Stress-free two-way shape memory polyurethane with high reversible strain and excellent mechanical properties for intelligent actuators

Abstract

Stress-free two-way shape memory polymers (2W SMPs), capable of reversible transformation between two distinct shapes, hold great promise as actuator components for applications in soft robotics and other intelligent devices. However, fabricating stress-free 2W SMPs with superior mechanical properties and high reversible actuation through a simple processing method remains a significant challenge. To address this issue, we propose a strategy that utilizes hydrogen bonds to construct the skeleton phase for the fabrication of stress-free 2W SMPs. By introducing N, N-bis(2-hydroxyethyl) oxamide (BHO) chain extenders rich in amide groups, strong hydrogen bond interactions are formed in the microdomains. After programming, these temperature-sensitive hydrogen bonds generate internal stress. This stress drives crystallization-induced elongation of the semi-crystalline polymer and enables reversible actuation without the need for external stress. The resulting materials exhibit both high reversible strain (22.6 %) and excellent mechanical properties (tensile strength of 45.78 MPa and fracture strain of 1992.52 %). Furthermore, the dynamic tunability of hydrogen bonds imparts the polymer network with outstanding two-way shape memory programmability. After erasing the original programmed state at high temperature and then reprogramming it, the material can be endowed with different reversible strain behaviors, thereby allowing reuse in various application scenarios. This strategy provides an effective approach for the fabrication of high-performance stress-free 2W SMPs, which is expected to advance the development of intelligent actuation materials.