**4.3. Thermal-gravimetric analysis**

**Figure 2. I.** XRD results. I PMAA/ClNa/PMAA-ClNa/Salecan/PMAA-Salecan/PMAA-Salecan-ClNa; **II.** PMAA/Cl30B/ PMAA-Cl30B/Salecan/ PMAA-Salecan/ PMAA-Salecan-Cl30B; **III.** PMAA/Cl20A/PMAA-Cl20A/Salecan/PMAA-Salecan/ PMAA-Salecan-Cl20A; **IV.** PMAA/Cl15A/PMAA-Cl15A/Salecan/PMAA-Salecan/PMAA-Salecan-Cl15A; **A**: Wide angle;

154 Current Topics in the Utilization of Clay in Industrial and Medical Applications

**B:** Low angle.

The thermal stability of the synthesized PMAA/Salecan hydrogel and hydrogel nanocomposites (PMAA/Salecan/clay) were examined by TGA analyses. Three steps of weight loss were registered (see **Figure 3**) with two maximum decomposition temperatures. The first weight loss is a result of water volatilization and the degradation of organic compounds [20].

The second and third steps are assigned to PMAA decomposition as well as the degradation of the main skeleton of Salecan. At this point, the polymer backbone is destroyed. Regarding the samples of nanocomposites obtained in the presence of simple/modified montmorillonite, some changes of the thermal stability of final materials were registered. These modifications were due to the presence of quaternary ammonium salts [4], more evident in the case of Cl15A and 20A. It is worth to mention that when Salecan was added, semi-interpenetrated networks were obtained, and the systems behave as homogenous architectures. Thus, the

TGA/DTG analyses provide information about deswelling behavior of the equilibrium swollen hydrogel samples. **Table 1** summarizes the characteristic values calculated from isotherm and derivative curves of the obtained samples. Derivative curves (see **Figure 5**) discover a slower weight change over time (50–80 min interval) in the following order: PMAA-Salecan, PMAA-Salecan-Cl20A, PMAA-Salecan-Cl15A and PMAA-Salecan-Cl30B/ PMAA-Salecan-ClNa. The samples with clay exhibit higher dehydration rate and released

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**Time (min) Weight (%) t1/2 (min) tend (min)**

**Sample Isotherm curves, 37°C Derivative curves**

**PMAA-Salecan** 74.92 91.43 72.27 79.22 **PMAA-Salecan-Cl Na** 134.81 98.83 134.10 138.82 **PMAA-Salecan-Cl 30B** 103.14 96.20 101.96 106.68 **PMAA-Salecan-Cl 20A** 93.69 97.53 92.03 96.05 **PMAA-Salecan-Cl 15A** 99.83 95.47 97.70 104.09

**Table 1.** TGA/DTG data for PMAA-Salecan and PMAA-Salecan-clay nanocomposites, respectively.

**Figure 5. I.** Weight loss as a function of time measured at constant temperature (37°C) of the equilibrium swollen hydrogel samples; **II**. Weight derivative as function of time for the equilibrium swollen hydrogel samples; PMAA-

Salecan; PMAA-Salecan–ClNa; PMAA-Salecan–Cl30B; PMAA-Salecan–Cl20A; and PMAA-Salecan–Cl15A.

**Figure 3. I.** TGA results on samples without Salecan; **II.** TGA results on samples with Salecan.

energy consumed for nanocomposites thermal destruction is within the same range regardless of Cloisite type. The growing residue is related to the presence of inorganic filler and proves the inclusion of clay into hydrogels.

#### **4.4. Swelling/deswelling behavior measurements**

The data resulted from the swelling studies are shown in **Figure 4**. The swelling kinetics demonstrated that in all the cases, the capability to absorb water is much higher for nanocomposites than for PMAA/Salecan hydrogels. In the case of nanocomposite hydrogel based on ClNa, the swelling degree is about eight times higher than for pure hydrogel. Also, as can be noticed, the swelling degree (SD) decreases as we move from the hydrophilic ClNa to the most hydrophobic modified montmorillonite, Cl15A. Increasing the hydrophobicity of the clay, we designed a highly cross-linked hydrogel with intercalated/exfoliated silica lamellae, which lowers the freedom of polymeric chains and blocks the inflow of water to the network. This aspect should also be taken into consideration as the capability of hydrogels to absorb water is decisive for the drug loading and release as well as for the applications where superabsorbency is necessary.

**Figure 4. I.** Time-dependent swelling profiles of the hydrogels; **II.** Representation of final degree ratio as a function of the clay type.

TGA/DTG analyses provide information about deswelling behavior of the equilibrium swollen hydrogel samples. **Table 1** summarizes the characteristic values calculated from isotherm and derivative curves of the obtained samples. Derivative curves (see **Figure 5**) discover a slower weight change over time (50–80 min interval) in the following order: PMAA-Salecan, PMAA-Salecan-Cl20A, PMAA-Salecan-Cl15A and PMAA-Salecan-Cl30B/ PMAA-Salecan-ClNa. The samples with clay exhibit higher dehydration rate and released


**Table 1.** TGA/DTG data for PMAA-Salecan and PMAA-Salecan-clay nanocomposites, respectively.

energy consumed for nanocomposites thermal destruction is within the same range regardless of Cloisite type. The growing residue is related to the presence of inorganic filler and

**Figure 3. I.** TGA results on samples without Salecan; **II.** TGA results on samples with Salecan.

The data resulted from the swelling studies are shown in **Figure 4**. The swelling kinetics demonstrated that in all the cases, the capability to absorb water is much higher for nanocomposites than for PMAA/Salecan hydrogels. In the case of nanocomposite hydrogel based on ClNa, the swelling degree is about eight times higher than for pure hydrogel. Also, as can be noticed, the swelling degree (SD) decreases as we move from the hydrophilic ClNa to the most hydrophobic modified montmorillonite, Cl15A. Increasing the hydrophobicity of the clay, we designed a highly cross-linked hydrogel with intercalated/exfoliated silica lamellae, which lowers the freedom of polymeric chains and blocks the inflow of water to the network. This aspect should also be taken into consideration as the capability of hydrogels to absorb water is decisive for the drug loading and release as well as for the applications where super-

**Figure 4. I.** Time-dependent swelling profiles of the hydrogels; **II.** Representation of final degree ratio as a function of

proves the inclusion of clay into hydrogels.

absorbency is necessary.

the clay type.

**4.4. Swelling/deswelling behavior measurements**

156 Current Topics in the Utilization of Clay in Industrial and Medical Applications

**Figure 5. I.** Weight loss as a function of time measured at constant temperature (37°C) of the equilibrium swollen hydrogel samples; **II**. Weight derivative as function of time for the equilibrium swollen hydrogel samples; PMAA-Salecan; PMAA-Salecan–ClNa; PMAA-Salecan–Cl30B; PMAA-Salecan–Cl20A; and PMAA-Salecan–Cl15A.

more water compared to the pure hydrogel are as follows: 91.43% water released versus 98.83% (ClNa sample), 96.2% (Cl30B sample), 97.53% (Cl20A sample) and 95.47% (Cl15A) at constant weight. We can notice that the samples with clay lost more water than PMAA-Salecan hydrogel, and PMAA-Salecan-ClNa lost the highest amount of water. This result is well correlated with swelling studies where the PMAA-Salecan-ClNa sample absorbed the largest amount of water. It was previously proved that clay affects the swelling by crosslinking the polymer chains which restricts the mobility of the hydrogel chains [21]. This is a possible explanation for nanocomposite hydrogels which release water later than pure hydrogel. It is worth noting that the time for swelling of the hydrogels is much longer than for deswelling at the same temperature. The collapse time is shifted toward higher values for samples with clay, especially for PMAA-Salecan-ClNa (138.82 min) versus PMAA-Salecan (79.22 min). The conclusion is that the presence of clay layers acts as a barrier in hydrogel networks restricting the release of water over time. This behavior is beneficial for hydrogels where controlled drug release is crucial.

When analyzing the SIPN prepared with Cloisite and Salecan, the microstructure of the examined surfaces consists of dimensionally equivalent voids separated by thin walls. All hydrogels demonstrated homogeneous and porous architectures with the pore size significantly smaller and the pore number became greater [24]. These observations are supported by swelling studies. Thus, when the pore size and number increased, the specific surface became greater resulting in a higher swelling capacity. Differences can be seen between the samples, as another type of Cloisite is used. The morphology of the sample with unmodified clay (ClNa) is a clear and uniform network (similar to the one analyzed before the sample without Salecan). The morphology changes as the hydrophobic (modified) Cloisites participate in the process of developing the nanocomposites. The main change consists of unorganized, crystalline-luminous aggregates of clay, visible on the surface of the samples. From the analysis of the surface, ClNa seems to have a more intercalated or even exfoliated construction, the presumption being confirmed also by XRD analysis. TEM analyses (see **Figure 7**) prove the existence of intercalated/exfoliated silica lamella within the hydrogel matrix, which was previously presumed by analyzing the XRD results. The internal intercalated structure of the nanocomposites, with the presence of areas with exfoliated clay nanosheets sustains as well the conclusions from the swelling-deswelling studies

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that showed superior properties for the PMAA-Salecan-ClNa sample.

**Figure 7.** TEM micrographs for PMAA-Salecan-ClNa samples. Exfoliated lamella highlighted with blue circle.

## **4.5. Scanning electron microscopy and transmission electron microscopy**

For the SEM analysis, the synthesized hydrogel cuts were freeze dried. The necessity of performing SEM on lyophilized hydrogels comes from the idea of having the microstructure and morphology of the samples well-retained.

When the samples (see **Figure 6**) were prepared without Salecan, the microstructure of the PMAA/clay hydrogels consists of nonhomogenous areas. A more detailed analysis shows some differences between the nanocomposites, as a function of clay type. When the unmodified montmorillonite (ClNa) is used, the network is clear and more uniform. This confirms the compatibility of the hydrophilic character of clay with the similar nature of the polymer. As for the modified montmorillonite, the more hydrophobic it became, the worse the compatibility with the polymer. Therefore, it can be noticed in case of Cl30B, Cl20A and Cl15A agglomerates of clay layers [22, 23] within the polymeric structure.

**Figure 6.** SEM morphology of the PMAA-clay and PMAA-Salecan-clay nanocomposites, respectively. Comparative images obtained for samples with different types of clay.

When analyzing the SIPN prepared with Cloisite and Salecan, the microstructure of the examined surfaces consists of dimensionally equivalent voids separated by thin walls. All hydrogels demonstrated homogeneous and porous architectures with the pore size significantly smaller and the pore number became greater [24]. These observations are supported by swelling studies. Thus, when the pore size and number increased, the specific surface became greater resulting in a higher swelling capacity. Differences can be seen between the samples, as another type of Cloisite is used. The morphology of the sample with unmodified clay (ClNa) is a clear and uniform network (similar to the one analyzed before the sample without Salecan). The morphology changes as the hydrophobic (modified) Cloisites participate in the process of developing the nanocomposites. The main change consists of unorganized, crystalline-luminous aggregates of clay, visible on the surface of the samples. From the analysis of the surface, ClNa seems to have a more intercalated or even exfoliated construction, the presumption being confirmed also by XRD analysis.

TEM analyses (see **Figure 7**) prove the existence of intercalated/exfoliated silica lamella within the hydrogel matrix, which was previously presumed by analyzing the XRD results. The internal intercalated structure of the nanocomposites, with the presence of areas with exfoliated clay nanosheets sustains as well the conclusions from the swelling-deswelling studies that showed superior properties for the PMAA-Salecan-ClNa sample.

**Figure 7.** TEM micrographs for PMAA-Salecan-ClNa samples. Exfoliated lamella highlighted with blue circle.

**Figure 6.** SEM morphology of the PMAA-clay and PMAA-Salecan-clay nanocomposites, respectively. Comparative

more water compared to the pure hydrogel are as follows: 91.43% water released versus 98.83% (ClNa sample), 96.2% (Cl30B sample), 97.53% (Cl20A sample) and 95.47% (Cl15A) at constant weight. We can notice that the samples with clay lost more water than PMAA-Salecan hydrogel, and PMAA-Salecan-ClNa lost the highest amount of water. This result is well correlated with swelling studies where the PMAA-Salecan-ClNa sample absorbed the largest amount of water. It was previously proved that clay affects the swelling by crosslinking the polymer chains which restricts the mobility of the hydrogel chains [21]. This is a possible explanation for nanocomposite hydrogels which release water later than pure hydrogel. It is worth noting that the time for swelling of the hydrogels is much longer than for deswelling at the same temperature. The collapse time is shifted toward higher values for samples with clay, especially for PMAA-Salecan-ClNa (138.82 min) versus PMAA-Salecan (79.22 min). The conclusion is that the presence of clay layers acts as a barrier in hydrogel networks restricting the release of water over time. This behavior is beneficial for hydrogels

images obtained for samples with different types of clay.

where controlled drug release is crucial.

158 Current Topics in the Utilization of Clay in Industrial and Medical Applications

morphology of the samples well-retained.

**4.5. Scanning electron microscopy and transmission electron microscopy**

agglomerates of clay layers [22, 23] within the polymeric structure.

For the SEM analysis, the synthesized hydrogel cuts were freeze dried. The necessity of performing SEM on lyophilized hydrogels comes from the idea of having the microstructure and

When the samples (see **Figure 6**) were prepared without Salecan, the microstructure of the PMAA/clay hydrogels consists of nonhomogenous areas. A more detailed analysis shows some differences between the nanocomposites, as a function of clay type. When the unmodified montmorillonite (ClNa) is used, the network is clear and more uniform. This confirms the compatibility of the hydrophilic character of clay with the similar nature of the polymer. As for the modified montmorillonite, the more hydrophobic it became, the worse the compatibility with the polymer. Therefore, it can be noticed in case of Cl30B, Cl20A and Cl15A

#### **4.6. Thermomechanical properties**

In order to appreciate the influence of clay addition, moreover, the effect of clay type upon the mechanical properties of the nanocomposites, dynamic mechanical analyses were performed and are presented in **Figures 8** and **9**. As known from the literature, the storage modulus (G') is considered a way of appreciation of the extent of gel network formation. Thus, a higher G' value means a stronger gel structure.

*in house* advanced, modified clays by edge covalent bonding [13]. Differences in storage modulus values along frequency-time cycles can be observed due to the hydrogels dehydration. More challenging to analyze were the results obtained from the samples with Salecan (**Figure 9**). Generally, higher values of storage modulus are noted with the increase in hydrophobicity of the clay used. Thus, even if the storage modulus was lower than for the samples obtained with modified clay, the SIPN containing Cloisite Na were proved to be more stable with frequency and time, in comparison with those obtained with hydrophobic montmorillonites, as observed from the

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**Figure 9.** DMA results: storage modulus as a function of frequency and time; stiffness as a function of frequency and time; Tan Delta as a function of time; storage modulus as a function of temperature for: PMAA-Salecan; PMAA-Salecan-

ClNa; PMAA-Salecan-Cl30B; PMAA-Salecan-Cl20A; and PMAA-Salecan-Cl15A.

In absence of Salecan, a simple conclusion can be conceived from the data registered (**Figure 8**). In matter of storage modulus, the inclusion of the inorganic filler leads to stronger microstructure of the hydrogels as function of frequency and time, respectively. The presence of montmorillonite increases the intermolecular forces, and the hydrogel nanocomposite behaves as an elastic solid with an enhanced capacity of storing energy and not breaking under the stress applied. For example, the hydrogels obtained in the presence of Cl20A and Cl15A exhibit higher storage modulus than the neat hydrogel. It is worth mentioning that due to unexpected swelling and disintegration phenomenon that occurred in the washing stage, the samples obtained with ClNa and Cl30B could not be analyzed. Moreover, the stiffness increased with the addition of Cl20A and Cl15A clays proving once again the enhanced mechanical stability of nanocomposite hydrogels induced by the hydrophobic modified silicate nanosheets [25]. The same behavior was observed in our recent study that refers to the obtaining of newly advanced nanocomposite hydrogels based on poly(methacrylic acid) with

**Figure 8.** DMA results: storage modulus as a function of frequency and time; stiffness as a function of frequency and time; for: PMAA; PMAA-Cl20A; PMAA-Cl15A.

*in house* advanced, modified clays by edge covalent bonding [13]. Differences in storage modulus values along frequency-time cycles can be observed due to the hydrogels dehydration.

More challenging to analyze were the results obtained from the samples with Salecan (**Figure 9**). Generally, higher values of storage modulus are noted with the increase in hydrophobicity of the clay used. Thus, even if the storage modulus was lower than for the samples obtained with modified clay, the SIPN containing Cloisite Na were proved to be more stable with frequency and time, in comparison with those obtained with hydrophobic montmorillonites, as observed from the

**Figure 9.** DMA results: storage modulus as a function of frequency and time; stiffness as a function of frequency and time; Tan Delta as a function of time; storage modulus as a function of temperature for: PMAA-Salecan; PMAA-Salecan-ClNa; PMAA-Salecan-Cl30B; PMAA-Salecan-Cl20A; and PMAA-Salecan-Cl15A.

**Figure 8.** DMA results: storage modulus as a function of frequency and time; stiffness as a function of frequency and

In order to appreciate the influence of clay addition, moreover, the effect of clay type upon the mechanical properties of the nanocomposites, dynamic mechanical analyses were performed and are presented in **Figures 8** and **9**. As known from the literature, the storage modulus (G') is considered a way of appreciation of the extent of gel network formation. Thus, a higher G'

In absence of Salecan, a simple conclusion can be conceived from the data registered (**Figure 8**). In matter of storage modulus, the inclusion of the inorganic filler leads to stronger microstructure of the hydrogels as function of frequency and time, respectively. The presence of montmorillonite increases the intermolecular forces, and the hydrogel nanocomposite behaves as an elastic solid with an enhanced capacity of storing energy and not breaking under the stress applied. For example, the hydrogels obtained in the presence of Cl20A and Cl15A exhibit higher storage modulus than the neat hydrogel. It is worth mentioning that due to unexpected swelling and disintegration phenomenon that occurred in the washing stage, the samples obtained with ClNa and Cl30B could not be analyzed. Moreover, the stiffness increased with the addition of Cl20A and Cl15A clays proving once again the enhanced mechanical stability of nanocomposite hydrogels induced by the hydrophobic modified silicate nanosheets [25]. The same behavior was observed in our recent study that refers to the obtaining of newly advanced nanocomposite hydrogels based on poly(methacrylic acid) with

time; for: PMAA; PMAA-Cl20A; PMAA-Cl15A.

**4.6. Thermomechanical properties**

160 Current Topics in the Utilization of Clay in Industrial and Medical Applications

value means a stronger gel structure.

flatness of the specific ClNa curve. These data were in good agreement with swelling-deswelling results where the SIPN with ClNa proved the highest swelling degree and the slowest water release capacity. As a consequence, given the fact that the samples "lose" water when mechanically stressed, the smallest storage modulus values are registered for hydrophilic Cloisites. This phenomenon can also be explained by the presence of a smaller number of silicate sheet in the same volume of the semi-interpenetrated network (the same size of the samples subjected to DMA analysis) as a consequence of the swelling process during washing, which is although an insufficient contribution of silicate lamellae in order to assure an enhanced mechanical behavior.

more intercalated structure for the ClNa, which was confirmed by the swelling-deswelling analyses. The presence of exfoliated sheets within the SIPN network prepared with pristine clay was noticed from TEM images. TGA definitely showed that the introduction of Salecan enhanced the thermal stability of the nanocomposites, but no significant distinction was noticed between the samples of different types of inorganic filler. Speaking of mechanical properties, Salecan had a significant impact on the energy dissipation within the semi-IPNs due to its outstanding viscosity properties. The effect of Cloisite and Salecan was expressed

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The more compatible nature of ClNa with the hydrophilic character of PMAA/Salecan hydrogel provided higher swelling capacity, thus slower release in time, which is highly favorable for the designing of a drug release mechanism. As a fact, the smooth architecture of the networks developed with sodium montmorillonite is another reason to believe that the unmodified ClNa would serve as most suitable inorganic filler for the development of an efficient drug release system. But neither SIPNs obtained with more hydrophobic clays are not to fall and may be employed in controlled *co*-delivery of polar-unpolar drugs. The nontoxicity of the components used sustains the development of semi-IPN architectures with improved mechanical features and adjustable release properties for specific applications in the extensive biomedical domain.

This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS/CCCDI-UEFISCDI, project number PN-III-P2-2.1-PED-2016-

, Ioana Catalina Gifu2

and Raluca Ianchis2

, Cristian Petcu2

\*

, Silviu Preda3

1 Faculty of Applied Chemistry and Materials Science, Politehnica University of Bucharest,

2 National R-D Institute for Chemistry and Petrochemistry ICECHIM, Bucharest, Romania

3 Institute of Physical Chemistry "Ilie Murgulescu", Romanian Academy, Bucharest,

, Bogdan Trica2

,

,

by the improved storage modulus and stiffness of the samples.

**Acknowledgements**

1896, within PNCDI III.

**Conflicts of interest**

**Author details**

Tatiana Munteanu<sup>1</sup>

Elvira Alexandrescu2

Bucharest, Romania

Romania

Cristina Lavinia Nistor2

The authors declare no conflict of interest.

, Claudia Mihaela Ninciuleanu2

, Sabina Georgiana Nitu2

, Augusta Raluca Gabor2

\*Address all correspondence to: ralumoc@yahoo.com

The DMA analyses obtained for SIPN with Salecan showed increased values for storage modulus and stiffness in comparison with hydrogel nanocomposites. This fact is due to the elastic properties of the Salecan [3, 26, 27] and its –OH groups' interactions with oxygen atoms of the PMAA and the amine protons of the clay as well as between the clay surface hydroxyls and the carbonyl of the polymer. These data are well correlated with FTIR results, peak shifts, respectively, that proved the obtaining of a unique complex structure. Previous studies demonstrated that clay nanoparticles behave as a support for polymer chains which absorb/desorb onto clay sheet by thus inducing a continuous movement in the system [28]. In our case, when applying frequency or time, the whole system reacts trying to withstand, consequently, increased mechanical stability is registered even if the storage modulus was lower than for the samples obtained with modified clay.

For all the hydrogels, tanδ subunitary values indicated that the storage modulus exceeded the loss modulus independent of time, which confirms the elastic solid behavior of the nanocomposites and the dissipation of energy within the whole structure [20].

When temperature increased, the freeze-dried samples evidenced storage modulus changes around 75 and 230°C. These changes are attributed to the movement of methacrylic acid units and are related with the polymer molecular weight. The loss modulus (inset **Figure 9**) registered for SIPN-functionalized clay samples, indicated transition around 70°C due to the melting of quaternary ammonium salts, more obvious in Cl20A and Cl15A cases. The glass transition temperature (around 230°C) does not significantly change when clays were added in the system unlike other cases where the mobility of hydrogel networks was affected, indicating a reinforcing effect of the modified clay [13].
