Properties Of Shape Memory Polymer Research Articles

4D printing of fiber-reinforced auxetic structures: the building blocks: A review

Abstract Four-dimensional (4D) printing has recently received much attention in the field of smart materials. It concerns using additive manufacturing to obtain geometries that can change shape under the effect of different stimuli. Such a technique enables the fabrication of 3D printed parts with the additional functionality of scalable, programmable, and controllable part shapes over time. 
This review provides a comprehensive examination of advances in the field of 4D printing, emphasizing the integration of fiber reinforcement and auxetic structures as crucial building blocks. 
The incorporation of fibers enhances structural integrity, while auxetic design principles contribute unique mechanical properties, such as negative Poisson's ratio and great potential for energy absorption due to their specific deformation mechanisms. Therefore, they present potential applications in aerospace, drones, and robotics.
The objective of this review article is first to describe the distinctive properties of shape memory polymers (SMPs), auxetic structures, and composite (fiber-reinforced) materials. A review of applications that use combinations of such materials is also presented when appropriate. The goal is to get a grip on the delicate balance between the different properties achievable in each case. The paper concludes by describing recent advances in 4D printing of fiber-reinforced auxetic structures.

Insight into single-helix intelligent shape memory polymer cables: modeling and optimization under finite sliding contact

This pioneering study focuses on the finite element analysis (FEA) of thermomechanical properties of shape memory polymer (SMP) wire ropes and their components under both small- and finite-sliding contact deformation. To validate the FEA, we need to validate both geometric modeling and non-linear material behavior. Owing to intricate geometry, as well as excessive wire interactions in the structure, this part is studied by simulating a 1 × 37 steel wire rope and then comparing it with existing experimental data. To evaluate the response of non-linear material behavior, we employ the available numerical results to model the thermomechanical property of an SMP rectangular bar under a uniaxial test and then verify both constrained and unconstrained recovery behavior. After rigorous validation, two configurations of 1 × 7 and 1 × 27 SMP cables are modeled based on the thermo-visco-hyperelastic constitutive framework for acrylate polymer systems. Upon exerting an axially tensile load on these 1 × 7 and 1 × 27 SMP wire ropes, the response of force and shape recovery, as well as the normal and shear stress distributions, are measured under constrained and unconstrained conditions. For a deeper physical understanding, the influences of different temperature rates (5 and 1 °C min−1), inter-wire sliding frictional coefficient (0.1–0.6), and multiple-shape programming on the stress-strain-temperature relations of these SMP cables are also investigated. Furthermore, based on optimizing two cable factors of diameter and helix angle, and using the design of experiments method, the specific energy of a 1 × 6 SMP cable is maximized. Under different thermomechanical loadings, this study tries to cast light on the remarkable features and possible potential applications of these newly developed SMP actuators which may foster unparalleled advancements in various industries.

Thermal-mechanical enhancement investigation of shape memory polymer nanocomposite for anti-/deicing

The oriented arrangement of graphene oxide (GO) and magnetic graphene oxide (MGO) in shape memory epoxy resin (EP) by employing the coupling effect of electric and magnetic fields were investigated. The heat transfer experiments showed that the thermal conductivity of the aligned shape memory MGO/GO/EP composites was 90.29% higher than that of the random doped composites. Through the two-dimensional aligned of MGO, the tensile strength of the shape memory MGO/GO/EP composite was significantly improved. The tensile strength reaches 24.52 and 22.67 MPa along the X and Y direction respectively, which increased of 53.92% and 42.31%. The shape recovery experiments showed that the shape recovery and shape fixity of the two-dimensional aligned composites were 95.55% and 91.49%, which increased by 4.44% and 7.82%. The results demonstrated that the two-dimensional directional arrangements of MGO layers significantly enhance the heat transfer and mechanical properties of shape memory polymer. The improvement of thermal conductivity and mechanical properties directly affected the anti-deicing performance of the composites. Hence, the deicing time of the shape memory MGO/GO/EP composite was 324 s, which represented an 18.2% improvement of the deicing efficiency.

Shrinking Devices: Shape-Memory Polymer Fabrication of Micro-and Nanostructured Electrodes.

Since their discovery in the 1940s, shape memory polymers (SMPs) have been used in a broad spectrum of applications for research and industry.[1] SMPs can adopt a temporary shape and promptly return to their original form when submitted to an external stimulus. They have proven useful in fields such as wearable and stretchable electronics,[2] biomedicine,[3] and aerospace..[4] These materials are attractive and unique due to their ability to "remember" a shape after being submitted to elastic deformation. By combining the properties of SMPs with the advantages of electrochemistry, opportunities have emerged to develop structured sensing devices through simple and inexpensive fabrication approaches. The use of electrochemistry for signal transduction provides several advantages, including the translation into inexpensive sensing devices that are relatively easy to miniaturize, extremely low concentration requirements for detection, rapid sensing, and multiplexed detection. Thus, electrochemistry has been used in biosensing,[5] pollutant detection,[6] and pharmacological[7] applications, among others. To date, there is no review that summarizes the literature addressing the use of SMPs in the fabrication of structured electrodes for electrochemical sensing. This review aims to fill this gap by compiling the research that has been done on this topic over the last decade.

4D printing of shape memory polymer nanocomposites for enhanced performances and tunable response behavior

The emergence of four-dimensional (4D) printing introduces a novel approach to the design and manufacturing of high-performance multifunctional shape memory polymers (SMPs). Herein, two types of nanomaterials were blended or embedded into SMPs matrix to enable 4D printing of shape memory polymer composites (SMPCs) via photocuring. The incorporation of hexagonal boron nitride (HBN) nanoparticles enhanced the thermal stability and mechanical properties of SMPs, and accelerated their thermally responsive shape recovery speed. HBN nanoparticles also significantly improved the performance stability of SMPs, even after undergoing multiple shape memory cycles. Electrically responsive shape memory behavior in SMPCs was achieved by embedding a spray-coated polyaniline: polystyrene sulfonate /graphene (PANI: PSS/GR) electrothermal layer (ETL). The applied voltage, ETL resistance, HBN blend content, and sample thickness had a significant regulation effect on electrically responsive shape recovery behavior and shape recovery force. PANI: PSS improved adhesion between ETL and SMPCs matrix, filled the gaps between graphene platelets, which resulted in rapid, uniform, and stable heating upon energization. The presented electrically responsive gripper was capable of grasping objects of various shapes and sizes. This strategy broadens the potential applications of 4D printed SMPCs in challenging environments while propelling the advancement of 4D printing.

Effect of Thread Count on the Shear Mechanical Properties and Dynamic Mechanical Properties of Shape Memory Polymer Reinforced by Single-Ply Weave Fabric

Effect of Thread Count on the Shear Mechanical Properties and Dynamic Mechanical Properties of Shape Memory Polymer Reinforced by Single-Ply Weave Fabric

Elastocaloric effect of shape memory polymers in elastic response regime

Solid-state cooling/heating technology based on the elastocaloric effect is one of the promising alternatives to vapor compression systems. Large elastocaloric temperature modulation is often generated through the non-linear strain-induced structural transition by applying large strain and/or stress to ferroelastic materials. Recently, an unconventional approach to expand the application possibilities of the elastocaloric effect was demonstrated by processing elastocaloric materials into kirigami structures, which was inspired by the art of paper cutting. Using this approach, only a small stretch of processed conventional plastics can locally provide more efficient performance of elastocaloric temperature modulation than that of ferroelastic materials. To further improve such a unique functionality, it is necessary to find plastic or polymeric materials showing large elastocaloric effects in the linear elastic response regime that can be driven by a MPa-order weak stress application, where the non-linear structural transition is irrelevant. In this work, by means of a recently developed measurement technique for the elastocaloric effect based on the lock-in thermography, we found that shape memory polymers (SMPs) show prominent performance for elastocaloric temperature modulation that is larger than conventional plastics. SMPs enable the control of crystallinity by changing the cross-linking agents, melting temperature by changing the degree of polymerization, and orientation of the polymer chain segment by the shape memory effect. By utilizing the unique properties of SMPs, we manipulated their elastocaloric performance. The experimental results reported here will highlight the potential of smart polymers for flexible and durable elastocaloric applications.

Temperature dependence analysis of mechanical properties and bending behaviors of shape memory programmable composites

Temperature significantly affects the mechanical properties of shape memory polymers (SMPs). The temperature dependence of mechanical properties and bending behaviors of shape memory polymer composites (SMPCs) was investigated in this study. Dynamic mechanical analysis and static mechanical tests were performed to investigate the temperature dependence on the mechanical properties of SMP and SMPC. The tensile strength and shear strength of SMP declined as the temperature rose, while the elongation at break increased first and subsequently reduced. SMPC exhibited brittle fracture at low temperatures and fiber buckling at high temperatures, respectively. Subsequently, a temperature-dependent model of the bending behavior of SMPC was developed. Theoretical model revealed the effect of temperature and fiber volume content on fiber buckling. The higher the temperature and fiber volume content is, the easier the fiber buckles. The effects of temperature, thickness and fiber volume content on critical buckling curvature, strain energy, half-wavelength and amplitude were investigated. Temperature and fiber volume content were positively related to critical buckling curvature, strain energy and amplitude, but negatively related to half-wavelength. Thickness was positively related to strain energy, half-wavelength and amplitude, but negatively correlated to critical buckling curvature, in which thickness does not determine whether SMPC buckling occurs. Eventually, the theoretical model was verified using the four-point bending test to ensure its credibility. The research results are expected to provide theoretical guidance for the design and application of SMPC.

New Approach to Shape Memory Polymer Composite Production Using Alkaline Lignin-Reinforced Epoxy-Based Shape Memory Polymers.

In the past few decades, there has been continued interest in shape memory polymers (SMPs), and tremendous efforts have been made to develop multifunctional composites of these SMPs to enhance the existing properties of SMPs. Although fossil-based sources are widely used in the production of shape memory polymer composites (SMPCs), the depletion of fossil-based resources and associated environmental problems increase interest toward renewable biobased products synthesized from natural resources. This study aims to produce alkaline lignin-reinforced SMPCs by using alkaline lignin in the SMP matrix. Thermo-mechanical, morphological, and shape memory tests are performed in order to reveal the effect of alkaline lignin usage in the SMP matrix on SMPC production. Differential scanning calorimetry analysis results show that adding alkaline lignin into the SMP matrix with 1 and 3% ratios led to an increase in T g values, while raising the alkaline lignin ratio to 5% decreased the T g value. According to the DMA results, increasing the alkaline lignin ratios caused an increase in the storage modulus of SMPCs, and the best storage modulus value was obtained at the 5% alkaline lignin ratio. The results of the three-point bending test also confirmed the results obtained from the DMA analysis, showing that an increasing alkaline lignin ratio caused an increase in the bending modulus. Scanning electron microscopy analysis showed a rough structure in 1 and 3% alkaline lignin supplementation, while a smoother structure was observed in 5% alkaline lignin supplementation. The smoother structure of the sample containing 5% alkaline lignin indicates that alkaline lignin supplementation exhibits a smoother surface by showing a plasticizing effect. As a result, it was observed that increasing the lignin ratio increased the polymer/alkaline lignin interaction, resulting in a harder structure and an increase in the flexural modulus value.

Physical cross-linked aliphatic polycarbonate with shape-memory and self-healing properties

As one of the stimulus-responsive intelligent materials, shape memory polymers (SMPs) gained increasing attention for possessing the benefits of lightweight, low density, easy processing, and large shape variables. It is of great significance to enhance self-healing properties and mechanical properties of SMPs. Herein, we synthesized an aliphatic polycarbonate monomer named MMC (5-methyl-5-[(4-methoxy)-benzyloxycarbonyl]-1,3-dioxan-2-one) by a two-step method. The cross-linked network was constructed by ring-opening copolymerization of 5-methyl-5-benzyloxycarbonyl-1, 3-dioxan-2-one (MBC) and MMC with mPEG43 as an initiator and Sn(Oct)2 as a catalyst. The acquired elastomer achieves a stretchability of up to 777.7 %, which is significantly higher than that of the homopolymer of MBC. The elastomer presents a shape recovery rate of 97.8 within 42 s at 37 °C and can maintain excellent stretchability up to 772.0 % which is 99.3 % of the initial value after repair at 37 °C for 6 h. The introduction of MMC heightens the viscosity of the cross-linked network and effectively improves its stretching and self-healing performance, which offers new candidates for future body-sensors, stimuli-responsive materials, and wound healing.

4D printing of shape memory composites with remotely controllable local deformation

Shape memory polymers with multiple temporary shapes have shown great potential for applications in biomedical, aerospace, and intelligent robotics fields. However, the realization of the multiple shape memory properties of shape memory polymers is currently mostly dependent on formula design. Here, we report a strategy to remotely program the local bending behavior of shape memory composites using an infrared light source. The infrared light source is used to locally activate a single side of the sample with a temporary shape, which induces an asymmetric shape recovery due to the temperature gradient along the thickness direction, and achieves the phototropic response on-demand bending deformation. We exerted control over the bending deformation of the composite prototypes by devising the 3D printing parameters, infrared light intensity, and temporary shape programming. As a demonstration case, we designed and prepared a hand-like structure that can be bent sequentially, a flower whose petals can be excited to fold sequentially, and a digital logic circuit for sequential lighting of LEDs.

Modelling strategy for tailorable mechanics and thermochemically driven shape memory effect in amorphous polymers

Thermochemically responsive shape-memory polymers (SMPs) with biocompatibility and variable stiffness have shown great potential in biomedical applications. However, it is difficult to characterize the relaxation behavior and visco-elastoplastic deformation of this kind of SMPs due to the complex interactions between polymer segments and solvent molecules. Herein, a thermo-chemo-mechanical coupling model was proposed to predict the shape-memory behavior and thermomechanical properties of SMPs at different solvent volume fractions and temperatures. By combining the Vrentas–Duta free volume theory and the Flory-Huggins theory, the effect of free volume change on the Gibbs free energy, chemical potential, thermal expansion coefficient, and relaxation time of SMPs was investigated. Subsequently, these functions were substituted into the Maxwell and Boyce models to characterize the visco-elastoplastic mechanical behavior of thermochemically responsive SMPs under large deformation. Based on the phase transition model, the chemical plasticizing effect of solvent molecules on the glass transition temperature, shape-recovery strain, and yield strength of SMPs was further investigated. The numerical results are in good agreement with the experimental results. The proposed model is expected to provide theoretical guidance for designing SMPs with tailorable mechanics.

The Current Status, Prospects, and Challenges of Shape Memory Polymers Application in Bone Tissue Engineering.

Bone defects can occur after severe trauma, infection, or bone tumor resection surgery, which requires grafting to repair the defect when it reaches a critical size, as the bone's self-healing ability is insufficient to complete the bone repair. Natural bone grafts or artificial bone grafts, such as bioceramics, are currently used in bone tissue engineering, but the low availability of bone and high cost limit these treatments. Therefore, shape memory polymers (SMPs), which combine biocompatibility, biodegradability, mechanical properties, shape tunability, ease of access, and minimally invasive implantation, have received attention in bone tissue engineering in recent years. Here, we reviewed the various excellent properties of SMPs and their contribution to bone formation in experiments at the cellular and animal levels, respectively, especially for the repair of defects in craniomaxillofacial (CMF) and limb bones, to provide new ideas for the application of these new SMPs in bone tissue engineering.

Online reinforcement learning for the shape morphing adaptive control of 4D printed shape memory polymer

Combining 3D printing and smart materials, 4D printing technologies enable the printed actuators to further change their shapes or other properties after prototyping. However, the shape morphing of 4D printed actuators suffers from poor controllability and low precision. One of the main challenges is that the 4D printed actuators are hard to be modeled and it is difficult to develop an appropriate controller for them. In this study, various popular reinforcement learning (RL) methods are applied to address the problem of online and adaptive model-free control of 4D printed shape memory polymer (SMP). Their training efficiencies are compared and an adaptive LQR controller based on Q learning is developed to realize efficient online learning. The RL controller achieves precise and quick shape control within 2−−3 learning episodes and is adaptive to the changing properties of SMP. The RL controller performance is then compared with a model-based LQR controller and shows high control precision and excellent adaptability to the varying control plant.

Experimental analysis of high-temperature shape memory polymers for deployable structures

Shape memory polymers (SMPs) continue to capture interest in the aerospace industry due to their unique properties. In general, the focus has been on SMPs with low transformation temperatures below 100 °C. However, certain aerospace applications require shape memory polymers with higher transformation temperatures to prevent undesirable actuation upon heat exposure. Moreover, and despite significant work on characterizing mechanical and recovery properties of shape memory polymers, the literature lacks full-field characterization of recovery properties, in particular at elevated temperatures. In this work, a high-temperature thermoset epoxy SMP (Tg ≈ 130 °C), named EPON SMP, is thoroughly investigated. This study provides a comprehensive experimental analysis of the mechanical and shape memory properties of the aforementioned high-temperature epoxy SMP. In addition, this study uses a full-field characterization technique, Digital Image Correlation (DIC), to provide further insight on localization, heterogeneity, and local recovery of shape memory strains. Furthermore, the study evaluates the recovery properties of the EPON SMP at different loading conditions (i.e., bending and tensile) and proposes shape-recovery evaluation methods that can be later implemented on other SMPs. • An epoxy-based shape memory polymer with high switching temperature (Tg ≈ 130 °C) was considered in this work. • The temperature dependant mechanical properties were evaluated across a wide temperature range around Tg. • The shape recovery properties were studied under simple tensile and complex bending conditions. • The heterogeneities in the accumulated and recovery strains were evaluated using full-field deformation measurements. • A deep insight into the local accumulation of programmed, thermal, mechanical, and recovery strains is provided.

Effect of process parameters on mechanical properties of additive manufactured SMP structures based on FDM

Abstract As a new concept and multidisciplinary research area that combines advanced manufacturing, materials science and mechanics, 4D printing has attracted academic interest in the recent years. Shape Memory Polymer (SMP) is widely used in 4D printing due to its high stiffness and fast response. In order to make better analysis and research on additive manufacturing of SMP structures based on FDM, the DMA, DSC and TMA tests were conducted first to determine the basic thermodynamic properties of SMP. And then nine parameters, divided into three groups including printing parameters, temperature parameters and structural parameters were examined to character and assess the effect on the mechanical properties of SMP samples, respectively. The results show that the best parameters combination of temperature and printing parameters for the SMP printing is that platform temperature T P = 65 °C, nozzle temperature T N = 210 °C, feed rate Fr = 50 mm/s, layer thickness Lt = 0.18 mm. Compared with temperature and printing parameters, the structural parameters directly affect the tensile strength of the specimen with improve of 17.298 MPa while these two are 3.639 and 5.528 MPa. The tensile strength of the specimen can be effectively improved through adjusting the structural parameters.

Shape-Memory Polymers Hallmarks and Their Biomedical Applications in the Form of Nanofibers.

Shape-Memory Polymers (SMPs) are considered a kind of smart material able to modify size, shape, stiffness and strain in response to different external (heat, electric and magnetic field, water or light) stimuli including the physiologic ones such as pH, body temperature and ions concentration. The ability of SMPs is to memorize their original shape before triggered exposure and after deformation, in the absence of the stimulus, and to recover their original shape without any help. SMPs nanofibers (SMPNs) have been increasingly investigated for biomedical applications due to nanofiber’s favorable properties such as high surface area per volume unit, high porosity, small diameter, low density, desirable fiber orientation and nanoarchitecture mimicking native Extra Cellular Matrix (ECM). This review focuses on the main properties of SMPs, their classification and shape-memory effects. Moreover, advantages in the use of SMPNs and different biomedical application fields are reported and discussed.

Experimental study on rheological and settling properties of shape memory polymer for fracture sealing in geothermal formations

This paper studies the rheology and annular flow of a smart lost circulation material (LCM) at 160 °C, using a high-temperature flow loop. The smart LCM is a shape memory polymer (SMP) that activates at high temperatures, and its particle size increases to seal fractures in geothermal formations. SMP was mixed with the base fluid in two different concentrations, 1.0 and 3.0 wt%, to study rheology, wellbore hydraulics, activation process, and settling behavior under different testing conditions. The results of this study showed that the SMP could be activated at high temperatures. An increase of 80–100% in the particle size was observed at 160 °C. The mud samples showed a high shear-thinning behavior at the two concentrations with a Power-law flow index (n) ranging between 0.025 and 0.101. No additional frictional pressure losses were observed when SMP was added to the base fluid. SMP particles showed an excellent suspension at 1.0 wt% while, at 3.0 wt%, a bed was formed at a low flow rate and without pipe rotation. Increasing the drill pipe rotational speed or flow rate effectively removed the bed and homogeneously dispersed the SMP particles, ensuring a better sealing efficiency. SMP particle dispersion in inclined wells was better than in horizontal wells. Moreover, the findings of this study help optimize the lost circulation treatment by considering a wide range of operating parameters that can further be extended to different systems and geometries.

Effects of networks composed of epoxy/dual thiol‐curing agents on properties of shape memory polymers

AbstractProfound understanding of relationships between molecular networks with properties can make it more convenient for people to prepare and employ shape memory polymers (SMPs). In this paper, a series of epoxy SMPs were developed based on epoxy (E‐51), trimethylolpropane tris(3‐mercaptopropionate) and pentaerythritoltetrakis (3‐mercaptopropionate), and 2‐methylimidazole, through “epoxy‐thiol” click reaction. The effect of networks on key properties (heat resistance and mechanical properties shape recovery performance) was systematically investigated. Experiment results indicated that there were positive correlations between network regularity with both thermal–mechanical properties and heat resistance. In addition, regularity of network structure had an effect to restrict shape recovery speed.

An insight into mechanical & thermal properties of shape memory polymer reinforced with nanofillers; a critical review

The advent of smart material has open new avenues for multifarious applications in engineering and technology. A shape memory polymer (SMP) has ability to return from deformed shape to original shape induced by an external stimulus is one such example of smart material. Shape memory polymer in neat form exhibit inherently poor mechanical & thermal properties. The poor mechanical & thermal properties hinder its applications in engineering such as pick & place gripper robots, coronary heart stent and other applications. Several researchers have attempted to reinforce different SMP with nanofillers to augment mechanical & thermal properties. Important mechanical and thermal properties augmented were tensile strength, hardness, and thermal conductivity which further influence other properties like wear resistance. The current study critically exonerates the work carried out and delineates the technical gap which provides a strong scope for future research.