Thermomechanical Shape Memory Research Articles

Electroactive shape memory properties of graphene/epoxy-cyanate ester nanocomposites

PurposeThe purpose of this paper is to prepare novel electroactive shape memory nanocomposites based on graphene and study the thermomechanical property and shape memory behavior of nanocomposites.Design/methodology/approachGraphene was dispersed in N,N-dimethylformamide, and the mixture was spooned into epoxy-cyanate ester mixtures to form graphene/epoxy-cyanate ester nanocomposites. The nanocomposites were deformed under 150°C, and shape recovery test was conducted under an electric voltage of 20-100 V.FindingsGraphene is used to improve the shape recovery behavior and performance of shape-memory polymers (SMPs) for enhanced electrical actuation effectiveness. With increment of graphene content, the shape recovery speed of nanocomposites increases significantly.Research limitations/implicationsA simple way for fabricating electro-activated SMP nanocomposites has been developed by using graphene.Originality/valueThe outcome of this study will help to fabricate the SMP nanocomposites with high electrical actuation effectiveness and improve the shape recovery speed of the nanocomposites.

Strategic design and fabrication of acrylic shape memory polymers

Modulation of thermomechanics nature is a critical issue for an optimized use of shape memory polymers (SMPs). In this study, a strategic approach was proposed to control the transition temperature of SMPs. Free radical vinyl polymerization was employed for tailoring and preparing acrylic SMPs. Transition temperatures of the shape memory tri-copolymers were tuned by changing the composition of monomers. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses were carried out to evaluate the chemical structures and compositions of the synthesized SMPs. The thermomechanical properties and shape memory performance of the SMPs were also examined by performing dynamic mechanical thermal analysis. Numerical simulation based on a finite element method provided consistent results with experimental cyclic shape memory tests of the specimens. Transient shape recovery tests were conducted and optical transparence of the samples was identified. We envision that the materials proposed in this study can help develop a new type of shape-memory devices in biomedical and aerospace engineering applications.

Highly Stretchable, Shape Memory Organohydrogels Using Phase-Transition Microinclusions.

Shape memory effect in polymer materials has attracted considerable attention due to its promising applications in a variety of fields. However, shape memory polymers prepared by conventional strategy suffer from a common problem, in which high strain capacity and excellent shape memory behavior cannot be simultaneously achieved. This study reports a general and synergistic strategy to fabricate high-strain and tough shape memory organohydrogels that feature binary cooperative phase. The phase- transition micro-organogels and elastic hydrogel framework act synergistically to provide excellent thermomechanical performance and shape memory effect. During shape memory process, the organohydrogels exhibit high strain capacity, featuring fully recoverable stretching deformation by up to 2600% and compression by up to 85% beneath a load ≈20 times the organohydrogel's weight. Furthermore, owing to the micro-organogel and hydrogel heterostructures, the interfacial tension derived from heterophases dominates the shape recovery of the organohydrogel material. Simple processing and smart surface patterning of the shape memory behavior and multiple shape memory effects can also be realized. Meanwhile, these organohydrogels are also nonswellable in water and oil, which is important for multimedia applications.

Influence of the soft segment nature on the thermomechanical behavior of shape memory polyurethanes

Shape‐memory polyurethanes (SMPUs) represent a highly interesting class of materials due to their applications in different sectors such as biomedical, textile, and aerospace. Moreover, it is possible to synthesize a variety of polyurethanes with different molecular architectures just choosing properly the chemical structure of their components. In this work, it is described the influence of the soft segment on the thermomechanical properties and shape memory behavior of shape memory polyurethanes. The synthesis, based on two‐step polymerization, was prepared by two different soft segments: poly(oxytetramethylene) glycol (PTMG) or poly(ethylene glycol) (PEG). Depending on the molecular architecture achieved, the materials present different properties that were studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Furthermore, the shape memory response of thermally activated SMPUs is determined qualitative and quantitatively by visually monitoring the shape recovery process and by thermomechanical analysis (TMA), respectively. All developed compositions have shown good shape memory behavior, with recovery ratios higher than 99.8%. POLYM. ENG. SCI., 58:238–244, 2018. © 2017 Society of Plastics Engineers

Parametric analysis and temperature effect of deployable hinged shells using shape memory polymers

Shape memory polymers (SMPs) are a class of intelligent materials, which are defined by their capacity to store a temporary shape and recover an original shape. In this work, the shape memory effect of SMP deployable hinged shell is simulated by using compiled user defined material subroutine (UMAT) subroutine of ABAQUS. Variations of bending moment and strain energy of the hinged shells with different temperatures and structural parameters in the loading process are given. The effects of the parameters and temperature on the nonlinear deformation process are emphasized. The entire thermodynamic cycle of SMP deployable hinged shell includes loading at high temperature, load carrying with cooling, unloading at low temperature and recovering the original shape with heating. The results show that the complicated thermo-mechanical deformation and shape memory effect of SMP deployable hinge are influenced by the structural parameters and temperature. The design ability of SMP smart hinged structures in practical application is prospected.

A coupled layered thermomechanical shape memory alloy beam element with enhanced higher order temperature field approximations

This article describes the development and validation of a new thermomechanically coupled multi-layered shape memory alloy beam finite element. The finite element is formulated, assuming coupled equilibrium equations for the mechanical and thermal problems. The constitutive shape memory alloy model of Lagoudas and coworkers is implemented in the formulation. Multi-field kinematic hypotheses are proposed, combining a first-order shear displacement field with a sixth-order polynomial temperature field through the thickness of the beam, enabling adequate representation of the temperature and phase transformation profiles due to rapid thermal loading, uneven thermal loading, and boundary conditions and multi-layered configurations with variable thermal properties. The non-linear transient discretized equations of motion of the shape memory alloy beam are synthesized and solved using the Newton–Raphson method with an implicit time integration scheme. Numerical results illustrate the time response of uniform and bi-layered NiTi beams under various thermomechanical loads predicted by the developed finite element. Correlations of the beam element predictions with those of plane stress two-dimensional finite element shape memory alloy models demonstrate excellent agreement in the calculated displacement, temperature, and phase transformation fields. Additionally, the developed beam finite element yields computationally fast simulations providing an effective tool for the design and simulation of rod, beam, and strip shape memory alloy actuators and active structures.

Investigation of thermomechanical couplings, strain localization and shape memory properties in a shape memory polymer subjected to loading at various strain rates

This paper presents experimental and modeling results of the effects of thermomechanical couplings occurring in a polyurethane shape memory polymer (SMP) subjected to tension at various strain rates within large strains. The SMP mechanical curves, recorded using a testing machine, and the related temperature changes, measured in a contactless manner using an IR camera, were used to investigate the polymer deformation process at various loading stages. The effects of thermomechanical couplings allowed the determination of the material yield point in the initial loading stage, the investigation of nucleation and development of the strain localization at larger strains and the estimation of the effects of thermoelastic behavior during the unloading process. The obtained stress–strain and thermal characteristics, the results of the dynamic mechanical analysis and estimated values of the shape fixity and shape recovery parameters confirmed that the shape memory polymer (Tg = 45 °C) is characterized by good mechanical and shape memory properties, as well as high sensitivity to the strain rate. The mechanical response of the SMP subjected to tension was simulated using the finite element method and applying the large strain, two-phase model. Strain localization observed in the experiment was well reproduced in simulations and the temperature spots were correlated with the accumulated viscoplastic deformation of the SMP glassy phase.

Use of Materials with Thermomechanical Shape Memory for Sollution of the Applied Problems in Construction Engineering Complex

The article deals with the unique alloy on the basis of titanium nickelide with thermomechanical shape memory. It is spoken in detail about the alloy's memory effect phenomenology. The main reasons constraining the application of alloys with shape memory in the production technologies in different branches of technology, including constructing industry are reflected. The efficient area of possible application of that kind of alloys is shown. In this article is also proposed a method of calculation of power elements, executed from titanium nickelide alloy, and also the examples of alloy's application in practice. It is shown that the offered technique of a preliminary temperature cycling of power elements made of titanium nickelide allows to reach the maximum possible straining and power characteristics.

A unified modeling approach for amorphous shape memory polymers and shape memory polymer based syntactic foam

Shape memory polymers (SMPs) and shape memory polymer composites have drawn considerable attention in recent years for their shape memory effects. A unified modeling approach is proposed to describe thermomechanical behaviors and shape memory effects of thermally activated amorphous SMPs and SMP-based syntactic foam by using the generalized finite deformation multiple relaxation viscoelastic theory coupled with time–temperature superposition property. In this paper, the thermoviscoelastic parameters are determined from a single dynamic mechanical analysis temperature sweep at a constant frequency. The relaxation time strongly depends on the temperature and the variation follows the time–temperature superposition principle. The horizontal shift factor can be obtained by the Williams–Landel–Ferry equation at temperatures above or close to the reference temperature (Tr), and by the Arrhenius equation at temperatures below Tr. As the Arruda–Boyce eight-chain model captures the hyperelastic behavior of the material up to large deformation, it is used here to describe partial material behaviors. The thermal expansion coefficient of the material is regarded as temperature dependent. Comparisons between the model results and the thermomechanical experiments presented in the literature show an acceptable agreement. Copyright © 2016 John Wiley & Sons, Ltd.

Rotor–bearing system integrated with shape memory alloy springs for ensuring adaptable dynamics and damping enhancement—Theory and experiment

Helical pseudoelastic shape memory alloy (SMA) springs are integrated into a dynamic system consisting of a rigid rotor supported by passive magnetic bearings. The aim is to determine the utility of SMAs for vibration attenuation via their mechanical hysteresis, and for adaptation of the dynamic behaviour via their temperature dependent stiffness properties. The SMA performance, in terms of vibration attenuation and adaptability, is compared to a benchmark configuration of the system having steel springs instead of SMA springs.A theoretical multidisciplinary approach is used to quantify the weakly nonlinear coupled dynamics of the rotor–bearing system. The nonlinear forces from the thermo-mechanical shape memory alloy springs and from the passive magnetic bearings are coupled to the rotor and bearing housing dynamics. The equations of motion describing rotor tilt and bearing housing lateral motion are solved in the time domain. The SMA behaviour is also described by the complex modulus to form approximative equations of motion, which are solved in the frequency domain using continuation techniques.Transient responses, ramp-ups and steady-state frequency responses of the system are investigated experimentally and numerically. By using the proper SMA temperature, vibration reductions up to around 50 percent can be achieved using SMAs instead of steel. Regarding system adaptability, both the critical speeds, the mode shapes and the modes’ sensitivity to disturbances (e.g. imbalance) highly depend on the SMA temperature. Examples show that vibration reduction at constant rotational speeds up to around 75 percent can be achieved by changing the SMA temperature, primarily because of stiffness change, whereas hysteresis only limits large vibrations. The model is able to capture and explain the experimental dynamic behaviour.

A molecular dynamics investigation of the deformation mechanism and shape memory effect of epoxy shape memory polymers

Following deformation, thermally induced shape memory polymers (SMPs) have the ability to recover their original shape with a change in temperature. In this work, the thermomechanical properties and shape memory behaviors of three types of epoxy SMPs with varying curing agent contents were investigated using a molecular dynamics (MD) method. The mechanical properties under uniaxial tension at different temperatures were obtained, and the simulation results compared reasonably with experimental data. In addition, in a thermomechanical cycle, ideal shape memory effects for the three types of SMPs were revealed through the shape frozen and shape recovery responses at low and high temperatures, respectively, indicating that the recovery time is strongly influenced by the ratio of E-51 to 4,4’-Methylenedianiline.

Development of drug-releasing shape-memory polyurethane/hydroxyapatite composites - smart biomaterial for bone tissue implants

Event Abstract Back to Event Development of drug-releasing shape-memory polyurethane/hydroxyapatite composites - smart biomaterial for bone tissue implants Monika Bil1, Marcin Heljak1 and Wojciech Swieszkowski1 1 Warsaw University and Technology, Faculty of Materials Science and Engineering, Poland Introduction: Shape-memory polymers (SMPs) are mechanically active or smart materials, which can be deformed and fixed in a temporary shape and are able to return to their original, permanent shape when exposed to a suitable external stimulus. The unique attributes of shape memory effect in conjunction with biodegradability and drug delivery present enormous opportunities for the design of next generation, resorbable less invasive self-fitting medical implants, tissue scaffolds and medical devices[1]-[3]. The ultimate goal of the study is to develop self - deploying drug delivery system for bone tissue regeneration utilizing thermoplastic SMPs which could self-fitting to a target tissue defect and ensure a local drug release at the site of administration. Materials and Methods: Hydroxyapatite/polyurethane (HA/PU) composites were synthesized through in-situ polymerization with biodegradable hydroxyl terminated oligomers of polycaprolactone and poly(lactide-co-glycolide), 1,6-hexamethylene diisocyanate as a coupling agent and dibutyltin dilaurate as the catalyst. HA nanoparticles (Merck) and gentamicin sulfate (GS) ( 5% wt.) ultrasonically dispersed in dehydrated tetrahydrofuran (THF) were added to synthesis mixture. Chemical structure of the composites were analysed by Raman spectroscopy. Drug release study were performed in vitro in PBS and analyzed using UV spectrophotometer at a wavelength of 332 nm. Shape memory behavior was analyzed in thermomechanical test by dynamic mechanical analyzer (DMA) in water. The shape recovery ratio Rr and the fixity ratio Rf were calculated based on DMA measurement. The relationship between amounts of HA, drug content and shape memory properties were analyzed. Results and Discussion: A series of thermoplastic nano-HA/PU/GS composite with the content of HA in the range 3, 5 10 wt. % were synthesized. Quantitative assessment of the shape memory performance through thermomechanical shape memory cycles verified that all materials exhibited a shape-fixing ratio Rf from 80% to 85% and shape recovery ratio (Rr 80-100%). The best shape recovery ability Rf -100% in water at 37oC was observed for composite with 3% wt. of HA. The shape recovery ratio decreased slightly with an increment in HA nanoparticles weight fraction, due to formation of agglomerates that could be a hindrance in recovery of the macromolecule structure. In order to evaluate the effect of shape recovery on drug release profile the drug release study were performed for the samples subjected thermomechanical shape memory cycle. The results revealed sustained drug release over two weeks. However, initial burst release increased with increasing of HA weight fraction what could be connected with easier water penetration into polymer matrix through HA nanoparticles polymer matrix interface. Conclusion: We successfully prepared series of thermoplastic PU/HA composites based on biodegradable oligoesters combining controlled drug release and shape memory effect. All the drug-loaded samples have satisfactory shape memory properties with shape recovery within body relevant temperature, and drug release behavior, therefore appear to be potentially useful for biomedical applications. The authors would like to thank the National Centre for Research and Development (Grant no: LIDER/037/673/L-4/12/NCBR/2013) for providing financial support to this project

Shape memory-based tunable resistivity of polymer composites

A conductive composite in bi-layer structure was fabricated by embedding hybrid nanofillers, namely carbon nanotubes (CNTs) and silver nanoparticles (AgNPs), into a shape memory polyurethane (SMPU). The CNT/AgNP-SMPU composites exhibited a novel tunable conductivity which could be facially tailored in wide range via the compositions or a specifically designed thermo-mechanical shape memory programming. The morphologies of the conductive fillers and the composites were investigated by scanning electron microscope (SEM). The mechanical and thermal measurements were performed by tensile tests and differential scanning calorimetry (DSC). By virtue of a specifically explored shape memory programming, the composites were stretched and fixed into different temporary states. The electrical resistivity (Rs) varied accordingly, which was able to be stabilized along with the shape fixing. Theoretical prediction based upon the tunneling model was performed. The Rs–strain curves of the composites with different compositions were well fitted. Furthermore, the relative resistivity and the Gauge factor along with the elongation were calculated. The influence of the compositions on the strain-dependent Rs was disclosed. The findings provided a new avenue to tailor the conductivity of the polymeric nano-composites by combining the composition method and a thermo-mechanical programming, which may greatly benefit the application of intelligent polymers in flexible electronics and sensors fields.

Nanoscale Order and Crystallization in POSS–PCL Shape Memory Molecular Networks

The angstrom- and nanometer-scale crystallization and long-range ordering behavior of polyhedral oligosilsesquioxane–poly(e-caprolactone) (POSS–PCL) shape memory nanocomposites under thermomechanical shape memory cycles and uniaxial stretching were studied by simultaneous wide-angle and small-angle X-ray scattering (WAXS/SAXS). POSS/PCL cross-linked molecular networks featuring a single POSS moiety centered between two PCL network chains, previously reported [Alvarado-Tenorio Macromolecules 2011, 44, 5682−5692], with molecular weight of 2600 g/mol and exhibiting shape memory behavior, were synthesized with variation of cross-linking molar ratio (POSS–PCL diacrylate/tetrathiol cross-linker, 2:1, 2:1.5, and 2:2). The photocured networks exhibited a morphology consisting of POSS crystals embedded in an amorphous PCL matrix, and the POSS crystals were ordered in a cubic nanostructure. However, under tensile stress afforded by a shape memory cycle, the networks yielded a double-induced orientation (90° and 180...

Controlled Shape Memory Behavior of a Smectic Main-Chain Liquid Crystalline Elastomer

A smectic main-chain liquid crystalline elastomer (LCE), with controlled shape memory behavior, is synthesized by polymerizing a biphenyl-based epoxy monomer with an aliphatic carboxylic acid curing agent. Microstructures of the LCEs, including their liquid crystallinity and cross-linking density, are modified by adjusting the stoichiometric ratio of the reactants to tailor the thermomechanical properties and shape memory behavior of the material. Thermal and liquid crystalline properties of the LCEs, characterized using differential scanning calorimetry and dynamic mechanical analysis, and structural analysis, performed using small-angle and wide-angle X-ray scattering, show that liquid crystallinity, cross-linking density, and network rigidity are strongly affected by the stoichiometry of the curing reaction. With appropriate structural modifications it is possible to tune the thermal, dynamic mechanical, and thermomechanical properties as well as the shape memory and thermal degradation behavior of LCEs.

Experimental and modelling studies of the shape memory properties of amorphous polymer network composites

Shape memory polymer composites (SMPCs) have become an important way to leverage improvements in the development of applications featuring shape memory polymers (SMPs). In this study, an amorphous SMP matrix has been filled with different types of reinforcements. An experimental set of results is presented and then compared to three-dimensional (3D) finite-element simulations. Thermomechanical shape memory cycles were performed in uniaxial tension. The fillers effect was studied in stress-free and constrained-strain recoveries. Experimental observations indicate complete shape recovery and put in evidence the increased sensitivity of constrained length stress recoveries to the heating ramp on the tested composites. The simulations reproduced a simplified periodic reinforced composite and used a model for the matrix material that has been previously tested on regular SMPs. The latter combines viscoelasticity at finite strain and time-temperature superposition. The simulations easily allow representation of the recovery properties of a reinforced SMP.

An analytic investigation on behavior of smart devices consisting of reinforced shape memory polymer beams

In this paper, structural behavior of a shape memory polymer (SMP) actuator is analytically studied. This system consists of a reinforced SMP beam and a spring which will be attached to the beam in a part of thermomechanical shape memory effect (SME) cycle. The 3D phenomenological constitutive model for SMP composites, recently proposed by Baghani et al. (2012b), is used as the basis for the beam model employing the Euler–Bernoulli beam theory. We implement and study this model for a reinforced SMP beam undergoing different external transverse loadings, and also under different recovery conditions during heating, e.g., partial stress-strain recovery through utilizing a spring in the heating process. Consequently, the proposed analytical solution can be used as an efficient tool for studying the effects of changing any of the material or geometrical parameters on smart structures consisting of SMP composite beams and actuators for their design and optimization where a large number of simulations is involved.

Analysis of intelligent hinged shell structures: deployable deformation and shape memory effect

Shape memory polymers (SMPs) are a class of intelligent materials with the ability to recover their initial shape from a temporarily fixable state when subjected to external stimuli. In this work, the thermo-mechanical behavior of a deployable SMP-based hinged structure is modeled by the finite element method using a 3D constitutive model with shape memory effect. The influences of hinge structure parameters on the nonlinear loading process are investigated. The total shape memory of the processes the hinged structure goes through, including loading at high temperature, decreasing temperature with load carrying, unloading at low temperature and recovering the initial shape with increasing temperature, are illustrated. Numerical results show that the present constitutive theory and the finite element method can effectively predict the complicated thermo-mechanical deformation behavior and shape memory effect of SMP-based hinged shell structures.

Analysis of chaotic non-isothermal solutions of thermomechanical shape memory oscillators

Shape memory materials exhibit strong thermomechanical coupling, so that temperature variations occur during mechanical loading and unloading. In previous works the nonlinear dynamics of pseudoelastic oscillators subject to an harmonic force has been studied and the possibility of non-regular chaotic responses has been thoroughly documented. Instead of the standard Lyapunov exponent treatment, the statistical 0–1 test based on the asymptotic properties of a Brownian motion chain was successively applied to reveal the chaotic nature of trajectories in the special case in which temperature variations were neglected. In this work, the 0–1 test is applied to fully non-isothermal trajectories. To improve its reliability the test has been applied to the time-histories of maxima and minima of each trajectory, in each component. The obtained results have been validated and confirmed by the corresponding Fourier spectra. Non-regular solutions with different levels of chaoticity have been analyzed and their qualitative difference is reflected by the different values to which the control parameter K asymptotically converge.

Experimental characterization and thermoviscoelastic modeling of strain and stress recoveries of an amorphous polymer network

An acrylate polymer network was submitted to thermomechanical shape memory cycles. The set of experiments characterized the material stress-free strain recovery and the strain-constrained stress recovery in uniaxial tension. Experimental parameters like temperature of strain fixation, amount of strain and heating rate, were varied in order to provide a relatively complete set of experimental data. A model combining the amorphous polymer viscoelasticity and its time–temperature superposition property was used to predict the shape memory behavior of the acrylate polymer network. All the model parameters were characterized using classical tests for mechanical characterization of polymers, which do not include shape memory tests. Model predictions obtained by finite element simulations compared very well to the experimental data and therefore the model relevance for computer assisted application design was assessed.