Thermomechanical Shape Memory Research Articles

Shape memory behaviors of three-dimensional five-directional braided composites with different axial yarns arrangements

The effect of braided structure on shape memory behaviors of 3D braided composites is critical to design the responses of the composite under external fields. Here we report the effect of axial yarn on shape memory behaviors of three-dimensional five-directional (3D5d) braided shape memory polymer composites (SMPCs) under bending. It was found that the axial yarns will improve bending stiffness and electro-thermal behaviors significantly. The thermomechanical deformation and shape memory effect of SMPCs are influenced by the braiding structural parameters. Specifically, the 3D5d SMPCs exhibits a 53.6% increase in bending recovery force and a faster shape recovery speed than those of 3D four-directional SMPCs. We found from finite element analyses (FEA) that the axial yarns positions influence the inner stress distribution and shape memory behaviors of SMPCs. The axial yarns distributed on both sides of SMPC influence the shape memory behaviors more than those of the middle layer.

Effect of silane coupling agent modification on shape memory behavior and mechanical properties of knitting fabric reinforced composites

Continuous glass fiber fabrics can improve the shortcomings of shape memory polymers in terms of mechanical properties. However, the mechanical properties of shape memory composites will not be optimal if the interface between the glass fibers and the matrix is poorly bonded. The purpose of this study is to explore the thermomechanical properties and shape memory behavior of shape memory epoxy resin and composites before and after the chemical modification of glass fiber. The effects of the ratios of epoxy to curing agent and whether the knitting fabrics are modified with silane coupling agent on the properties of the composites are investigated. The results show that the thermal decomposition temperature and glass transition temperature of the shape memory epoxy polymers increase with the increasing mass ratio of diaminodiphenylmethane. The shape fixing ratios and shape recovery ratios of the knitting fabric reinforced composites with different mass contents and silane coupling agent modification are above 98.0% at 18.0% tensile strain. After 10 cycles, the shape memory performance of the composites modified with KH560 is still good. Besides, the addition of silane coupling agent to the composites can reduce the shape recovery time by 17.9% and increase the recovery force with 30.0% subjected to 18.0% tensile strain. The tensile strength of the shape memory composites modified with silane coupling agent can increase 32.8% as compared with that of the composites without modification.

Space-deployable device based on shape memory cyanate ester composites

Shape memory cyanate ester is a class of shape memory polymers with high thermal stability and excellent space environmental adaptability. In this study, a space-deployable device based on cyanate ester-based shape memory polymer composite (CE-SMPC) was proposed, and its thermomechanical properties and shape memory behavior were experimentally evaluated. The space-deployable device maintained a high shape fixity rate even after prolonged exposure to high or low temperatures, which ensured its locking reliability in space applications. The results of self-deployment experiments indicated that the space-deployable device exhibited good shape recovery performance in both atmospheric and vacuum environments. The SMPC-based space-deployable device presented in this study was applied to China's first Mars exploration mission to achieve autonomous locking and release of the deployable engineering measurement subsystem on the probe.

Thermomechanical shape memory testing of 4D printed novel material rhombus-shape structure

4D printing of functional energy generation/absorption structures by material extrusion technique can capitalize on the exciting applications in intelligent damping devices and patterns to deform spontaneously. This paper investigated the capitalization of this acquired knowledge by studying the shape memory effects of a functional rhombus shape structure. Initially, the shape memory characteristics of energy absorption and dissipation capacity were analyzed using dynamic mechanical testing to develop databases to gather thermophysical data before expressing the material behavior and recovery performances of printed shape memory structures. Then a complete thermomechanical cycle (shape programming and recovery) on 4D printed amorphous and semi-crystalline shape memory polymers exhibited deep insights into shape memory performance. The performance factors (i.e., recovery and fixity ratios) are influenced by variable printing parameters (i.e., layer height, printing temperature, and speed) and stimuli-based testing conditions. Results reveal both materials have significant shape memory effects with a maximum recovery ratio of up to 92.30 ± 0.32% from the programmed configuration. The amorphous polymer was extremely affected by printing temperature, whilst the semi-crystalline was influenced heavily by the interaction of all three parameters. Finally, shape memory effects predicted by a high-order design model and compared with experimental results showed negligible error. The analyses and assessments presented in this paper are adequate to understand the shape memory behavior under process control parameters to establish a data-driven model of a 4D printed semi-crystalline and amorphous polymer reactive to thermal stimuli.

Thermo-mechanical modeling of semicrystalline triple shape memory polymers

Compared with amorphous shape memory polymers (SMPs), semicrystalline SMPs have more diverse shape memory effects (SMEs) which promotes their application in smart structures. To reveal the driving mechanism of the triple SMEs of semicrystalline SMPs, our study focuses on developing the constitutive model under the condition of finite deformation. In the paper, a thermo-mechanical constitutive model under consideration of the second law of thermodynamics is developed based on the theory of thermodynamics with internal state variables. The model can be used to describe the nonequilibrium response of the amorphous and semicrystalline components in the vicinity of the glass transition, melting, and crystallization. To verify the validity of model, numerical simulation is carried out for a thermo-mechanical shape memory cycle which can be divided into a two-step programing process and a two-step recovery process. The comparison between the model results and the test data shows good agreement.

Investigation of thermo-mechanical shape memory signatures of 3D printed and Injection molded polymers

This research paper investigates the shape memory signatures (SMSs) of SMPs fabricated by two different manufacturing processes, namely Fused Filament Fabricated (FFF) and Injection-molding (IM). The SMSs of each manufacturing route is first investigated by rating the thermophysical and thermodynamic properties of amorphous and semicrystalline-type shape memory polymers. Secondly, the capturing of SMSs is completed by a series of thermomechanical shape programming/recovery cycles on FFF printed specimens, and the results are correlated with the IM-produced samples of similar nature. Furthermore, the thermomechanical cycles are programmed at varying heating rates (1, 2, 3, and 5 °C min−1) to analyze their influence on shape memory behavior and cycling time. Results depict that an increase in induced pre-strain driven by small to large deformations (10% → 25%) in the elastic range leads to maximum shape memory performance and the highest performance was reported while heating at 2 and 3 °C min−1, respectively. Finally, the IM-produced samples showed slightly better shape recovery compared to the 3D-printed samples. The shape memory signatures of 3DP specimens are indeed up to 86% like the ones produced by injection molding, which is mostly governed by the distinct residual stress/strain induced during the two different manufacturing processes.

Shape memory property of polybutylene adipate-co-terephthalate

PurposeThe purpose of this study is to investigate the thermomechanical condition on the shape memory property of Polybutylene adipate-co-terephthalate (PBAT). PBAT is a widely researched and rapidly developed biodegradable copolyester. In a tensile test, we found that the fractured PBAT samples had a heat-driven shape memory effect which piqued our interest, and it will lay a foundation for the application of PBAT in new fields (such as heat shrinkable film).Design/methodology/approachThe shape memory effect of PBAT and the effect of the thermomechanical condition on its shape memory property were confirmed and systematically investigated by a thermal mechanical analyzer and tensile machine.FindingsThe results showed that the PBAT film had broad shape memory transform temperature and exhibited excellent thermomechanical stability and shape memory properties. The shape memory fixity ratio (Rf) of the PBAT films was increased with the prestrain temperature and prestrain, where the highest Rf exceeded 90%. The shape memory recovery ratio (Rr) of the PBAT films was increased with the shape memory recovery temperature and decreased with the prestrain value, and the highest Rr was almost 100%. Moreover, the PBAT films had high shape memory recovery stress which increased with the prestrain value and decreased with the prestrain temperature, and the highest shape memory recovery stress can reach 7.73 MPa.Research limitations/implicationsThe results showed that PBAT had a broad shape memory transform temperature, exhibited excellent thermomechanical stability and shape memory performance, especially for the sample programmed at high temperature and had a larger prestrian, which will provide a reference for the design, processing and application of PBAT-based heat shrinkable film and smart materials.Originality/valueThis study confirmed and systematically investigated the shape memory effect of PBAT and the effect of the thermomechanical condition on the shape memory property of PBAT.

Evaluation of the thermomechanical Shape memory polymers in equi-biaxial condition by hydraulic bulge test

Shape Memory Polymers (SMPs) are materials capable of changing their primary shape to a secondary shape thanks to the so called Shape Memory Effect (SME) phenomenon. The shape-shifting is achieved through the action of an external stimulus, such as heat, electricity, pH, etc. In this paper, experiments on a thermally actuated thin film of a Shape Memory Thermoplastic Polyurethane (SMPU) were performed to calibrate the parameters of a constitutive model which accounts the rubbery/glassy phase transition mechanism behind the shape memory behaviour. In particular, thermomechanical uniaxial tensile tests have been carried out in order to the Young modulus and Poisson’s ratio above/under glass transition temperature and the fixity/recovery ratio. Additionally, the hydraulic bulge test (HBT) in a thermally controlled loading/unloading cycle was used to study the behaviour of the SME at large strains under equi-biaxial stress state. The corresponding outcomes were, therefore, employed to validate the results of the initial calibration by means of a Finite Element (FE) simulation of the HBT.

Shape memory polymers for additive manufacturing: An overview

Additive manufacturing (AM), or 3D printing, has arisen as a booming technology for the development of more creative devices and potential functional applications in biomedical, electronics, sensors, and tissue engineering through the pioneering combination of shape memory polymers (SMPs). However, restricted printable materials and poor qualities of existing 3D printing materials in terms of mechanical strength, curing speed, and thermomechanical shape memory have impeded the adoption of SMPs in 3D printing. Several SMPs are available in the market in response to these drawbacks. This paper is contributing a review in advancement of polyurethanes as SMP polymers in 3D printing.

Thermo-responsive and electro-active shape memory poly(styrene-b-isoprene-b-styrene)/poly(ethylene-co-1-octene)/graphene composites: Effect of size of graphene nanoplatelets

Thermally and electrically triggered poly(styrene-b-isoprene-b-styrene) (SIS) and poly(ethylene-co-1-octene) (PEO) thermoplastic elastomer blend composites having a SIS/PEO weight ratio of 40/60 were prepared by melt blending method. Two different graphene nanoplatelets (GnPs) with different sizes (small size (SG) and big size (BG)) were used as conductive fillers at different amounts. The thermo-responsive and electro-active shape memory performances of the composites as well as their morphological, mechanical and thermal properties were investigated by paying attention to the effect of size and loading degree of the graphene nano-fillers. Both the BG and the SG improved elastic modulus with increase in amount of the fillers. The modulus values increased by 553 % and 507 % for the 50BG and 50SG composites having 50 phr GnPs respectively, as compared to the blend. Heat-triggered thermo-mechanical shape memory tests showed that the BG filler is more advantageous for shape fixing whereas the SG filler is favorable to shape recovery ability. The electrical percolation threshold was achieved at relatively lower content for the BG filler. The 50BG composite exhibited electro-active shape recovery under 25 V, while the SG composite recovered its shape under 65 V. The thermal stability of the blend significantly improved by the use of the both fillers. The degradation onset temperatures of the 50SG and the 50BG composites increased by 25 °C and 10 °C, respectively, compared to the blend.

Thermo-mechanical modeling of viscoelastic crystallizable shape memory polymers

Shape memory polymers (SMPs) are a class of soft active materials that have the ability to retain one or multiple temporary shapes and exhibit deformation as a response to stimuli. The temporary shapes involved can be very complex but SMPs can recover large amounts of strains as compared to other smart materials. They have a wide range of applications in fields as diverse as healthcare, transportation, energy generation, etc. and their mechanism can be easily altered. Hence, there is an increasing need to model their behavior. This paper discusses the behavior of a thermally activated subclass of SMPs in which the temporary shape is fixed by a crystalline phase and the return to original shape is due to the transition of this crystalline phase. We simulate its thermo-mechanical behavior using a framework based on the theory of multiple natural configurations. Viscoelastic effects have been incorporated using a rate type model along with the anisotropy of the crystalline phase. The model has been applied to boundary value problems of homogeneous deformations such as uniaxial extension of dual shape and triple shape memory polymers under controlled stress and strain conditions. The model has also been applied to boundary value problems related to inhomogeneous deformations such as circular shear, inflation, and extension of a hollow cylinder. The effects of parameters such as temperature, external stress, crystallinity, shear modulus and relaxation time on the nonlinear deformation have been studied. The results are found to be consistent with the experimental data and show how the complicated thermo-mechanical behavior and shape memory effect are influenced by these parameters.

Efficient sensitizer-supported electron beam irradiation induced crosslinking of polyvinylidene fluoride for shape memory applications

Polyvinylidene fluoride (PVDF) preferentially crosslinks upon exposure to ionizing electron beam irradiation, leading to substantial changes in its physicomechanical and thermophysical properties. However, low dose electron beam irradiation produces inefficient crosslinking in pure PVDF while high dose irradiation tends to induce the concomitant process of transverse bond degradation. This has been addressed by incorporating a sensitizer into PVDF to increase crosslinking efficiency under low dose irradiation. However, conventional sensitizers typically suffer from self-polymerization and the release of toxic volatile gases at high temperature, which diminishes the crosslinking efficiency and is toxic. This issue is addressed here via a series of well-designed bismaleimide (BMI) sensitizers and incorporating these into PVDF films for facilitating chemical crosslinking reactions under low electron beam irradiation doses. Analysis indicates that the sensitizing action of the proposed BMI sensitizers may be due to the formation of transverse bonds between vinylene groups and PVDF chains. Versus conventional triallyl isocyanurate and trimethylolpropane triacrylate sensitizers, the proposed BMI sensitizers facilitate high radiation crosslinking efficiency and exhibit good compatibility. Accordingly, the proposed sensitizers leading to irradiated PVDF films with outstanding thermomechanical performance and shape memory functionality.

A comparative study of force fields for predicting shape memory properties of liquid crystalline elastomers using molecular dynamic simulations

Molecular dynamic (MD) simulation techniques are increasingly being adopted as efficient computational tools to design novel and exotic classes of materials for which traditional methods of synthesis and prototyping are either too costly, unsafe, and time-consuming in laboratory settings. Of such class of materials are liquid crystalline elastomers (LCEs) with favorable shape memory characteristics. These materials exhibit some distinct properties, including stimuli responsiveness to heat or UV and appropriate molecular structure for shape memory behaviors. In this work, the MD simulations were employed to compare and assess the leading force fields currently available for modeling the behavior of a typical LCE system. Three force fields, including Dreiding, PCFF, and SciPCFF, were separately assigned to model the LCE system, and their suitability was validated through experimental results. Among these selected force fields, the SciPCFF produced the best agreement with the experimentally measured thermal and viscoelastic properties compared to those of simulated steady-state density, transition temperature, and viscoelastic characteristics. Next, shape fixity (Rf) and shape recovery (Rr) of LCEs were estimated using this force field. A four-step simulated shape memory procedure proceeded under a tensile mode. The changes in molecular conformations were calculated for Rf and Rr after the unloading step and the reheating step. The results revealed that the model LCE system exhibits characteristic behaviors of Rf and Rr over the thermomechanical shape memory process, confirming the suitability of selected force field for use in the design and prediction of properties of typical LCE class of polymers.

Biphasic organohydrogels based on phase change materials with excellent thermostability for thermal management applications

Abstract Innovative thermal management materials are in high demand for construction of energy-efficient systems. Evaporative cooling hydrogel has been extensively investigated in passive building cooling owing to its cost-effective and environmental-friendly characteristics. However, poor mechanical properties and shape instability severely restrict its scope and lifespan. In this study, we report an ingenious and effective thermal management material, the PCMs organohydrogel, with advanced thermal performance and outstanding shape and thermal stability, which integrates the advantages of evaporative cooling performance of water and latent heat storage capability of eicosane. The biphasic PCMs organohydrogel was synthesized by the CAA-HRP-H2O2 ternary enzyme-mediated polymerization of the eicosane-in-water emulsion stabilized by the cellulose acetoacetate (CAA). CAA played important roles in the system not only as the emulsifier but also as the polymerization mediator. The encapsulation of PCMs by the organohydrogel provides the composites outstanding thermal stability as the decomposition temperature of the eicosane was elevated by 101 °C. Furthermore, the PCMs organohydrogel exhibits excellent thermomechanical performance and shape memory effect because of the wrapped phase-transition micro-inclusions and the interfacial tension of heterophases. The application of the PCMs organohydrogel as passive cooling material on the model house roof was demonstrated. The cooling effectiveness of the PCMs organohydrogel was investigated, indicating that the biphasic PCMs organohydrogel is expected to be an efficient and reliable passive thermal management material in building thermoregulations.

Simulation of Shape Memory Alloy (SMA)-Bias Spring Actuation for Self-Shaping Architecture: Investigation of Parametric Sensitivity.

Parametric complexity of the thermomechanical shape memory alloy (SMA) model is one of the major barriers to advanced application of the SMA actuation in adaptive architecture. This article seeks to provide architectural practitioners with decision-making information about SMA actuator design parameters. Simulation-based global sensitivity analysis of an SMA-bias spring actuation model reveals that the SMA spring index (a spring’s outer diameter divided by its wire diameter) and stiffness of the bias spring are significant factors in both displacement and force exertion. Among all parameters, maximum output stroke and force largely depend on the temperature range at which the SMA spring operates. These findings also indicate a trade-off between the spring diameter and wire thickness, demonstrating that the output stroke and force tend to counter one another. Appropriate preloading and choice of an optimal spring index should be considered for desirable SMA motion.

3D-printed attachable kinetic shading device with alternate actuation: Use of shape-memory alloy (SMA) for climate-adaptive responsive architecture

To find a low-cost and flexibly adaptive building design and construction method in the field of sustainable architecture, the authors attempted to propose a user-fabricable 3D-printed kinetic shading device that is selectively actuatable by a switch between a geared DC motor and a thermomechanical shape memory alloy (SMA) actuator. This approach leverages additive manufacturing, SMA, and origami to suggest a lightweight, motorless, and silently operable kinetic building module with compact actuation parts. User-customization is prioritized in its manufacturing, installation, and operation: the device is made by 3D-printed thermoplastic components and is self-supportively installable. User-engaged operation is considered by involving an app-based remote control, along with sensor-integrated automation. The results of responsive building performance simulation and mockup tests demonstrate that the thermo-responsive building module enables control of solar radiation and light, reducing room temperature dynamically. The study findings speak to the limitations and potential of material-based actuation for adaptive building technologies.

Shape memory hydroxypropyl cellulose-g-poly (ε-caprolactone) networks with controlled drug release capabilities

Hydroxypropyl cellulose-g-poly (e-caprolactone) (HPC-g-PCL) derived networks from different lengths of PCL side chains have been prepared by crosslinking HPC-g-PCL, providing (X(HPC-g-PCL)) films with different gel contents, crystallinity, and morphology. Two networks with different e-cCaprolactone (CL) content (95 and 98%) named X(HPC-g-PCL)-95 and X(HPC-g-PCL)-98 were used for further study of thermo-mechanical properties and shape memory behavior. Young’s modulus (E), elongation at break (eb %) and tensile strength (σm) were examined at three different temperatures 22, 37 and 65 °C. The results revealed that E values significantly were controlled by crystallinity as well as crosslink density depending on the temperatures. Complete shape recovery was observed for both samples once the degree of crosslinking was increased over 95%, while a narrower recovery temperature ranges from 39 to 41 °C was found for the X(HPC-g-PCL)-98 sample. The shape fixity results showed a great dependence on the molecular weight of PCL. Therefore, the X (HPC-g-PCL)-98 sample demonstrated an excellent shape fixation. Naproxen-loaded shape memory films prepared using either the in situ (before crosslinking) or swelling (after crosslinking) methods (3 and 10 wt%), were also described as a model for controlled drug release device. The chemical composition of drug-loaded networks, drug concentration and the incorporation method greatly affected the crosslink density, morphology and mechanical properties. Importantly, loading via the in situ method resulted possibility to adjust the amount of incorporated drug and fast subsequent release, while drug-loaded films by the swelling method exhibited an initial burst release. Both networks exhibited slow sustained release of naproxen over two months.

Development of High-Strength, Fine, and Ultrafine-Grained Shape Memory Alloys

A brief review of the systematic studies of the structural and phase transformations and properties of alloys based on titanium nickelide and alloyed with Fe, Co, Cu, Zr, Hf, Pd, Pt, and Au is presented. Generalized phase diagrams of martensitic transitions are built. It is shown that alloying with a third component in a quasi-binary composition increases or decreases the critical temperatures of the thermoelastic martensitic transformations and shape memory effects (SMEs) to a different degree with respect to the base binary titanium nickelide. The structure of thermoelastic martensite is established to have a packet–pyramidal morphology of thin coherent nano- and submicrometer crystals. Martensite crystals are twinned over several systems of a “soft” twinning shear in an isotropic elastic–soft austenite. It is this mechanism that provides a unique coherent accommodation for the arising elastic stresses. The fine-grained (FG) and ultrafine-grained (UFG) structure hardens the alloys and increases their plasticity, while retaining the thermomechanical shape memory in them.

Investigation of ultraviolet radiation effects on thermomechanical properties and shape memory behaviour of styrene-based shape memory polymers and its composite

In recent years, shape memory polymers (SMP) have been researched extensively for space applications, such as deployable solar panels and antenna reflectors. Space applications cause SMP components to be severely exposed to ultraviolet (UV) light which may results in material degradation which may causes catastrophic failures and costs substantial amount of public money. This paper investigates the effect of UV light exposure on thermomechanical properties and shape memory effect (SME) of the Styrene-based SMP and its Glass fibre shape memory polymer composites (SMPC). Dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA), have been used to investigate the thermomechanical properties, SMEs and thermal stability before and after the UV exposure. Further, Fourier transform infrared spectroscopy (FTIR) was performed to analyse the after effect of UV exposure on the polymer's chemical structure. Results have revealed that UV exposure had different impacts on the SMP samples. UV exposure have degraded the mechanical properties, lowered the glass transition temperature (Tg), considerably reduced shape recovery rate, and programming and recovery stresses in all samples. However, the exposure had no considerable effect on the fixity ratio and relaxation modulus of the neat SMP sample, and it slightly increased the fixity ratio of the SMPC samples.

Effect of the addition of polyester-grafted-cellulose nanocrystals on the shape memory properties of biodegradable PLA/PCL nanocomposites

In this work the thermally-activated shape memory response of biodegradable nanocomposites based on PLA/PCL blend reinforced with different type of cellulose nanocrystals has been reported, and compared with those of the neat matrix, at the same transition temperature of 55 °C and at the same different deformations, 50%, 100% and 150%. In particular, cellulose nanocrystals have been synthesized and then functionalized by “grafting from” reaction by ring opening polymerization of both PLLA and PCL using the OH groups onto the cellulose nanocrystals surface as initiators for the reaction. The morphology, thermal and mechanical analysis have been performed in order to obtain the parameters for the thermo-mechanical shape memory cycles. Moreover, the addition of the CNC-based nanofillers on the compatibility of PLA-PCL blends in 70:30 proportion has been evaluated. All the biodegradable nanocomposite formulations showed excellent shape memory response, similar to those of the neat matrix, with strain recovery ratio and strain fixity ratio higher than 80% and 90%, respectively. This fact indicates that in this case, the shape memory response of the nanocomposites is mainly controlled by the response of the neat blend and they are slightly influenced by the increase of compatibility between the components of the blend. In addition, all nanocomposite films were fully disintegrated under composting conditions confirming their biodegradable nature, obtaining that the presence of CNC-based nanofillers speeds up the disintegration rate of the nanocomposites in comparison with the pure matrix.