Designing a Self-Healing Shape Memory Polymer with High Stiffness and Toughness: The Role of Nonuniform Chain Networks
Jiaxin Shi, Tianze Zheng, Jing Yuan, Chong Qiu, Yuxuan Ha, Hao Zhang, Baohua Guo, Jun Xu
Abstract
It has long been believed that uniformly cross-linked networks are beneficial to enhancing the toughness of elastomers and hydrogels by reducing stress concentration. However, in cross-linked semicrystalline polymers, it is not clear yet whether the uniformly cross-linked network is beneficial or worse for mechanical properties, since the crystallinity associated with the cross-linking network is also a key factor influencing mechanical properties. In this work, we designed chemically cross-linked polyurethane with crystalline segments either of nonuniform length or of uniform length. The nonuniform cross-linked networks are easier to crystallize and demonstrate higher strength and ductility than the uniform networks. The enhanced crystallinity caused by nonuniform cross-linking leads to stronger mechanical properties, resulting in a balance between stiffness and toughness. The obtained crystalline polyurethane plastic with nonuniform networks is biodegradable, with a strength of 68 MPa, Young’s modulus of 178 MPa, and a toughness of 365 MJ/m 3 . Additionally, the obtained polyurethane exhibits shape memory-assisted self-healing of cracks with a width of millimeter scale, which is conducive to expanding its functionality and extending its lifespan. As a result, the molecular design strategy utilizing nonuniform long chain networks to promote crystallization in chemically cross-linked polymers is established, which provides a new method for the design of high-performance polymers.