Epoxy-based Shape Memory Polymer Research Articles

High-performance shape memory epoxy resin with high strength and toughness: Prepared by introducing hydrogen bonds through polycaprolactone and low melting point alloy

Epoxy resin (EP) as common polymers have attracted much attention in the field of shape memory materials due to their excellent properties. However, epoxy-based shape memory polymers suffer from a trade-off between strength, toughness, variable stiffness capability and shape memory properties, and thus are subject to significant limitations in their applications. In this paper, we propose a method to prepare epoxy composites with high strength and toughness and high shape memory properties by introducing a lot of hydrogen bonds. Hydrogen bonding is generated through polycaprolactone (PCL) induced EP phase separation. A low melting point alloy (L) was also added to enhance the shape memory properties of the composites. The synthesis mechanism of EP and the formation mechanism of hydrogen bonding were revealed. The effects of the introduction of hydrogen bonding on the mechanical properties, stiffness and shape memory properties of the composites were investigated. New ideas and design directions are provided for the optimisation of modification of high-performance shape memory epoxy resins.

Thermally Triggered Multilevel Diffractive Optical Elements Tailored by Shape-Memory Polymers for Temperature History Sensors.

The morphological transitions induced by external stimuli in shape-memory polymers (SMPs) can be exploited with the real-time response of far-field diffraction patterns in diffractive optical elements (DOEs). In this paper, we combine the temperature characteristics of SMPs and the display characteristics of DOEs to obtain an optical temperature sensing film where the temperature information is taken as a change of far-field diffraction images. This process was achieved by imprinting the micropatterns of the DOEs on the epoxy-based SMP film, which can be programmed to hold a temporary optical image and revert to its original image upon exposure to a specific temperature. Furthermore, the specific temperature at which the image transformation occurs can be customized by varying the chain flexibility of the SMP. Based on a range of transition points, by imprinting the desired combination of SMP-DOEs on a film, a sensor that can record and inform the temperature history is demonstrated. As for the feasible application of this technique, it can be used for the compact and reliable optical temperature indicators, which can be applied in temperature-sensitive industries such as food and pharmaceuticals.

Space deployable parabolic reflector based on shape memory polymer composites

In this paper, a deployable parabolic reflector based on epoxy-based shape memory polymer (SMP) is investigated. A deployable SMP composite parabolic reflector with a diameter of 400 mm is designed and fabricated according to laboratory equipment conditions. The storage ratio and collapsed surface curvature of the three folding patterns are compared using finite element analysis (FEA). Then FEA is used to analyze the reflector's folding and deploying process by investigating the shape memory behaviors during these two processes as well as predicting the deploying time. Finally, the reflector folding and deploying tests are completed. The reflector’s shape fixation ratio, recovery ratio, and deploying time are measured to verify this pattern’s feasibility and rationality.

Photodegradation mechanisms and physico-chemical properties of EPON-IPD epoxy-based polymers

Shape memory polymers are increasingly being utilized in aviation and biomedical industries. However, full realization of their potential is limited due to the lack of mechanistic understanding of their environmental degradation. To address this issue, the chemical structure, thermo-mechanical, morphological, and optical properties of an epoxy-based shape memory polymer during UV-induced aging were studied using FTIR, DSC, nanoindentation and SEM techniques after exposure to UV radiation up to 600 h. The proposed mechanism involves competition between chain scission and crosslinking. SEM and DSC analysis confirmed that an increased UV exposure not only increases the number and dimension of microcracks on the surface but also increases the glass transition temperature (Tg) of epoxy-amine based polymer. Similarly, elastic modulus (E) and hardness (H) were increased by ~22 and 33%, respectively, after 600 h of UV radiation.

Theoretical and Experimental Investigation of Shape Memory Polymers Programmed below Glass Transition Temperature.

An epoxy-based shape memory polymer (SMP) is synthesized and examined for its deterioration in shape fixity due to springback and isothermal viscoelastic recovery at different ambient temperatures. Shape fixity depends not only on material properties but also on programming conditions. A constitutive finite deformation model is incorporated to predict the behavior of the proposed SMP and find maximum shape fixity. A programming approach is followed in which, in contrast to hot programming, the SMPs are neither heated before deformation nor cooled afterward but are deformed at ambient temperature and then stress-relaxed. The proximity of the programming temperature to the glass transition temperature plays a crucial role in determining the shape fixity of SMP. It has been found that the SMP with a glass transition temperature of 42.9 °C can achieve maximum shape fixity of 92.25% when programmed at 23 °C with 100 min stress relaxation time. Thermal contraction and dynamic tests are performed in the Dynamic Mechanical Analyzer (DMA) to determine structural relaxation properties and distinguish the programming temperature in the cold, warm or hot temperature zone. The shape memory tests are carried out using temperature-controlled UTM to determine the shape fixity and shape recovery of SMP. The SMPs are subjected to a full thermomechanical cycle with different stress relaxation times and programming temperatures.

Near infrared light responsive surface with self-healing superhydrophobicity in surface chemistry and microstructure

Self-healing superhydrophobic surfaces have attracted considerable attention for enhancing the durability of superhydrophobic coatings, which have applications in various fields. However, it is difficult to find safe and effective strategies for self-healing superhydrophobic coatings to recover both microstructure and surface chemistry simultaneously. Herein, we demonstrate a high-efficiency self-healing hydrophobic surface with remote control ability. Through near infrared light (NIR) irradiation, the collapsed surface microstructure and altered surface chemistry can be recovered easily and simultaneously in a very short period. The extraordinary performance is fully discussed and can be attributed to the excellent photothermal properties of candle soot (CS). In addition, the surface demonstrates repeatable self-healing ability thanks to the good shape memory of the epoxy-based shape memory polymer (SMP) and the continued release of fluorinated alkyl silane encapsulated in microcapsules. The presented surface shows great potential and broad impacts owing to its simplicity and efficiency.

Experimental analysis of high-temperature shape memory polymers for deployable structures

Shape memory polymers (SMPs) continue to capture interest in the aerospace industry due to their unique properties. In general, the focus has been on SMPs with low transformation temperatures below 100 °C. However, certain aerospace applications require shape memory polymers with higher transformation temperatures to prevent undesirable actuation upon heat exposure. Moreover, and despite significant work on characterizing mechanical and recovery properties of shape memory polymers, the literature lacks full-field characterization of recovery properties, in particular at elevated temperatures. In this work, a high-temperature thermoset epoxy SMP (Tg ≈ 130 °C), named EPON SMP, is thoroughly investigated. This study provides a comprehensive experimental analysis of the mechanical and shape memory properties of the aforementioned high-temperature epoxy SMP. In addition, this study uses a full-field characterization technique, Digital Image Correlation (DIC), to provide further insight on localization, heterogeneity, and local recovery of shape memory strains. Furthermore, the study evaluates the recovery properties of the EPON SMP at different loading conditions (i.e., bending and tensile) and proposes shape-recovery evaluation methods that can be later implemented on other SMPs. • An epoxy-based shape memory polymer with high switching temperature (Tg ≈ 130 °C) was considered in this work. • The temperature dependant mechanical properties were evaluated across a wide temperature range around Tg. • The shape recovery properties were studied under simple tensile and complex bending conditions. • The heterogeneities in the accumulated and recovery strains were evaluated using full-field deformation measurements. • A deep insight into the local accumulation of programmed, thermal, mechanical, and recovery strains is provided.

Synergistic water-driven shape memory performance and improving mechanism of grading photo-thermal curing shape memory composite

Shape memory materials draw much attention due to various promising potential applications. This work prepares a novel shape memory composite via grading photo-thermal curing method, which has not yet been reported. Expectedly, it merges advantages of epoxy-based shape memory polymer (EP) and polyvinyl alcohol-based shape memory polymer (PVA), which has applicable stiffness and multi-stimuli responsive performance. The unsatisfactory mechanical properties of most water-driven shape memory polymer (SMP) is expected to be solved by this fabrication strategy, so that the application range of water-driven SMP could be further extended. The synergistic plasticization of PVA and EP dominates water-driven shape memory behavior of this composite. Interestingly, its water-driven shape memory performance is able to be further improved by immersed in HCl/EtOH solution. The glassy-to-rubbery modulus ratio (E′g/E′r) of the PVA/EP-HCl/EtOH is about 650 after immersed in 0.1 mol/L HCl/EtOH for 6 h. In addition, this immersed method also can solve the post curing problem of such cationic photocuring. It is an easy, efficient, economical strategy for improving synergistic water-driven shape memory performance. Furthermore, this shape memory composite is expected to provide new perspective and practical approach to realize the intellectualization of underwater deployment and release structure.

Thermal-Regulated Adhesion Enhancement and Fast Switching within the Viscoelastic Glass Transition Zone of a Shape Memory Polymer

The effects of temperature and preload on adhesion of an epoxy-based shape memory polymer (ESMP) were investigated experimentally. The ESMP sheet and polydimethylsiloxane (PDMS) control sample were prepared by mold casting. The adhesion measurements were carried out on a home-built adhesion tester using a hemispherical glass indenter as the counter-surface. The reduced modulus was determined by the Oliver-Pharr method and Hertz theory. The viscoelastic energy dissipation was extracted, and the effective work of adhesion was calculated by the fitting method and JKR theory. In the vicinity of glass transition temperature (Tg), the viscoelastic enhancement factor (1 + f(v, T)) is as high as 955 because of the strong viscoelastic effect of the ESMP sheet. The adhesion strength of the ESMP sheet is about 589 kPa under a relatively smaller contact displacement condition (∼44.6 μm). The strong viscoelastic effect induces more viscoelastic energy dissipation that contributes to the effective work of adhesion and leads to strong preload dependence of the adhesion. The pull-off force Fpull-off is demonstrated to linearly depend on Fm1/3Er2/3. In the sharp half glass transition zone (T > Tg), the viscoelasticity and rigidity rapidly decrease with the temperature increasing about 10 °C, leading to a 6-fold reduction in adhesion. The results indicate that the adhesion of the ESMP sheet can be significantly enhanced and meanwhile rapidly switched within the viscoelastic glass transition zone.

Investigation of the Long‐Term Storage Stability of Shape Memory Epoxy Prepolymer

Epoxy‐based shape memory polymers (ESMPs) are important types of thermoset shape memory polymers due to their unique thermomechanical properties, which have great potential in applications ranging from smart actuators to deformable structures. The long‐term storage stability is important for ESMPs, especially when pursuing the same batch of materials for alternatives. Stable storage of ESMP preplomyers in the long term and taking it out for curing when needed are of great benefit to engineering applications which highly require the same batch of materials for high production efficiency, such as in deployable space structures, semiconductor, and electronic packaging. However, this important storage stability of ESMP has not been studied so far. Herein, a series of ESMPs with tunable glass transition temperatures (Tgs) ranging from 100 to 170 °C is prepared. The effect of long‐term storage at low temperatures on rheological, thermal, thermomechanical, mechanical, and shape memory properties is systematically investigated. The ESMP after storage shows an improvement of crosslinking density, Tgs, and mechanical strength, while maintaining chemical structures and excellent shape memory properties. This feasible method of low‐temperature storage realizes overall long‐term stability and enhancement of strength in ESMPs, making it more promising for engineering applications.

Bio-inspired smart surface to achieve controllable locomotion through adjustable anisotropic friction

Anisotropic friction generated by microstructured surfaces is crucial for performing functions such as directional locomotion and adhesion in biological systems. Hence, an epoxy-based shape memory polymer (SMP) incorporating Fe3O4 nanoparticles is used in this study to create a smart surface with oriented structures to mimic anisotropic friction and exploit human-developed controllable locomotion systems. Applying the specific properties of the epoxy-based SMP, fast switching friction can be achieved by adjusting the topography and stiffness of the microstructures on the surface. In addition, the photothermogenesis effect of Fe3O4 nanoparticles induces changes in the asymmetric topography and stiffness on the SMP surface under the irradiation of near-infrared (NIR) light, thereby inducing a rapid switching of the friction force. Furthermore, a microbot is created to demonstrate remotely controlled locomotion, such as unidirectional and round-trip movements, and braking by switching the friction force under NIR light. These results are promising for the design of new intelligent surfaces and interfaces; additionally, they may facilitate the investigation of biological structures and processes.

Molecular dynamics simulations of thermodynamics and shape memory effect in CNT-epoxy nanocomposites

Epoxy based shape memory polymers have attracted considerable attention in engineering applications. To achieve excellent thermo-mechanical performance, carbon nanotube (CNT) is used as reinforcement for epoxy matrix. The improvement of mechanical properties and shape memory effect in CNT-epoxy nanocomposites are investigated using molecular dynamics simulations in this study. The two representative systems, neat epoxy and CNT-reinforced epoxy nanocomposite are constructed, and the corresponding physical properties, such as the density and the glass transition temperature are obatained. The mechanical properties and shape memory behaviors within the selected temperature range are characterized by applying tensile loading. The segmental dynamics are also captured during deformation process to investigate how epoxy chains are activated and changed that leads to final conformations. In addition, the free volume during recovery process is tracked to study shape recovery properties. The results can provide better understanding of the reinforcing mechanism of CNT on mechanical properties and shape memory effect of epoxy nanocomposites, which help to enrich the fundamental knowledge of shape memory epoxy nanocomposites and enlightens the design of shape memory materials.

Effect of loading fraction of three-dimensional graphene foam (3D-C) on thermal, mechanical, and shape memory properties of 3D-C/SMP composite

Here, we have studied the effect of loading fraction of 3D-graphene (3D-C) on the properties of epoxy-based shape memory polymer (SMP). 3D-C has already been proven to be an effective filler to improve the thermal conductivity, resulting in increased shape transformation efficiency of the SMP. We prepared a series of 3D-C/SMP composite with different volume fractions of 3D-C and compared their thermal, mechanical, and shape memory properties. We observed that as the 3D-C content increases, the thermal conductivity of the composite increases along with shape transformation performances. However, interestingly, in conflict with typical nanofillers, we observed that the mechanical strength decreases at higher loading fraction due to the obstruction of polymer chains by interconnected branches. Hence, to leverage the extraordinary improvement of thermal conductivity by 3D-C, a careful design based on the amount of loading is essential to obtain a balance between thermal conductivity, mechanical strength and shape memory performance.

The Effect of POSS Type on the Shape Memory Properties of Epoxy-Based Nanocomposites.

Thermally activated shape memory polymers (SMPs) can memorize a temporary shape at low temperature and return to their permanent shape at higher temperature. These materials can be used for light and compact space deployment mechanisms. The control of transition temperature and thermomechanical properties of epoxy-based SMPs can be done using functionalized polyhedral oligomeric silsesquioxane (POSS) additives, which are also known to improve the durability to atomic oxygen in the space environment. In this study, the influence of varying amounts of two types of POSS added to epoxy-based SMPs on the shape memory effect (SME) were studied. The first type contained amine groups, whereas the second type contained epoxide groups. The curing conditions were defined using differential scanning calorimetry and glass transition temperature (Tg) measurements. Thermomechanical and SME properties were characterized using dynamic mechanical analysis. It was found that SMPs containing amine-based POSS show higher Tg, better shape fixity and faster recovery speed, while SMPs containing epoxide-based POSS have higher crosslinking density and show superior thermomechanical properties above Tg. This work demonstrates how the Tg and SME of SMPs can be controlled by the type and amount of POSS in an epoxy-based SMP nanocomposite for future space applications.

Cold programming of epoxy-based shape memory polymer

Shape Memory Polymers (SMPs) are the class of smart materials that show ability to memorize different shapes and regain original shape. The conventional method is to heat SMP above its glass transition temperature (Tg) and deform it to obtain the desired shape. It reverts to original shape upon heating. This is known as hot programming. Recent advances in the programming of SMP have shown promising results of a new programming method also known as cold programming. The SMP is plastically deformed at a temperature below Tg to impart the desired shape. This paper aims at the experimental investigation of shape memory properties of Epoxy-based SMPs using the cold programming method. The polymers of Epoxy used in this work are Diglycidyl Ether of Bisphenol-A (DGEBA) and Neopentyl Glycol Diglycidyl Ether (NGDE). The mechanical properties like elastic modulus, tensile strength, and failure strain have been found for different ratios of DGEBA and NGDE at various temperatures. The Tg of different compositions has been measured using Differential Scanning Calorimetry (DSC). In this work, a candidate material has been identified based on its mechanical and shape memory properties. It has been demonstrated that important properties of material like glass transition temperature, elastic modulus, and strength can be tailored according to the need by changing the ratio of constituents. A dedicated experimental setup consisting of Uniaxial Tensile Machine (UTM), temperature control chamber, and a camera for Digital Image Correlation (DIC) has been used to carryout thermomechanical cycles of SMP. The parameters that affect the performance of cold programming like stress-relaxation time, structural relaxation time, and operating temperature have been studied. The stress-strain response for a complete shape memory cycle has been measured. Parameters determining the shape memory capabilities like shape fixity, recovery ratio, recovery speed of the material were evaluated. The merits and demerits of cold programming over other programming methods have been discussed by carrying out required experiments. The differences in both the programming methods and the underlying mechanism of shape memory effect have been discussed briefly in this paper. Present work is limited to the study of SMP for uniaxial tensile cases undergoing large deformation with small strain-rate and low rate of heating and cooling.

Shape memory epoxy composites with high mechanical performance manufactured by multi-material direct ink writing

Using 3D printing to manufacture shape memory polymers (SMPs) becomes popular, since the technique endows SMPs the ability to shape into desired structures according to their applications. Among various types of SMPs, epoxy-based shape memory polymer and their composites are known for their high modulus and strength. However, limited by their rheological behavior, it is still hard to prepare high-quality printable epoxy materials. Here, by carefully tuning of rheological properties, we can prepare printable ink showing good shape retention, excellent mechanical performances below and above the glass transition temperature of epoxy, as well as good shape memory effect. The prepared ink can also be suitable for printing 3D models with spanning structures by the multi-material direct ink writing technique, which extends potential applications of epoxy inks in shape memory and other fields.

Electro-active Variable-Stiffness Corrugated Structure Based on Shape-Memory Polymer Composite.

Shape-memory polymers (SMPs) can adjust their stiffness, lock a temporary shape, and recover the permanent shape upon an appropriate stimulus. They are applied in the field of morphing skins. This work presents a variable-stiffness corrugated sheet based on a carbon fiber felt (CFF)-reinforced epoxy-based SMP composite that shows variable stiffness and extreme mechanical anisotropy for potential morphing skin applications. The corrugated sheet exhibits a variable stiffness with a change in temperature, which can help the skin adjust its stiffness according to different service environments. The corrugated sheet can be electrically heated rapidly and homogeneously due to its high electrical conductivity and enhanced heat transfer efficiency. Its Joule-heating effect acts as an effective active stimulation of the variable stiffness and shape-memory effect. The CFF-reinforced epoxy-based SMP composite was manufactured into a corrugated shape to obtain extreme mechanical anisotropy. The corrugated sheet shows a low in-plane stiffness to minimize the actuation energy, while it also possesses high out-of-plane stiffness to transfer the aerodynamic pressure load. Its mechanical properties, electrical heating performance, and shape-memory effect were investigated using experiments. The results show that the proposed SMP composite exhibits extreme mechanical anisotropy, considerable deformation ability, and variable stiffness induced by Joule heating without an external heater.

Study of low earth orbit ultraviolet radiation and vacuum thermal cycling environment effects on epoxy-based shape memory polymer

Shape memory polymers have been studied as the matrix of lightweight self-deployable space structures for decades. The epoxy-based shape memory polymer, which is a potential material for aerospace application, has been tested about their resistance abilities to the ultraviolet radiation and vacuum thermal cycling by using ground-based simulation facilities. The wavelength of ultraviolet radiation is 250–400 nm with not less than five times the solar constant, and the irradiation times are 80, 160, and 240 h, respectively. The vacuum level, temperature range, and cycling times of vacuum thermal cycling are 10−5 Pa; −100°C to +100°C; and 0 cycle and 15, 30, and 45 cycles, respectively. The shape memory polymer specimens are compared for mass loss ratio, chemical compositions, surface morphology, and mechanical properties before and after the ultraviolet radiation and thermal cycling. The experimental results indicate that the glass transition temperature (Tg), mass loss ratio, modulus, and yield and break strength increase after experiencing 240h ultraviolet radiation or 45 thermal cycles, which shows great potential of epoxy-based shape memory polymer in aerospace structures and device applications.

Stress relaxation properties of an epoxy-based shape-memory polymer considering temperature influence: experimental investigation and constitutive modeling

Epoxy-based shape-memory polymer (ESMP), belonging to viscoelastic thermosets, exhibits strong temperature-dependent relaxation property caused by the glass transition process. Thoroughly experimental investigation and accurate numerical prediction are indispensable to be performed in the structures that demand for high precise structural controls. In this work, key parameters of an ESMP in stress relaxation process including initial modulus, elastic modulus, viscous modulus and relaxation time were experimentally investigated and constitutive-model-based analyzed to study the influence of the temperature, where the constitutive model was established by modifying the constant parameters of a neo-Hookean hyper-viscoelastic model to be functions of the temperature. A numerical simulation method was developed by coding UMAT in ABAQUS using the constitutive model and was verified by finite element simulations of stress relaxation experiments. The maximum deviation of 4.5% between simulations and experiments indicated the reasonability of the model in depicting stress relaxation process considering the influence of the temperature.

Degeneration and damage mechanism of epoxy-based shape memory polymer under 170 keV vacuum proton irradiation

Shape memory polymers are the promising candidates for the next generation space deployable structures. However, to design highly reliable, long-life space deployable structures, it is essential to study the performance of shape memory polymers in the harsh space environments. In this work, the epoxy-based shape memory polymer was irradiated by 170 keV proton beam in vacuum condition. Multiple characterization techniques including dynamic mechanical analysis, electron paramagnetic resonance spectra, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used to study the properties and microstructure changes after irradiation. Experimental results showed that the shape fixity rate remained almost unchanged at approximately 99.6% after proton irradiation while the shape recovery rate decreased sharply from its original 98.6% to 76.3% with increasing fluence up to 10 × 1015 cm−2. Moreover, the decrease of glass transition temperature and the accelerated growth of free radical concentration indicated dominant chain scissions. Further research found that the cleavage of aliphatic chemical bonds, especially C–N and C–O groups of aromatic ether and aliphatic ether were responsible for the significant drop of the shape recovery rate, suggesting that epoxy-based shape memory polymers with more aromatic rings are thought to be better choices when designing the materials for applications in space deployable structures.