Hierarchically Porous Stimuli-Responsive Chitosan/MXene (Ti₃C₂T_x) Foams by Two-Step Crosslinking Mechanism

The ability of intelligent polymer foams to change their structure and volume phase in response to external stimuli, including temperature, pressure, light, pH, solvents, and electrical and magnetic fields, have provided new possibilities for various advanced technologies and significant research. In order to optimize the responses to external stimuli, geometric design is essential. However, the parameters of available manufacturing techniques frequently constrain the structural geometric design. Therefore, hierarchically porous structured foams will be synthesized here because the unique 3D structure can exhibit the integrated properties of low density, capability to recover from large deformation, and high mechanical properties resulting from the synergistic effects of internal structures and material incorporations. Moreover, by manipulating their internal structure, the desired properties can be delivered to the materials in fields requiring different properties. Therefore, hierarchically structured foams were designed by incorporating chitosan and MXene to optimize stimuli-response capabilities.<br/> <br/> Chitosan is a natural linear polysaccharide derived from the exoskeletons of crustaceans, such as crabs, shrimps, and others. Since over 10 gigatons of chitin are constantly present in the biosphere, chitosan is one of the most important green and renewable materials. In addition, it can generate self-crosslinking between polymer chains by employing the reaction between amino groups of chitosan, resulting in porous structures. It also has many hydrophilic groups, including amino and hydroxyl groups, generating desirable properties and qualities, including biodegradability, biocompatibility, and non-toxicity. Because of these advantages, extensive research has been reported to investigate chitosan-based materials for diverse environmental and biomedical engineering applications. <br/><br/> In addition, MXene (Ti₃C₂T_x) is a new large family of two-dimensional materials that have attracted rapidly growing scientific attention due to its extraordinary chemical and physical properties. However, it remains a challenge to keep their outstanding properties in polymeric composites. Therefore, chitosan, a biocompatible polymer, was used to develop chitosan/MXene foams. Furthermore, since MXene has hydrophilicity and high surface charges generated during the synthesis process, it allows the incorporation of MXene with chitosan through covalent bonds, hydrogen bonds, or electrostatic interactions. Therefore, the desirable combination of structure and property with versatile surface functionalities can make MXene suitable for diverse applications.<br/><br/> Chitosan/MXene foams were prepared by a two-step crosslinking reaction followed by freeze-casting and freeze-drying methods. A two-step crosslinking reaction provided a well-defined network structure exhibiting solvent stability and novel compressibility by providing strong bonding interactions. Their chemical compositions, structural morphologies, thermal, mechanical, and viscoelastic properties were characterized by FT-IR, SEM, DSC, TGA, compression, and DMA tests. In addition, their moisture- and solvent-responsive performance were observed from swelling measurement.<br/><br/> Inspired by the unique properties and structures of chitosan and MXene, this study investigates how to replicate them in stimuli-responsive composites, allowing enhanced mechanical properties and exploiting the promising stimuli-responses. Therefore, investigating the developed chitosan/MXene foams by optimizing the structure-property relationship and stimuli-responsive performance can provide flexible design characteristics. Furthermore, smart responses (e.g., shape or color transformation, optical or electrical property changes, and others) coupled with sensing or actuation can propel the stimuli-responsive materials to a critical component with various potential engineering applications.