Shape Memory Ability Research Articles

Metallohydrogel with Tunable Fluorescence, High Stretchability, Shape-Memory, and Self-Healing Properties.

Aiming at the problem that the reported smart optical metallohydrogels were limited with poor mechanical properties, we reported here a novel smart optical metallohydrogel (Al-hydrogel) with excellent elongation, shape-memory ability, self-healing property, and controllable fluorescence intensity. The Al-hydrogel was obtained by the HHPMA-Al3+ and carboxylate-Al3+ coordination after one-pot micellar copolymerization of acrylic acid (AAc), acrylamide (AAm), and hydrophobic arylhydrazone-based ligand (HHPMA). This hydrogel was able to extend up to 5000% of its original length without fracture. Its emission intensity was tunable by OH-/H+ or Zn2+/AAc and increased by 500% with 0.1 M OH- or Zn2+. Its tunable fluorescence enabled us to repeatedly pattern it. A reversible system consisting of Fe3+/H+, was implemented to control the shape of the Al-hydrogel, endowing the Al-hydrogel with shape-memory ability. This highly stretchable and multifunctional Al-hydrogel has potential applications in information transmission, wearable devices, and flexible sensors.

Reprocessable and Shape Memory Thermosetting Epoxy Resins Based on Silyl Ether Equilibration

AbstractA novel epoxy thermoset is synthesized with dynamic SiOSi bonds. The thermosetting epoxy resins possess excellent reprocessing, reshaping, and shape memory ability due to the special molecular structure. The material exhibits high tensile strength and a fast stress relaxation process. Surprisingly, it can achieve complete healing and keep stable mechanical property after multi‐processing.

Robust and stretchable self-healing polyurethane based on polycarbonate diol with different soft-segment molecular weight for flexible devices

The self-healing polyurethane films have received significant attention due to their unique shape memory behaviors and the hydrogen bonding arising from the urethane linkages. However, it is still a challenge to develop self-healing polyurethane with transparent, high impact resistance, and excellent shape recovery and mechanical properties. In particular, polycarbonate-based polyurethane shows transparency, weatherability and excellent mechanical properties. In this work, transparent self-healing polyurethane film with robust mechanical properties (breaking stress of 26 MPa and elongation at break of 630%) was synthesized by controlling the different molecular weight of the polycarbonate diol and incorporating dynamic disulfide bonds for reversibility. As a result, the self-healing polyurethane film showed an excellent self-healing ability with 98% healing efficiency and excellent shape memory ability with a recovery efficiency of ˃92%. In addition, we have identified a potential application for the self-healing flexible substrate in flexible electronics because of their stable electrical resistance even upon mechanical deformation.

Influence of free PCL in PCL/PBA-a copolymers and blends on morphology, thermo-mechanical and shape memory properties

Poly(ε-caprolactone) (PCL) and bisphenol A based polybenzoxazine (PBA-a) blends and copolymers differing in the amount of PCL and bonding mode are reported aiming at studying the role of free PCL. Copolymers containing and lacking free PCL were used to understand the influence of PCLs bonding mode on morphology and mechanical properties. XRD and DSC studies revealed that free PCL in the samples increased the degree of crystallinity in contrast to samples containing similar but bound amounts of PCL. Free PCL tends to phase separate leading to discontinuous phases in the appropriate blends in contrast to smooth surfaces and homogeneous copolymer-like samples as confirmed by AFM and SEM micrographs. Reduced crystallinity due to the absence of free PCL results in a less stabilized network and thereby in a diminished material stiffness. In view of shape memory properties, this study revealed that both a widened network due to covalent PCL incorporation and the existence of free and motile PCL facilitating crystallization are important parameters for the occurrence of shape memory abilities in PCL/PBA-a copolymers.

Tunable and Processable Shape-Memory Materials Based on Solvent-Free, Catalyst-Free Polycondensation between Formaldehyde and Diamine at Room Temperature.

Compared with traditional thermosets, malleable thermosets have more applications in aerospace, biotechnology, and construction. Here we report a one-step, solvent-free, catalyst-free polycondensation method between diamine and formaldehyde to prepare a series of malleable hemiaminal dynamic covalent networks (HDCNs). The materials have excellent malleability and reprocessability by hot pressing. The Young's modulus and breaking strength of HDCNs obtained by the polycondensation of formaldehyde and 4,4-diaminodiphenylmethane (MDA) are as high as 1.6 GPa and 60 MPa, respectively, which can be facilely adjusted through the introduction of polyetheramine-400 (PEDA). Moreover, the HDCNs feature the shape memory ability with a recovery ratio above 93.5% and can be recycled by the addition of different monomers. This promising HDCN, prepared from economical raw materials, may have vast applications in industries.

Tough supramolecular hydrogels with excellent self-recovery behavior mediated by metal-coordination interaction

Based on the strategy of dynamic metal-coordination for improving mechanical properties, a further ideal was taken to prepare the tough hydrogels with the combination of self-crosslinking monomer and metal-coordination complexes. Herein, a series of hydrogels of poly(acrylamide-co-acrylic acid-co- N-hydroxymethyl acrylamide) (P(AM-co-AAc-co-NMAM)) consisting the chemical crosslinking induced by NMAM units and the physical crosslinking derived from the coordination complexes of carboxyl-Fe3+ were prepared. The molecular structure was investigated with the ATR-FTIR, Raman and UV-vis spectra. These hydrogels with different water content of 57–93% possess good mechanical performances. The optimal hydrogels possess high tensile strength (8.56 MPa), prominent modulus (15.5 MPa), remarkable toughness (37.85 MJ/m3) and superb tearing energy (7062 J/m2). The tough hydrogels also display excellent self-recovery (95% toughness recovery within 50 min), pH-triggered healing, shape memory and plasticity abilities. These hydrogels having high strength and toughness may broaden range of potential applications in load-bearing soft actuators, flexible electronics, etc.

Smart Superhydrophobic Shape Memory Adhesive Surface toward Selective Capture/Release of Microdroplets.

Controllable manipulation of microdroplets is significant for the microfluidics, biomedical areas, microreactors, and so on; however, until now, reports about no-loss and selective capture/release of different microdroplets are still rare. Herein, we report a new superhydrophobic shape memory adhesive surface that can solve this problem. The surface is prepared by sticking a pillar-structured superhydrophobic polyurethane layer onto a shape memory polyurethane-cellulose nanofiber (PU-CNF) layer. Because of the good shape memory performance of the PU-CNF layer, the obtained surface can memorize and display various microstructure arrangements during the stretching/releasing process. Meanwhile, multiple superhydrophobic adhesive states from low-adhesive rolling performance to high-adhesive pinning performance can be observed on the surface, and all these adhesive states can be reversibly controlled between each other. Based on the smart shape memory ability in surface adhesion, not only traditional in situ capture/release of one microdroplet but also selective capture and release of different microdroplets can be realized. This work reports a new superhydrophobic shape memory adhesive surface; it is envisioned that this smart surface would be a powerful platform for microfluidics systems, complex droplet transportation, biological analysis, and so on.

High strength hydrogels with multiple shape-memory ability based on hydrophobic and electrostatic interactions.

Hydrogels with multiple shape-memory ability have aroused great interest due to their promising applications in various fields. Nevertheless, the weak mechanical performance of most shape-memory hydrogels seriously impedes the practical application in more complex environments. Herein, we reported a novel hydrogel with both high mechanical and multi-shape memory properties composed of stearyl methacrylate (SMA), acrylic acid (AA) and quaternary chitosan (QCH). The electrostatic interactions between AA and QCH together with the hydrophobic interactions of alkyl chains in SMA endowed the hydrogel with great strain-stress and fatigue resistance. Furthermore, due to the reversible destruction and construction of physical cross-links, the prepared hydrogels also exhibited the shape-memory ability in response to different stimuli, such as temperature, pH and NaCl solution. Additionally, the multiple shape-memory effect could be accomplished via programmable combination because of the relatively independent physical interactions in the hydrogels.

Smart behavior of collagen skin: water-sensitive shape memory

Abstract Collagen skin (CS) composed of collagen fiber is environment friendly biomaterial may be responsive to water for shape recovery form their temporarily fixed shape. This water induced smart behavior is regarded as shape memory (SM) ability. In this study, the water induced SM effect of CS and its mechanism regarding to structural component was first time investigated. The fixability under both axial and non-axial deformation are high with the fixation rate reach up to 100% while the shape recovery rate (ranged from 60 % to 98 %) is related to the temporary shape and cycle numbers. The effect of water on mechanical behavior, thermal properties, and structural component (hydrogen bonds) of CS the were systematically examined. The reversible cleavage and reforming of hydrogen bonds within collagen was thereafter proposed as “switch” to trigger the shape deformation and recovery in the hypothesized SM mechanism model. This research is expected to reveal a sophisticated structure not found in literature for extremely smart materials and demonstrate an excellent example of adding value to unwanted, discarded by-products by turning into desired and functional materials

A skin inspired bio-smart composite with water responsive shape memory ability

A skin inspired bio-smart composite was prepared by a facile green “paper-making” method, and it exhibits excellent water responsive shape memory.

A facile and environmentally friendly approach to fabricate hybrid crosslinked nitrile butadiene rubber with comprehensively improved mechanical performances by incorporating sacrificial ionic bonds

It is a challenge to develop advanced elastomers with a combination of high strength, stiffness and toughness by a relatively convenient and environmentally friendly approach. Herein, we reported a facile and environmentally friendly method to prepare hybrid crosslinked nitrile butadiene rubber (NBR) comprised of covalent and ionic bonds, and this hybrid crosslinked rubber possesses comprehensively improved mechanical properties, higher crosslinking density, deformation recovery and shape memory ability. The hybrid crosslinked NBR was prepared by incorporating 2-acrylamido-2-methyl propane sulfonic acid (AMPS) and zinc oxide (ZnO) into NBR via internal milling together with dicumyl peroxide (DCP), and then vulcanization, which is a very simple and environmentally friendly method without extra procedure or treatment. The hybrid crosslinked network in NBR has randomly distributed covalent and ionic bond crosslinks, the strong covalent bonds serve as permanent crosslinks, imparting the elasticity, whereas the weak ionic bonds reversibly rupture and reform, dissipating the energy. Because of the restricting effect of ionic bonds on segmental movement, the hybrid crosslinked NBR shows higher glass transition temperature, lower loss factor peak value and longer relaxation time. By tuning the ionic bond strength via temperature, a much smaller range of linear viscoelastic region is observed for hybrid crosslinked NBR with high content of AMPS and ZnO at a relatively higher temperature, which endows the sample with shape memory ability. This design principle of incorporating ionic bonds to strengthen vulcanized NBR should be applicable to other diene rubber materials.

Effect of percentage of graphene oxide on phase transition of water induced shape memory behavior of PVA-go-PEG hydrogel

Since supramechanical hydrogel was reported, hydrogel has been becoming a popular material to construct specialty features or functions. In this paper, a functional hydrogel made of poly (vinyl alcohol)/graphene-oxide/polyethylene-glycol (PVA-go-PEG) was investigated for the percentage effect of graphene-oxide (GO) on the phase transition of water induced shape memory ability. Experimentally, the hydrogel showed high shape fixed ability at below −5 °C and high shape recovered ratio at above 5 °C for GO content of 0.6%, 0.9% and 1.2%. The temperature range from −5 to 5 °C gives rise to a fixation and recovery dynamics of hydrogel for each GO content, thus the state of aqueous molecules inside hydrogel dominates the dynamic behavior. Characterizations from XRD, FTIR and NMR indicated that the crystalline phase in polymers and the generated hydrogen bonds between GO nanoparticles and macromolecules play key role of netpoints for maintaining hydrogel shape intact. DSC and XRD measured spectra revealed that the phase transition of aqueous molecules in the hydrogel act as switch in the shape memory program. A model for such shape memory mechanism was proposed to explain the effect of GO content on the phase transition of water induced shape memory behavior for the prepared hydrogel. This study inspires to explore high strength and adaptability of hydrogels in application areas of temperature conversion conditions.

Ternary Polymeric Composites Exhibiting Bulk and Surface Quadruple‐Shape Memory Properties

We report the design and characterization of a multiphase quadruple shape memory composite capable of switching between 4 programmed shapes, three temporary and one permanent. Our approach combined two previously reported fabrication methods by embedding an electrospun mat of PCL in a miscible blend of epoxy monomers and PMMA as a composite matrix. As epoxy polymerization occurred the matrix underwent phase separation between the epoxy and PMMA materials. This created a multiphase composite with PCL fibers and a two-phase matrix composed of phase-separated epoxy and PMMA. The resulting composite demonstrated three separate thermal transitions and amenability to mechanical programming of three separate temporary shapes in addition to one final, equilibrium shape. In addition, quadruple surface shape memory abilities are successfully demonstrated. The versatility of this approach offers a large degree of design flexibility for multi-shape memory materials.

A Robust, Self-Healable, and Shape Memory Supramolecular Hydrogel by Multiple Hydrogen Bonding Interactions.

A versatile double-network (DN) hydrogel with two noncovalent crosslinked networks is synthesized by multiple hydrogen bonding (H-bonding) interactions. The DN hydrogels are synthesized via a heating-cooling photopolymerization process by adding all reactants of agar, N-acryloyl glycinamide (NAGA) and N-benzylacrylamide (NBAA) monomers, UV initiators to a single water pot. Poly(N-acryloyl glycinamide-co-N-benzyl acrylamide) (P(NAGA-co-NBAA)) with a triple amide in one side group is synthesized via UV-light polymerization between NAGA and NBAA, forming a strong intermolecular H-bonding network. Meanwhile, the intramolecular H-bonding network is formed between P(NAGA-co-NBAA) and agars. The sol-gel phase transition of agars at 86 °C generates the molecular entanglement network. Such a double network enables the hydrogel high self-healing efficiency (about 95%), good shape memory ability, and high mechanical strength (1.1 MPa). Additionally, the DN hydrogel is completely crosslinked by multiple hydrogen bonds (H-bonds) and the physical crosslinking of agar without extra potential toxic chemical crosslinker. The DN hydrogels find extensive applications in the biomedical materials due to their excellent biocompatibility.

Biobased multiblock copolymers: Synthesis, properties and shape memory behavior of poly(hexamethylene 2,5-furandicarboxylate)-b-poly(ethylene glycol)

A series of novel multiblock copolymers poly(hexamethylene 2,5-furandicarboxylate)-b-poly(ethylene glycol) (PHFEGs) were synthesized from available biobased 2,5-furandicarboxylic acid (FDCA), 1,6-hexanediol (HD), and poly(ethylene glycol) (PEG) by a two-step melt polycondensation method. A systematic investigation of microstructure, molecular weight and its distribution, crystallization behavior, thermal stability of PHFEGs was performed using 1H NMR, GPC, DSC, and TGA, respectively. Moreover, effects of composition on mechanical and shape memory properties of PHFEGs were investigated systematically. Results showed that the weight percentage of PEG in PHFEGs was consistent with that in feed. DSC data showed most PHFEGs were double-crystalline block copolymers. All PHFEGs exhibited the excellent thermal stability. In summary, compared with PHF, the addition of PEG toughened PHF to the extent and endowed the double-crystalline block copolymers with excellent shape-memory ability, suggesting PHFEGs with excellent shape-memory properties have great potential in application.

Shape Memory Properties and Enzymatic Degradability of Poly(ε-caprolactone)-Based Polyurethane Urea Containing Phenylalanine-Derived Chain Extender.

Biodegradable shape memory polymers are promising biomaterials for minimally invasive surgical procedures. Herein, a series of linear biodegradable shape memory poly(ε-caprolactone) (PCL)-based polyurethane ureas (PUUs) containing a novel phenylalanine-derived chain extender is synthesized. The phenylalanine-derived chain extender, phenylalanine-hexamethylenediamine-phenylalanine (PHP), contains two chymotrypsin cleaving sites to enhance the enzymatic degradation of PUUs. The degradation rate, the crystallinity, and mechanical properties of PUUs are tailored by the content of PHP. Meanwhile, semicrystalline PCL is not only hydrolytically degradable but also vital for shape memory. Good shape memory ability under body temperature is achieved for PUUs due to the strong interactions in hard segments for permanent crosslinking and the crystallization-melt transition of PCL to switch temporary shape. The PUUs would have a great potential in application as implanting stent.

Poly(N,N-dimethylacrylamide-octadecyl acrylate)-clay hydrogels with high mechanical properties and shape memory ability

As a type of important intelligent materials, shape memory hydrogels (SMHs) have gathered a lot of interest due to their promising applications. However, preparing SMHs with excellent mechanical properties still remains a big challenge. In this study, a new type of SMHs with excellent mechanical strength is created. The SMHs were prepared by free radical micellar polymerization of hydrophobic monomer (octadecyl acrylate) with gelatin as emulsifier in a aqueous system containing hydrophilic monomer (N,N-dimethylacrylamide, DMA) and clay as reinforcing filler. The polymerization provided physically cross-linked network structures constructed by two non-covalent interactions, i.e. hydrophobic association formed by monomer units and multiple H-bonds among inorganic clay, gelatin and DMA. A judicious combination of the two physically cross-linked networks significantly improved the mechanical strength of hydrogels. More interestingly, the hydrogels demonstrated shape memory behavior due to the hydrophobic poly(octadecyl acrylate) domains. The novel SMHs are expected to find practical applications as biomaterials.

Cellulose nanofibers/polyurethane shape memory composites with fast water-responsivity.

Water-responsive shape-memory materials, especially cellulose nanofibers (CNF)-polymer composite materials, have been rapidly developed in recent years for designing new generation water-responsive sensors, actuators, biomedical devices, etc. However, achieving fast-responsivity is still a challenge. Herein, by simply incorporating CNF into a thermoplastic elastomer polyurethane (PU) matrix, we report a material, namely, PU/CNF nanocomposite, which has a good shape memory effect (SME) and fast responsivity. The effects of the CNF content on water absorption, shape memory behavior and mechanical properties of the composites are studied, and it can be found that when the CNF content reaches 30%, materials with good SME, such as high shape fixing and shape recovery ratio (higher than 90%) and fast responsivity (less than 1 min), can be obtained. The excellent SME can be ascribed to the combined effect of the good elastic recovery property of the PU matrix, high modulus of CNF and the rapid water uptake behavior of PU/CNF, based on which a rigid CNF percolation network in the elastomeric PU can be formed/destroyed by the desorption/absorption of water molecules repeatedly. Finally, based on the excellent rapid water-triggered shape memory performance, we also demonstrate some applications of the material, for example, as a "mechanical hand" to grasp solid objects in water and as a smart sealing film to build a controllable release system.

Collagen skin, a water-sensitive shape memory material.

By collagen skin (CS) we mean a material with only collagen fibres in a natural animal skin. Following continuous research of collagen biochemistry, CS shows its superiority in various applications such as leather, wound dressing and absorbing materials. This study discovered the water-induced shape memory effect of CS and exploited the corresponding mechanisms in terms of its molecular and morphological structure. It is revealed that CS can completely fix into a temporary shape and retain a high shape recovery ratio (>90%) under repeatable memory cycles. The reversible cleavage and re-formation of interpeptide hydrogen bonds can work as a "switch" to trigger the shape deformation while peptide chains together with cross-links within collagen work as a "netpoint" for shape recovery. Such water-responsive adaptable mechanical behaviour (a switchable modulus) allows CS to be either used as a robotic handle for loading or as a flexible material for covering. This discovery will provide new opportunities and guidance in emerging research areas such as shape memory devices, biomimetic actuators and artificial skins as well as the physiological function of an animal skin.

Hydrogen bond reinforced poly(1-vinylimidazole-co-acrylic acid) hydrogels with high toughness, fast self-recovery, and dual pH-responsiveness

As a promising structural material, various tough hydrogels have been developed in recent years by incorporating different kinds of noncovalent bonds as the energy-dissipating segments. Among these systems, it is challenging to use hydrogen bonds due to their instability in aqueous environment. Here we report a tough hydrogel of poly(1-vinylimidazole-co-acrylic acid) (P(VI-co-AAc)) reinforced by the robust hydrogen bonds between imidazole and carboxylic acid groups. The hydrogels with different composition possess excellent mechanical performances, with tensile breaking stress, breaking strain, Young's modulus, and tearing fracture energy being 0.3–1.8 MPa, 920–1400%, 0.1–0.7 MPa, and 0.7–5.6 kJ/m2, respectively, which are well maintained over a wide pH range (3 ≤ pH ≤ 10). The high stability of hydrogen bonds should be rooted in the strong proton donor-acceptor pair. However, when pH ≤ 2 or pH ≥ 11, the gels become highly swelled due to the dissociation of imidazole and carboxylic acid groups, respectively, accompanying with dramatic decrease in mechanical properties. The tough hydrogels also show temperature- and deformation rate-dependent mechanical properties, as well as shape memory and self-recovery abilities, due to the dynamic nature of noncovalent bonds. Such hydrogels with remarkable mechanical properties and dual pH-responsiveness should find applications in load-bearing soft actuators, flexible electronics, etc.