Innovative polyimide-based materials with self-healing properties for space applications

Self-healing polymers are smart materials that can heal their original structure and properties after mechanical, thermal, or radiation damage. Currently, polymer-based materials with self-healing properties are emerging in the aerospace field, where they can extend the useful life of structures, starting a self-repair at the onset of damage. Indeed, a spacecraft’s surface can be damaged by impact with micrometeoroids and lunar dust, and this compromises the astronaut's safety and the success of the mission. Polyimides (PI) are polymers with suitable properties for space applications. PI-based materials possess high-temperature and chemical stability, outstanding mechanical properties, flexibility, and resistance to UV radiation, making them ideal candidates for the realization of structures in space environments. Polyimides with self-healing properties were synthesized in a non-toxic bio-based solvent (dimethyl isosorbide) following a green synthesis approach that was recently developed by our group. During synthesis, selected additives were incorporated to enhance the intrinsic self-repair ability after mechanical damage. Samples were tested before and after mechanical damage to evaluate the self-repair properties. The thermal stability was evaluated by differential scanning calorimetry (DSC). An optical tensiometer was used to determine the surface hydrophobicity and surface free energy, whereas the mechanical properties were investigated by dynamic mechanical analysis (DMA). In this work, a balance between the mechanical stiffness of the polyimides and their self-healing properties is pursued. In particular, we investigate an intrinsic and autonomous repair mechanism, which is based on either reversible supramolecular interactions or dynamic covalent bonding, and is not triggered by external stimuli. Polyimides were synthesized in a two-step procedure using different concentrations of aromatic diamines and dianhydrides. The obtained poly(amic acids) were functionalized to achieve suitable self-healing properties and then imidized by heat treatment. The reversibility and dynamicity of the chemical bonds, which are crucial for self-healing, were analysed by FTIR spectroscopy. Results in terms of the degree of imidization, chemical structure, thermal stability, hydrophobicity level, and mechanical properties were used to assess the strength and durability of the polyimide-based materials in the space environment, as well as to evaluate their self-repair ability under mechanical damage.