Tuning Self-Healing Property of Stiff Supramolecular Polymer for Flexible Electronics
Authors: Qin Jiaxu , Lin Francis , Wang Yujia , Hubble Dion , Li Yun , Yang Jihui , Jen Alex 
Published: February 2020

Organic electronics are receiving increasing attention for the development of flexible devices, due to the generally easier processability and higher flexibility of organic, compared to inorganic, materials. However, although many suitable organic materials possess some measure of elasticity, flexible electronic devices are often prone to cracking or breaking over time, resulting in poor durability and reliability. As a result, the burgeoning field of self-healing materials for electrochemical devices has begun to attract attention. Self-healing polymers can be designed to recover their original mechanical properties after physical damage, increasing their useful lifetime. Therefore, development of self-healing polymers is highly desirable to accommodate increasing demand for flexible devices.

For decades, soft gels, elastomers, and hybrid organic-inorganic systems with self-healing properties have been designed based on reversible interactions and dynamic covalent bonds. However, previous approaches typically sacrifice stiffness to achieve self-healing properties, mostly resulting in materials with tensile moduli less than 1 MPa. These soft and deformable materials are less likely to maintain reliable and stable pathways for electron or ion movement in devices. Additionally, considering different application conditions, how to modulate these self-healing and mechanical properties in response to various environment cues, such as temperature, pH, or light is vital for improving long-term mechanical integrity as well. In particularly, tunability based on temperature is widely useful for initiating self-healing ability and controlling stiffness for flexible electronics, which will widen their ultimate potential applications.

Herein, we report a supramolecular polymer system based on pi-pi interaction between naphthalenediimide (NDI) and pyrene (Py) derivatives, which possesses high stiffness (Young’s modulus > 69 MPa) while also retaining mild self-healing temperature (< 50oC) and inherent ionic conductivity (> 10-6
S/cm at 50°C when doped with LiTFSI). Additionally, we demonstrate the tunability of the pi-pi interaction modes by doping small molecule additives into the polymer, which can modulate the interaction strength and crosslinking density in the system. The self-healing temperature and Young’s modulus are tunable in a wide range (30-60oC, 69-219 MPa) to satisfy different potential applications, such as artificial skin and wearable batteries. We demonstrate our polymer possesses decent stretchability (100% strain without cracking), flexibility, and ability to heal cracking at human body temperature, which shows application potential for wearable electronics. Compared to softer self-healing polymers reported previously, our system not only transcends the currently-available regime of mechanical properties in self-healing systems, but also provides a general strategy for tuning self-healing and mechanical properties in supramolecular polymers.