Multi-shape Memory Effect Research Articles

Hygroscopic Tunable Multishape Memory Effect in Cellulosic Macromolecular Networks with a Supramolecular Mesophase

Tunable multishape memory polymers offer intriguing opportunities for memorizing multiple temporary shapes with tunable transition temperatures from one material composition. However, such multishape memory effects have been exclusively correlated with the thermomechanical behaviors of polymers, significantly limiting their applications in heat-sensitive scenarios. Here we report a nonthermal tunable multishape memory effect in covalently cross-linked cellulosic macromolecular networks, which spontaneously organize into supramolecular mesophases by water evaporation induced self-assembly. The supramolecular mesophase endows the network with a broad, reversible hygromechanical response combined with a unique moisture memory effect at ambient temperature, enabling diverse multishape memory behaviors (dual-, triple-, and quadruple-shape memory) under highly tunable and independent control of relative humidity (RH) alone. Significantly, such a hygroscopic tunable multishape memory effect readily extends the implications of shape memory polymers beyond the conventional thermomechanical regimes with potential advantages for biomedical applications.

Catalyst-Free Vitrimer Cross-Linked by Biomass-Derived Compounds with Mechanical Robustness, Reprocessability, and Multishape Memory Effects.

Vitrimerization of thermoset polymers plays an important role in addressing resource recovery and reuse. Vitrimer elastomers with good mechanical properties often require well-designed crosslinking agents or fillers, but this increases processing complexity or reduces vitrimer dynamic properties. In this report, a simple green strategy to build a strong vitrimer elastomer is designed. Commercially available epoxidized natural rubber (ENR) is cross-linked with biomass-derived D-Fructose 1,6-bisphosphoric acid to get a vitrimer elastomer cross-linked by β-hydroxy phosphate ester bonds and has abundant hydrogen bonds. Hydrogen bonds can preferentially break and dissipate energy under external forces, which makes the sample robust. The topological network can be reformed at high temperatures through the dynamic exchange of β-hydroxy phosphate ester bonds, which gives the material malleability and recyclability. In addition, through the strategy of combining reprocessing and welding, multiple shape memory effects can be achieved in one postprocessing step. Considering that a variety of commercially available epoxy polymers are easily available, it is believed that this strategy can be a simple and versatile way to enable commercial epoxy polymers to achieve green crosslinking through biomass crosslink agents, which results in robust and recyclable vitrimers based on β-hydroxy phosphate bonds.

Negatively thermodynamic toughening in double network hydrogel towards cooling-triggered multi-shape memory effect

This paper explores fundamental mechanisms of negatively thermodynamic toughening during microphase separations of double network (DN) hydrogels, which have dynamically coordinating components with adjustable swelling behavior and designable multi-shape memory effect (multi-SME). Based on the Flory–Huggins theory and Fick’s second law, a thermodynamic model is formulated to study the diffusive dynamics, cooling-triggered multi-SME and mechanical toughness of the DN hydrogels. The negatively thermodynamic toughening effect of the DN hydrogels is strongly dependent on their water concentration, hydrophobic transition and microphase separation. Finally, effectiveness of this new model is demonstrated by applying it to predict dual-SME, quadruple-SME and cooling-triggered shape memory behaviors of DN hydrogels. This study provides a methodology for the design of shape memory DN hydrogels with tunable, giant and programmable cooling-triggered multi-SME.

Shape Memory Polymer Fibers: Materials, Structures, and Applications

Shape memory polymer (SMP) is a kind of material that can sense and respond to the changes of the external environment, and its behavior is similar to the intelligent reflection of life. Electrospinning, as a versatile and feasible technique, has been used to prepare shape memory polymer fibers (SMPFs) and expand their structures. SMPFs show some advanced features and functions in many fields. In this review, we give a comprehensive overview of SMPFs, including materials, fabrication methods, structures, multifunction, and applications. Firstly, the mechanism and characteristics of SMP are introduced. We then discuss the electrospinning method to form various microstructures, like non-woven fibers, core/shell fibers, hollow fibers and oriented fibers. Afterward, the multiple functions of SMPFs are discussed, such as multi-shape memory effect, reversible shape memory effect and remote actuation of composites. We also focus on some typical applications of SMPFs, including biomedical scaffolds, drug carriers, self-healing, smart textiles and sensors, as well as energy harvesting devices. At the end, the challenges and future development directions of SMPFs are proposed.

Multi-modal commutative dynamics in semi-crystalline polymers undergoing multiple shape memory behavior

Semi-crystalline polymers offer great opportunities for design and tuning of multi-shape memory effect (multi-SME) through their programmable melting transitions. However, coexistence of amorphous and crystalline components as well as their multiple interfaces results in complex cooperative dynamics. In this study, we propose a one-dimensional multi-modal dynamic model to describe the commutative and cooperative dynamics in semi-crystalline shape memory polymers undergoing multi-SME. A three-phase model and Takayanagi principle are firstly applied to study the cooperative dynamics of amorphous/crystalline components and their interfaces. Phase transition theory and modified Avrami theory are used to model the cooperative dynamics of glass and melting transitions, respectively. Commutative dynamics and glass/melting transitions are further investigated to achieve on-demand multi-SME and shape recovery behaviors. Finally, effectiveness of the newly established model was demonstrated to predict triple-SMEs and quadruple-SMEs in semi-crystalline polymers reported in literature, and the theoretically obtained results show good agreements with the experimental ones.

Fe3+, NIR light and thermal responsive triple network composite hydrogel with multi-shape memory effect

Recently, artificial intelligence has driven the development of shape memory hydrogels (SMHs) with complicated environmental sensitive abilities. However, current SMHs possess fundamental limitations in terms of their responsive performance and mechanical properties, which severely limits their practical applications. Herein, a triple-network composite hydrogel with Fe3+, NIR light, and thermal responsive abilities is fabricated and developed as multi-SMHs, named as PDA/CMC-Fe3+/PVA hydrogel. In this hydrogel system, the interpenetrating triple network are formed by chemically-physically co-crosslinked polyvinyl alcohol (PVA) networks, and physically crosslinked carboxymethyl cellulose (CMC)-Fe3+ network. In addition, polydopamine (PDA), which is introduced into the hydrogel system by in-situ polymerization way, could not only serve as an energy converter to endow the hydrogel with NIR-based responsive ability, but also enhance its mechanical strength. Remarkably, the as-prepared PDA/CMC-Fe3+/PVA hydrogels with the optimized compositions exhibit relatively excellent mechanical properties (tensile strength = 163.48 ± 10.42 kPa, elastic modulus = 45.42 ± 3.96 kPa, toughness = 165.02 ± 7.01 kJ/m3) due to the synergistic effect of the triple-network and PDA doping. Moreover, the composition of the hydrogel is optimized to realize a stable temporary shape and rapid recovery to the permanent shape, which is controlled by Fe3+, NIR irradiation, and thermal stimulus. The resultant hydrogels exhibit not only multi-stimulus responsive abilities, but also programmable multi-shape memory properties. The versatile shape memory properties endow potential for the materials to be used in the field of artificial intelligence.

Epoxy-vitrimer composites based on exchangeable aromatic disulfide bonds: Reprocessibility, adhesive, multi-shape memory effect

Vitrimers are a class of polymers integrating the advantage of thermosets and thermoplastics. The dynamic crosslinking networks endow vitrimers with interesting rheological behavior and functional applications, such as self-healing, thermally plasticity and shape memory properties. Here, based on the dynamic aromatic disulfide bonds, a series of vitrimer composites with reconfigurability and tunable mechanical properties were successfully prepared. Using the time-temperature superposition, we prove that the relaxation of vitrimer composites should be divided into two distinct parts, including the topological rearrangement of epoxy network and the relaxation of thermoplastic components. Meanwhile, the physical entanglement of thermoplastics and the reconfigurability of chemical cross-links enable vitrimer composites to have excellent adhesive property. Based on the reconfigurability of vitrimer composites, the multi-shape memory effect was readily realized through blending the vitrimer composites of different glass transition by a simple hot-pressing method. It is anticipated that this novel strategy will pave way for the fabrication of thermosetting polymers with both reconfigurability and functional applications.

Transparent, High-Strength, and Shape Memory Hydrogels from Thermo-Responsive Amino Acid-Derived Vinyl Polymer Networks.

Herein, the formation of unique shape-memory hydrogels that are composed of thermo-responsive amino-acid-derived vinyl polymer networks is reported; these are readily prepared by radical copolymerization of N-acryloyl glycinamide with commercially available cross-linkers, namely, methylenebis(acrylamide) and poly(ethylene glycol) diacrylate. These hydrogels are transparent (>90% transmittance at 600 nm) and are comprised of 97-70 wt% water. Furthermore, these contain both chemical and physical cross-linkages that are based on the multiple hydrogen bonds attained via amino acid units; this composition is aimed at generating opposing stimuli-responsive characters, namely, chemically stable and thermo-sensitive properties. A cooperative interplay of these two networks enables the hydrogels to exhibit a decent mechanical toughness (breaking strength ≈0.3 MPa and breaking elongation >600%) and a shape fix/memory capability. The temporary shape is easily fixed by cooling at 4 °C after deformation at high temperature, and it instantly recovers its original shape through reheating. Furthermore, a multi-shape memory effect is achieved by incorporating the pH-responsive N-acryloyl alanine unit into the hydrogel system as a comonomer; in this system, three distinct shapes can be fixed through temperature and pH manipulations. This facilely attainable shape memory hydrogel has significant potential in various fields, such as soft actuators, sensors, and biomedical materials.

Recyclable, Self-Healing, Thermadapt Triple-Shape Memory Polymers Based on Dual Dynamic Bonds

Fabricating a single polymer network with a combination of multi-shape memory effect (multiple-SME), solid-state plasticity, recyclability and self-healing behavior remains challenges. We designed imine bonds and ionic hydrogen bonds dual crosslinked polybutadiene (PB) networks. The resultant PB networks showed triple-shape memory effect (triple-SME), where imine bonds could be used to fix the permanent shape and ionic hydrogen bonds and glass transition acted as the transition segments for fixing/releasing the temporary shapes. Additionally, the dual dynamic bonds offered PB networks outstanding solid-state plasticity, recyclability and self-healing behavior. This strategy provides some insights for preparing shape memory polymers integrating multiple-SME and multi-functionality.

Collective and cooperative dynamics in transition domains of amorphous polymers with multi-shape memory effect

The multi-shape memory effect (multi-SME) in amorphous shape memory polymers (SMPs) linked with collective and cooperative rearrangements and accommodation of monomeric segments leads to the generation of complex thermodynamic modes. In this study, an extended domain size model is initially formulated to describe variations in temperature-dependent relaxation behavior and domain transitions in amorphous SMPs. According to the Adam–Gibbs theory, a cooperative model is employed to identify the principle role of domain size in the collective dynamics of multi-SMEs in amorphous SMPs. The phase transition theory is then combined with the multi-branch Kelvin model to describe the collective and cooperative relaxation behavior of the SMPs with multiple transition domains. It is shown that the proposed model is able to characterize the thermomechanical transitions and multiple shape recovery processes. Finally, the model is applied to predict the shape recovery behavior of SMPs with triple- and quadruple-SMEs, respectively, and the theoretical results are well validated by the experimental results.

On the free-volume model of multi-shape memory effect in amorphous polymer

Free volume theory, which is a popular approach to solve the problems associated with glass transition in polymers, is applied to investigate multi-shape memory effect (multi-SME) of amorphous shape memory polymers (SMPs) as well. To understand the phenomena of multiple shape recovery behaviors, theories about the SME and multi-SME in amorphous SMPs are firstly developed. The thermodynamics in the SMPs with dual- and triple-SME is then investigated in the frameworks of the free volume model and Eyring equation, taking account of the dependencies of free volume and activation energy on relaxation time. Finally, the proposed free volume model is evaluated and compared with results from the amorphous SMPs with dual- and triple-SMEs. Results from our newly proposed model show a good agreement with experimental results obtained at different temperatures.

A coupling model for cooperative dynamics in shape memory polymer undergoing multiple glass transitions and complex stress relaxations

Modelling multi-shape memory effect (multi-SME) of shape memory polymers (SMPs) is a critical challenge for fields of mathematics/statistics and condensed-matter physics. These SMPs have a huge number of segments and their thermomechanical behaviors are determined by heating history and cooperative relaxations (e.g., relaxation of all segments occurs simultaneously). In this study, a one-dimensional coupling model was proposed to investigate the cooperative dynamics of multiple glass transitions and thermomechanical behaviors of the SMPs. The overall relaxation behaviors of different tangled segments in the SMPs were formulated based on the Boltzmann's superposition principle by coupling the highest transition temperature (Tmax) and initial transition temperature (Tmin) of all segments. Dependences of thermomechanical properties and relaxation strains upon the parameters of Tmax, Tmin, relaxation time and heating rate were theoretically investigated. Multiple glass transitions, thermomechanical and shape memory behaviors of the SMPs have been well described using this newly proposed coupling model. Finally, the simulation results were compared with the experimental data, and good agreements between them were obtained.

Emulsion Lyophilization as a Facile Pathway to Fabricate Stretchable Polymer Foams Enabling Multishape Memory Effect and Clip Application.

Solvent freezing is an important method to produce polymer foams with highly tunable pore structure. However, foams prepared from aqueous solution precursors commonly suffer from poor water resistance, whereas those organo-phase systems are not environmental friendly. Here, we present that using an emulsion lyophilization method can overcome such a contradiction and synthesize multifunctional polymer foams. Commercially available polyacrylate-based emulsions with various targeted glass transition temperatures (Tgs) were applied. Adipodihydrazide molecules contained in the water phase of the emulsions reacted with the acetyl groups on the polymers during the freeze-drying, forming elastic networks to maintain the pore structure. The foams can tolerate a 650% elongation without failure and are notch insensitive. The porosity of the foams can be tuned from approximately 45 to 90% via lyophilization of diluted emulsions. The facile blending of emulsions with different targeted Tgs enabled foams with multishape memory capability. Moreover, the foams showed an excellent mechanical damping property, and the slow recovery nature enabled a clip application of clamping extremely weak objects.

A thermodynamic model for tunable multi-shape memory effect and cooperative relaxation in amorphous polymers

By integrating phase transition theory and Takayanagi principle, we have developed a thermodynamic model to describe working mechanism and thermomechanical behavior of shape memory polymers (SMPs) with tunable multi-shape memory effect (multi-SME). Thermodynamics and state/phase transitions of different segments in the SMPs were modeled by considering their cooperative relaxation, which is defined as a process where a group of segments undergo relaxation simultaneously, based on the Takayanagi principle. Dependences of thermomechanical behavior of amorphous polymers on volume fraction, cooperative relaxation and glass transition temperatures of both hard and soft segments were theoretically investigated. Working mechanisms of the multi-SME and cooperative relaxation of the amorphous polymers have well been explained using this newly proposed model, which offers an effective approach for designing new SMPs with multi-segments.

Multilayered assembly of poly(vinylidene fluoride) and poly(methyl methacrylate) for achieving multi-shape memory effects

The alternately-organized poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) multilayer materials were prepared through layer-multiplying coextrusion. With the multiplication of layers, the thickness of each layer was reduced in proportion and the layer interfaces were enriched generating a broader and more continuous thermal transition temperature (Ttrans) from PVDF to PMMA layers as mapped by in-situ thermal analysis. The low-Ttrans side originated from the glass transition of PMMA, whereas the high-Ttrans side was dominated by the melting of PVDF crystals based on the heating curves of DMA and DSC. The dielectric spectroscopy and 2D-SAXS were performed and demonstrated that the compositional diffusion not only broadened the relaxation distribution of amorphous chains, but also strengthened the interaction between amorphous and crystalline domains. Therefore, a unique multilayer network, where the crystals in PVDF layers acting as physical networks connected the neighboring amorphous layers, was fabricated and its potential application in obtaining multi-shape memory effect (MSME) was disclosed for the first time. The results exhibited that the 1024-layer specimen owned a better triple- and quadruple-shape memory capacity than conventional blend which possessed the same compositions and a similar Ttrans range. The latter one even failed to successively memorize more than two temporary shapes. A possible mechanism was proposed through polarized IR and creeping-recovery measurements. Higher phase continuity which benefited for the stress transfer was revealed to play a significant role in strengthening the shape-fixing and -recovering ability during each shape-memory progress. Accordingly, a new physically-compounding strategy was addressed to achieve outstanding MSME for meeting complex demands in smart applications.

Liquid crystalline polyurethane composites based on supramolecular structure with reversible bidirectional shape memory and multi-shape memory effects

In this study, a novel series of supramolecular liquid crystalline (LC) polyurethane composites, named SMPU–#HOBA (# represents the molar ratio of HOBA/BINA), were successfully prepared by incorporating hexadecyloxybenzoic acid (HOBA) into pyridine-containing polyurethane (PU).

The influence of liquid crystals on the properties of sisal fibre polyurethanes with multi-shape memory effects

LC-SF-SMPUs show excellent multi-shape memory properties.

A thermomechanical model of multi-shape memory effect for amorphous polymer with tunable segment compositions

Multi-shape memory effect (multi-SME) in amorphous polymers has attracted great attention due to their complex thermal transitions and multi-step recovery behavior. Many experimental studies have been reported for synthesis, characterization, testing and demonstration of the multi-SME. However, theoretical approaches have seldom been applied although they are critically needed to understand the principles and predict the behavior of the multi-SME in various amorphous polymers. In this study, a new thermomechanical model was proposed to describe the thermo-/chemo-responsive multi-SME of amorphous polymers with tunable segment compositions. Based on the Weibull statistical model, a new constitutive framework was established to describe temperature-, stretch ratio- and strain-dependent behaviors of thermo-/chemo-responsive multi-SME. Finally, this newly proposed model was applied to quantitatively identify the crucial factors for the thermo-/chemo-responsive shape recovery behaviors, which have been verified by the reported experimental data.

Gradient Polydopamine Coating: A Simple and General Strategy toward Multishape Memory Effects.

Multi shape memory polymers (multi-SMPs) exhibit many potential applications such as aerospace, soft robotics, and biomedical devices because of their unique abilities. Although many works are done to broaden the preparations of multi-SMPs, the desire to a simple and versatile strategy as well as more complex shapes still exists. Moreover, a light-induced SMP shows more advantages than a thermal-induced one in many practical working circumstances. Herein, inspired by strong adhesion and efficient photothermal conversion of polydopamine (PDA) coating, we report a more simple and facile approach to prepare light-induced multi-SMPs by introducing a gradient PDA coating on a dual-SMP through time-controlled dipping. The photothermal converting properties with varying thicknesses of PDA under the tunable near-infrared light source are investigated. Then, light-induced multishape memory effects based on gradient PDA coatings are illustrated, where three designs of multi-SMPs - rectangle, triangle, and cross are prepared and demonstrated. Also, the evolutions of coating morphology during shape shifting are carefully studied. Finally, we present few complex designs of patterns and shapes as well as a design of potential application for the highly controllable smart devices. This strategy demonstrates a very simple and general strategy to design and prepare the light-induced multi-SMPs with complex shapes based on any thermal-responsive dual-SMPs.

Polyurethane networks based on disulfide bonds: from tunable multi-shape memory effects to simultaneous self-healing

With the prompt development in intellectualization nowadays, the smart materials with multifunctionality or multi-responsiveness are highly expected. But it is a big challenge to integrate the different actuating units into a single system in a synergy pattern. Herein, we put forward a new strategy to develop the polyurethane networks which can present shape-memory effect and self-healing effect in independent way as well as simultaneous acting mode. To realize this goal, poly(tetremethylene ether) glycol was chosen as the soft segment to ensure the polymer chains a good mobility, and disulfide bond as the dynamic covalent bond was embedded in the backbone of polyurethane to endow it with desirable self-healing capacity under mild condition. Moreover, a rational control of the architecture of the networks by adjusting the content of disulfide bond and the degree of cross-linking, a broad glass transition temperature ( T g) was achieved, which enabled the network a versatile shape-memory effect, covering from dual-, triple- so far as to quadruple-shape memory effect. More importantly, the shape recovery and healing process can be realized simultaneously because of the highly matched actuating condition in this system.