**Acknowledgements**

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. 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

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

if the storage modulus was lower than for the samples obtained with modified clay.

posites and the dissipation of energy within the whole structure [20].

affected, indicating a reinforcing effect of the modified clay [13].

features of the fabricated materials as function of Cloisite composition.

**5. Conclusion**

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 nanocom-

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

Inspired by the recent studies regarding Polymer/Salecan networks and taking into consideration the numerous researches that highlight the outstanding mechanical properties of the clays, we decided to bring an improvement and study the possibility to synthesize PMAA-Salecan hydrogel nanocomposites based on pristine and modified montmorillonite. Further, the effect of the clay type on the final properties of PMAA/Salecan/clay nanocomposites was followed. With the other parameters kept constant, the morphological and compositional analysis performed gave us the possibility of explaining the physicochemical and thermal

The designed structures were investigated with the FT-IR technique, which revealed the presence of the specific partner peaks. X-Ray data, supported by TEM images pointed out a 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- 1896, within PNCDI III.
