3D printing of shape memory poly(d,l-lactide-co-trimethylene carbonate) by direct ink writing for shape-changing structures

Four-dimensional (4D) printing of shape memory materials has attracted increasing interests for personalized structures. In this study, a biocompatible poly(d ,l -lactide-co-trimethylene carbonate) (PLMC) is utilized to fabricate 4D shape-changing structures with customized geometries through direct ink writing. The printed objects show shape transformations at different dimensions under thermal programming. The influence of the printing parameters on the properties including rheological, solvent evaporation, and static mechanical behavior are systematically investigated. A printing map is further depicted to achieve high-quality printing with high viscous ink flowed from micronozzle to construct various structures. The printed structures in one-dimensional, two-dimensional, and three-dimensional (3D) exhibit shape-changing behavior with fast response around body temperature. The fast responsive time shows potential in the field of surgical suture (4 s), nonwoven fabric (3 s), and self-expandable stent (35 s). The feasibility of 3D printing of PLMC opens the way for applications in shape-changing devices with small diameter. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48177.     

Review on the temperature memory effect in shape memory alloys

Shape memory alloys (SMAs) are well known for their unique shape memory effect (SME) and superelasticity (SE) behavior. The SME and SE have been extensively investigated in past decades due to their potential use in many applications, especially for smart materials. The unique effects of the SME and SE originate from martensitic transformation and its reverse transformation. Apart from the SME and SE, SMAs also exhibit a unique property of memorizing the point of interruption of martensite to parent phase transformation. If a reverse transformation of a SMA is arrested at a temperature between reverse transformation start temperature (A _s) and reverse transformation finish temperature (A _f), a kinetic stop will appear in the next complete transformation cycle. The kinetic stop temperature is a ‘memory’ of the previous arrested temperature. This unique phenomenon in SMAs is called temperature memory effect (TME). The TME can be wiped out by heating the SMAs to a temperature higher than A _f. The TME is a specific characteristic of the SMAs, which can be observed in TiNi-based and Cu-based alloys. TME can also occur in the R-phase transformation. However, the TME in the R-phase transformation is much weaker than that in the martensite to parent transformation. The decrease of elastic energy after incomplete cycle on heating procedure and the motion of domain walls have significant contributions to the TME. In this paper, the TME in the TiNi-based and Cu-based alloys including wires, slabs and films is characterized by electronic-resistance, elongation and DSC methods. The mechanism of the TME is discussed.     

Mechanical Assembly of Thermo‐Responsive Polymer‐Based Untethered Shape‐Morphing Structures

Shape‐morphing robotic structures can provide innovative approaches for various applications ranging from soft robotics to flexible electronics. However, the programmed deformation of direct‐3D printed polymer‐based structures cannot be separated from their subsequent conventional shape‐programming process. This work aims to simplify the fabrication process and demonstrates a rapid and adaptable approach for building stimulus‐responsive polymer‐based shape‐morphing structures of any shape. This is accomplished through mechanically assembling a set of identical self‐bending units in different patterns to form morphing structures using auxiliary hard connectors. A self‐bending unit fabricated by a 3D printing method can be actuated upon heating without the need for tethered power sources and is able to transform from a flat shape to a bending shape. This enables the assembled morphing‐structure to achieve the programmed integral shape without the need for a shape‐programming process. Differently assembled morphing structures used as independent robotic mechanisms are sequentially demonstrated with applications in biomimetic morphing structures, grasping mechanisms, and responsive electrical devices. This proposed approach based on a mechanical assembling method paves the way for rapid and simple prototyping of stimulus‐responsive polymer‐based shape‐morphing structures with arbitrary architectures for a variety of applications in deployable structures, bionic mechanisms, robotics, and flexible electronics.     

Controllable shape‐memory recovery regions in polymers through mechanical programming

A temperature memory effect means a shape‐memory material can remember its deformation temperature. In general, a higher deformation temperature requires a higher activation temperature for shape recovery. In this work, we demonstrate that the unloading temperature can also significantly influence the shape‐memory performance. A series of shape‐memory tests are performed on Nafion while varying the loading and unloading temperatures. The results show that the loading temperature determines the final shape‐recovery region, while the unloading temperature influences the onset recovery region. We also develop a finite‐deformation viscoelastic model to investigate the above findings. The simulation results show good agreement with the experimental data, though the model predicts the recovery region occurring at a lower temperature. The model is further used to study the effects of the loading rate, unloading rate, and the holding time on the shape‐memory behaviors. The results suggest that a smaller loading rate and unloading rate and a longer holding time can shift the recovery region to a higher temperature. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45909.     

Shape memory polymers as actuators: Characterization of the relevant parameters under constrained recovery

In this work, a shape memory polyurethane is characterized through constrained recovery experiments performed in a tensile testing equipment. The most relevant results obtained are those concerned with the stress released over time during the recovery stage, since they provide quantitative information that can be used in the design of actuators. For this sake, design guidelines are proposed based on the effect of: (i) the programming temperature; (ii) the deformation imposed during the programming stage; (iii) the recovery temperature; and (iv) the manufacturing process used to produce the samples tested (compression molding and Fused Filament Fabrication). The set of experiments performed with compression-molded samples put in evidence a considerable variety of material responses: (i) the maximum released stress varied from 0.74 to 1.68 MPa; (ii) the time required to attain this stress varied from 47 to 600 s; and (iii) the stress was released as a peak value that relaxed rapidly, or, contrarily, had a lasting effect. Another relevant conclusion is that the 3D printing technique does not affect the shape memory behavior of the material. Having this in mind, the conclusions provided by the compression-molded samples study can be extended to printed ones.     

Polyethylenimine‐Based Shape Memory Polyurethane with Low Transition Temperature and Excellent Memory Performance

Flexible shape memory polyurethanes (SMPUs) are the favorable candidates as a coating or substrate for wearable smart textiles, electronics, and biomedical applications. However, conventional SMPUs (e.g., 1,4 butanediol (BDO)‐based) are not suitable in these applications due to high rigidity, poor mechanical properties, low shape recovery, and high transition temperature. Herein, a polyethylenimine (PEI)‐based SMPU with low transition temperature and tailored properties are reported. The synthesized SMPU are characterized, and their properties are compared with BDO‐SMPUs. The chemical structure of PEI is explored to improve thermal and mechanical properties and to assess their effect on shape memory behavior. The bulky nature of PEI plays a critical role in lowering transition temperature and introduces flexibility in the structure at room temperature. A drop in Young's modulus is found from 13.6 MPa in BDO‐SMPU to 6.2 MPa in PEI‐SMPU. Simultaneously, tensile strength is increased from 3.77 MPa in BDO‐SMPU to 11.85 MPa in PEI‐SMPU. Owing to the improved mechanical properties in PEI‐SMPU, 100% shape recovery is observed, which displays a reproducible trend in ten repetitive cycles due to the presence of reversible physical crosslinks. Therefore, it is envisioned that this can serve as a potential shape memory material in smart wearable technologies.     

Shape memory effect and properties memory effect of polyurethane

The relationship between shape and properties memory effect, especially viscoelastic properties of polyurethane under study is the main aim of this research work. Tensile tests have been performed in order to introduce 100% of deformation in the polyurethane samples. Under this deformation, stress–relaxation experiments have been performed in order to eliminate the residual stresses. This deformation of the samples has been fixed by cooling. Recovery tests, then, were carried out at different isothermal temperatures that varied from 30°C to 60°C. Viscoelastic behavior has been studied by a biparabolic model and by using the Cole–Cole method. It was shown that this model describes the behavior of the polymer at the different states of shape memory tests. The constants of this model then have been determined. This study leads to a better understanding of the mechanism of shape memory effect. The comparison between the virgin polymer and the polymer after a recovery test by DMTA (dynamic mechanical thermal analysis) and by Cole–Cole method has illustrated that the polymer does not obtain its initial properties even when it was totally regained its initial shape. These results have been confirmed by three successive shape memory tests on the same sample and by comparing the mechanical characteristics of different cycles because “shape memory effect” and “properties memory effect” do not follow the same mechanisms. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

A simulation method to analyze chemo‐mechanical behavior of swelling‐induced shape‐memory polymer in response to solvent

A network of thermally responsive shape‐memory polymers (SMPs) could imbibe a quantity of solvent molecules to swell, and subsequently induces a chemical potential change in polymer. When an equilibrium is reached between the mechanical load and the chemical potential of polymer network and solvent, the SMP polymer usually swells with a field of inhomogeneous and anisotropic deformation, which is considered to be equivalent to a hyperelastic field. We implement this theory in the free‐energy function equation, and analyze examples of swelling‐induced deformation and shape recovery behavior. This work may provide a powerful tool to study complex swelling‐induced shape‐memory behavior of SMPs in response to the immersing solvents. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Shape memory segmented polyurethanes: dependence of behavior on nanocellulose addition and testing conditions

The response of synthesized shape memory segmented polyurethanes (PUs) was affected by the addition of cellulose nanocrystals, as well as by the various conditions selected to carry out thermomechanical cyclic tests. The PUs were synthesized from an α‐hydro‐ω‐hydroxy‐poly(ethylene oxide), tolylene‐2,4‐diisocyanate and 1,4‐butanediol as chain extender. Nanocomposites were prepared by mixing a suspension of cellulose nanocrystals in N,N‐dimethylformamide with the thermoplastic PU dissolved in the same organic solvent. The thermal properties of the neat PU and resulting composites were examined using differential scanning calorimetry. It was found that cellulose addition increases the PU soft segment melting and crystallization temperatures and the degree of crystallinity of this phase. Shape memory behavior was studied using cyclic thermal tensile tests. Both neat PU and composites exhibit shape memory properties, with fixity and recovery values that depend on heating temperature, imposed deformation, deformation rate and nanofiller addition. Copyright © 2011 Society of Chemical Industry     

Advances in Shape Memory Polyurethanes and Composites: A Review

Polyurethanes are synthetic smart materials having exquisite property to regain original shape from temporary shape when an external force (heat, light, electricity, and entropy driven deformation) is applied. Shape memory polyurethanes have ability to replace shape memory alloys due to cut-rate, easy manufacturing, programing, and high shape recovery ratio. The review focused on polyurethane types exhibiting shape memory effect and various categories of shape memory effects in polyurethane. Moreover, compound structure, modeling structure, applications, and related synthetic methods for shape memory polyurethanes are discussed. The strategies for induction of cross-linking and post cross-linking at high, medium, and low temperature are surveyed.     

Nanoindentation of shape memory polymer networks

This work examines the small-scale deformation and thermally induced recovery behavior of shape memory polymer networks as a function of crosslinking structure. Copolymer shape memory materials based on diethylene glycol dimethacrylate and polyethylene glycol dimethacrylate with a molecular weight of 550 crosslinkers and a tert-butyl acrylate linear chain monomer were synthesized with varying weight percentages of crosslinker from 0 to 100%. Dynamic mechanical analysis is used to acquire the bulk thermomechanical properties of the polymers, including the glass transition temperature and the elastic modulus over a wide temperature range. Instrumented nanoindentation is used to examine ambient temperature deformation of the polymer networks below their glass transition temperature. The glassy modulus of the networks measured using nanoindentation is relatively constant as a function of crosslinking density, and consistent with values extracted from monotonic tensile tests. The ambient temperature hardness of the networks increases with increasing crosslinking density, while the dissipated energy during indentation decreases with increasing crosslinking density. The changes in hardness correlated with the changes in glass transition but not changes in the rubbery modulus, both of which can scale with a change in crosslink density. Temperature induced shape recovery of the indentations is studied using atomic force microscopy. For impressions placed at ambient temperature, the indent shape recovery profile shifts to higher temperatures as crosslink density and glass transition temperature increase.     

Fast Chemo‐Responsive Shape Memory of Stretchable Polymer Nanocomposite Aerogels Fabricated by One‐Step Method

A new type of stretchable poly(caprolactone)/graphene oxide (PCL/GO) aerogel with fast chemo‐responsive shape memory effect is fabricated by one‐step method of a sol‐gel procedure. The PCL/GO aerogels show uniformly circular and interconnected pores formed by twisted PCL nanolayers. GO platelets improve the crystallinity of PCL and increase the fracture stress and strain by 150% and 300% respectively, although the GO loading is only 0.5%. The dramatic increment of break strain is attributed to the uniform and circular pores that can afford large deformation and the interaction of GO and PCL. The aerogels can be programmed by external stress at ambient temperature without heating and recover upon ethyl acetate (EA) in 1 s. The fast chemo‐responsive shape recovery is ascribed to the fast wrinkle of the PCL nanolayers that decrease the diffusion time greatly and the interconnected micrometer pores that are in favor of penetrating for EA molecules.     

Shape‐memory behavior of high‐strength amorphous thermoplastic poly(para‐phenylene)

The purpose of this study was to investigate the shape‐memory behavior of poly(para‐phenylene) (PPP) under varying programming temperatures, relaxation times, and recovery conditions. PPP is an inherently stiff and strong aromatic thermoplastic, not previously investigated for use as a shape‐memory material. Initial characterization of PPP focused on the storage and relaxation moduli for PPP at various frequencies and temperatures, which were used to develop continuous master curves for PPP using time–temperature superposition (TTS). Shape‐memory testing involved programming PPP samples to 50% tensile strain at temperatures ranging from 155°C to 205°C, with varying relaxation holds times before cooling and storage. Shape‐recovery behavior ranged from nearly complete deformation recovery to poor recovery, depending heavily on the thermal and temporal conditions during programming. Straining for extended relaxation times and elevated temperatures significantly decreased the recoverable deformation in PPP during shape‐memory recovery. However, PPP was shown to have nearly identical full recovery profiles when programmed with decreased and equivalent relaxation times, illustrating the application of TTS in programming of the shape‐memory effect in PPP. The decreased shape recovery at extended relaxation times was attributed to time‐dependent visco‐plastic effects in the polymer becoming significant at longer time‐scales associated with the melt/flow regime of the master curve. Under constrained‐recovery, recoverable deformation in PPP was observed to have an exponentially decreasing relationship to the bias stress. This study demonstrated the effective use of PPP as a shape‐memory polymer (SMP) both in mechanical behavior as well as in application. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42903.     

An “Off‐the‐Shelf” Shape Memory Hydrogel Based on the Dynamic Borax‐Diol Ester Bonds

An “off‐the‐shelf” hydrogel with high‐efficiency shape memory property is designed on the basis of the dynamic borax‐diol chemistry. The system is facilely prepared from only several unmodified commercially available components: acrylamide (AAm), bis‐acrylamide (Bis), poly(vinyl alcohol) (PVA), and borax. The chemically crosslinked poly(acrylamide) network works to fix the permanent shapes of the hydrogel, while the dynamic PVA–borax boronate ester bonds serve as the reversible crosslinks to memorize the deformed temporary shapes. Retreatment of the hydrogel in acid/glucose solutions dissipates the PVA–borax ester bonds to recover its permanent shape. Because of the highly invertible nature of borax‐diol chemistry, the developed hydrogel system is characterized by high shape memory/recovery ratios, continuously adjusted shape memory/recovery rates, thus having a wealth of potential applications.     

Shape‐memory effects of radiation crosslinked poly(ϵ‐caprolactone)

Poly(?‐caprolactone) (PCL) with different molecular weight were crosslinked by γ‐radiation. The radiation crosslinking features were analyzed by Soxhlet extraction with toluene and the Charlesby–Pinner equation. The crosslinking degree is relative to molecular weight and radiation dose; the relation between sol fraction and dose follows the Charlesby–Pinner equation. All the samples were crystalline at room temperature, and the radiation crosslinking had a little effect on the crystallinity and the melting behavior of PCL. The shape‐memory results indicated that only those specimens that had a sufficiently high crosslinking degree (gel content is higher than about 10%) were able to show the typical shape‐memory effect, a large recoverable strain, and a high final recovery rate. The response temperature of the recovery effect (about 55°C) was related to the melting point of the samples. The PCL shape‐memory polymer was characterized by its low recovery temperature and large recovery deformation that resulted from the aliphatic polyester chain of PCL. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1589–1595, 2003     

Numerical study of smart honeycomb core using shape memory polymers

Shape memory polymers (SMPs) attract widespread attention because they are able to maintain a temporary deformation after unloading and recover the initial shape under high temperature conditions. Based on a three‐dimensionally constitutive equation of SMPs, a finite element program is followed by compiling user‐defined material subroutine, which describes the shape memory behavior of thermo‐mechanical experiment. A honeycomb core using SMP is designed, which has the ability to recover the initial shape after deformation and be used as a smart core for sandwich structures. To prove their advantages in the engineering application, a series of thermodynamic behaviors of the SMP honeycomb core are simulated, including loading at high temperature, cooling, unloading at the low temperature, and recovering original shape on heating. Shape memory behaviors of tensile, compressive, bending, and locally sunken deformations are demonstrated and the effect of time and temperature on the recovery process is discussed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45672.     

Rolling of 3D Printed Dual‐Layer Beam into a Cylinder by Ethanol Absorption

4D printing is an extension to 3D printing whereby a printed shape programmatically undergoes shape transformation through external stimulations. There is an increasing interest in this field because of its potential applications. However, many demonstrated shape transformations work on simple and limited geometrical shapes. In this work, the formation of a cylinder from a printed flat dual‐layer beam when exposed to ethanol is demonstrated. The newly formed cylinder retains its shape even when the ethanol is removed through the use of a locking mechanism. The proposed method can be used for building medical stents or various sensors and actuators that require cylindrical shapes.     

Characterization of nanocellulose‐ reinforced shape memory polyurethanes

BACKGROUND: Shape memory polymers are capable of fixing a transient shape and of recovering their original dimensions by the application of an external stimulus. Their major drawback is their low stiffness compared to smart materials based on metals and ceramics. To overcome this disadvantage, nanocellulose was utilized as reinforcement. RESULTS: Composites were prepared by casting stable nanocellulose/segmented polyurethane suspensions. The heat of melting of the polyurethane soft segment phase increased on cellulose addition. Composites showed higher tensile modulus and strength than unfilled films (53% modulus increase at 1 wt% nanocellulose), with higher elongation at break. Creep deformation decreased as cellulose concentration increased (36% decrease in 60‐minute creep by addition of 1 wt% nanocellulose). The nanocomposites displayed shape memory properties equivalent to those of the neat polyurethane, with recoveries of the order of 95% (referred to second and further cycles). CONCLUSIONS: It is possible to markedly improve the rigidity of shape memory polymers by adding small amounts of well‐dispersed nanocellulose. However, this improvement did not have substantial effects on the material shape fixity or recovery. Shape memory behavior seems to continue to be controlled by the polymer properties. Copyright © 2007 Society of Chemical Industry     

Studies on thermally stimulated shape memory effect of segmented polyurethanes

The shape memory behavior of a series of polycaprolactone/methane diisocyanate/butanediol (PCL/MDI/BDO) segmented polyurethanes of different composition was studied. The molecular weight of PCL diols was in the range of 1600–7000, and the hard segment content varied from 7.8 to 27% by weight. Film specimens for shape memory measurements were prepared by drawing at temperatures above the melting temperature of the soft segment crystals and subsequent quick cooling to room temperature under constrained conditions. The shape memory process was observed and recorded in a heating process. Parameters describing the recovery temperature, ability, and speed were used to study the influence of structure and processing conditions on the shape memory behavior of the sample. It was found that the high crystallinity of the soft segment regions at room temperature and the formation of stable hard segment domains acting as physical crosslinks in the temperature range above the melting temperature of the soft segment crystals are the two necessary conditions for a segmented copolymer with shape memory behavior. The response temperature of shape memory is dependent on the melting temperature of the soft segment crystals. The final recovery rate and the recovery speed are mainly related to the stability of the hard segment domains under stretching and are dependent on the hard segment content of the copolymers. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1511–1516, 1997     

Near‐Infrared Light Triggered Soft Actuators in Aqueous Media Prepared from Shape‐Memory Polymer Composites

Light triggered soft actuator in aqueous media has applications in operating underwater objects, creating liquid flow, and adjusting reaction velocity, etc. Here, composites prepared from commercial materials, poly[ethylene‐ran‐(vinyl acetate)] (EVA) and aniline black (AB), are reported as one cost efficient material for preparing such actuator, where EVA and AB work respectively as shape‐memory polymer matrix and near‐infrared light triggered photothermal filler. Upon irradiation, the temperature of the composites increases greatly with light power density and AB content. Light‐induced shape‐memory effect (SME) with recovery ratio >98%, temperature‐memory effect (TME), and reversible bidirectional shape‐memory effect (rbSME) of the prepared composites in air are realized. Higher light power density is required to trigger the shape recovery in aqueous media, while good SME, TME, and rbSME are also achieved. Releasing device and gripper are used to indicate the feasibility of the composites as light triggered soft underwater actuators.