A highly self-healable polymeric system with enhanced mechanical properties is prepared by blending conventional polyurethane (PU) with functional polyimide (PI). PU and PI synthesized in this study are miscible with each other and the thermal stability of the self-healable blend (PUPI) is improved by incorporation of PI into PU. Interestingly, on adding only a small amount of PI to PU, PUPI exhibits higher self-healing efficiency and faster self-healing kinetics. Furthermore, unlike conventional self-healing materials, PUPI also has superior surface and bulk mechanical properties. A model for the mechanism for the improvement of self-healing and mechanical properties is derived by analyzing FT-IR spectra. The outstanding self-healing and mechanical properties are attributed to the unique intermolecular networks resulting from the strong supramolecular interactions between urethane groups in PU and imide groups in PI. As a result, the PI chain acts as a polymeric glue inside the PU matrix of PUPI, which results in significant enhancements in both properties mentioned previously.

The development of flexible displays and electronics is hindered by the disadvantages of polymeric window films, such as low mechanical strength, poor optical properties, and lack of folding and rolling reliability. To overcome these critical limitations, a novel colorless polyimide window film is prepared in this study using the concept of charge‐transfer complex (CTC) intensification. The resulting window film has a tensile modulus of 8.4 GPa, total transmittance of ≈90%, and yellow index below 3, which is the best recorded balance between mechanical strength and optical properties for a highly flexible optical film. Unlike commercially available optical‐grade engineering plastic films and glass substrates, the prepared window film has both pencil hardness grade over 2H and folding reliability over 200 000 folding/unfolding cycles. These remarkable properties are attributed to the unique supramolecular structure with multiple hydrogen bonding and salt complexation interactions, which exhibits CTC intensification. The CTC intensification mechanism is also proposed in this study.

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A functional polyurethane based on the heterocyclic group was synthesized and its self-healing and mechanical properties were examined. To synthesize a heterocyclic polyurethane, a polyol and a heterocyclic compound with di-hydroxyl groups at both ends were blended and the blended solution was reacted with a crosslinker containing multiple isocyanate groups. The heterocyclic polyurethane demonstrates better self-healing efficiency than the conventional polyurethane with no heterocyclic groups. Furthermore, unlike the conventional self-healing materials, the heterocyclic polyurethane examined in this study shows an outstanding recovery of the mechanical properties after the self-healing process. These results are attributed to the unique supramolecular network resulting from the strong hydrogen bonding interaction between the urethane group and the heterocyclic group in the heterocyclic polyurethane matrix.Polymers 2020, 12, 968 2 of 11 several decades [7]. While they are more susceptible to damage caused by mechanical stress because of their low mechanical surface properties, these materials can effectively self-heal the areas damaged by the mechanical stress. In light of the critical issues mentioned above, self-healable polymeric coating materials will be preferred to hard coating materials for application in flexible displays. Therefore, endowing conventional polymeric materials with the ability to self-heal both dents and scratches is considered to be vital for the development of next-generation flexible displays [8,9].Traditionally, both extrinsic and intrinsic strategies have been employed to introduce the self-healing capability to polymeric materials. Extrinsic systems depend on embedded capsules or vascular networks [10][11][12]. When the material is damaged, the embedded container will release the contained healing agents to heal the damage by polymerization or chemical reaction. However, repeated healing at the same site is inherently difficult due to the limited amount of the healing agent inside the container, and the healing ability will disappear immediately after the healing agent is depleted [7]. By contrast, intrinsic systems utilize the reversible interaction between the components of the matrix [13][14][15][16], allowing them to repeatedly self-heal the same damaged areas. It is also noted that in contrast to intrinsic systems, it is difficult for the extrinsic systems to maintain the initial optical properties after the self-healing process because of the large difference in the color and refractive index between the intact matrix and the healed region. For these reasons, it is concluded that intrinsic systems are a more appropriate solution for applications, such as in display technologies, that require excellent optical properties.Polyurethane is a widely used intrinsic self-healing material that has been extensively studied because it can effectively recover from the dents on the surface due to its high elasticity [17][18][19][20][21]. However, while polyurethane can self-heal the dents on...

Polymeric coating materials (PCMs) are promising candidates for developing next-generation flexible displays. However, PCMs are frequently subjected to external stimuli, making them highly susceptible to repeated damage. Therefore, in this study, a highly self-healing PCM based on a charge transfer complex (CTC) was developed, and its thermal, self-healing, and mechanical properties were examined. The self-healing material demonstrated improved thermal stability, fast self-healing kinetics (1 min), and a high self-healing efficiency (98.1%) via CTC-induced multiple interactions between the polymeric chains. In addition, it eliminated the trade-off between the mechanical strength and self-healing capability that is experienced by typical self-healing materials. The developed PCM achieved excellent self-healing and superior bulk (in-plane) and surface (out-of-plane) mechanical strengths compared to those of conventional engineering plastics such as polyether ether ketone (PEEK), polysulfone (PSU), and polyethersulfone (PES). These remarkable properties are attributed to the unique intermolecular structure resulting from strong CTC interactions. A mechanism for the improved self-healing and mechanical properties was also proposed by comparing the CTC-based self-healing PCMs with a non-CTC-based PCM.