A comparative study on microstructure, physical-mechanical properties, and self-healing performance of two differently synthesized nanocomposite double network hydrogels based on κ-car/PAm/GO

Abstract

Physical-chemical double network (DN) hydrogels have been well recognized as new tough materials with unique structural platforms demonstrating a collection of various properties including superior mechanical strength, remarkable toughness recovery and self-healing property into a single material. However, a comprehensive study on the effect of synthesis methods of DN hydrogels is very critical as they influence their properties and performances. This study is aimed at investigating the effect of two different synthesis methods of thermal-curing and UV-curing on microstructure and physical-mechanical properties including self-healing capability, swelling and water-retaining capacity, as well as electrical conductivity of the κ-carrageenan (κ-Car)/polyacrylamide (PAm) DN hydrogel. Compared to the thermal-cured DN hydrogel, the UV-cured DN hydrogel has exhibited different mechanical behavior (plastic-like with localized necking vs. elastomeric-like), excellent mechanical properties (failure stress = 0.42 MPa vs. 0.12 MPa, failure strain = 2079%, vs. 722%, elastic modulus = 0.1 MPa vs. 0.069 MPa, and fracture energy = 3.54 MJ/m3 vs. 0.50 MJ/m3), and much greater self-healing capability. These could be explained in terms of reducing the contribution of the κ-Car physical network and also reversible interactions in the favor of increasing the cross-link density of PAm chemical network as a consequence of grafting reaction in the thermally-cured hydrogel, although the non-uniform lamellar-like structure in thermally-cured hydrogel could also play a role. By focusing on the linear and nonlinear rheological measurements, it was also demonstrated that the UV-curing method has resulted in much better thermo-reversibility of the hydrogel as a valuable property that can significantly extend the application of the κ-Car/PAm DN hydrogels. An attempt was also made to explore the effect of synthesis methods on the microstructure and physical-mechanical performance of the k-Car/PAm DN hydrogels in presence of GO nanosheets. In both synthesis methods, the presence of GO was found to have an enhancing effect on the properties of DN hydrogels. In the case of thermally-cured hydrogels, a partial reduction of GO was occurred which could, in turn, affect the performance of the hydrogels. It is hoped that this knowledge can contribute to a better understanding of the correlation between microstructure and different properties of hydrogels to precisely design a structural DN hydrogel with a special performance for the desired applications.