Self-Healing Polyimide Containing Dynamic Covalent Bonds and Aliphatic Diamine

Polyimide is an engineering plastic that has excellent thermal and mechanical properties and is widely used in various applications with electronic devices, and aerospace. Once converted from the precursor to an imidized structure with an imide ring through thermal or solution process, it is insoluble, infusible, and chemical resistant, allowing it to have advantages for a variety of applications as it can be used in high temperatures and hazardous environments. Most polyimides are processed in the form of thin films and used for electronic materials such as flexible devices, the importance of durability is increasing. If the flexible polyimide film is damaged by scratches or impacts, it is harmful to the mechanical strength of the polyimide, and device performance may deteriorate. Including soft and flexible segments in the polyimide chain lowers the glass transition temperature and increases chain mobility, making it easier to recover from damage to the film, but mechanical strength could be reduced. Therefore, efforts have been made to impart self-healing performance to polyimide while maintaining thermal and mechanical stability, and the introduction of dynamic covalent bonds (DCB) with various structures in the network has received attention.<br/>Dynamic covalent bonds include disulfide, imine, and boronate ester, and these are covalent bonds that allow reversible exchange reactions by external stimuli such as temperature, light, or catalysts. When an external stimulus is applied to the network, original bonds at exchangeable dynamic covalent bonds dissociate and new bonds are formed nearby. This process enables the material to have reprocessability and self-healing properties. Polyimide with dynamic covalent bonds exchange bonds by external stimulation but can exhibit close to the original value of thermal and mechanical properties. Therefore, when the materials are damaged or fatigued, self-healing and recovery are possible in a simple method, thereby increasing the sustainability of the material.<br/>In this study, diamine monomers with disulfide bonds and siloxane groups were incorporated into polyimide networks. the thermal, mechanical, and optical properties of polyimide can be adjusted by selecting the ratio of two types of diamine. In particular, the self-healing properties of polyimide film are related to the amount of disulfide bonds contained in the polymer network, and aliphatic diamine changes the glass transition temperature of polyimide, making dynamic covalent bond exchange reactions possible at lower temperatures. In addition, the mechanical strength of polyimide increases by the rigid aromatic segments with disulfide bonds, it is possible to prepare a self-healing polyimide film with desired thermal and mechanical properties. Polyimides with dynamic disulfide bonds can recover from mechanical damage through short-time processing at temperatures near T_g. By performing a stress-relaxation test on polyimide, the activation energy was calculated from the relaxation time during which the network was reorganized. By setting self-healing conditions of the polyimide film, and compared with the properties of the original film after process under those conditions. We confirmed the application of self-healing polyimide for use in electronic devices. Electrodes were deposited using polyimide as a substrate, and the effects of damage and recovery through self-healing to the substrate on device operation were confirmed. Self-healing polyimide can have controlled thermal and mechanical properties by disulfide-containing and aliphatic diamine monomer content, and is expected to be used in various applications using rapid self-healing through an easy process.