Epoxy Shape Memory Polymer Research Articles

Enhanced shape memory performance and numerical simulation of knitted‐fabric reinforced polymer composites with weft yarns

AbstractFabric‐reinforced shape memory polymeric composites (SMPC) have shown great potential in the design of intelligent deformation structures. In this work, the weft‐knitted fabric inserted with weft yarns was developed as reinforcement to fabricate the shape memory epoxy polymer (SMEP) composites. The effects of weft yarn, loop density and direction on the shape memory performance of SMPC under different bending radii were experimentally investigated. The results show that the SMPC have good shape fixity ratios and shape recovery ratios of around 98%. As compared with those of SMPCs without inserting weft yarns, the SMPCs with weft yarns have shorter shape recovery time, and the recovery force of SMPCs with weft yarns in the 0° and 90° directions shows an increase of 86.4% and 79.5%, respectively. The recovery force of SMPC improve 3.7 times compared to SMEP. The multiscale models of SMPC were established with basis of the results of micro‐computed tomography scanning of specimens and viscoelastic theory. The surface buckling behaviors of SMEP and SMPC specimens after U‐bending load were discussed. Finally, the deformation of loops and weft yarns of SMPC were analyzed to reveal the shape memory mechanism.Highlights Shape memory polymeric composites (SMPC) with weft yarns has shorter shape recovery time. Recovery force of SMPC with weft yarns was obvious enhanced. Multi‐scale viscoelastic models of SMPC were established. Surface buckling behaviors of SMPC after U‐bending load were revealed.

Effect of crosslinking density on the self-healing and self-reporting properties of epoxy anti-corrosion coatings

This work developed a novel coating with self-healing and self-reporting performance by embedding polyurethane/poly-(urea-formaldehyde) (PU/UF) microcapsules in a shape memory epoxy polymer. 2’7’-Dichlorofluorescein (DCF) and linseed oil were applied as the coloring agent and healing agent, respectively, which are encapsulated into the PU/UF microcapsules (DL@PU/UF microcapsules) via a facile one-step method. To adjust the shape memory effect and the self-reporting performance of epoxy system, two kinds of curing agents (D230 and D400) were reacted with bisphenol A diglycidyl ether epoxy monomer to design a series of coating samples, finally the optimal coating system was selected by three indexes including barrier performance, residual amino content and coating hardness. When the composite coating was damaged, DCF can flow out of the broken microcapsules and react with the residual amino groups in the epoxy coating matrix, which can be recognized red signals in natural light and yellow fluorescent signals under ultraviolet light irradiation. More importantly, the excellent self-reporting property of the coating can be achieved on various metal substrates that broaden the application range of the coating. As for self-healing performance, the damaged coating can be healed quickly due to the shape memory effect of the coating as well as the filling of linseed oil in the scratched region. The Rct value of the healed coating containing 6 % DL@PU/UF microcapsules after immersing in 3.5 wt% NaCl solution for 5 days was more than 3 times that of the scratched coating, and the Rc value was about 16 times that of the scratched coating, indicating its corrosion protection performance. The novel coting not only achieve timely self-reporting ability of color and fluorescence, but also rapidly repair the coating damage, which is essential to practical applications.

Bending shape memory properties and multi-scale viscoelastic behaviors of knitted-fabric reinforced polymer composites

Knitted fabrics with easily deformable loop structure have the potential in the development of shape memory polymeric composites with large recovery deformations. The knitted fabric reinforced shape memory epoxy polymer composites (SMPC) were prepared in this work. The effects of loop densities, orientations and bending radii on shape memory properties of SMPC were investigated. The shape fixity ratio and shape recovery ratio of SMPC subjected to U-shaped bending radius of 5 mm are above 98 %. The shape recovery force of SMPC can reach up to 5.9 N. The thermodynamic properties of SMP were also characterized to obtain mechanical parameters and a user-defined material subroutine (UMAT) of shape memory epoxy polymer (SMEP) was written. Based on viscoelastic theory and the multi-scale geometrical structures, the macroscopic homogeneous thermodynamic model and mesoscopic thermodynamic model of knitted fabric reinforced SMPC were established to study the macro-scale stress distribution and meso-scale deformation evolution during shape memory process, respectively. The neutral surface position of SMPC during bending deformation is offset inner. The unique anisotropic loop structure of knitted fabric determines the shape memory behavior of the SMPC. Finally, micro-CT characterizations of knitted fabric reinforced SMPC were conducted to further understand the loop deformation mechanism during shape memory process. This study will provide important theoretical and technical support for large deformation structure design and deformation prediction of smart composites.

Experimental and molecular dynamics study on thermal‐mechanical properties of epoxy shape memory polymers

AbstractExperimental and molecular dynamics simulations were employed to investigate epoxy shape memory polymers' thermal‐mechanical properties and shape memory characteristics, utilizing polyether amine and modified hexanediamine as curing agents following the material prerequisites for variable stiffness fixtures. Glass transition temperature and Young's modulus of epoxy shape memory polymers are characterized by dynamic mechanical analysis. The consistency of the glass transition temperature between the simulation model and the experimental findings verified the accuracy of the molecular dynamics simulation. The uniaxial tensile simulation revealed that non‐bond energy predominantly exerts its influence in stretch. After reaching the yield stress, the weight fraction of the curing agent affects the generation of internal pores, with the yield stress of systems exhibiting substantial pores being minor. Systems characterized by a higher weight fraction of curing agents are more prone to pore formulation. Simulation of shape memory properties demonstrates that the entropy resulting from differences in molecular weight impacts the simulated system's shape fixation ratio and shape recovery ratio.

Viscoelasticity, stiffness gradient and their effects on adhesion of an epoxy shape memory polymer

The shape memory polymer based adhesives have demonstrated excellent programmable switchable adhesion, in which the material properties play an important role. Here, the viscoelasticity, stiffness gradient, and their effects on adhesion of an epoxy shape memory polymer (ESMP) were studied. The ESMP sample and polydimethylsiloxane control sample were fabricated firstly by mold casting and curing techniques. The sample was fixed on an electric heating plate with double-sided tape to conduct the adhesion measurements against a hemispherical glass indenter under different temperature and displacement conditions by using an adhesion tester and the temperature measurements on the double-sided tape and ESMP sample surfaces by using a digital thermodetector. Based on the measured force–displacement–time data, the hysteresis behavior and internal dissipation were evaluated; the peak force relaxation was studied; the reduced modulus was calculated and the stiffness gradient features were evaluated. The results show that the ESMP sample exhibits strong viscoelasticity and significantly enhanced adhesion that strongly depends on the compressive displacement in the glass-rubber transition zone near it is glass transition temperature ( Tg ), while weaker viscoelasticity and lower adhesion in the soft rubbery state. The reduced modulus of the ESMP sample obviously decreases with the increasing compressive displacement at a preset temperature below 70 °C. It is found that there exists a linear relation between the pull-off force, effective work of adhesion, reduced modulus, and contact area. This study helps deeply understand the material properties and adhesion mechanism of the ESMP, which can guide the design of switchable adhesives.

A novel self-healing coating with mechanically-triggered self-reporting properties: Color and fluorescence dual damage indications

Smart coatings with self-reporting and self-healing functions are essential to prolong their service life. In this work, a novel strategy was proposed to fabricate a dual-function coating with both mechanically triggered self-reporting ability and shape memory effect activated self-healing function. Polyurethane/poly-(urea-formaldehyde) microcapsules containing 2′,7′-dichlorofluorescein (DCF) were incorporated into the shape memory epoxy polymer (SMEP) coatings, and the molar ratios of curing agent and epoxy monomer were adjusted to modify the residual amino groups in the coating so as to strengthen both the self-reporting and self-healing performances. Once the coating was mechanically damaged, DCF would release from the ruptured DCF@PU/UF microcapsules and react with the residual amino groups in the coating to illustrate prominent red color. Even with a topcoat of polyurethane varnish or black paint, yellow fluorescence signals of DCF could be facilely observed upon ultraviolet light irradiation. For the self-healing performance, the high mobility of polymer networks endowed the coating with shape memory effect to recover the original shape and the barrier property. This strategy exhibits excellent potential to prolong the service lifetime of smart coatings.

Characterization and Selection of Tailorable Heat Triggered Epoxy Shape Memory Polymers for Epoxy Debondable Adhesives

AbstractThe increasing concern for environmental and economic sustainability has resulted in a growing interest in recycling the hybrid structures used in modern industries. To address the challenge of easy separation without damaging the substrates, a debondable‐adhesive incorporating shape memory polymer (SMP) filler is proposed. This study synthesizes seven types of epoxy SMPs at different stoichiometric ratios of the same chemicals. The thermomechanical, mechanical, and recovery characteristics of the SMPs are evaluated, and the most suitable candidate for debondable‐adhesive application is selected based on the selection criteria of SMP for embedding in the debondable adhesive. The results of this study offer valuable insights into the potential SMPs which can be incorporated in debondable adhesives used in modern industries.

Electrically and Thermally Triggered Three-Dimensional Graphene-Foam-Reinforced Shape Memory Epoxy Composites.

Shape memory polymer (SMP) epoxy composites have attracted significant attention due to their easy processing, lightweight nature, and ability to recover strain. However, their limited recovery rate and inferior mechanical properties have hindered their functional applications. This research explores the potential of three-dimensional (3D) graphene foam (GrF) as a highly efficient reinforcement for SMP epoxy composites. We demonstrated that the incorporation of a mere 0.13 wt.% GrF into mold-cast SMP epoxy leads to a 19% increase in the glass transition temperature (Tg). To elucidate the reinforcing mechanism, we fabricated and extensively analyzed composites with varying weight percentages of GrF. The GrF-based SMP epoxy composite exhibits a 57% increase in thermal conductivity, measuring 0.296 W mK-1 at 70 °C, due to the interconnected 3D graphene network within the matrix. Notably, this composite also demonstrates remarkable electrical conductivity, making it suitable for dual-triggering applications. The GrF-SMP epoxy composite achieves a maximum shape recovery ratio and a significant 23% improvement in the recovery rate, effectively addressing the issue of slow recovery associated with SMPs. We investigated the effect of switching temperatures on the shape recovery rate. We identified the optimal triggering temperature to initiate shape recovery for epoxy SMP and GrF-epoxy SMP as thermal energy equivalent to Tg + 20 °C. Additionally, we fabricated a bird-shaped composite using GrF reinforcement, which showcases self-healing capabilities through the crack opening and closure and serves as a tangible demonstration of the transformative potential of the composite. These GrF-epoxy SMP composites, responsive to stimuli, hold immense promise for diverse applications, such as mechanical systems, wearable sensors, morphing wings, foldable robots, and antennas.

Role of heating rate on the triple-shape memory effect of amorphous polymers: A cooperative thermodynamic model

The triple-shape memory effect (triple-SME) in amorphous polymers arises from the staged structural rearrangement of their macromolecular chains upon heating. However, the working mechanism behind the heating-rate dependence of the triple-SME in shape memory polymers (SMPs) is not well understood due to the complex compositions. Herein, we adopt the extended free volume theory to investigate the effect of local vibration and structural rearrangement of macromolecular chains on the shape memory behavior of SMPs. By introducing the phase transition theory, the dependences of the glass transition, thermomechanical behavior, and triple-shape recovery behavior of SMPs on thermal history are then discussed. The accumulation of the remote free volume upon heating is identified as the driving force for the SME in SMPs. The effectiveness of the proposed model is further evaluated by applying it to predict the shape memory behavior of epoxy SMPs with dual- and triple-SMEs at various heating rates. This study is expected to provide a practical methodology for understanding the working mechanism of triple-SME in amorphous polymers.

Bulk Polymerization of Thermoplastic Shape Memory Epoxy Polymer for Recycling Applications.

Conventional epoxy polymers are thermo-set and difficult to recycle and reuse. In this study, a series of linear thermoplastic epoxy polymers (EPx) with shape memory properties were prepared by using a bifunctional monoamine diglycolamine (DGA) as a curing agent and an equivalent amount of bifunctional rigid epoxy resin (E-51) and bifunctional flexible epoxy resin (polypropylenglycol diglycidyl ether, PPGDGE) in a bulk polymerization reaction. The results showed that these samples can fully react under the curing process of, 60 °C/2 h, followed by 80 °C/2 h, followed by 120 °C/2 h. The introduction of different contents of PPGDGE can adjust the Tg of the material to adapt to different environmental requirements, and can significantly increase the fracture strain of the material and improve its micro-phase separation structure. Thus, Rf of the material is close to 100%, and Rr is increased from 87.98% to 97.76%. Importantly, this linear chain structure allows the material to be easily recycled and reprocessed by dissolving or melting, and also means the material shows potential for 3D printing or other thermoplastic remolding.

Dynamic equilibria with glass transition heterogeneity and tailorable mechanics in amorphous shape memory polymers

Modeling dynamic heterogeneity in amorphous shape memory polymers (SMPs) is a huge challenge due to the complex statistics of strain energy distributions during their thermodynamic relaxations. In this study, based on the dynamic heterogeneity of strain energy distribution, we have considered, for the first time, the influences of different temperature rates and strain rates on strain energy evolution as a dynamic equilibria, rather than a quasi-static problem. We propose a phase transition model incorporated with Gaussian distribution statistics to investigate the dynamic equilibria with glass transition heterogeneity and tailorable mechanics for the amorphous SMPs. The Gaussian distribution statistics is firstly applied to characterize the heterogeneity of strain energy distributions in the amorphous polymers. Phase transition theory is then developed to describe working principles of strain energy evolution, glass transition heterogeneity, thermodynamic relaxation and tailorable mechanics. Finally, the dynamic equilibria of heterogeneity about the statistics of strain energy distribution are formulated based on the one dimensional Maxwell multi-branch model. The analytical results are compared with the experimental data of epoxy, polyamide and vinylester SMPs reported in literature, and good agreements between them are demonstrated. This study provides a new insight into the dynamic heterogeneity in the mechanics of amorphous SMPs.

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.

Mechanical, Thermal and Shape Memory Characterization of a Novel Epoxy Shape Memory Polymer

In past 3 decades a large number of shape memory polymers are available for various applications. This paper is aimed to produce a novel epoxy polymer by combining an epoxy base polymer Diglycidyl Ether of Bisphenol-A (DGEBA-Araldite LY556) and Polypropylene Glycol Diglycidyl Ether (PPGDE). The mechanical, thermal and shape memory characterization of material is done. It was found that the tensile strength of the material is reduced linearly with increasing the percentage of PPGDE. The elongation at break of the material is increased from 10% to 60% with 25% addition of PPGDE. The glass transition temperature of the material decreases linearly with increasing PPGDE percentage. D-70-P-30 (DGEBA-PPGDE) combination of the material is chosen for shape memory characterisation as its glass transition is above and within the vicinity of room temperature which is the pre-requisite for cold programming. The material shows good shape memory properties such as shape fixity and shape recovery.

Development and performance evaluation of a morphing wing design using shape memory polymer and composite corrugated structure

ABSTRACT Shape memory polymers (SMPs) are used in various industries due to their excellent variable stiffness behavior and innate response to external stimuli. The morphing wing is one of the technologies that employed the SMPs to reduce fuel consumption. This study proposes a novel morphing wing design using epoxy SMP as a wing skin, and a corrugated shape SMP composite (SMPC) as a wing core. The Dynamic mechanical analysis, bending and tensile tests were performed to determine the variable stiffness behavior of the skin, and the anisotropic behavior of the corrugated core. To evaluate the performance of the developed wing, the skin out-of-plane deformation is assessed, and the wing morphology is evaluated using the shape fixity and shape recovery tests. The results revealed that the skin out-of-plane deformation is within the acceptable limit for flight speed up to 0.16 Mach. Moreover, the wing shape recovery was excellent; the trailing edge of the wing is able to restore up to 99% of its bent position.

Modification of the multiphase shape memory composites with functionalized graphene nanoplatelets: enhancement of thermomechanical and interfacial properties

Ameliorating the mechanical and thermomechanical properties of the shape memory polymers (SMPs) make them an important class of materials for new-age applications ranging from aerospace, electronics, and marine. To show the more recent attempts proof-of-concept in designing the SMPs with higher mechanical and thermomechanical properties, herein we had designed the multiscale shape memory hybrid composites (SMHCs) having known composition of functionalized graphene nanoplatelets (fGNPs) through dispersing the fGNPs uniformly using ultrasonication in shape memory epoxy polymer. Subsequently, hybrid composites were prepared through impregnation of carbon fiber in the fGNP ∼ epoxy solution. Mechanical and thermomechanical characterizations were performed to evaluate the effect of the fGNPs within the SMHC. Significant improvement in the mechanical and thermomechanical properties were achieved in the SMHC with amine functionalized fGNP compared with the carboxy functionalized fGNP and non-functionalized epoxy polymer. Compared with the unmodified carbon fiber reinforced polymer composites, SMHC with 0.4 and 0.6 wt% amine fGNP improved by 19.7% and 33.2%, whereas 5.9% and 17.4% by carboxyl fGNP. Morphological study indicated the improvement of wettability of carbon fiber and interfacial bonding between the fiber and matrix. The recovery of the modified polymer composites into the original shape primarily occurred due to the gradual release of internal energy harnessed in the frozen and active phase of the polymer network. Due to the incorporation of nanofillers, the molecular kinetic energy of the reversible active phase increases, which intensifies the micro-Brownian motion. Thereby, the recovery stage was achieved due to the return of the frozen phase to the disoriented entropy condition. The addition of nanoparticles increases the cross-linking, which in turn increases the volume of the frozen phase. Hence, higher volume of frozen phase would hamper the recovery of the samples in its original shape, resulting in the lower shape fixity. However, the shape memory parameters of shape fixity and shape recovery evaluated with the heat activation bending did not degraded significantly and remained ∼95 ± 2% and ∼96 ± 1%, respectively.

Effects of networks composed of epoxy/dual thiol‐curing agents on properties of shape memory polymers

AbstractProfound understanding of relationships between molecular networks with properties can make it more convenient for people to prepare and employ shape memory polymers (SMPs). In this paper, a series of epoxy SMPs were developed based on epoxy (E‐51), trimethylolpropane tris(3‐mercaptopropionate) and pentaerythritoltetrakis (3‐mercaptopropionate), and 2‐methylimidazole, through “epoxy‐thiol” click reaction. The effect of networks on key properties (heat resistance and mechanical properties shape recovery performance) was systematically investigated. Experiment results indicated that there were positive correlations between network regularity with both thermal–mechanical properties and heat resistance. In addition, regularity of network structure had an effect to restrict shape recovery speed.

Numerical Analysis of Space Deployable Structure Based on Shape Memory Polymers.

Shape memory polymers (SMPs) have been applied in aerospace engineering as deployable space structures. In this work, the coupled finite element method (FEM) was established based on the generalized Maxwell model and the time–temperature equivalence principle (TTEP). The thermodynamic behavior and shape memory effects of a single-arm deployment structure (F-DS) and four-arm deployment structure (F-DS) based on SMPs were analyzed using the coupled FEM. Good consistency was obtained between the experimental data and simulation data for the tensile and S-DS recovery forces, verifying that the coupled FEM can accurately and reliably describe the thermodynamic behavior and shape memory effects of the SMP structure. The step-by-step driving structure is suitable for use as a large-scale deployment structure in space. This coupled FEM provides a new direction for future research on epoxy SMPs.

A novel temperature-sensitive expandable lost circulation material based on shape memory epoxy foams to prevent losses in geothermal drilling

Lost circulation will inevitably increase non-productive time (NPT) and well cost. The application of high-efficiency lost circulation materials (LCMs) plays an indispensable role in preventing and mitigating loss. The development of new LCMs based on stimuli-responsive materials is an important research direction and it is expected to realize the revolutionary development of LCMs. In recent years, the temperature-responsive plugging agent based on shape memory polymers has caused a lot of attention. In this paper, based on the shape memory epoxy polymer (SMEP) and hollow glass beads (HGBs), a shape memory epoxy foam (SMEF) was successfully synthesized. The results show that the shape memory temperature (51.6 °C - 97.5 °C) of the SMEPs has a good linear relationship with the content of curing agent 4, 4′-diaminodiphenyl methane (DDM) (12% - 20%), and the expansion ratio (0% - 120.2%) of the SMEFs has a good linear relationship with the content of HGBs (0% - 50%). The mechanism of the SMEFs was proposed. A novel temperature-sensitive and expandable plugging agent (SMP-LCM) has been developed. The plugging experiment shows that SMP-LCM has a good plugging effect on high permeability loss zone simulated by sand beds. Compared with conventional LCMs (e.g., nutshells or fibers), SMP-LCMs can eliminate the risk of plugging of downhole drilling tools and enhance the plugging effect.

Effects of modified carbon nanotubes with different sizes on properties of epoxy shape memory polymers

AbstractShape memory polymers (SMPs) have occupied an important role in material application, in which the contribution of various fillers on the enhancement of their low mechanical and thermal properties is indispensable. Lots of excellent researches on carbon nanotubes improving comprehensive performance have been accomplished, while few comparative studies about carbon nanotubes with different sizes. In the present study, epoxy shape memory composites of multi‐walled carbon nanotubes/epoxy matrix (MWCNTs/epoxy) were prepared, in which MWCNTs with different sizes modified through epoxidation act as enhancement filler. The results indicated that MWCNTs with smaller size possess an efficient effect on improving mechanical properties, and MWCNTs with larger size achieve higher enhancement on increasing heat resistance as the filler content is low (0.1%, 0.2%). Furthermore, the effect of MWCNTs on shape recovery ratio and shape recovery speed of composites has also been systematically investigated.

Effect of Nanofiller Content on Dynamic Mechanical and Thermal Properties of Multi-Walled Carbon Nanotube and Montmorillonite Nanoclay Filler Hybrid Shape Memory Epoxy Composites.

The aim of the present study has been to evaluate the effect of hybridization of montmorillonite (MMT) and multi-walled carbon nanotubes (MWCNT) on the thermal and viscoelastic properties of shape memory epoxy polymer (SMEP) nanocomposites. In this study, ultra-sonication was utilized to disperse 1%, 3%, and 5% MMT in combination with 0.5%, 1%, and 1.5% MWCNT into the epoxy system. The fabricated SMEP hybrid nanocomposites were characterized via differential scanning calorimetry, dynamic mechanical analysis, and thermogravimetric analysis. The storage modulus (E’), loss modulus (E”), tan δ, decomposition temperature, and decomposition rate, varied upon the addition of the fillers. Tan δ indicated a reduction of glass transition temperature (Tg) for all the hybrid SMEP nanocomposites. 3% MMT/1% MWCNT displayed best overall performance compared to other hybrid filler concentrations and indicated a better mechanical property compared to neat SMEP. These findings open a way to develop novel high-performance composites for various potential applications, such as morphing structures and actuators, as well as biomedical devices.