Two-way Shape Memory Polymers Research Articles

A highly visible-transparent thermochromic smart window with broadband infrared modulation for all-season energy savings

Abstract Thermochromic smart windows effectively reduce the energy consumption for building through passive light modulation including the transmission of visible (TVis), near-infrared (TNIR), and the emissivity of mid-infrared (εMIR) in response to the ambient temperature change. However, thermochromic windows that maintain high TVis while modulating TNIR and εMIR simultaneously are highly desirable but still challenging. Here, we develop a thermochromic smart window based on a two-way shape memory polymer to enable reversible transformation and achieve TNIR modulation of 44.0% and εMIR modulation of 76.5% while maintaining high TVis (>50%). Compared to traditional windows based on silica glass, this device shows 4°C lower temperature in summer daytime, 2°C higher in winter daytime, and 1°C higher in spring nighttime. It is expected that our device can achieve greater annual energy savings in comparison with commercial glass anywhere in the world and promote the progress of thermochromic windows for energy-efficient buildings.

Intelligent Reversible Reconfigurable Metamaterials Based on a Two-Way Shape Memory Polymer.

The development of intelligent reversible reconfigurable metamaterials has great significance in constructing three-dimensional metamaterials and introducing reversible tunability into metamaterials. Here, we introduce an intelligent metamaterial consisting of a two-way shape memory polymer (2W-SMP) ethylene vinyl acetate copolymer (EVA) actuator substrate and a patterned flexible-rigid film. Mechanical buckling of the 2W-SMP substrate was controlled by thermal stimulation. This makes it possible to afford an ability to initiate 3D structure formation or shape reconfiguration remotely in an on-demand fashion. In addition, the shape of the 2W-SMP substrate is temperature-dependent, allowing repeatable reversible deformation through temperature control after a single programming. Therefore, the electromagnetic properties of metamaterials can also be repeatedly and reversibly tuned between 9.15 and 10.82 GHz. Experimental demonstrations include the deformation and tunable electromagnetic properties of intelligent reversible reconfigurable metamaterial cells. The results create many opportunities for advanced programmable three-dimensional metamaterials.

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.

Tuning the Properties of Multi-Stable Structures Post-Fabrication Via the Two-Way Shape Memory Polymer Effect.

Multi-stable elements are commonly employed to design reconfigurable and adaptive structures, because they enable large and reversible shape changes in response to changing loads, while simultaneously allowing self-locking capabilities. However, existing multi-stable structures have properties that depend on their initial design and cannot be tailored post-fabrication. Here, a novel design approach is presented that combines multi-stable structures with two-way shape memory polymers. By leveraging both the one-way and two-way shape memory effect under bi-axial strain conditions, the structures can re-program their 3D shape, bear loads, and self-actuate. Results demonstrate that the structures' shape and stiffness can be tuned post-fabrication at the user's need and the multi-stability can be suppressed or activated on command. The control of multi-stability prevents undesired snapping of the structures and enables higher load-bearing capability, compared to conventional multi-stable systems. The proposed approach offers the possibility to augment the functionality of existing multi-stable concepts, showing potential for the realization of highly adaptable mechanical structures that can reversibly switch between being mono and multi-stable and that can undergo shape changes in response to a change in temperature.

Ultra-tough and stress-free two-way shape memory polyurethane induced by polymer segment “spring”

Two-way shape memory polymers (2WSMP) are highly desired for artificial muscles, soft actuators and sensors, while the stress-free 2WSMP greatly expands the practical potential of shape memory polymers. However, traditional 2WSMP cannot show high strength or good mechanical properties, which limited its practical application. It is important to create a new reversible polymer system in order to obtain more intricate movements, precise control and great mechanical properties. Herein, a two-way shape memory polyurethane was created using a new molecular strategy, which shows great properties, i.e., 2342% super stretch strain, 591 MPa true tensile strength, 708.04 J/kg of energy density, and 667.96 W/kg of power density. In addition, the 2WSMP could achieve a maximum 24.53% reversible transformation under 1 MPa stress, and 13.7% two-way shape change without external stress, respectively. In this polyurethane, the great mechanical properties and stress-free two-way shape memory effect are caused by the elastic polybutadiene (PB) segments, which acts as a “molecular spring.” Furthermore, this 2WSMP could simulate reversible muscle-like motion, and be employed in ligaments. Consequently, this polyurethane offers a fresh design concept for stress-free two-way performance memory polymers and has a wide range of potential applications in the field of artificial muscles, soft actuators and ligaments.

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.

Photo-crosslinkable and ultrastable poly(1,4-butadiene) based organogel with record-high reversible elongation upon cooling and contraction upon heating

Two-way shape memory polymers with reversible elongation upon cooling (EUC) and contraction upon heating (CUH) have emerged as promising smart materials for use in soft actuators. However, it remains challenging to develop organogels with as high as possible actuation strain. Here, novel photo-crosslinkable poly(1,4-butadiene) (PBD) based organogels with exceptional stability and high reversible EUC and CUH have been developed for the first time. The conventional photo-initiator 2-hydroxy-2-methylpropiophenone and plasticizer bis(2-ethylhexyl) phthalate were utilized to achieve photo-crosslinking ability and to serve as the organic free phase, respectively. Owing to the stability of the plasticizer, the PBD based organogels are pretty stable in high vacuum environment and below 100 °C. Importantly, the PBD organogel with 60 wt % plasticizer exhibited the best overall performance, especially with the record-high EUC (156%) and CUH (151%), and the pretty high actuation reversibility (97%). Moreover, the high stretchability makes the PBD based organogels possess great potential in strain sensing application. The widely accessible raw materials and simple preparation method make PBD based organogels with good possibility in practical applications.

4D Printing of Single-Network Shape Memory Polyurethanes with Two-Way Actuation Properties

Two-way shape memory polymers (SMPs) have received much attention due to their ability to enable stimuli-responsive reversible structures without requiring repeated human intervention, which may be useful for application in many fields where multiple actuation cycles are desired. However, many layered or composite SMP systems are not integrated structures and suffer from several disadvantages, such as poor interlayer adhesion with limited actuation repeatability. In addition, two-way SMPs are limited by their manufacturing method to producing user-defined, complex, high-precision devices and parts. In this work, we developed a 3D-printable resin comprising two polyurethane-based oligomers with distinctly different transition temperatures─polypentadecalactone diacrylate (PPDLDA) and polycaprolactone diacrylate (PCLDA)─together with other monomers to fabricate single-network two-way shape memory smart materials by digital light projection 3D printing. The chemical, mechanical, and thermal properties of printed products can be easily tuned by facile manipulation of monomer and oligomer compositions. The PCLDA and PPDLDA switching segments were found to exhibit high mean shape fixity ratios (Rf) of >97% and high shape recovery ratios (Rr) of >89%. The strategy for achieving 3D-printable single-network two-way SMPs presented herein holds promise for applications in soft robotics, medicine, and so forth.

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.

Deep learning for predicting the thermomechanical behavior of shape memory polymers

Thermomechanical constitutive modeling is essential for shape memory polymers (SMPs) to be used in engineering structures and devices. However, the classical method of deriving constitutive models is difficult, time consuming, and relies heavily on trial and error. In this work, we aim to decrease the time and resources needed to develop new thermomechanical models for SMPs. The method proposed in this work uses deep learning (DL) to predict the thermomechanical behavior of SMPs under thermomechanical cycles. Particularly, a semicrystalline two-way shape memory polymer (2W-SMP) is selected as an example. Predicting such behavior will give insight on the SMP properties and help validate its characteristics. In this paper, we have compared several DL models to find which one can predict the experimental thermomechanical behavior with the highest accuracy within a reasonable time frame. The results reveal that the fully connected neural network (FCNN) and the convolutional neural network (CNN) were the most accurate DL models. Overall, using one of the selected DL models, we can predict the results of new iterations of the experiment without spending as much time and resources.

Two-way thermo-responsive thermoset shape memory polymer based on benzoxazine/urethane alloys using as self-folding structures

A novel two-way shape memory polymers derived from benzoxazine/urethane alloys (poly(BA-a/PU)) are developed in this research. The effect of urethane contents acted as spring elastomer on dynamic mechanical properties and two-way shape memory performance of the alloys was investigated. The experimental test and numerical simulation were also used to study and predict the ability of the developed 2WSMPs. The results showed that shape fixity at room temperature (RT) of poly(BA-a/PU) was enhanced in the range of 80–97% with the addition of PU contents. Shape recovery values to first temporary shape at Tg+20 °C of the specimen were 79–93%, whereas, those to second temporary shape at RT were 82–95%. Furthermore, 2WSMP based on poly(BA-a/PU) was reversely recovered between both temporary shapes more than 7 cycles. The experimental and numerical results revealed that poly(BA-a/PU) exhibited high 2WSMP performance for using as smart materials in self-folding structures application.

Insight in thermomechanical constitutive modeling of shape memory polymers

Shape memory polymers (SMPs) are a new member of the smart materials family. SMPs have found wide applications or potential applications in almost all manmade structures and devices. In order to better design smart structures and devices using SMPs, thermomechanical constitutive modeling is essential. In this insight paper, we will focus on presenting several multi-length-scale and multi-physics modeling frameworks, including the thermodynamics consistent model, elasto-viscoplastic model, statistical mechanics model, and phase evaluation law model. The SMPs modeled will include amorphous one-way shape memory polymers, semicrystalline one-way shape memory polymers, semicrystalline two-way shape memory polymers, and functional and mechanical damage effects on SMPs. Finally, we will give some in-depth perspectives on future development in this area of study.

A soft syntactic foam actuator with high recovery stress, actuation strain, and energy output

Novel syntactic foams were prepared by incorporating three types of glass microbubbles in a crosslinked cis poly(1,4-butadiene) (cPBD) matrix. The as-prepared semi-crystalline polymer-based syntactic foams displayed a clear two-way shape memory effect (2W-SME) with remarkable strain actuation of about 56% expansion when the temperature drops from 60 °C to − 40 °C, and about 40% contraction when the temperature rises from − 40 °C to 60 °C, comparable to most pure two-way shape memory polymers. Compared to the pure cPBD, which was actuated under 0.32 MPa tensile stress, the foams were actuated under a much larger external tensile stress of up to 1.6 MPa, signifying its superior actuation capability. The energy output is also much higher for the syntactic foam, up to 0.83 MJ/m3, compared to 0.28 MJ/m3 for the pure cPBD. Furthermore, it has a working temperature below zero Celsius and may be actuated at temperatures lower than most other available shape memory polymer foams. With a low density, decent mechanical properties, and great actuation capability, these syntactic foams can be adopted for applications such as artificial muscles, biomedical devices, soft robots, sealants, and aerospace structures, which can be potentially manufactured by 3D printing.

Mechanism of Behavior of Two-Way Shape Memory Polymer under Constant Strain Conditions

In this paper, we report detailed investigation of structural changes occurring in two-way shape memory polymers (2W-SMP) upon thermal cycling at constant strain conditions. We demonstrate that cooling of stretched chemically cross-linked polycaprolactone results first in a slow decrease of stress, the stress drops rapidly upon further cooling, and further cooling results in sharp increase of stress. In this paper, we investigated structural changes occurring in each of these three phases using in situ force measurements and XRD analysis (SAXS and WAXS). In the first phase, the stress decreases due to decrease of entropic factor in the elastic state. In the second phase, polymer chains start to crystallize in the stretched state as a result of the number of active springs producing stress decreasing. This results in the decrease of the stretching degree of other polymer chains, and finally, unstretched polymer chains crystallize that result in an increase of stress─the polymer simply tends to shrink.

Converse two-way shape memory effect through a dynamic covalent network design

A unique thermally driven two-way shape memory polymer is reported through a dynamic covalent network design. Its actuation direction relative to the programming force is reflected as cooling-induced contraction and heating-induced elongation.

Semicrystalline polymer networks with a swelling-enhanced water-triggered two-way shape-memory effect for programmable deformation and smart actuation

A simple, effective and universal strategy is proposed to fabricate a water-triggered two-way shape-memory polymer with the highest angle reversibility of 45.2%, which can be applied as a soft gripper and water level monitor.

Achieving multimodal locomotion by a crosslinked poly(ethylene-co-vinyl acetate)-based two-way shape memory polymer

Locomotion is a critically important topic for soft actuators and robotics, however, the locomotion applications based on two-way shape memory polymers (SMPs) have not been well explored so far. In this work, a crosslinked poly(ethylene-co-vinyl acetate) (cPEVA)-based two-way SMP is synthesized using dicumyl peroxide (DCP) as the crosslinker. The influence of the DCP concentration on the mechanical properties and the two-way shape memory properties is systematically studied. A Venus flytrap-inspired soft actuator is made by cPEVA, and it is shown that the actuator can efficiently perform gripping movements, indicating that the resultant cPEVA SMP is capable of producing large output force and recovering from large deformations. This polymer is also utilized to make a self-rolling pentagon-shaped device. It is shown that the structure will efficiently roll on a hot surface, proving the applicability of the material in making sophisticated actuators. With introducing an energy barrier, jumping can be accomplished when the stored energy is fast released. Finite element simulations are also conducted to further understand the underlying mechanisms in the complex behavior of actuators based on cPEVA SMP. This work provides critical insights in designing smart materials with external stimulus responsive programmable function for soft actuator applications.

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.