Stress memory materials and their fundamental platform

Abstract

Smart materials for stress applications are both sought after in the industry and are also of academic interest. Motivated by the unexpected drastic differences in the cyclic thermomechanical responses between T_g and T_m shape memory polyurethanes (SMPU), we discovered a new class of polymers known as stress-memory materials. We revealed that stress memory is not guaranteed by the shape-memory effect (SME), but instead manifests itself as a unique behaviour of shape memory polymers (SMPs) possessing the extra characteristic of an enthalpy switch. Stemming from our findings on a rubbery switch, memory stress is realized from the entropic elasticity within rubbery chains of the SMP soft segments. Enthalpy in a T_m-switch, crystal switch, can modulate this entropic energy leading to stress-memory, whereas the T_g-switch is a second-order thermodynamic transition. Thus, a model needs two basic elements: entropy domination for spring elasticity and enthalpy modulation of entropy as a switch for the stress-memory polymer networks. This forms a fundamental platform for materials development in energy, smart devices, artificial muscles, biological and physical massage systems with polymers, and high entropy ceramics and metals.