Stress memory of a thermoset shape memory polymer

Abstract

The performance of stress recovery and shape recovery are equally important for high performance shape memory polymers (SMPs) in emerging applications. However, unlike shape recovery, stress recovery does not always follow a monotonic behavior, i.e., “stress plateau,” “stress overshoot,” and “stress undershoot” can be observed. In order to reveal the complicated stress memorization and recovery behavior, this study employs a phenomenological model which considers the recovery stress as the sum of residual programming stress, memorized stress, thermal stress, and relaxed stress for amorphous crosslinked SMPs. This model is demonstrated by a stress recovery experiment in which a polystyrene based SMP was programmed at two prestrain levels above the glass transition temperature, i.e., 20% (neo‐Hookean hyperelastic region) and 50% (strain‐hardening region), and two fixation temperatures, i.e., 20°C (below Tg) and 45°C (within the Tg region), respectively. In addition, a clear distinction between the memorized stress and recovery stress is presented. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42112.

Cold, warm, and hot programming of shape memory polymers

“…As discussed above, this type of distinction is expected because the driving force is different and the level of damage in the specimens is also different. In addition, all the three specimens exhibit an “overshoot” in stress recovery, which can be explained by nonequilibrium volume change and temperature dependent stress relaxation …”

“…Step 6 is a constrained stress recovery. It consists of three components: thermal stress due to thermal expansion during heating process, release of memorized stress, and stress relaxation . The stress usually comes to a peak immediately before the onset of the glass transition zone due to the high stiffness of the SMP at glassy state.…”

“…As indicated by Wang and Li, the measured stress during the fully constrained stress recovery is not the true memorized stress. The memorized stress is the difference between the recorded or measured stress by the testing machine and the thermal stress and relaxed stress.…”

Structural origin of stress and shape recovery in shape memory starch

“…For the two samples, the shape of the stress recovery curve is the same. Firstly the stress decreases due to thermal expansion of the sample [24], then stress recovery occurs until reaching a maximum of stress recovery equal to 4 and 0.8 MPa at: 55 C and 71 C for temperatures of deformation of 55 C and 90 C respectively. It can be noticed that the maximum of recovered stress is substantially lower than the maximum of stress applied during SMCP and closer to the stress after relaxation for material deformed at 55 C. For material deformed above T g at 90 C, the recovered stress is approximately equal to stress applied during SMCP.…”

Temperature and rate dependent thermomechanical modeling of shape memory polymers with physics based phase evolution law

“…The relaxed stress -S›`oe normally takes the form of multiple relaxation procedures (Messé et al, 2001;Nguyen et al, 2010;Strobl, 2007;Wang and Li, 2015;Westbrook et al, 2011):…”