A comprehensive review on thermomechanical constitutive models for shape memory polymers

A thermodynamically-consistent 3 D constitutive model for shape memory polymers

A constitutive model of shape memory polymers based on glass transition and the concept of frozen strain release rate

Shape memory polymers (SMPs) are one of the intriguing functional materials and have been widely and intensively studied. In order to apply these new polymers to load bearing engineering structures and devices, developing physics-based thermomechanical constitutive models is mandatory. The aim of this Tutorial is to demonstrate how to establish a thermomechanical constitutive model for SMPs. It begins with classifications of SMPs, followed by a discussion on the underlying physics for different SMPs. After that, three classical SMP thermomechanical modeling frameworks are introduced, which include the visco-elasto-plastic based rheological framework, the storage strain-based phase transition framework, and the representative unit cell based multi-branch framework. Next, three commonly adopted new model establishment methods are presented within these frameworks with detailed examples. Finally, future perspectives on this research direction are discussed. We hope that this Tutorial will help readers understand the roadmap from physics to mathematical modeling of SMPs.

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.

Shape memory polymers (SMPs) have thermomechanical property drastically altering around the glass-transition temperature (Tg), temporary shape fixity and shape recovery property, and so on. To understand the mechanisms of such unique properties and design products including SMPs optimally, a mathematical model which is simple yet can represent these properties reasonably and accurately is necessary. Especially, it is important for the model to be able to consider the effect of temperature and strain history on the properties because the properties are considered to depend on not only the temperature and the strain themselves but also their history. Therefore, in this paper first the effect of the temperature and strain history on the shape fixity and shape recovery properties was examined. Then, a micromechanical model of SMP was developed to describe this effect. This model consisted of a lot of unit elements having four springs, two dash pots, one latch, and one thermal expansion element. Different Tg was assigned to each element because Tg depends on local condition inside of SMP and distributed in general. It was found from the experiment that the temperature and strain history applied to SMP when the shape is gradually frozen has an influence on the shape recovery process. With respect to the model, it was shown that the proposed micromechanical model can capture the thermomechanical behaviors of SMP and, especially, the temperature and strain histories dependence on the shape recovery process quite well.

4D printing is a novel concept and multidiscipline research area which integrates advanced manufacturing, material science and mechanics. As an active material with high stiffness and rapid response speed, shape memory polymer (SMP) is widely used in 4D printing structures. Accordingly, a simple but effective theoretical model becomes highly desirable to better assist the 4D printing design. In this paper, a phenomenological model is developed in which the concept of phase evolution is used to describe the glass transition behavior of SMP. Explicit iterative format has been derived where the stress–strain behaviors of phase generation and phase vanishment are distinguished. The model is further employed to predict the shape memory behavior of pure SMP and active bending behavior of SMP-elastomer bilayer composites. With its inherent conciseness and generality, the model is promising to be applicable in modelling other soft active materials with phase evolution behaviors.

Multifunctional shape memory composites composed of shape memory polymers and shape memory alloys exhibit superior shape memory properties; therefore, it is of great interest to researchers to model their thermo-mechanical behavior numerically. Although a number of constitutive models of shape memory alloys and shape memory polymers have been developed, very few models have been developed for shape memory composites and validated with experimental data. In this study, we first review separately constitutive models of shape memory alloys and shape memory polymers developed in previous studies. Both models were validated with thermo-mechanical tests conducted on a shape memory alloy fiber and shape memory polymer, respectively. A constitutive model for the shape memory composite was then developed utilizing the homogenization scheme. Shape memory composites containing 0.5% or 1% of the shape memory alloy fiber volume content embedded in the shape memory polymer matrix were fabricated. Thermo-mechanical tests were carried out on these shape memory composites to validate the proposed constitutive model. The experimental results showed that the proposed shape memory composite model was able to predict the general trend of the thermo-mechanical behavior of the shape memory composites.

Shape memory polymers (SMPs) are evolving intensively in smart materials research. SMPs can undergo large deformations, hold a temporary shape and then recover their original shape upon exposure to a particular external stimulus. The emerging development of fibre reinforced shape memory polymer composites (SMPCs) has improved SMPs’ mechanical properties to be comparable to modern fibre reinforced composites. Moreover, the inclusion of nanoparticles has enhanced the diversity in activation mechanisms and improved structural properties and durability.
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\nToday, the SMPCs are generally viewed as promising substances for engineered applications that have raised intensive interest among the scientists and engineers. Interestingly, stimulation by light enables unique and advanced shape memory effects (SMEs) in SMPCs that can fulfill the sophisticated demands of engineering. The ability to vary the light wavelength, intensity, periodicity, polarization and the irradiated position on the SMP component enables wavelength selective, reversible, sequential and multiple SMEs. Furthermore, light travel long distances in a vacuum, can be focussed onto a certain area, is safe to humans and, can be a control signal carrier and most notably, can be sent through optical fibres to extremely inaccessible areas.
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\nIncorporation of metallic, carbon and organic based photothermal fillers into a SMP matrix is a convenient proven method to prepare light activated shape memory polymer composites (LASMPCs). These photothermal fillers convert electromagnetic radiation into thermal energy that triggers the SMP matrix. The current study comprehensively reviewed the performances, mechanisms, challenges and limitations of the existing LASMPC systems and associated optical technologies. Predominantly this research was intended to develop reinforced LASMPC systems and allied optical technologies applicable for large scale structural engineering applications.
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\nIn this study, styrene and bisphenol A epoxy based thermosets and polylactic acid (PLA) based thermoplastic SMPs were used as polymer matrix materials. Carbon fibre, glass fibre, multiwalled carbon nanotube (MWCNT) and rare earth organic complexes of Nd(TTA)3Phen and Yb(TTA)3Phen were used as reinforcement and photothermal fillers. The samples were produced by means of vacuum bagging, mould casting and 4D printing. Morphological investigations were conducted to identify the microstructure and manufacturability of the LASMPCs. The samples’ mechanical and physical properties were tested by standard test methods for polymer matrix composites. Dynamic mechanical analysis was carried out to determine the thermomechanical characteristics. The photothermal effect was investigated through thermal imaging and associated shape memory behaviours were studied through shape fixing and recovery experiments. A finite element analysis was performed with ABAQUS to simulate the photothermal heating behaviour.
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\nThe current study established a material development framework for LASMPC materials and presented apt manufacturing methods for structural and large scale engineering applications. Different modes of light diffusion such as scattered low intensity light, focussed laser beams and light delivery through fibre optics were proved for structural LASMPCs. Sequential and selective wavelength activation of structural LASMPCs are presented. The FEA simulation provided additional understanding on light exposed LASMPCs’ 3-dimensional heating behaviour. Embedding a D-shaped optical fibre in an LASMPC created a single unit smart actuator, which was remotely activated by light. It was proven that flexible radiation shields can reduce the unanticipated shape recovery of LASMPCs components and protect the LASMPCs from polymer degradation due to radiation.
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\nThe improvements to their structural properties and innovative remote activation methodologies have justified LASMPCs’ aptness for space engineering applications. This study has proven its successful completion by three selected case studies of space applications. An LASMPC solar panel array model was sequentially deployed in four recovery steps. In a vacuum, a space habitat structure model was compressed to a third of its volume and then recovered its original shape. A 4D printed boom structure model was tested, proving customized LASMPC structures for sophisticated space equipment fabrication. The deployable LASMPC structures can reduce the occupied room in spacecraft. The improvements to the structural properties of LASMPCs while keeping the SME, understanding on thermomechanical and photothermal behaviours, remote actuation through optical fibres, radiation shields and the insight to space applications are the most significant novel outcomes of this study. This thesis opens up windows of opportunity for the scientific and engineering communities to provide innovative solutions to realize the sophisticated large scale engineering demands in aerospace and space applications and beyond.

ABSTRACTThis review is focused on the use of ionomers in shape memory polymers. Ionomers are polymers that contain less than ∼15% ionic groups. The incompatibility between the ion‐pairs and the polymer backbone drives microphase separation producing dispersed ionic aggregates, which can physically crosslink the polymer. Shape memory polymers are responsive materials that can be deformed to program a temporary shape and then recovered on application of an external stimulus. Through the review of the main types of ionomers used in shape memory polymers, polyurethanes and polyester ionomers, polyolefin and polyaromatic ionomers, and perfluorosulfonic acid ionomers (i.e., Nafion®) it will be shown that ionomers can produce robust thermoplastic shape memory polymers and in many cases impart unique properties which allow advanced shape memory materials to be obtained including antibacterial, high temperature, and multishape memory polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1389–1396

Poly(lactic acid)/polycaprolactone-based self-programmed photothermal responsive shape memory polymers

Personalized 4D printing of bioinspired tracheal scaffold concept based on magnetic stimulated shape memory composites

Shape memory polymers (SMPs) possess the remarkable ability to recover their original shape after being severely deformed, triggered by the right stimulus. This chapter reviews recent progress in shape memory materials, focusing on traditional types like shape memory alloys (SMAs) and SMPs, as well as emerging shape memory hybrids, which enable customizable properties for specific applications in a “do-it-yourself” manner. SMPs, in particular, show great promise for morphing aircraft applications due to their temperature- and time-dependent mechanical properties. Tests on SMPs under monotonic loading in high shear demonstrate their ability to recover original shapes while resisting large loads. Advances in SMP composites (SMPCs), which incorporate functional materials and nanostructures, further enhance their deformation recovery, mechanical properties, and controllable actuation. SMPs have broad applications across aerospace, biomedical devices, flexible electronics, and more. This chapter also provides an in-depth analysis of the shape recovery mechanisms, multifunctionality, and potential future directions for SMP development.

Shape memory effect in polymers: Experiment and theory

This paper explores the concept of reconfigurable compliant mechanisms. We define these to be fully or partially compliant mechanisms whose performance can be modified after they have been fabricated. Specifically, we are interested in the nature and extent of in situ reconfigurability in compliant mechanisms. In other words, we seek to understand the range of performance that can be achieved by these mechanisms without requiring significant reassembly. The material properties such as the storage modulus of a newly studied class of materials — shape memory polymers — vary by over an order of magnitude over a temperature range of 20 – 50 C. These polymers also allow the fixing of moderate to large strains (20 – 75%) experienced at high temperatures for extended periods of time, while retaining the ability to remember their original shape when reheated to the same high temperatures. These two properties make shape memory polymers a natural candidate for the fabrication of reconfigurable compliant mechanisms. We explore various means for introducing reconfigurability in compliant mechanisms, and from these, select a subset that is suitable for in situ reconfiguration. Quasi-static nonlinear finite element simulations are used to study the change in performance due to reconfiguration of four fully compliant mechanisms made of a shape memory polymer. Preliminary results indicate that noticeable qualitative and quantitative changes in performance can be achieved by these mechanisms.

Shape memory materials represent a promising class of dual-shape materials that can move from one shape to another in response to a stimulus such as light, heat, electricity or magnetism. In this regard, the biomedical field is showing large interest in this class of materials, especially in shape memory polymers (SMPs), whose mechanical properties make them extremely attractive for many biomedical applications. However, diverse characteristics including also the mechanical behaviour are still part of research. In this contribution the shape memory properties of polymers will be quantified by cyclic thermomechanical investigations. One cycle includes the "programming" of the sample and the recovery of its permanent shape. To describe this phenomenon, a three-dimensional thermomechanical coupled model is proposed. This macromechanical constitutive model is based on the physical understanding of the material behaviour and a mechanical interpretation of the stress-strain-temperature changes observed during thermomechanical loading. The main focus of this work is the influence of both, the material constants and heat transfer boundary conditions on the response of shape memory polymers. Therefore we illustrate different general simulations as well as examples of application.