Modeling the thermo-mechanical behavior and constrained recovery performance of cold-programmed amorphous shape-memory polymers

The low recovery stress has limited utilizing shape-memory polymers (SMPs) as actuators. In this work, we demonstrate that the resultant recovery stress of amorphous SMPs can be significantly increased through cold programming. A three-dimensional model is formulated to describe the thermo-mechanical behavior and shape-memory performance of amorphous polymers in large deformation. The constitutive relationship is derived based on the two-temperature thermodynamic framework employing an effective temperature as a thermodynamic state variable to describe the nonequilibrium structure of amorphous polymers. The model also incorporates the molecular orientation with a relaxation mechanism to describe strain hardening. The model is applied to simulate the stress-strain relationship and stress relaxation behaviors of an amorphous SMP in the programming process and the stress response in the constrained recovery process. The model can accurately reproduce the measured stress response at different temperatures and loading rates. Both experimental and simulation results show that polymers deformed at a larger loading rate exhibit a lower stress in the holding period, though simulation pronouncedly underestimates the magnitude of stress drop in the stress relaxation process. The model also captures the magnitude and location of the peak stress during the constrained recovery process for SMPs programmed at different temperatures and with different applied strains.