salt solutions is most appropriately described by Peleg's model (**Figure 7a**). However, to describe the kinetics of the hydrogel collapse with a semi-interpenetrating network—poly(AAm-co-AAc)/pectin, the pseudo-firstorder model is most suitable.

#### *3.2.5. Equilibrium sorption studies*

**Figure 6** shows the kinetic curves of collapse in a 1 M solution of calcium chloride. It can be seen from the figure that the collapse of composite gels occurs with both to bentonite, and to pectin. However, it can be noted that the presence of bentonite and pectin in polyelectrolyte hydrogel composites to some extent prevents collapse, reducing the rate of desorption of water into the solution. This effect is most typical for polymer hydrogels with higher filler content. **Figure 6** shows that the collapse curves have a high collapse rate at the initial section for an unfilled hydrogel than for a polymer composite with 5 wt.% bentonite and 4% pectin. A plausible explanation is that the spatial interactions between the bentonite plates exclude the further collapse of the gels. As can be seen from **Figure 6b**, a hydrogel with a semi-interpenetrating network has a lower collapse rate, which indicates large spatial interactions between the polymer network and the polysaccharide chains, as compared to ben-

The first-order collapse rate constants calculated from Eq. (6) are shown in **Table 3**. It can be seen from the table that the rate constants of collapse in various salts decrease with increasing percentage of bentonite and pectin filling in the polymer composite, which indicates a

Poly(AAm-co-AAc)/bentonite k1 0.06488 0.04719 0.02848 0.02904 0.01437

Poly(AAm-co-AAc)/pectin k1 0.02965 0.03337 0.08836 0.04875

Poly(AAm-co-AAc)/bentonite k1 0.029 0.04313 0.02999 0.03526 0.05486

Poly(AAm-co-AAc)/pectin k1 0.06199 0.04873 0.03695 0.03544

**Table 3.** The rate constants of the collapse, calculated from the Peleg's model and the first-order kinetic model.

**0 1 2 3 4**

k2 0.00764 0.00569 0.00706 0.00795 0.0077 R<sup>2</sup> 0.99596 0.98497 0.98842 0.99 0.99871

k2 0.00274 0.00308 0.00806 0.01068 R<sup>2</sup> 0.98704 0.99775 0.99516 0.9369

R<sup>2</sup> 0.86309 0.86428 0.88308 0.8132 0.97665

R<sup>2</sup> 0.99515 0.98231 0.99362 0.99761

**\* 0.11433 0.08432 0.04918 0.0348 R2\* 0.99147 0.99804 0.98799 0.96057**

**\* 0.08591 0.12385 0.14159 0.11425 0.07411 R2\* 0.92486 0.97008 0.97526 0.99654 0.99946**

decrease the rate of collapse depending on the content of fillers.

**Sample Percentage of filler in the composite (%)**

**k1**

**k1**

\*Kinetic constants are calculated at the initial stage of the collapse of hydrogels (0–50 min).

tonite particles.

108 Recent Research in Polymerization

Peleg's model

First-order kinetic model

It is known that bentonite possesses good sorption properties and is used to purify water from pollutants. Articles [30, 31] cite data that bentonite is added to composite materials in order to increase the sorption properties of the material. It is also known that pectin is an enterosorbent [32]. Pectins have complex-forming ability based on the interaction of a molecule of pectin with ions of heavy metals and radionuclides. Due to the presence of a large number of free carboxyl groups in molecules, it is the low-esterified pectins that are most effective.

In the present work, the absorption properties of the synthesized polymer compositions in aqueous salt solutions of Pb(NO3 )2 were to be studied. To observe the sorption of Pb2+ ions onto poly(AAm-co-AAc) hydrogel systems containing bentonite and pectin, the hydrogel systems were placed in aqueous solutions of Pb(NO3 )2 and allowed to equilibrate for 1 day at 25°C. The concentration of sorbed ions was determined by the voltammetric method.

Equilibrium adsorption isotherms of poly(AAm-co-AAm), poly(AAm-co-AAc)/bentonite and poly(AAm-co-AAc)/pectin hydrogel systems are presented in **Figure 8**.

**Figure 7.** Experimental data and data obtained with first-order models and Peleg's model for poly(AAm-co-AAc)/ bentonite 1% (a) and poly(AAm-co-AAc)/pectin 1% (b) hydrogel composites.

**Figure 8.** Sorption of lead ions from solutions of various concentrations of Pb(NO3 )2 poly(AAm-co-AAc) hydrogel (1) and poly(AAm-co-AAc)/pectin (a) hydrogels with 1% (2), 2% (3), 3% (4), and 4% (5) pectin and poly(AAm-co-AAc)/ bentonite (b) hydrogels with 1% (2), 2% (3), 3% (4), and 4% (5) bentonite.

In **Figure 8**, the heavy metal ions removal capacity (mg amount of sorption Pb2+ per unit mass) of the hydrogel systems is increased with the increasing concentration Pb(NO3 )2 sorbed onto unit dry mass of the gel. Adsorption of heavy metal ions occurs due to ionic and coordination interactions with charged hydrogel groups (chemisorption).

At first, complexation takes place mainly on the surface of the hydrogel, as evidenced by the high initial rate of sorption of metal ions, possible to observe inhomogeneous distribution of ionic groups, which leads to uneven swelling of the hydrogel by volume. The experimental results show that the hydrogel-filled composites filled with pectin have greater sorption ability than the unfilled hydrogel (**Figure 8a**). The increase in the sorption of lead ions by pectin-containing hydrogels is via additional complex formation due to the presence of a large number of free carboxyl groups in the pectin molecules. Thus, from a solution with a concentration of 0.1 M lead nitrate, composites with 1 and 3% pectin are adsorbed by 20% of lead ions more than the uncharged hydrogel. However, as **Figure 8b** shows, hydrogels containing bentonite absorb a larger amount of lead ions. Moreover, the sorption capacity of bentonite-containing hydrogels is 25% higher than that of polysaccharide-containing hydrogels (as moisture absorbing properties).

#### **4. Conclusion**

Nowadays, there is a trend in development of multifunctional nanocomposites based on different types of hydrogels acting as a matrix for various nanomaterials. Crosslinked hydrophilic polymers are capable of absorbing large volumes of waters and salt solutions. Therefore, most of the modern work is devoted to the development of new composite materials based on hydrogels with improved sorption properties.

In the chapter of the book, two representatives of hydrogel composites are considered—a composite with dispersed filler and a composite with a semi-interpenetrating network and their sorption properties are compared.

Thus, incorporation of hydrophilic group containing chemicals, such as acrylic acid, polymers such as pectin, and clay such as bentonite in AAm hydrogels, can be obtained successively by the free radical solution polymerization method. Multifunctional crosslinkers such as MBA were used in the polymerization process. Poly(AAm-co-AAc)/bentonite and poly(AAm-co-AAc)/pectin hydrogel systems have showed high water absorbency. Some swelling and diffusion properties were discussed for different semi-IPNs and hydrogels prepared under various formulations. To determine the sorption characteristics of heavy metal ions such as Pb into the hydrogel systems, some sorption parameters have been calculated.
