Development of Self-healable Sulfur-containing Polyimides and their Application as Multifunctional Nanocomposites

Abstract

Recent rapid advances in flexible and wearable electronics as well as 5G/6G communication technologies, have exacerbated demand for self-healing properties to enhance durability and multifunctional materials for electromagnetic interference (EMI) shielding. Thereby, interest in lightweight and flexible polyimide (PI) and PI composites for electronic applications has grown significantly. Chapter I, Introduction, summarizes self-healing polymers and their approaches, as well as the applications and recent advances of multifunctional PI composites for electronic applications. Chapter II presents the development of self-healing PIs by introducing dynamic covalent bonding, disulfide bonds (S-S). Using the disulfide bond-containing diamine, 4-aminophenyl disulfide (APD), the flexible aliphatic siloxane-containing diamine, 1,3-bis(3-aminopropyl)tetramethyldisiloxane (BAPDS), and the rigid aromatic dianhydride, 4,4'-biphthalic anhydride (BPDA), the novel PIs were synthesized and demonstrated to exhibit adjusted thermal and mechanical properties and excellent self-healing performance. The PI, which possessed satisfactory mechanical strength (66.3 ± 3.4 MPa) and toughness (4.4 ± 1.4 MJ m−3), rapidly and simply self-healed above its glass transition temperature (Tg, 155 ℃), exhibiting a tensile recovery of 92%. This PI was applied as a flexible electrode substrate, demonstrating high durability after self-healing. It also demonstrated rapid self-healing performance within 15 minutes via Joule heating. Chapter III presents the synthesis of a novel water-soluble precursor poly(amic acid) salt (PAS) using the PI developed in Chapter II, along with a strategy for fabricating multifunctional PI/CNT nanocomposites. The PAS was synthesized using two amines with different hydroxyl group contents, 3-(dimethylamino)-1,2-propanediol (DMAPD) and 4-hydroxy-1-methylpiperidine (HMP), to form salts with the carboxylate groups of PAA. PASs exhibited enhanced water solubility depending on the hydroxyl group content. This enabled PAS to act as a dispersant to improve the dispersibility of CNTs in water, as a binder for composite film formation, and as a good compatible matrix for PI/CNT nanocomposites through non-covalent and covalent interactions, thereby enhancing the mechanical, thermal, and electrical performance of the nanocomposites. The PI/CNT nanocomposite film, with an excellent electrical conductivity of 701.0 ± 70.9 S cm-1 at 50 wt% CNT content, achieved an excellent EMI SE of up to 73.9 dB at a thickness of 75 μm, outperforming the polymer/CNT composite system. This is attributed to the high conductivity enabled by excellent CNT dispersion and to the high electromagnetic wave absorption due to the layered microstructure within the film. The PI/CNT nanocomposite film developed from a precursor for self-healing PI was confirmed to have the potential for surface-damage repair via Joule heating, owing to its excellent conductivity.