Shape memory polymers (SMPs) are a subclass of smart materials, which are able to restore their permanent shapes from their temporary shapes under certain stimuli. SMPs can be divided into one-way SMPs (1W-SMPs) and two-way SMPs (2W-SMPs), and both 1W-SMPs and 2W-SMPs can exhibit multiple shape memory effects. Recently, two-way shape memory effect (2W-SME) has been demonstrated in several types of SMPs. Unlike classical 1W-SME, 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 semi-crystalline 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 semi-crystalline 2W-SMPs, there is currently a lack of effort towards modeling the true and advanced 2W-SME within the two-mechanism framework.
Therefore, in order to comprehend the intrinsic mechanisms for true and advanced 2W-SME, a constitutive model is highly desired, which can facilitate and enhance future design, synthesis, and application of 2W-SMPs in engineering structures and devices. On this account, Louisiana State University researchers: Dr. Cheng Yan, Dr. Qianxi Yang and led by Professor Guoqiang Li developed a new thermomechanical constitutive model which can capture both the entropic elasticity and melt/crystallization events. Their work is currently published in the International Journal of Mechanical Sciences.
In their approach, the research team first described the balance equations for mass, linear momentum, and angular momentum. Next, a general three-dimensional constitutive model was presented in detail. The researchers then validated the presented model by comparing with the experimental results for a chemically crosslinked cis poly(1,4-butadiene) (cPBD) 2W-SMP with quasi 2W-SME, true 2W-SME and advanced 2WSME, respectively. Lastly, a model sensitivity analysis was conducted to rank the relative importance of the parameters involved in the model.
The authors reported that the modeling results and test results showed reasonable agreement. In fact, it was found that the model captured the three types of 2W-SMEs: quasi 2W-SME, true 2W-SME, and advanced 2W-SME. Further, the researchers found out that proper tensile programming before the first thermomechanical cycle could make a semi-crystalline SMP exhibit all the three types of 2W-SMEs.
In summary, the study developed a new three-dimensional constitutive model within the two-mechanism framework for the quasi 2W-SME, true 2W-SME, and advanced 2W-SME of semi-crystalline polymers. The team reported that by comparing the single parameter effect and combined parameters effect, it was found that focusing on a single parameter can obtain only a limited understanding of the physics. In a statement to Advances in Engineering, Professor Guoqiang Li said their study may serve as a design tool to advance applications of semi-crystalline 2W-SMPs in engineering structures and devices.
Cheng Yan, Qianxi Yang, Guoqiang Li. A phenomenological constitutive model for semicrystalline two-way shape memory polymers. International Journal of Mechanical Sciences: Volume 177, 1 July 2020, 105552.