Designing a Self-Healing Shape Memory Polymer with High Stiffness and Toughness: The Role of Nonuniform Chain Networks

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/m3. 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.