Two-way Shape Memory Behavior Research Articles

Tailoring the stress-free two-way shape memory effect in sol–gel crosslinked poly(ϵ-caprolactone)-based semicrystalline networks

Two-way shape memory polymers are stimulus-responsive materials capable of changing their shape between two configurations based on an on/off thermal stimulus. While the traditional effect has been studied under the application of an external mechanical load, it was demonstrated also in the absence of an external load. Such a response only relies on a carefully tailored macromolecular architecture of the polymer combined with a specific thermo-mechanical protocol. In particular, semicrystalline networks, either consisting of a multi-phase copolymer network or a homopolymer based network with broad phase transitions, have been proposed to this aim under ad hoc thermo-mechanical histories. In this work, the two-way shape memory behavior is studied on a poly(ϵ-caprolactone)-based network, crosslinked by means of a sol–gel approach and tailored on the selection of the molecular weight of the precursor polymer. Changing the prepolymer precursor allowed to tune the melting/crystallization regions of the networks, thus the thermal region of the reversible shape memory effect. The application of properly designed thermo-mechanical cycles allowed to study the two-way shape memory effect without the application of an external load under tensile conditions. Given a specific network, the stress-free actuation of the reversible elongation-contraction cycle under tensile conditions was induced across its specific melting/crystallization region. The extent of the effect was found to depend on the crystalline fraction remaining for the given actuation temperature and on the tensile stretched state imposed on the materials during the training step. The results were compared with the response achieved under the traditional two-way shape memory protocol under stress. The stress-free two-way shape memory effect was also successfully demonstrated and emphasized, under flexural conditions, which suggests the potential of these materials as intrinsically reversible actuators, promising for applications in the biomedical field and/or for soft robotics.

A thermo-mechanical constitutive model for triple-shape and two-way shape memory polymers

Semicrystalline polymers often have diverse molecular configurations in response to the variation of temperature, making it easier to realize multiple or two-way shape memory effects (SMEs). In order to provide guidance for the design of relevant smart structures, it is necessary to develop corresponding constitutive models to better characterize the thermal-mechanical behavior of such shape memory polymers (SMPs). For the first time, a thermo-mechanical finite deformation constitutive model is presented to reveal the deformation mechanism of the triple-shape two-way SME of semicrystalline SMPs in our work, based on the theory of thermodynamics with internal state variables. To verify the validity of the model, the model results are compared with the test data for a typical shape memory cycle. It is found that the model can be employed to describe the non-equilibrium response of semicrystalline SMPs with two types of crystallites in the vicinity of crystallization and melting. Since the model results fit well with the test data, the effectiveness of the model in predicting the triple two-way shape memory behavior is demonstrated.

Laser deposition of titanium layer to enhance two-way shape memory behavior of Nitinol sheet

One of the long-standing mechanical challenges associated with shape memory alloys (SMAs) is achieving and stabilizing an evolving two-way shape memory effect (TWSME) through thermomechanical training/cycling. In most martensitic NiTi SMAs that exhibit zero TWSME at room temperature, the TWSMEs typically require hundreds of cycling repetitions to track trivial values. This demonstrates a promising solution to overcome this challenge. The surface of NiTi SMA sheets was treated by depositing a certain number of titanium (Ti) powder layers on the NiTi surface using a laser beam. The results showed that a unique TWSME of approximately 12% with good stability was achieved after 100 cycles when the optimum number of five Ti layers was deposited, which was much higher than that of the NiTi sheet (0.3% TWSME) and the sample with a single-time laser path irradiation (1.8%). This was attributed to the low coefficient of thermal expansion of the Ti vs. NiTi sheet, which prevented reverse bending during heating, and the formation of Ti-rich intergranular Ti2Ni(Ox) precipitates, which hindered dislocation movement. Meanwhile, in this sample, owing to the generation of a large residual tensile stress and a large heat-affected zone that led to a reduction in plastic zones, the one-way shape memory effect (OWSME) was retained at values above 90% with good stability over cycling. However, the introduction of additional Ti layers can cause problems in terms of oxygen accumulation within the NiTi sheet, which should be considered when designing a product.

Enhancing two-way shape memory behavior of polycaprolactone nanofibers through aligned crystallization optimization

Stress-free two-way shape memory polymers capable of reversible shifting between two distinct shapes offer versatile platforms for the development of future smart devices. However, improving the recovery performance of bidirectional shape memory polymers with good biocompatibility and stimulus temperature remains challenging. Studies have shown that enhancing oriented crystallization can effectively improve shape recovery performance. Herein, we demonstrate a simple approach to enhance the number of nanofibers aligned in the tensile stress direction by controlling the alignment direction of polycaprolactone nanofibers, which promotes the generation of oriented crystallization in the tensile state, thereby achieving a 230.8% improvement in recoverable strain. The optimization of the nanofiber structure offers a generalized method for enhancing the two-way shape memory behavior of semicrystalline polymers.

Two-Way Reversible Shape Memory Behavior of Chitosan/Glycerol Film Triggered by Water.

Reversible shape memory polymers (SRMPs) have been identified as having great potential for biomedical applications due to their ability to switch between different shapes responding to stimuli. In this paper, a chitosan/glycerol (CS/GL) film with a reversible shape memory behavior was prepared, and the reversible shape memory effect (SME) and its mechanism were systematically investigated. The film with 40% glycerin/chitosan mass ratio demonstrated the best performance, with 95.7% shape recovery ratio to temporary shape one and 89.4% shape recovery ratio to temporary shape two. Moreover, it shows the capability to undergo four consecutive shape memory cycles. In addition, a new curvature measurement method was used to accurately calculate the shape recovery ratio. The suction and discharge of free water change the binding form of the hydrogen bonds inside the material, which makes a great reversible shape memory impact on the composite film. The incorporation of glycerol can enhance the precision and repeatability of the reversible shape memory effect and shortens the time used during this process. This paper gives a hypothetical premise to the preparation of two-way reversible shape memory polymers.

An analytical approach to the stress relaxation behavior of a low temperature shape-memory fabric based on viscoelastic models

Shape-memory materials are a promising new class of smart materials with many applications such as strain sensors, artificial muscles, and smart breathing textiles. These materials are subjected to force and extension in situ. Thus, the time-dependent behavior of these materials can play an important role in their long-term performance. The present study was conducted to investigate the time-dependent behavior of a type of shape-memory fabric. Nanoclay-reinforced polylactic acid/thermoplastic polyurethane was used as the precursor. The yarn that was produced was highly twisted. The twisted yarn was then shaped into a coiled structure by mandrel annealing. This yarn was then used to produce knitted fabric. The fabric was examined under both cold (25 °C) and hot (50 °C) conditions. The fabric contracted in hot water in the course direction but did not show a significant contraction in the wale direction. It returned to its original width in cold water. This effect was observed repeatedly over several cycles. This shows that the knitted fabric composed of the precursor twisted-coiled yarn exhibited low-temperature actuation and reversible two-way shape-memory behavior. A value of %16 was calculated for the contraction stroke along the course direction. The stress relaxation behavior of the two-way shape-memory fabric was then studied and analyzed. For this purpose, four different viscoelastic models were considered: the standard linear model (a), Burgers model (b), Jeffrey model (c), and Kelvin-Voigt-Maxwell model (d). We used the curve fitting procedure to find the best fit to the experimental data based on the least-squares method. The results showed that the Kelvin-Voigt-Maxwell model (model d) exhibited a higher and more acceptable regression coefficient (R 2) than the other three models. The Jeffrey model showed the lowest regression coefficient (R 2), thus confirming that it is not suitable for explaining the relaxation behavior of the fabric. However, a limitation of Model (d) is that it is not in line with the experimental loading stage. To modify the model, we propose the replacement of the dashpot with a dynamic frictional element. The results indicate that the proposed dynamic friction model can eliminate the limitations of the dashpot body during the loading stage.

Shape memory behaviors of 3D printed liquid crystal elastomers

As soft active materials, shape-memory polymers (SMPs) and liquid crystal elastomers (LCEs) have attracted considerable research interest due to their potential applications in various areas. SMPs refer to polymeric materials that can return to their permanent shape in response to external stimuli, such as heat, light, and solvent. In this sense, LCEs can exhibit intrinsic shape-memory behaviors since LCEs can switch between two shapes with temperature change due to the order-disorder transition of liquid crystals. In this work, we fabricate both the polydomain and monodomain nematic LCEs through direct ink writing 3D printing. With increasing the temperature of the substrates, the printed LCEs change from the monodomain state to the polydomain state. For polydomain LCEs, a reversible shape change can occur upon constant loading, while the monodomain can switch the shape with temperature in the stress-free state. This two-way shape-memory behavior is caused by the nematic-isotropic phase transition. We further show that the printed LCEs exhibit a good one-way shape-memory effect due to glass transition. The shape recovery region increases with the programming temperature, which is a typical temperature memory effect. Finally, it is demonstrated that complex shape-memory performance can be designed by combining one-way and two-way shape-memory effects. Specifically, for the monodomain LCEs, with increasing temperature, the programmed shape first recovers, and a second shape change can further occur due to the nematic-isotropic transition.

A 1D physically based constitutive model for two-way shape memory effects in semicrystalline networks

Two-way shape memory effects (2 W-SMEs) have been recently demonstrated in several semicrystalline networks, the underlying mechanism of which is revealed as crystallization induced elongation (CIE) and melting induced contraction (MIC). In this article, a one-dimensional physically based constitutive model, considering the crystallization micro-mechanism at chain-scale, is proposed to describe the CIE and MIC phenomena, as well as the stress-strain-temperature relations of semicrystalline polymers in typical two-way shape memory cycles. The overall model is implemented into Mathematica and a quantitative evaluation of the model is performed by comparisons with the two-way shape memory behaviors of several kinds of semicrystalline networks.

NiTi shape memory alloy pipe couplers for ultra-high vacuum systems: development and implementation

Novel Nickel–Titanium (NiTi)-based shape memory alloy (SMA) pipe couplers were designed and developed. They are suitable for room-temperature ultrahigh vacuum (UHV) systems and provide a quick and compact solution at reasonable cost. Their use is particularly interesting for restricted-access areas of particle accelerators as their installation/dismounting can be performed remotely by temperature variations. A NiTi SMA with suitable composition was selected. NiTi couplers with different diameters in the range 30–135 mm were manufactured and thermo-mechanically trained to exhibit a proper two-way shape memory behaviour which comply with the strict operative constraints for coupling applications in room temperature vacuum sectors. The connectors are easily implementable as they were designed to be compatible with commercially available flanges (DN16, DN25, DN100) used worldwide in vacuum systems. The effect of the SMA joint geometry on the thermo-mechanical response and vacuum performance was investigated by numerical studies and experimental analyses such as strain-gauge, extensometer and leak-tightness tests performed under different operating conditions including static axial loads and multiple thermal cycles. It was demonstrated that NiTi-based connectors can be thermally mounted upon heating and can guarantee the leak tightness of the vacuum pipe within a suitable temperature window. The thermal dismounting was also verified by cooling the couplers down to subzero temperatures (lower than −40 °C). Possible use of these connections at European Organization for Nuclear Research (CERN) is foreseen in vacuum assemblies installed in high radioactive areas, like those nearby particle collision points and beam collimators. Thanks to their compactness, SMA couplers are also of great interest for connecting beam pipes with small aperture such as those studied for the electron–positron future circular collider (FCC-ee) and next-generation synchrotron light facilities.

Novel two-way multiple shape memory effects of olefin block copolymer (OBC)/polycaprolactone (PCL) blends

The shape memory properties of olefin block copolymer (OBC)/polycaprolactone (PCL) crosslinked blends at different blend compositions were investigated. This is the first OBC-based two-way shape memory blends processes without external load under thermal and vapor stimuli. The deformation temperature of 80 °C was selected far away between the respective melting temperatures of PCL (56.5 °C) and OBC (110.3 °C) for the thermo-triggered two-way multiple shape memory behavior. The internal stress developed between the unmelted OBC crystalline region and crystallized PCL during cooling under pre-strain conditions played an important role in further melting-induced contraction and cooling-induced elongation without external load, even though the deformation temperature caused the complete melting of the crystalline region of PCL and the crystallinity of unmelted OBC was very small. As for the solvent vapor-triggered cases, the THF and n-hexane solvents were used. The bent sample shrank in response to the vapor stimulation, followed by expanding in the air. The blends remarkably showed rare solvent vapor-triggered reversible shape memory behavior without any external stress and pre-soaking treatment. This interesting finding was not reported earlier until now, to the best of our knowledge. This current finding unveils numerous possibilities to prepare multi-triggered blending systems with reversible shape memory behavior.

Cyclic pseudoelastic training and two-way shape memory behavior of a NiTi alloy with small irrecoverable plastic strains:numerical modeling

Novel applications of shape memory alloys (SMAs) in various fields of medicine and engineering necessitate theoretical models that can capture non-inherent material behaviors such as the two-way shape memory effect (TWSME). Most often, this requires the use of completely different sets of models or parameters to distinguish the “virgin” material exhibiting the fundamental behaviors like superelasticity (also known as pseudoelasticity), pseudoplasticity, and one-way shape memory effect, from the “trained” alloy showing the TWSME. Although using a single model (or set of parameters) can provide much insight on the changes within the material during various loading paths and histories from the initial virgin state to the trained state, it is quite challenging. In this study, an attempt was made to use a developed general multi-mechanism material model and a single set of parameters to predict both the inherent pseudoelastic behavior and non-intrinsic two-way shape memory effect characteristics of a NiTi shape memory alloy. To this end, several simulations were carried out to investigate the effect of pseudoelastic training conditions on the corresponding actuation strain, cyclic stability, and transformation temperatures of the NiTi under zero-load TWSME. Whilst previous studies considered external indicators such as the magnitude of plastic or irrecoverable strain as a prerequisite for TWSME, the present model focused primarily on internal stresses. Based on the magnitude of the zero-load actuation strain (measured from the trained material under no external load), the model results indicated that the magnitude of the pseudoelastic training strain (in comparison to the training temperature, and the number of training cycles) has a pronounced effect on the cyclic TWSME actuation characteristics. Moreover, observations from the internal state (hidden) variables of the model, representing the intrinsic residual stress state in the trained material (under zero observable/external stress) showed the possibility for an SMA to generate TWSME even if there is no significant accumulation of permanent strains.

On the microstructure and mechanical properties of a two-way shape memory NiTi/NiTiCu bi-layer diaphragm

In this research, a NiTi/NiTiCu bi-layer composite consist of austenitic and martensitic layers was made by the diffusion bonding method, to make a diaphragm to show two-way shape memory actuation. The initial phase of the single sheets and the quality of the bonding after diffusion annealing were investigated using X-ray diffraction (XRD) and microstructural analyses, respectively. The effects of applied force on both sides of the diaphragm were studied using loading-unloading steps in the bending test. The observations implied the dependence of mechanical response on the loading mode; pseudo-elastic displacement and thermal recovery showed various figures when the loading mode changes. For both the loading modes, interestingly, the bi-layer showed two-way shape memory behavior in a diaphragmatic mode at a certain amount of force, without any complicated thermomechanical process and training and just by one cycle of loading-unloading. However, the results illustrated a more extensive range of total displacement in which the bi-layer showed two-way shape memory behavior when the load applied on the austenitic side. The stress and strain profile in the cross-section of the diaphragm was studied using finite element simulation. The results could be useful in the design of diaphragms with two-way shape memory behavior.

Shape Memory Behavior of Emulsion-Templated Poly(ε-Caprolactone) Synthesized by Organocatalyzed Ring-Opening Polymerization

Organocatalyzed ring-opening polymerization (ROP) of e-caprolactone (CL) and 4,4′-bioxepanyl-7,7′-dione as a bis-lactone cross-linker was performed within the oil-in-oil high internal phase emulsions (HIPEs) at 50 °C. In this way, the cross-linked poly(e-caprolactone) (PCL) polyHIPE foams of ∼85% porosity were synthesized. Thermomechanical properties of the prepared polyHIPEs were studied and proved to greatly depend on a degree of PCL cross-linking. The melting and crystallization temperatures as well as the degree of crystallinity of PCL polyHIPE foams decrease with an increasing cross-linking degree. Semi-crystalline polyHIPEs demonstrate shape memory behavior with excellent shape fixity and shape recovery. At an appropriate degree of PCL cross-linking, the polyHIPE temporary shape can be fixed at room temperature, while a transition to the permanent shape occurs upon heating at 40 °C. Moreover, a two-way shape memory behavior of the PCL polyHIPEs under constant stress was observed.

POSS-Induced Dynamic Cross-Links Produced Self-Healing and Shape Memory Physical Hydrogels When Copolymerized with N-Isopropyl acrylamide.

The hydrogel morphology and viscoelasticity and the two-way shape memory behavior of copolymers of N-isopropyl acrylamide-polyhedral oligomeric silsesquioxane (PNIPAm-POSS) were investigated. NIPAm-POSS copolymers are unentangled and exhibit a thermoplastic and thermoresponsive hydrogel behavior ( Romo-Uribe ; ; Albanil , Eur. Polym. J. 2018 , 99 , 350 - 360 ). Here, we demonstrated by high-resolution transmission electron microscopy that POSS segregated into crystals to form physical cross-links. The hydrogels exhibited significant swelling, higher than chemical hydrogels at the same POSS content. The morphology of the hydrogels consisted of a honeycomb structure, and the pore size and swelling were a decreasing function of POSS. Moreover, the hydrogels were self-healing, exhibiting a liquid-like behavior under large amplitude shear and rubber-like behavior immediately after cessation of shear, that is, the POSS aggregates break and re-form, acting as dynamic cross-linkers. Strikingly, the physical networks behaved as perfect networks, thus matching chemically cross-linked networks and holding the scaling |η*| ∼ ω-1. Furthermore, the physical hydrogels exhibited a thermally activated shape memory (SM) behavior, without external stress and the lower critical solution temperature (LCST) being the only stimulus. The SM process involved swelling-deswelling, effectively opening opportunities for drug delivery. Furthermore, the SM was reversible, and no external stress was involved, therefore behaving as the two-way SM. Finally, POSS modulated the shear elastic modulus G' and LCST; therefore, the PNIPAm-POSS copolymers open opportunities for reusable, thermoformable SM hydrogels with tuned mechanical modulus and activation temperature.

Two-way shape memory effects of sulfur vulcanized natural rubber (NR) and NR/paraffin wax (PW)/carbon nanotube (CNT) nanocomposites

Abstract In this study, we investigated the shape memory properties of natural rubber and paraffin wax (NR/PW) blends and composites (8:2, 6:4 and 4:6) vulcanized with sulfur/accelerators. Different loadings of paraffin wax in the natural rubber matrix were prepared to evaluate the one-way and two-way shape memory performance in the blends using the melting temperature of wax as the switching temperature. With the addition of multi-walled carbon nanotube (CNT), the prepared NR/PW-CNT nanocomposites not only have the improved wax dispersion but also could be remotely heated by near-infrared laser irradiation to trigger the shape memory process in a short time. The simple crosslinked NR with or without crystallizing wax also demonstrated high actuation and recovery magnitudes in the two-way shape memory behavior. Most surprisingly, to the best of our knowledge, the NR/PW blends and nanocomposites showed the first published solvent vapor-triggered reversible shape memory system without any external stress and pre-soaking treatment in the open literature. This approach paves the way for numerous possibilities to prepare multi-triggered systems with reversible shape memory properties.

Light-actuated reversible shape memory effect of a polymer composite

In the current study, a novel polymer composite with excellent light-actuated two-way shape memory effect (2W-SME) was fabricated using poly (ethylene-co-vinyl acetate) (EVA) as matrix and p-aminodiphenylimide (p-AP) as both light-absorber and heat source (EVA/p-AP). Unlike traditional thermally-driven two-way reversible shape memory polymers, in our concept, the EVA/p-AP featured remotely controlled and light-manipulatable reversible two-way shape memory behaviors. Utilizing the distinct light-responsive behavior of p-AP in the irradiation of 365 nm ultraviolet (UV) light, the EVA/p-AP composite exhibited excellent light-actuated reversible 2W-SME by a light-manipulated procedure. The results from this study indicated that the composite material could have greatly potential applications in soft reversible drivers.

Microstructural study and simulation of intrinsic two-way shape memory behavior of functionally graded Ni-rich/NiTiCu thin film

In this research, the gradient structure and mechanical properties of an austenitic/martensitic (Ni50.8Ti/Ni45TiCu5 (at.%)) bi-layer thin film was investigated. The bi-layer was deposited on a Si substrate using DC magnetron sputtering. After crystallization of the film at 773K for 60min, the microstructure and mechanical properties of the bi-layer thin film were characterized using X-ray diffraction (XRD), transmission electron microscopy (STEM and HRTEM) and nanoindentation, respectively. The diffraction pattern illustrated that the crystallized bi-layer was combined of martensitic and austenitic layers while secondary ion mass spectroscopy (SIMS) and high resolution transmission electron microscopy analysis demonstrated the existence of a compositional gradient through the thickness and residual strain in the interface of the bi-layer, respectively. Furthermore, the compositional gradient in the bi-layer led to gradual variations in the structure, hardness and Young's modulus through the thickness of the bi-layer. The intrinsic two-way shape memory effect due to its functionally graded structure and the induced stress was confirmed by experimental test and finite element simulation.

A Two-Way Shape Memory Study on Ni-Rich NiTi Shape Memory Alloy by Combination of the All-Round Treatment and the R-Phase Transformation

Ni4Ti3 precipitates are formed in Ni-rich NiTi shape memory alloys (SMAs) after a certain heat treatment. Such a treatment then results in the changed phase transformation behavior of the alloy switching from the one-step reversible phase change B2 ↔ B19′ to the two-step, B2 ↔ R ↔ B19′, phase change. The present study reports a two-way shape memory (TWSM) due to an all-round treatment followed by an R-phase constraint aging at room temperature. The enhanced TWSM behavior was observed upon temperature cycling between 273 K (R-phase) and 77 K (B19′ phase). The effect of various constraint strains in R-phase aging by employing different diameters of the constraint ring was studied. However, the TWSM effect due to constraining the R-phase will be eliminated after a temperature rise of specimen to the fully parent phase (373 K).

The role of residual stress states in modeling the cyclic two-way shape memory behavior of high-temperature NiTiPd alloys and actuation components

A multi-mechanism, three-dimensional, material model that captures the cyclic training and subsequent two-way shape memory behaviors of high-temperature ternary NiTiPd material is presented. To this end, available experimental test data on this alloy have been used for characterization of the model parameters and the subsequent validation of its predictions. Numerical simulations involving many thermal cycles and stress levels are used to investigate some of the key factors which influence the magnitude and stability of TWSM actuation strains. In particular, the developed model is used to provide quantitative measurements of the internal stresses induced in the NiTiPd SMA material after cyclic training, and how they predominantly affect the behavior of the material under zero-load TWSME. This is further substantiated by a detailed analysis of the global bending response of a trained beam strip, where emphasis is placed on the interplay between the two major agents responsible for the resulting TWSME actuation rotations: (1) intrinsic internal state variables, reflecting the microstructural changes occurring at the material point during thermal cycling, and (2) the extrinsic global residual state resulting from the inhomogeneous stress fields at different points over the section of the trained beam upon release of the training loads.

Vapor-grown carbon nanofiber/poly(ethylene-co-vinyl acetate) composites with electrical-active two-way shape memory behavior

Vapor-grown carbon nanofibers were filled into poly(ethylene-co-vinyl acetate) through solution mixing to achieve electrical-active two-way shape memory. The vapor-grown carbon nanofiber/poly(ethylene-co-vinyl acetate) nanocomposites were systematically and explicitly investigated through combined analytical techniques. Experimental results showed an increase in crystallization temperature ( Tc) and a decrease in melting temperature ( Tm) as the vapor-grown carbon nanofiber concentration in the nanocomposites increased. This result indicates that carbon nanofibers can act as nucleating agents during crystallization but may hinder the formation of thermodynamically stable crystals. The nanocomposites had a higher modulus but a lower strength than pure poly(ethylene-co-vinyl acetate). In two-way shape memory testing, a decrease in reversible strain and shape recovery occurred in the nanocomposites. Nevertheless, obvious two-way shape switching was observed in all samples during cyclic heating and cooling. The nanocomposite with 15% vapor-grown carbon nanofibers could be electrically heated rapidly and uniformly because of the high conductivity of vapor-grown carbon nanofibers and their uniform distribution in the matrix. Finally, the electrical-active two-way shape memory effect was achieved. This work contributes to the design of electrical-active two-way shape memory polymers in device applications for advanced functions.