Multi-shape Memory Research Articles

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.

Spatiotemporally Controlled Plasticity and Elasticity in 3D Multi‐Shape Memory Structures Enabled by Elemental Sulfur‐Derived Polysulfide Networks with Intrinsic NIR Responsiveness

Front Cover: In article 2000013, Dong-Gyun Kim, Ho Gyu Yoon, Yong Seok Kim, and co-workers report an effective and facile preparation of thermadapt shape-memory polymers based on elemental-sulfur-derived poly(phenylene polysulfide) networks (PSNs), which possess intrinsic near infrared (NIR)-induced photothermal conversion properties for enabling spatiotemporal control of their plasticity and elasticity. The NIR-controllable plasticity and elasticity of the PSNs eventually lead to the realization of versatile shape manipulation of 3D multi-shape memory structures, including building-block assembly, reconfiguration, shape fixing/recovery, and repair.

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.

Spatiotemporally Controlled Plasticity and Elasticity in 3D Multi-Shape Memory Structures Enabled by Elemental Sulfur-Derived Polysulfide Networks with Intrinsic NIR Responsiveness.

Thermadapt shape memory polymers (SMPs), utilizing a variety of dynamic covalent bond exchange mechanisms, have been extensively studied in recent years but it is still challenging to address several constraints in terms of limited accuracy and complexity for constructing 3D shape memory structures. Here, an effective and facile preparation of thermadapt SMPs based on elemental sulfur-derived poly(phenylene polysulfide) networks (PSNs) is presented. These SMPs possess intrinsic near-infrared (NIR)-induced photothermal conversion properties for spatiotemporal control of their plasticity and elasticity. The NIR-controllable plasticity and elasticity of the PSNs enable versatile shape manipulation of 3D multi-shape memory structures, including building block assembly, reconfiguration, shape fixing/recovery, and repair.

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.

Dual Cross‐linked Vinyl Vitrimer with Efficient Self‐Catalysis Achieving Triple‐Shape‐Memory Properties

As an emerging class of dynamic cross-linked network, vitrimers have attracted much attention due to the combination of mechanical advantages of thermosets and recyclability of thermoplastics at an elevated temperature. In particular, most vitrimers with multi-shape memory properties usually involve more than one thermal transition or molecular switch, which might pose a challenge for facile sample fabrication and potentially limits their applications. In pursuit of a more universal and simple route, utilizing commercially available and inexpensive reagents to prepare shape-memory vitrimers with dual cross-linked network from vinyl monomer-derived prepolymers is reported here. Copolymerization of desired vinyl monomers gives prepolymers containing carboxyl and zinc carboxylate groups, which are later converted into vitrimers in a single step by post-curing with diglycidylether of bisphenol A. The Zn2+ ions can not only act as physical crosslinking points through ionic coordination interactions, thus providing the triple-shape-memory properties, but also play the role of catalyst to activate transesterification in the dynamic covalent network. This new self-catalyzed vitrimer has excellent transesterification efficiency, triple-shape-memory properties, and can be sufficiently healed and reprocessed at an elevated temperature. The proposed molecular design of self-catalyzed materials opens a new avenue toward commercially relevant fabrication of high-performance vitrimers with multiple shape-memory properties.

Novel photo-thermal staged-responsive supramolecular shape memory polyurethane complex

Photo-responsive polymers attracted a great deal of interest in recent years. This paper reports a facile preparation of supramolecular shape memory polyurethane complexes from the pyridine-containing polyurethanes and azobenzene-containing mesogens. The results showed that the azobenzene-containing mesogens are grafted on the polyurethane networks via hydrogen bonds between pyridine rings and carboxyl of mesogens. The addition of azobenzene-containing mesogens greatly affected the intermolecular interactions, increasing the glass transition temperature, thermal stability, and storage modulus while promoting the micro-phase separation. The complexes exhibited multi-shape memory properties and excellent photo-thermal staged-responsive shape memory properties showing photo-responsive deformation, temporary shape fixation under visible light irradiation at room temperature and thermal-responsive shape recovery. Particularly, the photo-deformation was achieved even at 2 wt % mesogens added to the polymer network. However, the photo-response greatly increased when the content of mesogen reached 6 wt%. Finally, the proposed mechanism of photo-deformation revealed that this property is due to the photo-isomerization of azobenzene groups of the mesogens, whereas the shape recovery was ascribed to the thermal-responses of polymer networks. It is thus anticipated that these supramolecular polyurethane complexes could be widely applied in smart devices, photo-thermal sensors, and other fields.

A constitutive level-set model for shape memory polymers and shape memory polymeric composites

This work proposes a new constitutive model for shape memory polymers and shape memory polymeric composites under the use of level-set functions as additional variables of state. The model regards shape memory polymers as inhomogeneous bodies consisting of two different phases, an active (rubbery elastic) phase and a frozen (glassy elastic) phase. Introducing the level-set function provides the potential to describe the interface separating the two distinct phases in an implicit manner. Under proper thermodynamical arguments, an appropriate evolution equation is derived that provides the tool to describe phase transformations in shape memory polymers. Furthermore, an extended model version is developed that applies in shape memory polymeric composites by introducing two (or more) level-set functions so as to represent implicitly three (or more) material phases capturing the behavior of multi-shape memory polymers. The level-set constitutive equations are formulated in three dimensions, although the one-dimensional case is adopted and analyzed thoroughly. The reproduction of the shape memory thermomechanical cycles of polymers and polymeric composites provides validity and credibility to the current model.

Properties and characterization of near infrared-triggered natural rubber (NR)/carnauba wax (CW)/carbon nanotube (CNT) shape memory bio-nanocomposites

Very few studies have investigated on the blending of small molecules in the commercial biobased polymers with shape memory behavior. Herein, we investigated the shape memory properties of natural rubber and carnauba wax (S-NR/CW = 8:2, 6:4 and 5:5) melt-blends and nanocomposites vulcanized with sulfur/accelerators, using the melting temperature of wax as the switching temperature. With the addition of multi-walled carbon nanotube (CNT), the prepared S-NR/CW-CNT nanocomposites can not only enhance the mechanical properties but also enhance the shape fixity ratios, attributing to the unexpected improved dispersion of CW and the increased crystallinity of CW. In the one-way test, the shape fixity and recovery ratios for S-NR/CW/CNT 5:5 nanocomposites could reach up to 99.6% and 97.3% in the third thermomechanical cycle, respectively. In addition, CNT could be selectively and remotely heated by irradiation with near infrared laser to trigger shape memory process in a very short time. The additional paraffin wax was also blended with carnauba wax to form the wax mixture for preparing multi-shape memory behavior to exploit the various types of waxes with different melting temperatures (supplementary file). To the best of our knowledge, this is the first small molecule-filled NR nanocomposites with remotely-triggered shape memory properties.

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.

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.

Fabrication of remote controllable devices with multistage responsiveness based on a NIR light-induced shape memory ionomer containing various bridge ions

A novel NIR light responsive shape memory material exhibits excellent light-induced plasticity and light-induced multi-shape memory properties.

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).

Tunable Shape Memory Capacity of Polymeric Materials via Multilayer-Assembled Strategy

Abstract Shape memory polymers (SMPs), as a class of stimuli-responsive materials, have drawn much attention because of their specific capacity to fix temporary deformation and sequentially recover to their original shape upon exposure to an external stimulus. Based on the number of temporary shapes memorized, SMPs can possess dual- or multi-shape memory capacities, which greatly expands their potential applications. Among various methods developed to prepare SMPs, physical compounding recently received increasing concerns because of its ease of processability. Herein, a novel layer-multiplying co-extrusion technology was introduced. Benefiting from the characteristics of the multilayer architecture (i.e. regular layer distribution, continuous layer space and enriched layer interfaces), different kinds of multilayer-assembled SMPs with tunable shape memory capacities were successfully fabricated.

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.