Shape Memory Cycle Research Articles

Environmental Durability of Fabric-Reinforced Shape-Memory Polymer Composites

This study is a baseline assessment of the environmental durability of current state-of-the-art, fabric-reinforced shape-memory (SM) materials being considered for morphing applications. Tensile dog-bone-shaped specimens are cut along three different directions, namely, along (0°), perpendicular (90°), and oblique (45°) to the planar orientation of the fabric. The elastomeric response and shape memory properties before and after simulated environmental exposure to moisture, lubrication oil, and UV radiation are measured. Weight loss of the as-received and conditioned specimens is monitored and the dog-bone-shaped specimens are subjected to recovery following fixation. Parameters being investigated include modulus in the glassy and rubbery state, stored strain, shape fixity, recovery stress, and unconstrained shape recovery. There is a twofold decrease in the composite stiffness as the material is cycled between room and elevated (above the glass transition) temperature. At room temperature, the 0-degree specimen has the maximum stiffness (5.8 GPa), failure strength (94 MPa), and failure strain (5.4%), while above the Tg, the 90-degree specimen has the least stiffness (∼18 MPa) and largest strain to failure (>200%). Thus, the composite exhibits large deformation in its rubbery state. Parameters which are strongly affected by the damage developed during the first SM cycle include rubbery and glassy (or unloading) moduli values measured during the second SM cycle, while smaller changes are observed in shape fixity and recovery properties.

Study on the Two-Way Shape Memory Characteristics Induced by Compressive Loading Cycles

The NiTi alloy can be trained by repetitive loading or heating cycles. As a result of the training, a two-way shape memory effect (TWSME) can be induced. Considerable research has been reported regarding the TWSME trained by tensile loading, however the TWSME trained by compressive loading has not been investigated nearly as much. In this research, six types of specimens (one solid cylindrical and five tubular) were used to obtain the two-way shape memory strain and two-way recovery stress and to evaluate the actuating capacity. The two-way actuating strain showed a saturated tendency after several training cycles for the same maximum deformation. A maximum value of the two-way strain was obtained for 7% of maximum deformation, independently of the geometry of the tubular specimens. The two-way strains obtained by the shape memory cycles and two-way recovery stress linearly increase as a function of the maximum deformation and the two-way strain, respectively, and the geometry of specimen affects the two-way recovery stress. Although the results show that sufficient recovery stress can be generated by either the two-way shape memory process or by the one-way shape memory process, the two-way shape memory process can be applied more conveniently to actuating applications.

Tunable polymer multi-shape memory effect

Shape memory polymers are materials that can memorize temporary shapes and revert to their permanent shape upon exposure to an external stimulus such as heat, light, moisture or magnetic field. Such properties have enabled a variety of applications including deployable space structures, biomedical devices, adaptive optical devices, smart dry adhesives and fasteners. The ultimate potential for a shape memory polymer, however, is limited by the number of temporary shapes it can memorize in each shape memory cycle and the ability to tune the shape memory transition temperature(s) for the targeted applications. Currently known shape memory polymers are capable of memorizing one or two temporary shapes, corresponding to dual- and triple-shape memory effects (also counting the permanent shape), respectively. At the molecular level, the maximum number of temporary shapes a shape memory polymer can memorize correlates directly to the number of discrete reversible phase transitions (shape memory transitions) in the polymer. Intuitively, one might deduce that multi-shape memory effects are achievable simply by introducing additional reversible phase transitions. The task of synthesizing a polymer with more than two distinctive and strongly bonded reversible phases, however, is extremely challenging. Tuning shape memory effects, on the other hand, is often achieved through tailoring the shape memory transition temperatures, which requires alteration in the material composition. Here I show that the perfluorosulphonic acid ionomer (PFSA), which has only one broad reversible phase transition, exhibits dual-, triple-, and at least quadruple-shape memory effects, all highly tunable without any change to the material composition.

In situ evaluation of structural changes in poly(ester-urethanes) during shape-memory cycles

The relationship between shape-memory behavior and structure was studied using three series of poly(ester-urethanes) with varying amounts of hard segment. The materials were designed to display a three-phase structure consisting of a disperse phase of crystallites and hard domains embedded in an amorphous matrix based on soft segment. Structure and thermal properties of the resultant materials were investigated using techniques such as modulated differential scanning calorimetry (MDSC), dynamic mechanical analysis (DMA) and small angle X-ray scattering (SAXS). The results revealed morphological changes in the materials during a low temperature shape-memory cycle. To study shape recovery, a deformed specimen was evaluated on a heating stage mounted at the SAXS beamline. Furthermore, to study the effect of temperature during recovery, the specimens were subjected to different thermo-cycles. Under each set of conditions, the phase morphology and composition were investigated. Temporary shape was stored by the metastable structure formed during deformation. The recovery was triggered by the melting of crystallites and hydrogen bonding between hard domains. The recovery process was divided into three stages. Bulk incompatibility and entropic recovery determined the final polyurethane morphology.

Structure Evolution in Segmented Poly(ester urethane) in Shape-Memory Process

A segmented poly(ester urethane) shape-memory polymer with poly(e-caprolactone) (PCL) soft segment and urethane hard segment was studied by FTIR to investigate the structural evolution in the shape-memory cycle. It was revealed that in the cold drawing programming process, first the amorphous PCL chains start to orient along the drawing direction, and then, the hard segment and the crystalline PCL chains orient upon further extension, accompanied by the weakening of the hydrogen bonds along the drawing direction between the hard segments and stress-induced disaggregation and recrystallization of the crystalline PCL. In the recovery process, the hard segments restore first with the parallel hydrogen bonds strengthening and then the amorphous PCL chains restore the original random orientation, and finally the crystalline PCL chains lose their alignment to a less-oriented state.

The morphology and phase mixing studies on poly(ester–urethane) during shape memory cycle

Three series of shape memory poly(ester–urethane) with varying hard-segment contents were synthesized. The materials were designed to display a three-phase structure consisting of a disperse phase formed by crystallites and hard domains embedded in an amorphous matrix. The initial undeformed morphology was investigated using techniques such as modulated differential scanning calorimetry, Fourier transform infrared spectroscopy, and wide angle X-ray scattering. These techniques were used to determine the phase separation, hydrogen-bonding structure, and crystalline fraction of the specimens prior to thermo-mechanical treatments. The obtained information was correlated with small angle X-ray scattering investigations of morphological changes that occurred during shape memory cycling. The deformation cycle led to the formation of an oriented nanostructure derived from chain alignment. The nanostructure recovered was observed to be triggered by the melting of the crystallites and bulk incompatibility. A relationship between the ability of the studied poly(ester–urethane) specimens to recover their original shape and their original nanostructure was determined.

Kinetics and dynamics of thermally-induced shape-memory behavior of crosslinked short-chain branched polyethylenes

Systematic investigation of the relation between shape-memory (SM) behavior and characteristics of the covalent network and the crystalline domains of a crosslinked polymer, i.e., crosslink density and crystallinity, respectively, was performed using homogeneous ethylene-1-octenecopolymers (EOC) as model polymers. The EOCs have been crosslinked by 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (DHBP) decomposition. Two EOCs with a degree of branching of 30 and 60 hexyl side chains per 1000C atoms with each four different crosslink densities were employed. The investigated EOCs differ significantly in crystallinity, melting temperature (Tm) and crosslink density. The crosslinked EOC undergone the programming at a strain of 100% showed high strain fixity ratio (Rf) and strain recovery ratio (Rr). The Rf and Rr values increase with increasing crystallinity and crosslink density, respectively, and decline only slightly in a subsequent SM cycle. The switching temperature (Tsw) is strictly related to Tm and decreases with increasing degree of branching as well as crosslink density in the temperature range of 101–63°C. Tsw remains nearly unchanged when the programming temperature (Tpr) or the load during SM recovery is varied. The kinetics of SM recovery as characterized by the temperature dependence of recovery rate is controlled by the melting behavior. The specific work generated by the programmed specimen during thermally-induced recovery under constant load, gains with increasing crosslink density, and is proposed as dynamical characteristic of practical relevance. The opportunity of tailoring Tsw by variation of the degree of branching and crosslink density makes such polymers attractive candidates for applications requiring Tsw temperatures in the range from 60 to 100°C.

Novel synthetic strategy toward shape memory polyurethanes with a well-defined switching temperature

Shape memory polyurethanes (SMPUs) have been synthesised via a novel synthetic methodology, resulting in an improvement of the phase separation in the multi-block structure of the polyurethane and in its shape memory properties. ABA block copolymers based on semi-crystalline poly(ɛ-caprolactone) and amorphous poly(propylene oxide) (PPO) were used as precursor for the SMPUs. For their synthesis, poly(ɛ-caprolactone) diols have been converted into isocyanate end-capped prepolymers by using a mixture of 3(4) isocyanato-1-methyl-cyclohexylisocyanate isomers, after which a coupling with low-Tg poly(propylene oxide) oligomers is done. The shape memory polymers are obtained by reaction of the ABA block copolymers with hexamethylenediisocyanate and 1,4-butanediol as chain extender. Using this new strategy, a flexible segment (PPO) was introduced between the hard and the switching segments of the SMPU. For comparison, SMPUs without flexible segment have also been prepared with the conventional synthetic route. DSC, isostrain experiments and cyclic shape memory tests revealed narrower switching temperatures for the SMPUs including a flexible segment.

Combining Pattern Instability and Shape-Memory Hysteresis for Phononic Switching

We report a fully reversible and robust shape-memory effect in a two-dimensional nanoscale periodic structure composed of three steps, the elastic instability governing the transformation, the plasticity that locks in the transformed pattern as a result of an increase in glass transition temperature (T(g)), and the subsequent elastic recovery due to the vapor-induced decrease in T(g). Solvent swelling of a cross-linked epoxy/air cylinder structure induces an elastic instability that causes a reversible change in the shape of the void regions from circular to oval. The pattern symmetry changes from symmorphic p6mm to nonsymmorphic p2gg brought via the introduction of new glide symmetry elements and leads to a significant change in the phononic band structure, specifically in the opening of a new narrow-band gap due to anticrossing of bands, quite distinct from gaps originating from typical Bragg scattering. We also demonstrate that numerical simulations correctly capture the three steps of the shape-memory cycle observed experimentally.

Relationship Between Materials Properties and Shape Memory Behavior in Epoxy-Amine Polymers

AbstractAlthough epoxy-based polymers remain infrequently used as shape memory polymers (SMP’s), they are a promising base material for highly demanding applications due to their intrinsic physical properties and ease of processing. A series of epoxy SMP’s was synthesized with varying mechanical properties and with glass transition temperatures ranging from 31 to 93 °C, tunable via the variations of the molecular structures. The influence of chemical structures and physical properties of these epoxy SMP’s on their shape memory (SM) behavior is examined in detail along with the impact of the shape memory cycling conditions. While the results show that lower crosslink densities and/or higher molecular flexibility/mobility leads decreased SM performance, at low crosslink density the effect of molecular flexibility/mobility becomes dominant in influencing the SM response.

Side chain dendritic polyurethanes with shape-memory effect

In this study, thermally reversible polyurethanes (PUs) with shape memory effect were developed by using hydrogen bonding to enhance physical interactions. Two different types of PUs were synthesized: (1) a linear PU whose hard segment consists of methylene di-para-phenyl isocyanate (MDI) and di(ethylene glycol) (DEG); its soft segment is made of Tone 0260 polyol, a polyester polyol; (2) a side-chain dendritic PU which replaces DEG with different generations of dendritic chain extenders. By incorporation of the dendritic structure with peripheral long alkyl chains (strong van der Waals forces), the miscibility between hard and soft segments can be significantly improved. Consequently, the hydrogen bonding reinforced hard segments (malonamide linkages) of side chain dendritic PUs result in greatly enhanced mechanical properties and shape memory effect. During cyclic shape memory tests, one of the series can effortlessly achieve complete recovery in less than 10 second without any deficiency.

A constitutive theory for shape memory polymers. Part II: A linearized model for small deformations

A constitutive theory is developed for shape memory polymers. It is to describe the thermomechanical properties of such materials under large deformations. The theory is based on the idea, which is developed in the work of Liu et al. [2006. Thermomechanics of shape memory polymers: uniaxial experiments and constitutive modelling. Int. J. Plasticity 22, 279–313], that the coexisting active and frozen phases of the polymer and the transitions between them provide the underlying mechanisms for strain storage and recovery during a shape memory cycle. General constitutive functions for nonlinear thermoelastic materials are used for the active and frozen phases. Also used is an internal state variable which describes the volume fraction of the frozen phase. The material behavior of history dependence in the frozen phase is captured by using the concept of frozen reference configuration. The relation between the overall deformation and the stress is derived by integration of the constitutive equations of the coexisting phases. As a special case of the nonlinear constitutive model, a neo-Hookean type constitutive function for each phase is considered. The material behaviors in a shape memory cycle under uniaxial loading are examined. A linear constitutive model is derived from the nonlinear theory by considering small deformations. The predictions of this model are compared with experimental measurements.

A constitutive theory for shape memory polymers. Part I: Large deformations

A constitutive theory is developed for shape memory polymers. It is to describe the thermomechanical properties of such materials under large deformations. The theory is based on the idea, which is developed in the work of Liu et al. [2006. Thermomechanics of shape memory polymers: uniaxial experiments and constitutive modeling. Int. J. Plasticity 22, 279–313], that the coexisting active and frozen phases of the polymer and the transitions between them provide the underlying mechanisms for strain storage and recovery during a shape memory cycle. General constitutive functions for nonlinear thermoelastic materials are used for the active and frozen phases. Also used is an internal state variable which describes the volume fraction of the frozen phase. The material behavior of history dependence in the frozen phase is captured by using the concept of frozen reference configuration. The relation between the overall deformation and the stress is derived by integration of the constitutive equations of the coexisting phases. As a special case of the nonlinear constitutive model, a neo-Hookean type constitutive function for each phase is considered. The material behaviors in a shape memory cycle under uniaxial loading are examined. A linear constitutive model is derived from the nonlinear theory by considering small deformations. The predictions of this model are compared with experimental measurements.

A comparison of methods for the training of NiTi two-way shape memory alloy

The creation of an effective two-way shape memory alloy (TWSMA) requires appropriateheat treatment and optimal training considerations. In particular, the training method usedplays a key role. This work investigates different training methods for producing NiTiTWSMA wires with the hot shape of an arc and the cold shape of a straight line. Thesemethods are shape memory cycling, constrained cycling of deformed martensite,pseudoelastic cycling and combined shape memory and pseudoelastic cycling. Inorder to give a meaningful evaluation of their performance that is relevant totraining TWSMA for practical applications, these training methods are assessed interms of maximum two-way strain, changes in the original hot shape togetherwith the transformation temperatures after the training process, and the effectiveproduction of the cold shape. It was found that only the combined shape memoryand pseudoelastic cycling provides an effective training method for creating NiTiTWSMA with a non-uniaxial two-way shape change. The undesirable side effects oftraining are that the NiTi TWSMA wire loses partial memory of the originalhot shape and its transformation temperatures shift to lower values. There alsoexists an optimal number of training cycles, and possibly an optimal training loadfor obtaining the best cold shape memory and the greatest two-way recoverablestrain. These findings give future directions to advance the training technology forTWSMA.

Crystallography and interfacial structure of twinned shape memory martensite

This paper reports observations of the interface and surface relief of twinned martensite in a Cu-14wt%Al -3.4wt%Ni shape memory alloy. The transformation in this alloy is from ordered cubic DO3 parent phase (β 1 ) to 2H (orthorhombic) martensite (γ 1 ') with Type II transformation twins. Optical and atomic force microscopy were used to study single isolated plates. These plates consisted of lamellae of martensite parent (A) and twin (B) volumes. The average width fraction of Twin A was 0.667, close to the theoretical value of 0.698. Although twin relief was generally evident in the random sections examined, well defined interfacial facets corresponding to the terminating twin volumes were not observed. Instead, side-plates extending beyond the habit plane were common, being associated with the smaller of the two twin volumes (Twin B). This structural feature was common to both thermal and stress-induced martensite. The twin interface plane was close to a habit plane variant, and the extension of the twin ahead of the general interface, with its own twinned substructure, is inferred to be related to the need to accommodate localised interfacial misfit. Analysis of the crystallography of the transformation indicated that two distinct solutions are possible, adding to the flexibility ofthe transformation in accommodating forces applied during shape memory and superelastic cycling.

Stabilization of the shape memory effect in NiTi: an experimental investigation

Over the last 20 years or so, shape memory alloys have become well known for their ability to undergo large shape changes that traditional metals could never do. One such alloy, NiTi, has been of particular interest due to it’s high efficiency (when compared to other SMA’s) to convert thermal energy into mechanical energy. While NiTi is currently used in industry, its use is hindered by an extreme sensitivity to processing techniques. Certain parameters such as alloy composition, annealing procedures and various loading conditions can improve the shape memory effect or completely make it disappear. Many studies have focused on these variables. Of particular interest to this study is the one-way shape memory effect (OWSME) after the NiTi has been cycled (deformation followed by recovery) many times. It is very desirable to find a processing technique that would stabilize the OWSME and make it repeatable after many cycles. This paper explores these parameters in hopes of finding an optimal process to stabilize the shape memory effect in NiTi. Many studies have been conducted examining the cyclic behavior of NiTi. Most common are studies done on pseudoelastic NiTi (1,2,3) and on the two way shape memory effect (TWSME) (4,5,6,7). However, very few have explored these cyclic effects on the OWSME. One study that was conducted on the OWSME used wire samples and carried tests out to 100 cycles (8). It was reported that the recovery strain was reduced to 93.4% and 81% of the total strain when total strains of 6% and 8% were used, respectively over the 100 cycles. (Note, that these values will differ from ours since they used “hard” cycles with a fixed maximum strain where as we used “soft” cycles with a fixed maximum stress.) This displays the degradation of the shape memory effect that hinders NiTi’s use. After this work, another study was performed that investigated a conditioning technique to try to stabilize the shape memory effect (9). Their research recommended that after SME (strain-heat-recover-cool-repeat) cycling the NiTi 30 times at 6% strain, a stable strain range was obtained for strains less than 6%. This was only shown over a very few cycles and in wire samples. It is our hypothesis that some form of “conditioning” may be performed on NiTi after annealing to improve its cyclic shape memory properties. The result will be a protocol that can be applied to NiTi before its service use. In doing so, it is important to define a few performance criteria that are considered indicative of “good behavior” and clarify what will hopefully be stabilized:

Electro-thermomechanical behaviour of a Ti-45.0Ni-5.0Cu (at.%) alloy during shape memory cycling

In the present paper, electrical resistance (ER) changes are measured simultaneously with the stress-assisted two-way memory effect (SATWME) in Ti-45.0Ni-5.0Cu (at.%) wires during thermal cycling (max. 15 cycles) for several different stress levels. Interesting qualitative evolutions of the ε-ER-T loops during cycling are observed as a function of the applied stress. On cooling, for stresses higher than 175 MPa, a clear deviation of the ε-T curves is verified and the reversion of this anomaly is not observed during heating. After some cycles, serrations are frequently observed on the ER-T loops essentially below M f and above A f , indicating an interaction between the formation and reversion of oriented martensite variants with the defects introduced during the thermomechanical cycling. A linear relationship is observed between ER and ε for the direct and reverse transformation ranges. The characteristic slope d(Δ R / R )/dε is slightly dependent on the applied stress and on the number of thermal cycles.

Description of an Fe-based shape memory alloy thermomechanical behavior in terms of the synthetic model

The phenomenological two-level synthetic model of the non-linear deformation has been used for reflection of the interaction of deformational processes of principally different origin (martensitic, dislocational): the yield stress of a material in the two-phase state depends on the respective amount of the martensite. In the martensite deformation investigation the residual oriented microstresses action is taken into account. The principle advantages of the synthetic model – such as mathematical simplicity – are considered. The approach leads to the reliable description of the cyclic shape memory effect taking place under complex modes of temperature-stress investigations. Applicability of the model for description of deformational behavior of the modern Fe-based SME alloy has been demonstrated.

Transformation Characteristics of Ce-TZP during Shape Memory Cycles

Transformation characteristic of Ce-TZP ceramic in shape memory cycles was studied by MTS machine in the load and stroke control modes. The shape memory effect did not degrade up to 8 cycles in load control and the residual plastic strain reached 3.8% in stroke control. The yield stress decreased and the reverse transformation temperature of stress-induced martensite increased with the number of the cycles. The dependence of yield stress and transformation temperature on the shape memory cycles is attributed to the damages associated with the transformation.

A Model of Shape Memory Materials with Hierarchical Twinning : Statics and Dynamics

We consider a model of shape memory material in which hierarchical twinning near the habit plane (austenite-martensite interface) is a new and crucial ingredient. The model includes (1) a triple-well potential ({phi} model) in local shear strain, (2) strain gradient terms up to second order in strain and fourth order in gradient, and (3) all symmetry allowed compositional fluctuation induced strain gradient terms. The last term favors hierarchy which enables communication between macroscopic (cm) and microscopic ({Angstrom}) regions essential for shape memory. Hierarchy also stabilizes between formation (critical pattern of twins). External stress or pressure (pattern) modulates the spacing of domain walls. Therefore the ``pattern`` is encoded in the modulated hierarchical variation of the depth and width of the twins. This hierarchy of length scales provides a hierarchy of time scales and thus the possibility of non-exponential decay. The four processes of the complete shape memory cycle -- write, record, erase and recall -- are explained within this model. Preliminary results based on 2D Langevin dynamics are shown for tweed and hierarchy formation.