Excellent Shape Memory Properties Research Articles

Nanocomposite electrospun fibers of poly(ε-caprolactone)/bioactive glass with shape memory properties

Electrospun fibers of shape memory triethoxysilane-terminated poly(epsilon-caprolactone) (PCL-TES) loaded with bioactive glasses (BG) are here presented. Unloaded PCL-TES, as well as PCL/BG nanocomposite fibers, are also considered for comparison. It is proposed that hydrolysis and condensation reactions take place between triethoxysilane groups of the polymer and the silanol groups at the BG particle surface, thus generating additional crosslinking points with respect to those present in the PCL-TES system. The as-spun PCL-TES/BG fibers display excellent shape memory properties, in terms of shape fixity and shape recovery ratios, without the need of a thermal crosslinking treatment. BG particles confer in vitro bioactivity to PCL-based nanocomposite fibers and favor the precipitation of hydroxycarbonate apatite on the fiber surface. Preliminary cytocompatibility tests demonstrate that the addition of BG particles to PCL-based polymer does not inhibit ST-2 cell viability. This novel approach of using bioactive glasses not only for their biological properties, but also for the enhancement of shape memory properties of PCL-based polymers, widens the versatility and suitability of the obtained composite fibers for a huge portfolio of biomedical applications.

Catalyst-free self-healing bio-based vitrimer for a recyclable, reprocessable, and self-adhered carbon fiber reinforced composite

Carbon fiber reinforced composites (CFRPs) have the problems of over-reliance on petroleum-based products and difficult recycling of Carbon fibers (CFs). To solve these problems, a degradable Epoxidized menthane diamine–adipic acid (EMDA–AA) vitrimer matrix for CFRPs is obtained from bio-based menthane diamine and adipic acid. Owing to the rigid six-membered ring structure of EMDA, the EMDA–AA vitrimer matrix exhibits good mechanical properties. With the autocatalysis of tertiary amines in the EMDA–AA vitrimer, the vitrimer network can achieve topological rearrangement via dynamic transesterification reactions without additional catalysts. The fabricated vitrimer exhibits excellent self-healing, shape memory and reprocessing properties. Moreover, EMDA–AA–CF composite with EMDA–AA epoxy vitrimer as the matrix resin shows excellent reprocessing, shape memory and self-adhesive properties. In addition, the EMDA–AA vitrimer matrix resin can be rapidly degraded into polyols by amination reaction with ethanolamine under mild conditions, resulting in the efficient recycling of Carbon fibers (CFs). Scanning electron microscopy, Raman spectroscopy, and tensile tests show that the recycled CFs have almost the same chemical structure and mechanical properties as the virgin CFs. This study can assist in the preparation of green and high-strength CFRPs using recyclable bio-based epoxy vitrimer matrices and provide a strategy for efficient recycling of CFs from CFRPs.

Spinodal decomposition and martensitic transformation of the high manganese Mn‒xCu alloys fabricated by additive manufacturing

Mn‒Cu-based shape memory alloys have great potential owing to their excellent shape memory properties, high damping capacity, low production cost, and ease of fabrication. In this study, a selective laser melting (SLM) technique was used to design and manufacture a series of high-manganese Mn‒xCu (x=10–40 wt.%) alloys by blending the elemental powders. Spinodal decomposition and martensitic transformation of the alloys during SLM and heat treatment (HT) processes were investigated and correlated with the tensile, damping, and shape memory properties of the alloys. The results showed that γ‒(Mn, Cu) was the main constituent phase in all of samples subjected to SLM only (hereafter as-SLMed) and the SLMed + HTed Mn‒Cu alloys. The effect of the chemical composition and manufacturing process on the phase structural evolution of the Mn‒Cu alloys were monitored. The spinodal decomposition, martensitic transformation and α-Mn precipitation reactions occurred in the as-SLMed Mn‒xCu samples with x = 15–20 wt.%, x < 30 wt.%, and x < 15 wt.%, respectively. The tensile properties of the as-SLMed Mn‒xCu alloys followed a general downward trend with increasing Mn content, while the corresponding damping capacity and shape memory recovery strain showed the opposite trend. HT can promote spinodal decomposition and martensitic transformation to form an equiaxed γ‒(Mn, Cu) grain, parallel twin plate structure, tweed microstructure, and α-Mn precipitate in the alloys. The SLMed + HTed Mn‒30% Cu alloy displayed a better combination of tensile properties (764, 540 MPa, and 12.5%), damping capacity (tan δ = 0.072 at a strain amplitude of 900 × 10−6), and one-way and two-way (εsme = 1.6% and εtw = 0.7% under the pre-bending strain of 10%) shape memory properties. In addition, the performance of the as-SLMed Mn‒xCu (x = 15–20 wt.%) and SLMed + HTed Mn‒xCu (x = 30 wt.%) alloys is comparable with and even superior to those of multi-component and commercial Mn‒Cu-based alloys, respectively.

An investigation on the performance of epoxy vitrimers based on disulfide bond

AbstractThe research for self‐healing epoxy vitrimers has attracted a lot of attention over the years. However, due to the special structure of the epoxy molecular chain, which limits its movement ability, epoxy vitrimers usually need to be healed under high temperature and pressure. So it is relatively difficult to healing. Here, a new type of cross‐linked network is obtained by cross‐linking hydrogenated bisphenol A epoxy resin and polypropylene glycol diglycidyl ether (PPGDGE) as hard and soft segments with an amine curing agent containing disulfide bonds. The addition of PPGDGE can improve the mobility of the molecular chains in the cross‐linked network we designed, thereby obtaining a novel epoxy vitrimers with better healing properties. The effect of PPGDGE content on the mechanical properties and self‐healing properties of the material was studied. The ideal sample HEP‐3 has a good tensile strength (33.4 MPa) and a completely broken sample can achieve a good self‐healing rate (71.5%) at 100°C for 3 h without high pressure. In addition, the ideal sample also has excellent shape memory properties (shape memory fixity ratio Rf = 97.1%, shape recovery ratio Rr = 94.9%). Moreover, due to the existence of the dynamic disulfide bond introduced by the curing agent 2‐aminophenyl disulfide, compared with traditional thermosetting epoxy resins, epoxy vitrimers prepared by us can be reshaped or postprocessed like thermoplastic polymers under specific conditions. These characteristics will become the guarantee for expanding the application range of epoxy materials.

Shape recovery and reprocessable polyurethanes crosslinked with double decker silsesquioxane via Diels-Alder reaction

In this work, a novel organic-inorganic polyurethane thermoset crosslinked with a difunctional polyhedral oligomeric silsesquioxanes (POSS) was synthesized via Diels-Alder (DA) reaction. First, a linear multi-segmented polyurethane with furan rings in the main chain (PU–F) was synthesized via the step-growth polymerization of 4,4′-diphenylmethane diisocyanate (MDI) with poly(tetramethylene ether glycol) (PTMEG) and 2,5-furandimethanol. Second, 3,13-dimaleimido octaphenyl double decker silsesquioxane (3,13-dimaleimido DDSQ) was synthesized and used as the crosslinker of the linear PU-F. By adjusting the feed ratio of the difunctional POSS macromer (i.e., 3,13-dimaleimido DDSQ) to PU-F, the organic-inorganic PU thermosets (denoted DAPU-DDSQs) with variable densities of crosslinking were obtained. It was found that the DAPU-DDSQs were microphase-separated and that POSS cages were aggregated into the spherical microdomains, the sizes of which were 20–40 nm in diameter. It was demonstrated that the crosslinking in the DAPU-DDSQs had two aspects of contribution: i) the chemical crosslinking with D-A adducts between 3,13-dimaleimido DDSQ and PU-F and ii) the physical crosslinking with POSS microdomains via the aggregation of POSS cages. The DAPU-DDSQs displayed the improved thermomechanical properties with increasing 3,13-dimaleimido DDSQ incorporation. Owing to the formation of crosslinked structures, the DAPU-DDSQs displayed excellent shape memory properties. More importantly, the DAPU-DDSQs displayed the reprocessing (or recycling) properties at the temperature of retro DA reaction. The reprocessing properties of the DAPU-DDSQs can be utilized to re-edit the original shapes of shape memory PUs.

Digital Light Processing 4D Printing of Transparent, Strong, Highly Conductive Hydrogels.

Hydrogels have excellent biocompatibility, transparency, stretchability, and ionic conductivity, but their fabrication through photopolymerization-based 3D printing is limited due to the low solubility of hydrophobic photoinitiators and lack of efficient hydrophilic photoinitiators. Herein, a type of microemulsion is synthesized and the common hydrophobic photoinitiator can be adopted and finally, a series of transparent hydrogels with high strength (up to 22.9 MPa), elasticity (up to 583%), and ionic conductivity (up to 9.64 S m-1) are fabricated through digital light processing 3D printing technology. Objects with complex structures and a high printing resolution are printed. Hydrogels with both high strength and high ionic conductivity are obtained through chemical crosslinking and ion coordination effect. Dual-material 3D printing is applied to package the hydrogel with elastomers. Due to the high sensitivity and reliability under both stretching and compressive deformation, the hydrogel sensors are applied to monitor various human motions. In addition, the hydrogel exhibits solvent-induced dehydration and excellent water-activated shape memory properties, which are greatly beneficial for its storage and applications in the biomedical field.

Shape memory composite hydrogel based on sodium alginate dual crosslinked network with carboxymethyl cellulose

Based on the need for whole biomass hydrogels in biomedical applications, a dual-crosslinked hydrogel based on natural product sodium alginate (SA) were prepared. Dual-crosslinked was formed by SA crosslinking with glutaraldehyde (GA) and calcium ion (Ca2+), and the introduction of carboxymethyl cellulose (CMC) into crosslinked network to obtain excellent water-induced shape memory property. The chemical and physical crosslinking net points, formed by GA and Ca2+, as the stationary phase to hold the original shape; While hydrogen bonding interactions between CMC and SA as the switcher to fix the temporary shape in air and driving the recovery in water. The shape fixing and recovery ratio can reach 90%. The dual crosslinking effect and introduction of CMC, also strengthen the mechanical properties of hydrogels and change the size of pores in hydrogels. Cell culture experiment showed that these hydrogels are good at biocompatibility and can control the cell differentiation by adjusting the size of pores in hydrogel networks. For its excellent performance, these hydrogels can be potentially applied as biological device materials in the field such as soft tissue engineering, controlled drug release and bioactuators.

Polyrotaxane: New generation of sustainable, ultra-flexible, form-stable and smart phase change materials

The development of thermal energy storage materials is the most attractive strategy to harvest the solar energy and increase the energy utilization efficiency. Phase change materials (PCMs) have received much attention in this research field for several decades. Herein, we reported a new kind of PCM micro topological structure, design direction, and the ultra-flexible, form-stable and smart PCMs, polyrotaxane. The structure of polyrotaxane was fully confirmed by 1H nuclear magnetic resonance, attenuated total reflection-fourier transform infrared and X-ray diffraction. Then the tensile properties, thermal stability in the air, phase change energy storage and shape memory properties of the films were systematically analyzed. The results showed that all the mechanical performance, thermal stability in air and shape memory properties of polyrotaxanes were enhanced significantly compared to those of polyethylene oxide (PEO). The form stability at temperatures above the melting point of PEO significantly increased with the α-CD addition. Further with the high phase transition enthalpy and excellent cycle performance, the polyrotaxane films are therefore promising sustainable and advanced form-stable phase change materials for thermal energy storage. Notably, its ultra-high flexibility, remolding ability and excellent shape memory properties provide a convenient way for the intelligent heat treatment packaging of complex and flexible electronic devices. In addition, this is a totally novel insight for polyrotaxane application and new design method for form-stable PCMs.

Sustainable alternative for bisphenol A epoxy resin high-performance and recyclable lignin-based epoxy vitrimers

Epoxy vitrimers based on dynamic covalent bond chemistry have been developed in recent years as they can solve the problem of recycling and reuse for traditional thermosetting materials. Lignin as an aromatic raw material has recently used to partially replace commercial diglycidyl ether bisphenol A (DGEBA) in the epoxy vitrimers, however, it is difficult to achieve high lignin replacement rate, excellent mechanical property and reprocessability in a facile method at the same time. In this work, high mechanical strength and lignin replacement rate of lignin-based vitrimers were successfully synthesized by gradually replacing DGEBA with glycidyl ether of lignin. The mechanical properties of the lignin-based epoxy vitrimers can be tuned by different ratios of glycidyl ether of lignin and DGEBA, the Young’s modulus and tensile strength of those vitrimers can reach up to 2.00 GPa and 46.8 MPa, respectively. It's worth noting that the tensile strength and elongation at break were 39.5 MPa and 5.81 % when the lignin replacement rate was 100 %. All the lignin-based epoxy vitrimers exhibit excellent self-healing, reprocessing and shape memory property. The crack widths of the vitrimers were repaired over 80 % in just 5 min at 160 ℃, the tensile strength can reach up to 97 % healing efficiency after being hot reprocessed. The great properties of lignin-based epoxy vitrimers indicate that they can be used as renewable alternatives for bisphenol A epoxy vitrimers.

Superhydrophobic shape memory film with switchable adhesion to both water and solid

• The membrane can regulate the adhesion to both solid and water droplets. • Both microstructure and surface energy were crucial for special adhesion. • The membrane can be used to in-situ capture/release both solid and liquid. Recently, inspired by gecko’s smart adhesion, many intelligent adhesive surfaces have been reported. However, all of these are focused on single adhesion to liquid or solid, to obtain a surface with switchable adhesion to both water and solid is still a challenge. Herein, by coating a layer of polyurethane (PU) with micropillar structures onto a shape memory polymer (SMP) substrate to imitate the skin and muscle of the gecko’s foot, respectively, we report a new shape memory film that can repeatedly display curved/unfolded shapes with the help of the excellent shape memory property. Research results indicate that besides the variation of the contact area, a proper surface energy is also crucial for the special adhesion. Furthermore, based on the smart adhesive performance, in-situ capture/release of both solid and liquid were demonstrated by using the film. This research proposes a film with tunable adhesion to both solid and liquid, which provides a fresh idea for designing novel adhesive film, and meanwhile broadens the application of the adhesive materials.

Flexible copper foam-based phase change materials with good stiffness-toughness balance, electro-to-thermal conversion ability and shape memory function for intelligent thermal management

Flexible phase change composites (PCMs) show great potential for the thermal management in confined spaces. However, some defects such as low mechanical strength, low thermal/electrical conductivity have limited their advanced applications. In this work, flexible PCMs based on copper-coated melamine foam (MF@Cu) and polyethylene glycol (PEG) were prepared. The MF@Cu foams obtained by electroless copper plating on porous MF were used to encapsulate PEG. Due to the continuous copper network, MF@Cu foams not only enhanced the mechanical loading and encapsulation capacities of PCMs, but also imparted PCMs with electro-to-thermal conversion capability. Moreover, the MF@Cu/PEG PCMs exhibited excellent thermal-induced shape memory property by combining the elasticity of foams and the phase change of PEG. This multifunctional MF@Cu/PEG PCMs were further applied for the thermal management of electronic devices. Therefore, this study provides a strategy to prepare flexible PCMs with excellent integrated performances, which is beneficial to broaden the applications of PCMs.

Multi‐functional polyurethane composites with self‐healing and shape memory properties enhanced by graphene oxide

AbstractDevelopment of shape memory polymer materials with integrated self‐healing ability, shape memory property, and outstanding mechanical properties is a challenge. Herein, isophorone diisocyanate, polytetramethylene ether glycol, dimethylglyoxime, and glycerol have been used to preparation polyurethane by reacting at 80°C for 6 h. Then, graphene oxide (GO) was added and the reaction keep at 80°C for 4 h to obtain polyurethane/GO composite with self‐healing and shape memory properties. Scanning electron microscopy shows that the GO sheets were dispersed uniformly in the polyurethane matrix. The thermal stability was characterized by thermogravimetric analyses. The tensile test shows that the Young's modulus of the composites increases from 38.57 ± 4.35 MPa for pure polyurethane to 95.36 ± 10.35 MPa for the polyurethane composite with a GO content of 0.5 wt%, and the tensile strength increases from 6.28 ± 0.67 to 15.65 ± 1.54 MPa. The oxime carbamate bond and hydrogen bond endow the composite good self‐healing property. The healing efficiency can reach 98.84%. In addition, the composite has excellent shape memory property, with a shape recovery ratio of 88.6% and a shape fixation ratio of 55.2%. This work provides a promising way to fabricate stimulus‐responsive composite with versatile functions.

Preparation of chitosan biguanidine/PANI-containing self-healing semi-conductive waterborne scaffolds for bone tissue engineering

Electrically conducting self-healing scaffolds are known as a new series of intelligent biomaterial for regulating Human Adipose Mesenchymal Stem Cells biological behaviors, especially their differentiation to bone cells. Herein, we developed a novel hydrophilic semi-conductive chitosan derivative (CP) and loaded it into the self-healing waterborne polyurethane structure, as a new osteogenic agent. The fabricated scaffolds exhibited excellent shape memory properties with shape fixity (> 97 %) and shape recovery ratio (> 98 %) with excellent self-healing value (> 93 %) at a temperature close to the body temperature. The results of MTT, cell attachment, alkaline phosphatase activity, and alizarin red staining analysis demonstrated that the CP-contained scaffolds promote proliferation of hADSCs and matrix mineralization. Also, by introducing the CP the gene expression level of COL-1, ALP, RUNX2, and OCN were significantly enhanced, in line with matrix mineralization. These multifunctional engineered constructs are promising biomaterials for repairing various bone defects.

Synthesis of shape memory electroconductive polyurethane with self-healing capability as an intelligent biomedical scaffold for bone tissue engineering

Electrically conductive polymeric materials are known as a special class of intelligent bioactive materials capable of regulating cellular behaviors of human adipose-derived mesenchymal stem cells, especially their differentiation to bone tissue. Here the synthesis of electroactive self-healing polyurethane scaffolds (PUAT) with shape memory ability based on aniline trimer and disulfide bonds are reported. The prepared scaffolds were characterized by FTIR, tensile test, water contact angle, electrical conductivity, and degradation test. The PUAT5 scaffold (containing 5% aniline trimer) exhibited excellent self-healing (>95%) and shape memory properties with shape recovery ratio (>95%, ~30 s at 40 °C) and shape fixity ratio (>96%) at a temperature close to the physiological temperature. The PUAT5 significantly promote the human adipose-derived mesenchymal stem cell adhesion and proliferation compared to PUAT0 and TCP. Also, the gene expression level of RUNX2, COL1, ALP, and OCN of PUAT5 were significantly enhanced after 21 days, in line with the increased maturity of extracellular matrix and mineralization. Furthermore, almost 75% of the bone volume has been recovered at 8 weeks post-surgery while the control group showed 26% recovery. The In vitro and In Vivo results suggested that these multifunctional engineered constructs have great potential in repairing various bone defects.

Design and fabrication of thermoplastic elastomer with ionic network: A strategy for good performance and shape memory capability

Smart elastomeric materials with outstanding performance and multifunctional properties are highly desirable for many potential applications. Here, strong shape memory materials were prepared based on poly(vinylidene fluoride)/carboxylated nitrile butadiene rubber (PVDF/XNBR) blends containing ionic crosslinks. The ionic clusters in the blends improved mechanical properties and elasticity of the shape memory blends. Interestingly, the elongation at break of this blend increased with rubber content without deterioration of tensile strength. A remarkable improvement of mechanical performance of the blends arose from the reinforcing cite and dynamic mobility of ionic clusters in XNBR matrix. The combination of blending and ionic cross-linking not only improved mechanical properties but also improved shape memory performance. The blends prepared in this work exhibited thermally triggered shape memory because both phases are responsible for shape fixity and recovery. As a consequence, the blends with high XNBR content showed high shape recovery (90%). Finally, this work provides an efficient approach to design new ionic crosslinked blends with excellent mechanical properties and shape memory properties.

Thermo-Responsive Shape Memory Behavior of Methyl Vinyl Silicone Rubber/Olefin Block Copolymer Blends via Co-Crosslinking

Abstract Shape memory polymers (SMPs) are developed by blending and cross-linking polymers which include crystalline domains and cross-linked networks. In this paper, we describe the morphology, thermal and shape memory behavior of methyl vinyl silicone rubber (MVMQ)/olefin block copolymer (OBC) blends prepared by a melt-blending and chemical cross-linking method. MVMQ without crystalline domains could not hold its temporary shape. After introducing the OBC, the obtained blends exhibited excellent dual shape memory properties. The cross-linking networks of MVMQ acted as reversible domains, while crystalline regions of OBC worked as fixed domains. When the blending ratio of MVMQ/OBC was 50/ 50, the blend had both a high shape fixity ratio and shape recovery ratio.

Multi-recyclable Shape Memory Supramolecular Polyurea with Long Cycle Life and Superior Stability

Shape memory polymers (SMPs) have attracted a wide range of interest owing to their technological impact. It remains a great challenge to construct SMPs with both excellent shape memory recoverability and durability and satisfactory reprocessability. Herein, by incorporating low-content noncovalently bonded linkers into the covalent network of SMP, a shape memory supramolecular polyurea (SMSP) with excellent reprocessability and shape memory properties has been successfully constructed. The SMSP was endowed with remarkable shape memory performance and solvent resistance owing to its covalent cross-linking. It displayed outstanding shape memory cycle stability with above 92.7% of shape recovery ratio even after 20 consecutive shape memory cycles. In addition, SMSP could be recycled for 7 times without clear decline in mechanical properties, exhibiting excellent multiple reprocessability. It is anticipated that this work will provide a new insight into designing SMPs with excellent reprocessability and cycle stability.

A foldable compact actuator based on an oxetane liquid crystal network

The fabrication of a foldable, compact liquid crystal actuator based on oxetane reactive monomers is reported. The free-standing liquid crystal network films are optically clear, flexible, and have easily tunable anisotropic thermal and mechanical properties. The oxetane liquid crystal networks demonstrate excellent one-way shape memory properties, with good temporary shape fixation and complete shape recovery. In addition, the flexible polyether backbone of the oxetane liquid crystal networks results in large two-way bending actuation, with the actuators showing rotation inversion and making multiple rotations. Both actuation mechanisms could be combined to form actuators that can be temporarily folded before fully recovering to their original shape prior to use. As a result, the oxetane liquid crystal networks have great potential as foldable, compact actuators.

Microstructural behaviour and electrochemical characterization of Nitinol alloy subjected to constrained groove pressing and heat treatment

Nitinol, owing to its excellent shape memory properties, superelasticity and high corrosion resistance is an ideal choice for orthodontic implants. Its mechanical and electrochemical properties can be further improved by suitable mechanical processing and heat treatment. The present work shows the effect of Constrained Groove Pressing (CGP) and post CGP heat treatment at three different temperatures of 350°C, 450°C and 550°C on the electrochemical behaviour of Nitinol. The Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PDP) studies of water quenched, CGP processed and CGP and heat treated specimens were analysed and it was observed that the heat treated samples showed an improvement in polarization resistance in the order of 15%–42%, with the highest resistance observed for the specimen heat treated at 350°C. An overall improvement in the microhardness, post CGP treatment was also observed. The microstructures of all the samples were observed using optical microscopy to validate the change in mechanical and electrochemical properties. The results of these studies showed that CGP led to an increase in dislocation density and grain refinement, which were responsible for improvement of mechanical properties. The post CGP heat treatment led to nucleation and growth of intermetallic compounds Ni4Ti3 and Ni3Ti whose presence was confirmed by XRD and were responsible for the modification in Nitinol’s electrochemical behaviour.

SEBS-based composite phase change material with thermal shape memory for thermal management applications

The employ of phase change materials (PCMs) provides a potential selection for thermal energy storage (TES) and thermal management (TM). The main reasons hinder the application are the low thermal conductivity, leakage and strong rigidity of PCMs. In this paper, a thermal shape memory composite phase change material composed of triblock copolymer (SEBS), paraffin (PA) and expanded graphite (EG) is prepared. The composite phase change materials are characterized by SEM, FTIR, DSC and TGA. The shape stability and thermal shape memory mechanism are studied in detail through rheological analysis. The results show that the prepared composite exhibit superior shape stability and thermal stability. Furthermore, the existence of SEBS can effectively suppress the effect of natural convection and improve the thermal stratification of the composite along the direction of gravity, and the composite material exhibited excellent thermal shape memory property. In addition, the existence of EG can enhance the heat transfer performance of the composite material.