Reversible Shape Memory Effect Research Articles

Fabrication of shape memory polyimides with exploration of their activation mechanisms

The construction of polymer-based materials with reversible bidirectional shape memory effects, good thermal and electrically controlled properties is of paramount importance for the development of intelligent exploration in high-temperature fields. By adjusting the hydrogen bond contents of the structure to regulate the intermolecular strength, a series of random shape memory copolyimides have been synthesized with the triggering temperatures ranging from 275 to 384℃, which displayed good thermal stability, high shape fixation rate and shape recovery rate of over 95 % and 98 %, respectively, and excellent shape recovery speed (∼9 s). Visualized quantification of the microstructural phase evolution during stretching and the effects of hydrogen bonds on the properties of polyimide were deeply explored to better understanding the activation mechanisms of shape memory behaviors. Furthermore, flexible fabric composites with bidirectional shape memory effects motivated by electrical signals were also designed, providing a new sight of exploration for high-temperature shape-memory materials.

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.

Sustainable Approach for the Synthesis of a Semicrystalline Polymer with a Reversible Shape-Memory Effect

Shape-memory polymers (SMPs) have demonstrated potential for use in automotive, biomedical, and aerospace industries. However, ensuring the sustainability of these materials remains a challenge. Herein, a sustainable approach to synthesize a semicrystalline polymer using biomass-derivable precursors via catalyst-free polyesterification is presented. The synthesized biodegradable polymer, poly(1,8-octanediol-co-1,12-dodecanedioate-co-citrate) (PODDC), exhibits excellent shape-memory properties, as evidenced by good shape fixity and shape recovery ratios of 98%, along with a large reversible actuation strain of 28%. Without the use of a catalyst, the mild polymerization enables the reconfiguration of the partially cured two-dimensional (2D) film to a three-dimensional (3D) geometric form in the middle process. This study appears to be a step forward in developing sustainable SMPs and a simple way for constructing a 3D structure of a permanent shape.

Reverse shape memory effect in Cu-Mn-Ga-Mo alloys

Reverse shape memory effect in Cu-Mn-Ga-Mo alloys

Hierarchical network relaxation of a dynamic cross-linked polyolefin elastomer for advanced reversible shape memory effect.

Reversible shape-memory polymers (RSMPs) are highly desired for soft actuators due to the repeatability of deformation. Herein, a polyolefin elastomer vitrimer (POEV) was prepared by constructing a dynamic cross-linked network based on boronic ester bonds. POEV showed varied network relaxation in a wide temperature range due to hierarchical network relaxation, and then the entropy decreased and the relaxation of POEV chains was facilely controlled by temperature. The controllable relaxation of POEV by programming the temperature enabled the actuation domain with a reduction in entropy and the skeleton domain with a relatively high entropy can be built in POEV, greatly affecting the reversible shape memory effects (RSMEs). The topological rearrangement resulted from the activated exchange of dynamic covalent bonds, which enables POEV with good shape reconfigurability, and allows for complicated 3D shapes and shape-shifting on demand. More interestingly, combining the decreasing entropy of POEV chains and fully topological rearrangement tailored by temperature, hybrid aligned carbon nanotubes (CNTs) can be constructed in POEV via a two-stage training. Then, the aligned CNTs can enhance the elasticity and act as a hybrid skeleton for RSMEs, avoiding the negative impact of CNTs on the reversible actuation strain. The hierarchical network relaxation facilitates combining all these unusual properties in one shape memory network synergistically, paving new avenues for realizing smart materials with advanced RSME.

Narrow response temperature range with excellent reversible shape memory effect for semi-crystalline networks as soft actuators.

Complex and controlled reversible actuation inevitably relies on changing thermal fields (direct or indirect) for semi-crystalline reversible shape memory networks. Unfortunately, the non-tunability of thermal signals often brings potential limitations to actuators' applications. In practice, a wide response temperature range (T-range) formed by Thigh and Tlow in the remarkable reversible actuation is an obvious fact. Herein, we demonstrate the tunability of the transition temperatures while stably maintaining excellent actuation abilities. We further verified that the narrow T-range (24 °C) that had not been reported could present more than 17% reversible strain. Special parameter optimization provides opportunities for potential non-implantable biomedical applications. Therefore, based on target 2W-SMP, a vehicle concept with the drug release and vehicle recovery ability was proposed, proving our approach's feasibility.

Fabrication of controllable and reversible bidirectional shape memory fabric composites based on multiblock copolyimide and their properties

The construction of polymer-based materials with reversible bidirectional shape memory effects and good thermal properties is of paramount importance for the development of the intelligent exploration in high temperature fields. In this work, a series of multiblock shape memory copolyimides were synthesized with the introduction of soft segments containing aliphatic diamines into aromatic polyimide structures to control the mobility of the molecular chains. The number of blocks for copolyimide was finely regulated to optimize the shape memory behaviors with thermal stability and both of the shape fixity ratio and shape recovery ratio could reach higher than 97% with the triggering temperature around 260 °C. Then controllable fabric composites with high temperature and reversible bidirectional shape memory effects were firstly constructed by incorporating of deliberately designed block copolyimide with commercialized polyimide fabric. The flexible fabric composites exhibited variable fixed shapes by regulating the thickness discrepancies between upper and lower layers, which showed good two-way shape memory properties with the shape recovery ratio of 98% during the programming process, providing a new sight of the exploration for high temperature shape-memory materials.

Multiple/Two-Way Shape Memory Poly(urethane-urea-amide) Elastomers.

Multiple and two-way reversible shape memory polymers (M/2W-SMPs) are highly promising for many fields due to large deformation, lightweight, strong recovery stress, and fast response rates. Herein, a semi-crystalline block poly(urethane-urea-amide) elastomers (PUUAs) are prepared by the copolymerization of isocyanate-terminated polyurethane (OPU) and amino-terminated oligomeric polyamide-1212 (OPA). PUUAs, composed of OPA as stationary phase and PTMEG as reversible phase, exhibit excellent rigidity, flexibility, and resilience, and cPUUA-C7 -S25 exhibits the best tensile property with strength of 10.3MPa and elongation at break of 360.2%. Besides, all the PUUAs possess two crystallization/melting temperatures and a glass transition temperature, which endow PUUAs with multiple and reversible two-way shape memory effect (M/2W-SME). Physically crosslinked PUUA-C0 -S25 exhibits excellent dual and triple shape memory, and micro chemically crosslinked cPUUA-C7 -S25 further shows quadruple shape memory behavior. Additionally, both PUUA-C0 -S25 and cPUUA-C7 -S25 have 2W-SME. Intriguingly, cPUUA-C7 -S25 can achieve a higher temperature (up to 165°C) SME, which makes it suitable for more complex and changeable applications. Based on the advantages of M/2W-SME, a temperature-responsive application scenario where PUUAs can transform spontaneously among different shapes is designed. These unique M/2W-SME and high-temperature SME will enable the applications of high-temperature sensors, actuators, and aerospace equipment.

A constitutive model and its numerical implementation for reversible behavior of shape memory hydrogels

Shape memory hydrogels (SMHs) are kinds of smart materials with great importance in many fields, such as drug release and soft robotics. In order to design the structures base on SMHs, it is necessary to reveal the mechanism of the shape memory effect and establish the constitutive model of SMHs. However, the existing constitutive models can not describe some of important mechanical behavior of SMHs, such as reversible shape memory effects. In this paper, a three-dimensional finite deformation constitutive model is developed for SMHs with reversible shape memory effects caused by the conformation transition of N-isopropylacrylamide (PNIPAM). In order to well capture its reversible shape memory effect, the polymer network for PNIPAM is decomposed into two parts, coil PNIPAM and globule PNIPAM, with different reference configurations. Then this model is implemented into a user material subroutine (UMAT) and is used for simulating the equilibrium swelling, isothermal uniaxial tension, reversible shape memory behavior and multiple shape memory cycles of SMHs. Our model is validated by comparing the simulation results with experiments. The deformation of a more complex 3D structure and a bilayer structure containing SMHs are also numerically studied which demonstrates great potential of our model in exploring the application of SMHs.

Design and Development of High-Strength and Ductile Ternary and Multicomponent Eutectoid Cu-Based Shape Memory Alloys: Problems and Perspectives

An overview is presented on the structural and phase transformations and physical and mechanical properties of those multicomponent copper-based shape memory alloys which demonstrate attractive commercial potential due to their low cost, good shape memory characteristics, ease of fabrication, and excellent heat and electrical conductivity. However, their applications are very limited due to brittleness, reduced thermal stability, and mechanical strength—properties which are closely related to the microstructural features of these alloys. The efforts of the authors of this article were aimed at obtaining a favorable microstructure of alloys using new alternative methods of thermal and thermomechanical treatments. For the first time, the cyclic martensitic transformations during repeated quenching, methods of uniaxial megaplastic compression, or torsion under high pressure were successfully applied for radical size refinement of the grain structure of polycrystalline Cu-Al-Ni-based alloys with shape memory. The design of the ultra- and fine-grained structure by different methods determined (i) an unusual combination of strength and plasticity of these initially brittle alloys, both under controlled heat or hot compression or stretching, and during subsequent tensile tests at room temperature, and, as a consequence, (ii) highly reversible shape memory effects.

Self-Sensing Actuators Based on a Stiffness Variable Reversible Shape Memory Polymer Enabled by a Phase Change Material.

Soft actuators with integrated mechanical and actuation properties and self-sensing ability are still a challenge. Herein, a stiffness variable polyolefin elastomer (POE) with a reversible shape memory effect is prepared by introducing a typical phase change material, i.e., paraffin wax (PW). It is found that the variable stiffness of POE induced by PW can balance the reversible strain and load-bearing capability of actuators. Especially, carbon nanotubes (CNTs) are concentrated in a thin surface layer by spraying and hot pressing in the soft state of POE/PW blends, providing signal transductions for the strain and temperature perception for actuators. Taking advantage of tunable reversible deformation and mechanical transformation of the POE/PW actuator, different biomimetic robotics, including grippers with high load-bearing capability (weight-lifting ratio > 146), walking robots that can sense angles of joints, and high-temperature warning robots are demonstrated. A scheme combining the variable stiffness and electrical properties provides a versatile strategy to integrate actuation performance and self-sensing ability, inspiring the development of multifunctional composite designs for soft robotics.

Interpenetrating polymer network hydrogels using natural based dyes initiating systems: Antibacterial activity and 3D/4D performance

In the past few decades, interpenetrating polymer network (IPN) hydrogels have attracted huge attention due to their special cell-like structures, interpenetrating interface, and two-phase continuity, which has special synergistic effects on their performance and function. In this work, the best candidates were selected from the firstly synthesized 11 dyes based on 1-aryl-3-(2,4,5-trimethoxyphenyl)prop-2-en-1-one or 3-aryl-1-(3,4,5-trimethoxyphenyl)prop, and then combined with an amine (i.e., 1,4-(dimethylamino) ethyl benzoate, EDB) and an iodonium salts (bis(4-tert-butylphenyl)iodonium hexafluorophosphate, Iod) as a three-component visible light photoinitiating systems (PIS) to induce both free-radical photopolymerization (FRP) of poly(ethylene glycol) diacrylate (PEG-DA) and cationic photopolymerization (CP) of 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexanecarboxylate (EPOX) and in situ reduction of silver cations into silver nanoparticles to prepare interpenetrating network polymers (IPN) with antibacterial activity for Gram-positive bacteria (e.g. Staphylococcus aureus) and Gram-negative bacteria (e.g. Pseudomonas aeruginosa). More interestingly, stable 3D patterns were successfully produced through direct laser write (DLW) technique, and they exhibited reversible swelling properties and shape memory effects caused by swelling and dehydration processes, thus enabling 4D printing.

Arbitrarily Reconfigurable and Thermadapt Reversible Two-Way Shape Memory Poly(thiourethane) Accomplished by Multiple Dynamic Covalent Bonds.

The fabrication of a single polymer network that exhibits a good reversible two-way shape memory effect (2W-SME), can be formed into arbitrarily complex three-dimensional (3D) shapes, and is recyclable remains a challenge. Herein, we design and fabricate poly(thiourethane) (PTU) networks with an excellent thermadapt reversible 2W-SME, arbitrary reconfigurability, and good recyclability via the synergistic effects of multiple dynamic covalent bonds (i.e., ester, urethane, and thiourethane bonds). The PTU samples with good mechanical performance simultaneously demonstrate a maximum tensile stress of 29.7 ± 1.1 MPa and a high strain of 474.8 ± 7.5%. In addition, the fraction of reversible strain of the PTU with 20 wt % hard segment reaches 22.4% during the reversible 2W-SME, where the fraction of reversible strain is enhanced by self-nucleated crystallization of the PTU. A sample with arbitrarily complex permanent 3D shapes can be realized via the solid-state plasticity, and that sample also exhibits excellent reversible 2W-SME. A smart light-responsive actuator with a double control switch is fabricated using a reversible two-way shape memory PTU/MXene film. In addition, the PTU networks are de-cross-linked by alcohol solvolysis, enabling the recovery of monomers and the realization of recyclability. Therefore, the present study involving the design and fabrication of a PTU network for potential applications in intelligent actuators and multifunctional shape-shifting devices provides a new strategy for the development of thermadapt reversible two-way shape memory polymers.

Shape Memory Polymer Fibers: Materials, Structures, and Applications

Shape memory polymer (SMP) is a kind of material that can sense and respond to the changes of the external environment, and its behavior is similar to the intelligent reflection of life. Electrospinning, as a versatile and feasible technique, has been used to prepare shape memory polymer fibers (SMPFs) and expand their structures. SMPFs show some advanced features and functions in many fields. In this review, we give a comprehensive overview of SMPFs, including materials, fabrication methods, structures, multifunction, and applications. Firstly, the mechanism and characteristics of SMP are introduced. We then discuss the electrospinning method to form various microstructures, like non-woven fibers, core/shell fibers, hollow fibers and oriented fibers. Afterward, the multiple functions of SMPFs are discussed, such as multi-shape memory effect, reversible shape memory effect and remote actuation of composites. We also focus on some typical applications of SMPFs, including biomedical scaffolds, drug carriers, self-healing, smart textiles and sensors, as well as energy harvesting devices. At the end, the challenges and future development directions of SMPFs are proposed.

Photopolymerization and 3D/4D applications using newly developed dyes: Search around the natural chalcone scaffold in photoinitiating systems

Chalcones are well-known natural dyes, 18 different derivatives selected from the rational molecular design (i.e. through molecular orbital calculations) were synthesized. If used in conjunction with an amine (i.e. ethyl l, 4-(dimethylamino) benzoate, EDB) and an iodonium salt (bis(4-tert-butylphenyl) iodonium hexafluorophosphate, Iod) as photoinitiating systems (PISs), these dyes can initiate both the free radical polymerization of polyethylene glycol diacrylate (PEG-diacrylate) and the cationic polymerization of 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (EPOX) via oxidation–reduction reaction mechanisms. Furthermore, among the 18 different suggested structures, except for dyes 9 and 10, which have been synthesized and used in other fields, the remaining 16 dyes have never been synthesized before. The photopolymerization efficiency of acrylates carried out under LED@405 nm are excellent both for thin films (the thickness is about 0.2 mm) and thick films (the thickness is about 2 mm). Furthermore, all of them can also boost the cationic polymerization of EPOX upon LED irradiation at 405 nm, in the presence of an iodonium salt and an amine. More interestingly, stereoscopic 3D patterns were successfully fabricated by the direct laser write (DLW) technique which demonstrated reversible swelling properties and reversible shape-memory effects induced via swelling and dehydration for the access to 4D printing.

Embedding carbon nanotube to the surfaces of poly(ε-caprolactone) film for multi-responsive actuations

Although incorporation of carbon nanotube (CNT) may endow reversible shape-memory polymer (rSMP) with electric actuation, it degrades the rSMP's intrinsic actuation performance (thermally responsive reversible shape-memory effect). It remains a challenge to add CNT to rSMP without degrading its intrinsic thermal actuation. Herein, a simple approach was developed to prepare a rSMP-based multi-responsive soft actuator by embedding CNT to the surfaces of a photo-crosslinked poly (ε-caprolactone) (cPCL) film via hot pressing. Scanning electron microscopy result reveals the presence of thin CNT conductive layers on the surfaces of the cPCL film. The CNT-embedded cPCL shows significantly improved thermal stability and mechanical properties over bare cPCL. The incorporation of CNT to the surfaces endows the cPCL film with good electric actuation while maintaining its good thermal actuation. In addition, it also exhibits a much faster recovery rate and a higher recovery force when compared to cPCL and a cPCL film with single CNT-embedded surface. This work develops a facile strategy to prepare the multi-responsive soft actuator with good actuation capability by embedding CNT to the rSMP film surfaces and the obtained actuator holds great potential for the applications in soft robots, smart switch, and so on.

A photo-driven metallo-supramolecular stress-free reversible shape memory polymer

Crosslinking of a crystalline network via metal ligand interactions gave rise to programmable actuation via a reversible shape memory effect. Simultaneously, the metallo-bonds offer remote light powering capability using their photo-thermal ability.

Thermoplastic Blend Exhibiting Shape Memory-Assisted Self-Healing Functionality

Here, we report on a polymer blend consisting of a soft thermoplastic polyurethane (TPU) elastomer and a low melting temperature thermoplastic healing agent (polycaprolactone, PCL). In this study, polymer blends containing up to 60 wt % PCL were prepared and the resulting mechanical, thermal, shape memory, and self-healing properties were studied. These immiscible polymers exhibit two well-separated transitions attributable to the melting of PCL and TPU hard segments. This viscoelastic behavior engendered shape memory capability at moderate processing temperatures (∼90 °C) and melt processability at elevated temperatures (>160 °C). The reversible plasticity shape memory (RPSM) effect was also characterized: when subjected to 125% strain at room temperature and subsequently heated to 90 °C, the samples nearly fully recovered to their original length. Moreover, upon heating to above PCL's melting temperature, the flow of PCL into an undeformed crack was shown to fill the crack void, thus promoting self-repair. Through the action of mild heating (90 °C/30 min), fracture surfaces are brought into intimate contact through the action of the RPSM effect and subsequently healed through the redistribution of molten PCL. The shape memory-assisted self-healing efficiency was evaluated by comparing the tensile force restoration after healing of a highly deformed, notched sample to its behavior prior to notching. It was observed that blends containing up to 30 wt % PCL showed nearly complete restoration of properties. In contrast, pure TPU showed only about 5% healing efficiency because of the absence of the PCL healing agent. Blends containing 50 and 60 wt % PCL likewise did not exhibit appreciable restoration of properties, and this was attributed to their propensity to neck during crack opening and poor mechanical properties at elevated temperatures. Blends may serve as a self-healing replacement for pure TPU in existing applications (e.g., automotive and sporting goods) or as a self-healing shape memory polymer in advanced products in soft robotic, biomedical, and microelectronic applications.

Microstructure, martensitic transformation and shape memory effect of polycrystalline Cu-Al-Mn-Fe alloys

In this study, two Cu-Al-Mn-Fe polycrystalline alloys were prepared, and their microstructure, reversible martensitic transformation, mechanical properties and shape memory effects were investigated. The results show that the reversible martensitic transformation temperatures of the studied alloys are between room temperature and 373 K, which are suitable for practical applications. Two typed martensites of 18R and 2H coexist both in two alloys. The bcc β (FeAl) nanoparticles are Fe-rich, Mn-rich and Cu-poor, whereas the martensite is Cu-rich, Fe-poor and Mn-poor. The size of nanoparticles ranges from tens to hundreds of nanometers. Full shape recovery property is displayed in Cu-12.9Al-4.5Mn-2.6Fe alloy all the time while applying different deformation from 5% to 8%. The maximum recoverable strain is up to 4.4% with a recovery rate of 100%.

A Review on Liquid Crystal Polymers in Free-Standing Reversible Shape Memory Materials.

Liquid crystal polymers have attracted massive attention as stimuli-responsive shape memory materials due to their unique reversible large-scale and high-speed actuations. These materials can be utilized to fabricate artificial muscles, sensors, and actuators driven by thermal order–disorder phase transition or trans–cis photoisomerization. This review collects most commonly used liquid crystal monomers and techniques to macroscopically order and align liquid crystal materials (monodomain), highlighting the unique materials on the thermal and photo responsive reversible shape memory effects. Challenges and potential future applications are also discussed.