A unified thermodynamic modeling approach for amorphous shape memory polymers

The programming of shape memory polymers (SMPs) for use in large-scale space structures and implantable medical devices is a process that is currently time-consuming, labor-intensive, and energy-intensive, particularly when carried out at high temperatures. Fortunately, SMPs can usually be induced to produce shape memory effects not only at high temperatures but also at low temperatures, which makes them a hotspot in the fields of biology, medicine, aerospace, etc. However, few studies clearly present a unified method for modeling the shape memory characteristics across disparate programming temperatures. In the paper, we develop a unified thermodynamic modeling approach for SMPs. The free energy is decomposed into a rubbery part and a glassy part with the introduction of the phenomenological theory. Consequently, the complex structure and stress relaxation mechanisms undergo significant simplification and innovation. The fully thermomechanically coupled constitutive equations are derived from the second law of thermodynamics. Subsequently, the constitutive model is employed to reproduce the shape memory effect (SME) under both high-temperature programming and low-temperature programming. The model findings are effectively compared with the thermo-mechanical experiments, resulting in a good agreement.