Shape Memory Mechanism Research Articles

Flexible multifunctional composite films for adjustable EMI shielding behavior under self-fixed deformation

With the widespread application of electromagnetic technology, the significance of intelligent electronic devices in modern communication is increasingly prominent. Despite this, the public focuses more on the harm caused by electromagnetic pollution. However, traditional EMI shielding materials struggle to meet the diverse and dynamic application requirements. Hence, developing materials capable of adjusting EMI shielding has become imperative to satisfy various scenarios. This paper presents the composite film TAPU-MXene, which is capable of adjustable EMI SE under self-fixed deformation. Introducing tannic acid increases the innermolecular interactions of TAPU. This enhancement leads to remarkable improvements in mechanical performance, achieving a stress level of 36.6 MPa at a strain of 560 %. Furthermore, the catechol groups can establish non-covalent interactions with multiple substrates, endowing the TAPU film with remarkable adhesion and self-healing capabilities. By combining the highly conductive nano-filler MXene with TAPU, the EMI shielding composite film TAPU-MXene is obtained. The EMI SE of TAPU-MXene was up to 56.7 dB when MXene loading was 3 mg cm−2. Most importantly, TAPU-MXene exhibits the ability to adjust the EMI shielding effectiveness from 56.7 dB to 2.75 dB under self-fixing deformation through shape memory mechanism, demonstrating promising applications in the field of adjustable EMI shielding materials.

Dynamically customizable 4D printed shape memory polymer biomedical devices: a review

Abstract There is an increased risk of complications and even surgical failures for various types of medical devices due to difficult to control configurations and performances, incomplete deployments, etc. Shape memory polymers (SMPs)-based 4D printing technology offers the opportunity to create dynamic, personalized, and accurately controllable biomedical devices with complex configurations. SMPs, typical representatives of intelligent materials, are capable of programmable deformation in response to stimuli and dynamic remodeling on demand. 4D printed SMP medical devices not only enable active control of configuration, performance and functionality, but also open the way for minimally invasive treatments and remote controllable deployment. Here, the shape memory mechanism, actuation methods, and printing strategies of active programmable SMPs are reviewed, and cutting-edge advances of 4D printed SMPs in the fields such as bone scaffolds, tracheal stents, cardiovascular stents, cell morphological regulation, and drug delivery are highlighted. In addition, promising and meaningful future research directions for 4D printed SMP biomedical devices are discussed. The development of 4D printed SMP medical devices is inseparable from the in-depth cooperation between doctors and engineers. The application of 4D printed SMP medical devices will facilitate the rapid realization of "smart medical care" and accelerate the process of "intelligentization" of medical devices.

A Novel Multiple Shape Memory Effect in PVDF‐Based Ferroelectric Copolymers and Terpolymers

AbstractShape memory polymers (SMPs) have been extensively investigated because of their wide range of biomedical and robot applications. In most of SMPs, only one temporary shape can be formed and recovered through the mechanism of melting or glass transition. Herein, a multiple shape memory effect (mSME), i.e., formation of at least two temporary shapes, can be realized in polyvinylidene fluoride (PVDF)‐based ferroelectric polymers by exploiting their expanded ferroelectric–paraelectric (F‐P) phase transition temperature range. Although P(VDF‐TrFE) (TrFE: trifluoroethylene) (55/45) copolymer is thought to be a normal ferroelectric, its ferroelectric phase transforms into a paraelectric phase through an intermediate relaxor ferroelectric‐like state and mSME is observed in this extended phase transition temperature range. By incorporating CTFE (chlorotrifluoroethylene) into P(VDF‐TrFE), P(VDF‐TrFE‐CTFE) becomes a relaxor ferroelectric with a further extended phase transition temperature range. The terpolymer exhibits improved mSME and at least three temporary shapes can be formed and recovered. A comparison of SME and structures of several PVDF‐based copolymer and terpolymers suggests that the amount of polar phase is a critical factor affecting the SME. This study not only demonstrates mSME in ferroelectric polymers, which expands their application potential, but also provides an in‐depth understanding of the shape memory mechanism of the polymers.

Understanding the shape-memory mechanism of thermoplastic polyurethane by investigating the phase-separated morphology: A dissipative particle dynamics study

Shape-memory polyurethanes (SMPUs) are promising materials that change shape in response to external heat. These polymers have a dual-segment structure: a hard segment for netpoint and a soft segment for molecular switch. Understanding the molecular behavior of each segment and microphase-separated morphology is crucial for comprehending the shape-memory mechanism. This study aimed to understand the shape-memory behavior by observing the phase separation of SMPU using mesoscale models based on dissipative particle dynamics (DPD) simulations. The SMPU copolymer was modeled using 4,4′-diphenylmethane diisocyanate (MDI, hard segment) and poly(ethylene oxide) (PEO, soft segment). By calculating segment solubility and repulsion parameters, we found that the hard-segment domain changes from isolated form to a lamellar and interconnected structure and eventually to a continuous form as its content increases. Combining these insights with shape-memory performance models can enhance our understanding of better SMPU design and contribute significantly to the optimization of smart stimuli-responsive materials.

Effect of curing agents n‐octylamine and m‐xylene diamine on the mechanical and memory properties of E51 epoxy resin

AbstractThe effects of curing agents n‐octylamine (OA) and m‐xylene diamine (MXDA) on the mechanical and memory properties of E51 epoxy resin (EP) are investigated in this work. The curing mechanism and thermal stability of OA and MXDA with EP are also investigated via Fourier‐transform infrared spectroscopy and thermogravimetric analysis, the influence of curing agent ratios on the thermo‐dynamic mechanical properties of EP are investigated through dynamic thermo‐mechanical analysis, and the relationship between the storage modulus, the hysteresis phase angle and the temperature are constructed. Moreover, the cross‐linking densities of the EP with different curing agent ratios are calculated using Archimedes's method. Besides, the effects of different proportions of curing agents on the mechanical properties of EP are studied. The results show that with the increases of MXDA ratio, the phase transition temperature, crosslink density, recovery temperature, recovery rate, the tensile strength, tensile modulus, strain strength, and strain modulus are enhanced, however, the hysteresis phase angle MXDA ratio and the tenacity decrease. Finally, the shape memory mechanism of E51 is discussed.

Regulating porous microstructure of polyimide aerogels toward efficient shape memory performance

The development of lightweight shape memory materials with exceptional thermal insulation properties is highly important in aerospace applications. Herein, shape memory polyimide (PI) aerogels was fabricated by molecularly designed PI molecular chains through freeze gelation and subsequent thermal imidization. The freezing conditions were controlled to regulate the porous microstructures of the aerogels. The interactions between PI chains formed physically crosslinked netpoints, which could overcome the expansive force and capillary force, thereby creating a stable porous structure. Because shape memory was thermally activated, the shape memory mechanisms associated with the thermal conductivity of the aerogels were comprehensively explored. Upon decreasing the pore size, dense frameworks were formed, contributing to the improved thermo-mechanical and compressive properties. Moreover, the heat transfer efficiency in the aerogel skeleton was enhanced, resulting in a rapid shape recovery response. Benefiting from the physically crosslinked structures, including entangled chains and π-π interactions, the fabricated PI aerogels demonstrated exceptional shape fixation and recovery capabilities. These findings may provide guidelines for designing smart aerogels for aerospace applications.

Shape memory polyimide/carbon nanotube composite aerogels with physical and chemical crosslinking architectures for thermal insulating applications

The development of lightweight smart materials with exceptional shape memory and thermal insulation properties is highly important in the aerospace industry. The reported shape memory polymer aerogels (SMPAs) are restricted to harsh environments because of their poor high-temperature resistance and large volume shrinkage at elevated temperatures. Herein, shape memory polyimide (PI) composite aerogels with superior thermal insulation were fabricated by combining molecularly designed PI chains with amino-functionalized carbon nanotubes (NH2-CNTs) through freezing gelation and subsequent thermal imidization. Because shape memory behavior was thermally triggered, the thermal conductivity associated with the shape memory mechanisms of the aerogels was comprehensively explored. The presence of CNTs promoted heat transfer in the aerogel skeleton, facilitating the shape recovery response. Benefiting from chemical-crosslinked and physical-crosslinked structures, the fabricated PI composite aerogels demonstrated desirable thermo-mechanical properties, exceptional shape memory and superior thermal insulation performance. This study may provide guidelines for designing shape memory PI composite aerogels for harsh environment applications in aerospace engineering.

Damage-indicating and self-healing anticorrosion coatings based on fluorescence resonance energy transfer and photothermal shape memory mechanism

Self-healing coatings, which possess the ability to repair damage and restore corrosion resistance without significant human intervention, have become a hot topic in corrosion protection research. In this paper, (±)-10-camphorsulfonic acid-doped polyaniline is synthesized and then combined with copolyurethane (copPU) to form the photothermal shape memory composite polymer (CSPA-copPU). An aggregation-induced emission agent, named N’,2-bis[(E)-5-chloro-2-hydroxybenzylidene] hydrazine-1-carbohydrazide, is synthesized and applied to create a synergistic fluorescence system with a prepared chelation-enhanced fluorescence (CHEF) agent, named Rhodamine Benzimidazole. Under the CHEF behavior in response to Fe3+ and the fluorescence resonance energy transfer effect, the system exhibits a strong and sensitive fluorescence response to corrosion-generated Fe3+. Using electrospinning technology, CSPA-copPU@Fl fibers are prepared with CSPA-copPU as the shell and a mixture of fluorescent agents as the core solution and applied to create the composite coatings. The coatings effectively indicate damage in the form of fluorescence, providing guidance for infrared laser repair. CSPA facilitates the passivation of exposed steel. Under irradiation by an infrared laser, the surface temperature reaches the glass transition temperature of copPU and the epoxy binder. Through softening expansion and diffusion entanglement of molecular chains, scratches in the coatings are closed and repaired, and the corrosion resistance is restored to a level of intact coatings.

Study of shape memory mechanism and performance of shape memory polyurethane prepared based on tetrahydrofuran

A series of shape memory polyurethane (SMPU) substrates were fabricated utilizing polyether polyol (PPG) with varying molecular weights, 4,4′-diphenylmethane diisocyanate (MDI), and4,4′-methylene-bis-(2-chloroaniline) (MOCA) as primary raw materials. Tetrahydrofuran (THF) served as the solvent. The investigation focused on the effects of different soft segment molecular weights and additional THF content on the properties of SMPU. The study aimed to explore the impact of varying soft segment molecular weights and THF additions on the shape memory properties of polyurethane, as well as the mechanism behind THF's influence. The findings demonstrated that the introduction of THF significantly enhanced the phase separation degree and ordered hydrogen bonding of polyurethane, leading to the acquisition of shape memory properties. Compared to specimens without THF addition, when PPG had a molecular weight of 500, hydrogen bonding increased by 50 %. Furthermore, at THF addition of 9 mL, the specimen exhibited the highest shape recovery degree, with a phase separation temperature of 28.21 °C, a degree of carbonyl-ordered hydrogen bonding of 49.08 %, a bending strength of 43.81 MPa, and an impact toughness of 38.19 KJ/m2. The shape memory performance of the specimens was evaluated at various temperatures, revealing that the models both achieved 100 % shape fixation at different temperatures. Additionally, the degree and rate of retraction increase with the increase in temperature. The shape memory performance of the prepared specimens was shown to be good by the DMA shape memory cycling test.

Shape fixing abilities of SEBS/Paraffin blends with different paraffins under conventional and reversible plasticity shape memory programming

Poly(styrene-b-[ethylene-co-butylene]-b-styrene) (SEBS)/paraffin system is a promising class of shape memory polymers and the transition temperature of such materials could be flexibly regulated by choosing different paraffins. However, the difference in shape memory performance of SEBS/paraffin blends containing paraffin with same content but different melting temperature (Tm) has not been comparatively studied. In this work, two series of SEBS/paraffin blends containing same content paraffin (30 and 70 wt%, respectively) with Tm of 28.5 and 61.3 °C (denoted as PL and PH, respectively) were prepared and their shape fixing ratios (Rfs) under conventional shape memory (CSM) and reversible plasticity shape memory (RPSM) programming methods were investigated. It was found that these blends with the same paraffin content showed different Rfs regardless of the programming method. For example, the 30 wt% PH-containing sample exhibited higher Rf under CSM programming, while the 70 wt% PL-containing sample showed higher Rf under RPSM programming. By investigating microstructural changes, a possible explanation for the shape memory performance of SEBS/paraffin blends containing paraffin with different Tm was proposed. This work was helpful to deepen the understanding of shape memory mechanism and broaden the application of SEBS/paraffin-based shape memory polymers.

Cryogenic temperature deformation behavior and shape memory mechanism of NiTiFe alloy

The excellent shape memory properties of NiTiFe alloys make them a popular choice for tube connection components and actuators in the aerospace industry. However, in order to obtain these properties, the alloy must undergo pre-deformation at specific cryogenic temperatures to induce the martensitic phase transformation. Despite their widespread use, the deformation behavior and shape memory mechanism of NiTiFe alloys at cryogenic temperatures remain unclear. To address this, we conducted a study investigating the deformation behavior and shape memory mechanism of Ni47Ti50Fe3 alloy at temperatures ranging from −190 °C to 150 °C using tensile tests, free recovery tests, and EBSD analysis. Our findings suggest that the deformation behavior of NiTiFe alloy can be classified into four classes corresponding to different deformation mechanisms. At temperatures above the Md (−15 to −30 °C), dislocation slip and {114}B2 deformation twinning occur without reversible strain. However, reversible and irreversible strains occur simultaneously at temperatures between −50 to −90 °C. At temperatures ranging from −110 to −150 °C, the reorientation/detwinning of the R-phase, stress-induced martensitic phase transformation, and yield stage of martensite occur in sequence. When the deformation temperature is within the range of −165–190 °C, the reorientation/detwinning of martensite occurs during the stress platform stage, and irreversible strain occurs during the yield stage of martensite. The best shape memory effect of the NiTiFe alloy is observed at deformation temperatures near Mf (−180 °C). Moreover, the reverse phase transformation temperatures increase with the increasing deformation amounts, even without plastic deformation. The temperature-dependent deformation map of NiTiFe alloy was obtained based on the tests conducted in this study, from which the influence of deformation temperatures on the different critical stresses and the deformation mechanism under different temperature conditions can be determined.

Wood as a hydrothermally stimulated shape-memory material: mechanisms of shape-memory effect and molecular assembly structure networks

Abstract This study aimed to evaluate the shape-memory effect (SME) of wood (Populus x beijingensis W. Y. Hsu) and identify the net-points and switches in its molecular and morphological structures. During several cycles of deformation and subsequent recovery, a high shape recovery rate and ratio were maintained. The transverse compression tests of wet and dry wood reveal that the hydrothermal coupling stimulation can considerably reduce the strength of wood. The X-ray diffraction characterization of wood under hydrothermal stimulation shows that the role of network nodes in the SME of wood is influenced by temperature. The wavenumber shifting and changes in the intensity ratio of the characteristic Fourier transform infrared peaks showed that hydrogen bonds acted as switches for the water-stimulated shape-memory behavior. By taking into account viscoelastic relaxation, a kinetic model derived from nonequilibrium thermodynamic fluctuation theory was used to describe the shape recovery process. The effects of hydration on recovery kinetics, activation, and dynamic mechanical behaviors were also studied. To explain the shape-memory mechanism of wood under hydrothermal stimulation, a hybrid-structure network model based on a single three-dimensional switch network was proposed in this study.

A Constitutive Model of Water-Triggered Shape Memory Hydrogels and Its Finite Element Implementation

AbstractShape memory hydrogel is a type of hydrogel whose shape can transform between a temporary shape and its initial shape when exposed to external stimuli, such as water, temperature, and pH. Over the last decade, shape memory hydrogels have gained increasing interest owing to their distinct properties; however, constitutive models to describe their shape memory mechanism are still lacking. In this paper, we propose a constitutive model for water-triggered shape memory hydrogels based on the transition between the sparse and dense phases. In the model, the shape memory process is identified using two internal variables: the frozen deformation gradient and dense phase volume fraction. To validate the model for describing shape memory effects, we implemented the model in the finite element method using a user-defined element (UEL) subroutine in ABAQUS. To verify the accuracy of the proposed UEL, we simulated the water-triggered shape memory effects in different recovery processes under different uniaxial loads. Furthermore, we investigated the water-triggered shape memory behavior of a self-bending bilayer structure and a four-arm gripper structure using both experiments and simulations. Good agreement was observed between the simulation and experimental results.

Reusability and energy absorption behavior of 4D printed continuous fiber-reinforced auxetic composite structures

This work proposed a strategy for fabricating intelligent continuous fiber-reinforced lattice structures (CFRLSs) for reusable energy absorption applications based on the cold-programming-induced shape memory effect. By integrating the smart material with continuous fiber, the CFRLSs can not only dissipate a considerable amount of energy but also recover their original shape after stimulation. Herein, two auxetic CFRLSs were developed to demonstrate their reusable energy absorption capacity. Specifically, the shape memory mechanism of the fiber-reinforced composites was first revealed, and the mechanical performance, reusability, and energy absorption capacity of the intelligent CFRLSs were investigated comprehensively. The result shows that CFRLSs present a preferable reusability with a shape recovery ratio above 90% in all test cycles and still exhibit a considerable energy absorption capacity compared to the pure Polylactic acid specimen, even after performing six test cycles. This approach provides a feasible way to design reusable protection devices and implementation in multi-functional applications.

Regenerative Living 4D Printing via Reversible Growth of Polymer Networks.

Living creatures possess complex geometries, exceptional adaptability, and continuous growing and regenerating characteristics, which are difficult for synthetic materials to imitate simultaneously. A living polymer network with these features is reported. The polymer can be digitally printed into arbitrary 3D shapes and subsequently undergoes growth via living polymerization of a monomer as the nutrient. This leads to macroscopic dimensional growth and transforms the printed amorphous network into a crystallizable network, resulting in geometric adaptability via a shape-memory mechanism. By controlling the localized growth, an initial homogeneous structure can be converted into a geometrically different heterogeneous structure composed of materials with different properties (crystallization and mechanical properties). After growth, the original network can be chemically regenerated for regrowth. With this regenerative living 4D printing, one 3D-printed seed template can be turned into different derivatives with distinct geometries and mechanical properties when repeated regeneration is conducted in different localized regions and the degree of regrowth is varied.

Magnetofriction—a new concept for shape memory composites

This paper introduces a new concept for shape memory based on elastic forces and magnetically induced friction forces in composite materials consisting of magnetoactive elastomers (MAEs). Magnetic attraction forces between two MAEs generate a contact pressure at their interface and the friction allows to maintain stable deformed shapes of the so-called magnetofriction shape memory polymers (MF-SMPs). When the contact is loosened, the friction forces vanish and the elastic forces in each part of the assembly bring the parts back into their initial state where the contact can be established once again. The shape memory effect is studied in three-point bending tests with two stacked MAEs. The global force-displacement relations reveal a hysteretic behavior due to local residual displacements after the test are observed by the help of digital image correlation. The test structure stores up to 25% of the applied displacement. The local contact state (sliding or sticking) is evaluated in different regions of the MF-SMP which gives an insight into the shape memory mechanism magnetofriction. Two methods for the shape-recovery of the MF-SMP by elastic forces in the MAEs are proposed, a manual separation and an air flow at the interface of the MF-SMP, and a comparison of magnetofriction to other shape memory mechanisms is performed.

Thermomechanical Constitutive Models of Shape Memory Polymers and Their Composites

Abstract Shape memory polymers (SMPs) and SMP composites (SMPCs) have been widely employed in several fields and exhibit excellent self-actuation, deformation, and self-adaption. Establishing reasonable constitutive models is vital for understanding the shape memory mechanism and expanding its applications. Moreover, the mechanical response of SMPs under different conditions can be predicted, facilitating their precise control. The internal mechanism for the shape memory behavior in most SMPs is thermal actuation. This study reviews the theories of thermally actuated SMPs, rheological and phase transition concept models, and models combining the rheology and phase transition concepts. Furthermore, the constitutive models of particulate-reinforced SMPCs, carbon-fiber-reinforced SMPCs, and the buckling behavior of SMPCs are summarized. This study is expected to help solve the remaining issues rapidly and contribute to the establishment of rational constitutive models for SMPs and SMPCs.

Stress-Free Two-Way Shape-Memory Mechanism of a Semicrystalline Network with a Broad Melting Transition

Stress-Free Two-Way Shape-Memory Mechanism of a Semicrystalline Network with a Broad Melting Transition

Thermal actuation shape memory of ultra-high-molecular-weight polyethylene (UHMWPE) with molecular orientation

• Preparation of shape memory UHMWPE/HDPE samples with molecular orientation. • Cyclic tensile shape memory property of UHMWPE/HDPE samples. • Movement and evolution of chain segments in shape-changing process. Conventional compression-molding method cannot illustrate the effect of molecular orientation on shape memory property. Herein, high-density polyethylene (HDPE) is incorporated into ultra-high-molecular-weight polyethylene (UHMWPE) to prepare samples with molecular orientation by the single screw extrusion and traction machine. Interestingly, the shape recovery rate (R r ) of samples with the same tensile direction as molecular orientation decreases from 66.13% to 60.71% with the traction rate, while the R r of samples with tensile direction perpendicular to molecular orientation exhibits an opposite tendency in cyclic tensile shape memory tests. Meanwhile, the shape fixation rate of these two samples does not change significantly. In addition, the possible mechanism of thermal actuation shape memory of samples with molecular orientation is interpreted.

Shape memory property and mechanism of the knitting-fabric reinforced epoxy composites subjected to tensile loading

Shape memory polymers (SMPs), as stimulus-responsive shape-shifting materials, are generally reinforced with particles and fibers to fabricate the shape memory polymer composites (SMPCs) for better mechanical and shape-recovery properties. However, many continuous fibers reinforced SMPCs have good shape recoverability subjected to bending loading but limited shape recoverability under tensile loading due to limitation of tensile deformation of continuous fibers. Here, we developed the continuous fiber knitting-fabric reinforced shape memory epoxy composites and investigated their shape memory property and mechanism subjected to large tensile strain. The effect of epoxy component ratios, tensile strains, and course(0°)/wale(90°) directions on the shape memory behavior, recovery stress, and mechanical property of the shape memory epoxy (SMEP) and its composite (SMPC) were studied. The results show that the SMEPs and SMPCs with the tensile strain up to 30% have good shape fixation rates of above 99% and final recovery ratios of above 98%. A mesoscopic finite element model of knitting-fabric reinforced SMPC was established to study the shape memory mechanism. The recovery stress of the SMPC can improve up to 5.8 times as compared with pure SMEP. The knowledge gained in this work will benefit future development and application for better actuators.