A phenomenological constitutive model for semicrystalline two-way shape memory polymers

Recently, two-way shape memory effect (2W-SME) has been demonstrated in several types of shape memory polymers (SMPs). Unlike classical one-way shape memory effect (1W-SME), these polymers can switch back and forth repeatedly between two different shapes without subsequent programming procedure. The reversible actuation can be realized either under a constant external tensile load, which is referred to as quasi 2W-SME, or under zero load, which is referred to as true 2W-SME. Most recently, actuation under an external compressive load, which is referred to as advanced 2W-SME, has also been discovered. Test results have shown that semicrystalline 2W-SMPs exhibit 2W-SME in two distinct regions due to two different mechanisms: one is due to entropic elasticity above the crystallization temperature, and the other is due to melt/crystallization transition below the crystallization temperature. While several constitutive models have been developed over the years for semicrystalline 2W-SMPs, there is currently a lack of effort towards modeling the true and advanced 2W-SME within the two-mechanism framework. In this study, we developed a thermomechanical constitutive model which captured both the entropic elasticity and melt/crystallization events. The modeling results and test results show reasonable agreement. It is found that the model captured the three types of 2W-SMEs: quasi 2W-SME, true 2W-SME, and advanced 2W-SME. It is also found that proper tensile programming before the first thermomechanical cycle can make a semicrystalline SMP exhibit all the three types of 2W-SMEs. This study may serve as a design tool to enhance applications of semicrystalline 2W-SMPs in engineering structures and devices.