Shape Memory Polymer Systems Research Articles

Ag-S coordination strategy for high recovery driving stress in recyclable shape memory polymers

To address the issue of low strength and recovery stress in recyclable shape memory polymer (SMP) materials with dynamic covalent bonds in practical applications, we propose a method based on Ag-S coordination interactions to improve dynamic covalent bonds, thereby preparing recyclable SMP with high recovery stress and large recovery strain. The introduction of in situ reduced silver nanoparticles (AgNPs) in the SMP system play multiple roles, serving as additional physical crosslinking points at the microscale and coordinating with disulfide bonds in the system to construct a multi-level dynamic network structure. This endows the material with exceptional mechanical properties (tensile strength of 25.93 MPa and fracture strain of 1466.83 %) and recovery driving stress (5.59 MPa), while maintaining more than 99 % shape recovery and fixity rate. Furthermore, due to the outstanding photothermal performance of AgNPs, the composite material has acquired remote photothermal driving capabilities. The proposed Ag-S coordination enhancement offers a new approach for SMP materials that achieve both large actuation distance and high recovery stress, holding significant potential for applications in soft robotics and actuators.

3D printing of photochromic and thermochromic shape memory polymers for multi-functional applications

The field of 3D printing is growing rapidly and offers a wide range of potential applications, particularly with the use of Shape Memory Polymers (SMPs). However, current studies on SMP-based 3D printing have mainly focused on structural design and deformation behavior. To enhance the practicality of 3D printed structures, the ability to change color is highly desirable, especially for applications such as anti-counterfeiting, encryption, and bioinspired camouflage. This paper presents the fabrication of a UV-curable PUA-based Shape Memory Polymer (SMP) system with thermochromic and photochromic capabilities. The system is acrylate-based, making it highly UV-curable and compatible with high-resolution 3D printing techniques. Thermo/photochromic effects are achieved by adding thermochromic microcapsules to the system, resulting in printed structures that can change color upon heating or UV) exposure. The printed parts with multi-color hidden information, such as QR codes and digital numbers, were successfully demonstrated through the printing of various anti-counterfeiting patterns.And the expriment results show the exceptional multi-functional performance including shape recovery and thermo/photochromic. The development of this UV-curable PUA-based SMP system represents a significant advancement in the application of SMP-based 3D printing for anti-counterfeiting and secure data recording.

4D Printing Of Shape Memory Epoxy For Adaptive Dynamic Components

AbstractProgrammed 4D printing shape‐memory polymers (SMPs) have been endorsed as an emerging paradigm for developing adaptive dynamic components with sophisticated architectures and biomimetic adaptation capacities. However, the species of SMPs systems are still rare while most of them are produced from expensive monomers which have restricted their mass production. Moreover, most SMPs show poor elongation at break and strain which cannot support a higher degree of bending during shape memory deformation. Herein, a kind of new epoxy vinyl ester resin ink is developed and improves the 4D printing process of shape memory epoxy‐based polymer. By adjusting the printing conditions to match the ink property, a series of precisely designed printing structures are successfully produced. The printed SMP system shows a favorable strain rate under heat‐treated conditions above its glass transition temperature (Tg) which leads to fine shape memory performance. Furthermore, the printed components obtained through micro to macro multi‐scale 4D printing structure design confirm its application potential as dynamic adaptive devices in aerospace and safety engineering.

Super-tough, self-sensing and shape-programmable polymers via topological structure crosslinking networks

Shape-memory polymers (SMPs) are smart materials capable of deforming actively under external excitation. Owing to their characteristics, including variable stiffness, programmable deformation, composability, and self-sensing, and its behavior is similar to the intelligent reflection of life, these materials are highly valuable for applications in many fields. We have developed a molecular engineering strategy based on topological cross-linking networks, which form hydrogen bonds, hyperbranching, ring network structure, etc. to obtain shape memory epoxy resin (SMEP) with super toughness, high-temperature resistance and triple shape memory effect (TSME). Additionally, the internal relationship between the molecular network structure and material performance is simulated by molecular dynamics. It has super toughness (1288 %) and high impact energy (216 MJ/m3) above its transition temperature, breaking through the previously reported thermosetting resin. Importantly, the developed SMPs system exhibited outstanding fatigue resistance and was successfully loaded repeatedly for more than 100 cycles. Finally, the intramolecular cyclization effect led to the generation of different linked structures in situ, which endowed SMEP with TSME, thereby rendering such materials potentially applicable in engineering fields, such as aerospace, smart furniture, and soft robotics.

Design of Shape Memory Thermoplastic Material Systems for FDM-Type Additive Manufacturing.

The work presented here describes a paradigm for the design of materials for additive manufacturing platforms based on taking advantage of unique physical properties imparted upon the material by the fabrication process. We sought to further investigate past work with binary shape memory polymer blends, which indicated that phase texturization caused by the fused filament fabrication (FFF) process enhanced shape memory properties. In this work, two multi-constituent shape memory polymer systems were developed where the miscibility parameter was the guide in material selection. A comparison with injection molded specimens was also carried out to further investigate the ability of the FFF process to enable enhanced shape memory characteristics as compared to other manufacturing methods. It was found that blend combinations with more closely matching miscibility parameters were more apt at yielding reliable shape memory polymer systems. However, when miscibility parameters differed, a pathway towards the creation of shape memory polymer systems capable of maintaining more than one temporary shape at a time was potentially realized. Additional aspects related to impact modifying of rigid thermoplastics as well as thermomechanical processing on induced crystallinity are also explored. Overall, this work serves as another example in the advancement of additive manufacturing via materials development.

Thermomechanically Triggered Reversible Multi‐Transformability of a Single Material System by Energy Swapping and Shape Memory Effects

AbstractThermally triggered active metamaterials with shape memory polymers (SMPs) show greater potential for structural applications with reconfigurability than other programmable structures owing to their temporally stiff condition with shape locking. However, most SMP‐based active metamaterials have not shown complex transformation, such as multi‐modal and asymmetric deformations, because of the lack of an adaptable strategy with reasonable mechanics models. Moreover, conventional SMP has a critical drawback – irreversible transformability, limiting its reconfigurability for active metamaterials. Herein, a thermomechanical tool that allows a single material system to transform with reversible, multi‐modal, and asymmetric deformations is constructed and demonstrated. Using transformation aids (TAs), a localized pre‐stress and a temperature‐dependent reverse stiffness effect to exchange energy with a lattice is conceived. The deformation of a single SMP system whose energy is swapped from TAs by localized pre‐stress and reverse stiffness can transform into reversible, multi‐modal, and asymmetric deformations with shape‐locking. The methods can be used for reconfigurable structures, tuning symmetry, and chirality, especially for active acoustic metamaterials, deployable devices, and biomedical devices. The mechanics‐inspired design approach of local deformation of TA and the interaction with the temperature‐dependent stiffness drop of the lattice open an avenue to the robust design of thermally triggered active metamaterials.

Mechanically Robust and UV-Curable Shape-Memory Polymers for Digital Light Processing Based 4D Printing.

4D printing is an emerging fabrication technology that enables 3D printed structures to change configuration over "time" in response to an environmental stimulus. Compared with other soft active materials used for 4D printing, shape-memory polymers (SMPs) have higher stiffness, and are compatible with various 3D printing technologies. Among them, ultraviolet (UV)-curable SMPs are compatible with Digital Light Processing (DLP)-based 3D printing to fabricate SMP-based structures with complex geometry and high-resolution. However, UV-curable SMPs have limitations in terms of mechanical performance, which significantly constrains their application ranges. Here, a mechanically robust and UV-curable SMP system is reported, which is highly deformable, fatigue resistant, and compatible with DLP-based 3D printing, to fabricate high-resolution (up to 2µm), highly complex 3D structures that exhibit large shape change (up to 1240%) upon heating. More importantly, the developed SMP system exhibits excellent fatigue resistance and can be repeatedly loaded more than 10000 times. The development of the mechanically robust and UV-curable SMPs significantly improves the mechanical performance of the SMP-based 4D printing structures, which allows them to be applied to engineering applications such as aerospace, smart furniture, and soft robots.

Color-Changeable Four-Dimensional Printing Enabled with Ultraviolet-Curable and Thermochromic Shape Memory Polymers.

Four-dimensional (4D) printing, which enables 3D printed structures to alter shapes over time, is attracting increasing attention because of its exciting potential in various applications. Among all the 4D printing materials, shape memory polymers (SMPs) have a higher stiffness and faster response rate and therefore are considered as one of the most promising 4D printing materials. However, the current studies of SMP-based 4D printing mainly focused on the deformation behavior and structural design of 4D printed structures. An additional function such as color change is desired for 4D printed structure, which would be potentially beneficial to the applications such as anti-counterfeiting, encryption, and bioinspired camouflage. In this paper, we report an ultraviolet (UV)-curable and thermochromic (UVT) SMP system that enables color-changeable 4D printing. The UVT SMP system is acrylate-based, thus highly UV-curable and compatible with PμSL-based high-resolution 3D printing technique. Thermochromism is imparted by adding the thermochromic microcapsules to the UVT SMP system, which allows the printed structures to reversibly change colors upon heating and cooling. To demonstrate its extraordinary thermochromic and mechanical performance, we use UVT SMP to print QR codes and multilevel anti-counterfeiting patterns which can hide the visible information at room temperature and visualize the information by encrypting, decrypting, and encrypting again steps with the shape-color recovery process. The development of UVT SMP will significantly advance current applications of SMP-based 4D printing, especially for anti-counterfeiting and safe data recording.

Two-stage reactive shape memory thiol–epoxy–acrylate system and application in 3D structure design

Shape memory polymers (SMPs) are widely applied in actuator, aerospace, and super wetting. Besides, the specific structures of SMPs are important to achieve such performance. However, the preparation process of specific structure of SMPs to achieve certain performance are always time-consuming and costly for the high crosslinking density exists in the resin matrix. Fortunately, two-stage reactive SMPs system can be used to prepare complex structure for the function of precise material design. To attain this goal and explore new application of SMPs, we designed a novel two-stage reactive shape memory polymer (SMP) system based on non-stoichiometric thiol, epoxy, and acrylate. In this novel system, intermediate polymer with shape memory performance can be prepared in the first thermal curing stage. With the shape memory property, the intermediate polymer can be designed to a certain structure. Then, final polymer with stable and complex shape can be further fabricated after the photo-curing in the second stage. Therefore, this novel two-stage reactive SMP system shows promising applications in 3D structure manufacturing, mold-free fabrication, and spatio-selective materials.

An experimental investigation on the energy storage in a shape-memory-polymer system

In this paper, the effect of thermomechanical loading on the behavior of deflection-based harvested energies from a shape memory polymer system is experimentally investigated. Samples are created with honeycomb cells from poly-lactic acid using additive manufacturing techniques. The shape memory effect in shape recovery and force recovery paths are studied under thermomechanical tests in bending and tensile modes. The maximum recoverable strain energy is computed as well. According to the conducted thermomechanical tests, it is shown that the thermal expansion coefficient is much more dominant in the tensile mode. Some procedures are proposed to reduce the thermal expansion effect on the force recovery and arrive at higher energy harvested from a shape memory system.

Hybrid Electroactive Shape Memory Polymer Composites with Room Temperature Deformability

AbstractPolymeric blend shape memory polymers (SMPs) can be constructed from two immiscible polymeric matrices. The shape recovery behavior of these composite systems can be easily controlled by varying the ratio of the polymer blends. It has been recently discovered that the functionality of SMPs can be further enhanced with electroactive ability through the use of conductive fillers. However, the fillers may negatively interact with the SMPs and cause a reduction in the elongation at failure thereby diminishing the shape recovery performance. It is proposed that a plasticizer can be utilized to alter the microstructure of the SMPs with conductive fillers. In this study, a new hybrid SMP is developed by combining single‐walled carbon nanotubes (SWCNT) into a poly(lactic acid) (PLA) and thermoplastic polyurethane (TPU) SMP system containing poly(ethylene glycol) (PEG) plasticizer. The incorporation of PEG is able to lower the activation temperature, while enhancing dispersion of SWCNT. The presence of SWCNT can stabilize the SMP system and significantly enhance the shape‐fixing capability after deformation at room temperature conditions. By carefully controlling the formulation, an electroactive SMP can be created by optimizing the amount of SWCNT and PEG plasticizer.

Self-Healing Four-Dimensional Printing with an Ultraviolet Curable Double-Network Shape Memory Polymer System.

Four-dimensional (4D) printing that enables 3D printed structures to change configurations over time has gained great attention because of its exciting potential in various applications. Among all the 4D printing materials, shape memory polymers (SMPs) possess higher stiffness and faster response rate and therefore are considered as one of most promising materials for 4D printing. However, most of the SMP-based 4D printing materials are (meth)acrylate thermosets which have permanently cross-linked covalent networks and cannot be repaired if any damage occurs. To address the unrepairable nature of SMP-based 4D printing materials, this paper reports a double-network self-healing SMP (SH-SMP) system for high-resolution self-healing 4D printing. In the SH-SMP system, the semicrystalline linear polymer polycaprolactone (PCL) is incorporated into a methacrylate-based SMP system which has good compatibility with the digital light processing-based 3D printing technology and can be used to fabricate complex 4D printing structures with high resolution (up to 30 μm). The PCL linear polymer imparts the self-healing ability to the 4D printing structures, and the mechanical properties of a damaged structure can be recovered to more than 90% after adding more than 20 wt % of PCL into the SH-SMP system. We investigated the effects of PCL concentration on the thermomechanical behavior, viscosity, and the self-healing capability of the SH-SMP system and performed the computational fluid dynamics simulations to study the effect of SH-SMP solution's viscosity on the 3D printing process. Finally, we demonstrated the self-healing 4D printing application examples to show the merits of the SH-SMP system.

Water-induced poly(vinyl alcohol)/carbon quantum dot nanocomposites with tunable shape recovery performance and fluorescence.

Water-induced shape memory polymers (SMPs) show promising applications in biomedicine, biosensing, anti-counterfeiting and intelligent actuating systems. However, the dual function of shape morphing and color switching has not been achieved in the water-induced SMP system. Herein, a novel water-induced SMP with both color-switching fluorescence behavior and shape memory performance is reported. The material is fabricated by crosslinking poly(vinyl alcohol) (PVA) and pH-responsive fluorescent carbon quantum dots (CQDs). The incorporation of CQDs with PVA not only improves the shape recovery performance but also endows the material with color-switching features. To our best knowledge, such smart ability is first realized in this PVA/CQD SMP system, and this report provides a novel strategy for fabricating smart water-induced SMPs with adjustable shape recovery performance and fluorescence.

Actuating and memorizing bilayer hydrogels for a self-deformed shape memory function.

A general strategy for fabricating a double layer self-deformed shape memory hydrogel which includes a thermo-responsive actuating layer and a pH-responsive memorizing layer is presented. Compared with traditional shape memory polymer systems, the temporary shape of the hydrogel could be generated by a thermo-responsive actuating layer upon the trigger of heat without the need for an external force, which could be further memorized by the pH-responsive memorizing layer.

Toward the low actuation temperature of flexible shape memory polymer composites with room temperature deformability via induced plasticizing effect.

Semi-crystalline/elastomer blends such as poly(lactic acid) (PLA) and thermoplastic polyurethane (TPU) have been well-studied as shape memory polymer (SMP) systems over the last decade. The SMP system is known to be reliable as the activation or transitional temperature (Ttrans) is well defined by its glass transition temperature (Tg). In this study, the possibility of altering SMPs'Ttrans by introducing plasticizer molecules, such as poly(ethylene glycol) (PEG), into the system was investigated. Significant changes in morphologies and thermal behaviours of the composites after the incorporation of plasticizer inferring the possible changes in shape memory behaviours were reported. It was concluded that the SMP performance, in terms of activation temperature, shape fixing (Rf), shape recovery (Rr) ratio, and recovery time, could be improved significantly due to the induced plasticizing effects. With only the incorporation of 5 wt% PEG, the Ttrans of PLA/TPU SMP was successfully decreased from the well-known 75 °C to 50 °C while achieving 85% Rr ratio. An improvement in 37.5% shorter recovery time can be observed with the incorporation of 10 wt% PEG when compared with a pristine PLA/TPU system under 50 °C. The results indicated that plasticizing is an effective method for altering the polymer chain structures and a step closer toward the development of fast response SMPs that can be activated under a low temperature.

Development of siloxane-based amphiphiles as cell stabilizers for porous shape memory polymer systems

HypothesisPolyurethane foaming surfactants are cell stabilized at the polymer-gas interface during foam blowing to prevent bubble coalescence. Siloxane-based surfactants are typically used to generate a surface tension gradient at the interface. The chemical structure of the hydrophobic and hydrophilic units affects surfactant properties, which can further influence foam morphology. ExperimentsSiloxane-polyethylene glycol (PEG) ether amphiphiles were synthesized in high yield via hydrosilylation to serve as surfactants for shape memory polymer (SMP) foams. Hydrophobic units consisted of trisiloxane and polydimethyl siloxane, and PEG allyl methyl ether (n=8 or 25) was the hydrophilic component. Upon confirming successful synthesis of the surfactants, their surface tension was measured to study their suitability for use in foaming. SMP foams were synthesized using the four surfactants, and the effects of surfactant structure and concentration on foam morphology were evaluated. FindingsSpectroscopic data confirmed successful siloxane-PEG coupling. All surfactants had a low surface tension of 20–21mN/m, indicating their ability to reduce interfacial tension. SMP foams were successfully fabricated with tunable cell size and morphology as a function of surfactant type and concentration.

Preparation of 2‐(dimethylamino) ethyl methacrylate copolymer micelles for shape memory materials

ABSTRACTResearchers are actively developing shape memory polymers (SMPs) for smart biomaterials. This paper reports a new SMP system synthesized from biocompatible 2‐(dimethylamino) ethyl methacrylate (DMAEMA), butyl acrylate (BA) and tri(ethylene glycol) divinyl ether (TDE). Preliminary results show that the DMAEMA‐co‐BA‐co‐TDE copolymers form micelles in aqueous solution due to chemical crosslinking and hydrophobicity. The micelle size decreased with the increase in the BA content since the hydrophobicity of copolymers increases with the increase of BA content. The resulting polymer films contain–N(CH3)2 functional groups for further biomaterial applications. The thermal stability of DMAEMA‐co‐BA‐co‐TDE copolymers is determined by the DMAEMA structure and content. Moreover, the copolymers form micro‐phase‐separated structures containing a reversible amorphous soft phase, and the storage moduli decreases significantly around Tg. Therefore, good thermal‐induced shape memory effects are achieved in the DMAEMA‐co‐BA‐co‐TDE copolymers by adjusting the BA content. This work proposes a new strategy for designing smart biomaterials using a biocompatible monomer. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42312.

A Processable Shape Memory Polymer System for Biomedical Applications.

Polyurethane shape memory polymers (SMPs) with tunable thermomechanical properties and advanced processing capabilities are synthesized, characterized, and implemented in the design of a microactuator medical device prototype. The ability to manipulate glass transition temperature (Tg ) and crosslink density in low-molecular weight aliphatic thermoplastic polyurethane SMPs is demonstrated using a synthetic approach that employs UV catalyzed thiol-ene "click" reactions to achieve postpolymerization crosslinking. Polyurethanes containing varying C=C functionalization are synthesized, solution blended with polythiol crosslinking agents and photoinitiator and subjected to UV irradiation, and the effects of number of synthetic parameters on crosslink density are reported. Thermomechanical properties are highly tunable, including glass transitions tailorable between 30 and 105 °C and rubbery moduli tailorable between 0.4 and 20 MPa. This new SMP system exhibits high toughness for many formulations, especially in the case of low crosslink density materials, for which toughness exceeds 90 MJ m(-3) at select straining temperatures. To demonstrate the advanced processing capability and synthetic versatility of this new SMP system, a laser-actuated SMP microgripper device for minimally invasive delivery of endovascular devices is fabricated, shown to exhibit an average gripping force of 1.43 ± 0.37 N and successfully deployed in an in vitro experimental setup under simulated physiological conditions.

Effect of the crosslinking density and programming temperature on the shape fixity and shape recovery in epoxy–anhydride shape‐memory polymers

ABSTRACTIn addition to the fabrication of thermoset epoxy–anhydride shape‐memory polymers (SMPs), a systematic experimental investigation was conducted to characterize the crosslinking density, micromorphology, thermal properties, mechanical properties, and shape‐memory effects in the epoxy SMP system, with a focus on the influence of the crosslinking density and programming temperature on the shape‐fixity and shape‐recovery behaviors of the polymers. On the basis of the crosslinking density information determined by NMR technology, we concluded that the effect of the crosslinking density on the shape‐fixity behaviors was dependent on the programming temperature. The advantage of a nice combination of crosslinking density and programming temperature provided an effective approach to tailor the actual shape recovery within a wide range. The increasing crosslinking density significantly improved the shape‐recovery ratio, which could be further improved through a decrease in the programming, whereas the crosslinking density was more fundamental. This exploration should play an important role in the fabrication and applications of SMP materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40559.

A Structural Approach to Establishing a Platform Chemistry for the Tunable, Bulk Electron Beam Cross-Linking of Shape Memory Polymer Systems.

The synthetic design and thermomechanical characterization of shape memory polymers (SMPs) built from a new polyurethane chemistry that enables facile, bulk and tunable cross-linking of low-molecular weight thermoplastics by electron beam irradiation is reported in this study. SMPs exhibit stimuli-induced geometry changes and are being proposed for applications in numerous fields. We have previously reported a polyurethane SMP system that exhibits the complex processing capabilities of thermoplastic polymers and the mechanical robustness and tunability of thermomechanical properties that are often characteristic of thermoset materials. These previously reported polyurethanes suffer practically because the thermoplastic molecular weights needed to achieve target cross-link densities severely limit high-throughput thermoplastic processing and because thermally unstable radiation-sensitizing additives must be used to achieve high enough cross-link densities to enable desired tunable shape memory behavior. In this study, we demonstrate the ability to manipulate cross-link density in low-molecular weight aliphatic thermoplastic polyurethane SMPs (Mw as low as ~1.5 kDa) without radiation-sensitizing additives by incorporating specific structural motifs into the thermoplastic polymer side chains that we hypothesized would significantly enhance susceptibility to e-beam cross-linking. A custom diol monomer was first synthesized and then implemented in the synthesis of neat thermoplastic polyurethane SMPs that were irradiated at doses ranging from 1 to 500 kGy. Dynamic mechanical analysis (DMA) demonstrated rubbery moduli to be tailorable between 0.1 and 55 MPa, and both DMA and sol/gel analysis results provided fundamental insight into our hypothesized mechanism of electron beam cross-linking, which enables controllable bulk cross-linking to be achieved in highly processable, low-molecular weight thermoplastic shape memory polymers without sensitizing additives.