Shape Recovery Properties Research Articles

Study of energy absorption and recovery characteristics of shape memory material zero Poisson’s ratio gradient structures

The integration of smart materials with metamaterials offers significant research value in additive manufacturing. This paper proposes a novel approach combining shape memory polymers (SMP) with zero Poisson’s ratio (ZPR) metamaterials to create better space-filling, lightweight, and high-energy absorbing materials. A cellular structure with three deformation processes was designed by adjusting the rib arm lengths of the ZPR lattice AuxHex, forming a 3D gradient structure. The cellular units were also prepared using the dual-nozzle fusion filament printing technique, and the energy absorption and shape recovery properties of various hierarchical gradient structures, compared to the uniform structure, were simulated and analyzed using the viscoelastic intrinsic model. The results indicate that all gradient structures exhibit superior energy absorption capabilities compared to uniform materials, with the energy absorption performance of the optimal gradient material being 44.32% higher than that of the nongradient material, and they also enhance the shape recovery performance to some extent. Furthermore, it was observed that when a lattice layer, which avoids internal rib contact throughout the compression process, is positioned in the middle layer, it harmonizes the deformation of the upper and lower layers, thereby improving overall stability. This configuration enhances both energy absorption and partial shape recovery performance, offering a novel approach for the design of smart material assemblies with graded metamaterial structures. This study provides a new perspective and methodology for the future design of supergradient smart material collections.

Fully biobased and robust antibacterial cellulose aerogel for uranium extraction.

Developing efficient adsorbent is imperative for the utilization of uranium resources in seawater. Marine microorganisms and bacteria play an important role in the process of adsorption of uranium. In this work, a completely bio-based antimicrobial aerogel (quaternary cellulose/chitosan aerogel-QCNF/CS) was prepared by cross-linking quaternary cellulose nanofibers (QCNF) and chitosan (CS) via citric acid (CA). The QCNF/CS aerogel has a high adsorption capacity of 565.97mgg1, high selectivity (Kd=1.6×104mLg-1). Moreover, the incorporation of CS improved the mechanical properties and enhanced the shape recovery property. The adsorption system reached equilibrium within 300min and had good recovery of 93% for UO22+. After seven cycles of adsorption-desorption, QCNF/CS aerogel still maintained its original structure and retained 80.7% of its initial adsorption capacity. The introduction of quaternary ammonium salt groups gives the QCNF/CS aerogel strong antimicrobial activity, and the inhibition rate can be up to 96%. The aerogel also showed good adsorption performance in spiked natural seawater. DFT results and XPS analysis further indicated that -COOH of CA and -OH of CNF functional groups could enhance the adsorption capacity of QCNF/CS aerogel. Therefore, QCNF/CS aerogel was a potential adsorbent for the extraction of uranium from seawater.

Effect of hierarchical structure on the shape recovery properties and load-carrying capacity of 4D-printed kirigami-inspired honeycomb

Kirigami-inspired honeycomb structures demonstrate outstanding self-expanding capabilities and exceptional mechanical properties. To further enhance the shape recovery properties and load-carrying capacity of kirigami-inspired honeycomb, the hierarchical structure is introduced by importing porous structures into cell walls of 4D-printed kirigami-inspired honeycomb (Structure II), and then an innovative honeycomb structure (Structure I) is designed for achieving the hierarchical effect. Additionally, three hierarchical structures with different shape configurations (Structure III–V) are also designed for comparing with traditional Structure II. The finite element analysis and experiments are conducted to compare the compression deformation behavior and energy absorption capacity of Structure I–V. It is found that Structure I exhibits significantly improved properties compared with the traditional Structure II. The stiffness of Structure I is increased by 50.43%, and the energy absorption performance is also increased by 65.00%. The recovery time has been shortened by 27.27% and it has a better shape recovery rate. Structure III–V can also enhance the performances compared with traditional Structure II. Structure IV exhibits the best energy absorption capacity, with an increase of 76.25%. Thus, the developed hierarchical kirigami-inspired honeycombs have broad application prospects for the multifunctional applications of honeycomb structures in the future.

Injectable ultrathin porous membranes harnessing shape memory polymers for retinal tissue engineering.

Age-related macular degeneration (AMD) is a leading cause of vision loss, characterized by the progressive degeneration of retinal cells, particularly retinal pigment epithelial (RPE) cells. Conventional treatments primarily focus on slowing disease progression without providing a cure. Recent advances in tissue engineering and cell-based therapies offer promising avenues for regenerating retinal tissue and restoring vision. In this study, we developed ultrathin, nanoporous membrane scaffolds designed to mimic Bruch's membrane (BrM) for RPE cell transplantation using vapor-induced phase separation. These scaffolds, fabricated from a blend of poly(L-lactide-co-ε-caprolactone) (PLCL) and poly(lactic-co-glycolic acid) (PLGA), exhibited favorable topography, biocompatibility, and shape-memory properties. In vitro experiments confirmed that the nanoporous topography effectively supports the formation of RPE monolayers with intact tight junctions. Additionally, the shape-memory characteristic enables the membrane to self-expand at body temperature (37 °C), facilitating minimally invasive delivery via injection. ARPE-19 cell-attached nanothin membranes successfully demonstrated shape-recovery properties and were deliverable through a catheter in an ex vivo model. Our findings suggest that the developed scaffolds provide a promising approach for retinal tissue engineering and could significantly contribute to advanced treatments for AMD and other retinal degenerative diseases.

Shape Memory PLA/TPU Blend Using High-Speed Thermo-Kinetic Mixing.

In this study, a thorough examination of the chemical, thermal, and mechanical characteristics, as well as shape memory behavior at low temperatures, of blends consisting of polylactic acid (PLA) and polyurethane (TPU) is conducted. The research involves the preparation of PLA/TPU mixtures with varying concentrations of TPU using a high-speed thermo-kinetic mixing approach. Chemical, morphological, and thermal analyses were conducted on pure PLA, TPU, and PLA/TPU mixtures by using Fourier Transform Infrared (FTIR), X-ray diffraction pattern spectroscopy (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). Mechanical properties were assessed through tensile and three-point bending tests. The achievement of a uniform mixture is confirmed through SEM images, reduction in the glass transition temperature according to DSC and DMA, and an improvement in mechanical properties compared to results documented in the literature, implying a more effective mixing method for the compounds. To assess the practical applicability of this blend, an investigation into the shape memory properties of the mixture when deformed at low temperatures, i.e., cold programming) is carried out. Gray relational analysis (GRA) is employed to identify the optimal TPU content for the mixture, considering both mechanical and shape memory properties. The results indicate that a mixture with a 20% volume fraction of TPU exhibits mechanical properties comparable to those of pure PLA, along with sufficient flexibility at room temperature and notable shape recovery properties.

Programable shape recovery properties and snap-through instabilities of viscoelastic and shape-memory NS metamaterials

The combination of smart material and mechanical metamaterial has the potential to bring novel properties and additional functionalities, which, however, has been ignored for a long time. This work numerically, experimentally, and analytically examines the negative stiffness (NS) metamaterials based on shape memory polymers (SMPs) with a focus on the tunable and temperature-dependent properties induced by the interaction of material and geometry nonlinearity. A universal discrete model of a series of shape-memory NS metamaterial is developed by separating the bending effect and stretching effect, where the viscoelastic material is described by the generalized Maxwell-Wiechert model, and an SMP constitutive model is proposed based on multiplicative decomposition of the deformation gradient. The mechanical and shape-memory properties influenced by temperature, geometry, material and loading rate are experimentally observed through relaxation tests and cyclic compression tests. Then the discrete model and finite element analysis (FEA) is adopted to examine the novel mechanical responses in a wide range of parameters. Results show that viscoelasticity and loading rate play a significant role in mechanical responses, and tunable load-capacity and stability can be achieved for shape-memory metamaterial. A phase diagram of different stabilities is constructed showing strictly monotonic, S-shaped, and snapping responses. Additionally, repeated compression and recovery performance proves the possibility of reusable applications. This work provides new opportunities for functional metamaterial to obtain programmable shape recovery and mechanical responses.

Mussel-inspired oxidized sodium alginate/cellulose composite sponge with excellent shape recovery and antibacterial properties for the efficient control of non-compressible hemorrhage

Enhancing the hemostatic efficacy and minimizing blood loss in the body has consistently been a primary objective for researchers. This study improved the hemostatic efficacy and tissue adhesion strength of the hemostatic material by augmenting the aldehyde groups in the side chains of sodium alginate. Additionally, it immobilized the aldehyde-modified sodium alginate onto the surface of the hemostatic material through complexation with iron ions, thereby enhancing its antibacterial properties. The mechanical performance results demonstrated that the composite hemostatic sponge can rapidly absorb water, swell to counteract arterial blood pressure, and seal the bleeding wound. The results of tissue and blood cell adhesion and in vivo hemostasis showed that the composite hemostatic sponge can enhance the rate of blood clot formation and the adhesion strength of tissue, thereby reducing the hemostasis time and blood loss. The bacterial and cell growth results demonstrated that the composite hemostatic sponge displays excellent biocompatibility and antibacterial properties. Based on the excellent water absorption, swelling properties, and softness of cellulose sponge, the composite hemostatic sponge offered a significant advantage for achieving rapid hemostasis in penetrating wounds or deep wounds with substantial bleeding.

Lignin Nanofiber Flexible Carbon Aerogels for Self-Standing Supercapacitors.

Renewable feedstocks are sought for clean technology applications, including energy storage applications. In this study, LignoForce™ lignin, a biobased aromatic polymer commercially isolated from wood, was fractioned into two parts using acetone, and the resulting lignin fractions had distinct thermo-rheological behavior. These two fractionated lignins were combined in various ratios and transformed into nanofibers by electrospinning. Subsequently, electrospun fiber materials were disrupted by agitating the mats in water, and the materials were transformed into ultralight 3D aerogels through lyophilization and post-process controlled heating. Using only this combination of two fractions, the morphology of lignin nanofibers was tailored by heat treatment, resulting in lignin aerogels with high flexibility and significant shape recovery properties. Various microscale structures of lignin fibers impacted the resulting physical properties of the elastic aerogel materials, such as resilience, compressive strength, and electrical conductivity for the corresponding carbonized samples. By exploiting lignin's sensitivity to heat and tailoring the thermal properties of the lignin through fractionation, the work provided an interesting path to form robust lignin-derived functional materials without any toxic chemical additives and significant ability to serve as free-standing electrodes with specific capacitance values better than some graphene-based supercapacitors.

Effect of Structural Configurations on Mechanical and Shape Recovery Properties of NiTi Triply Periodic Minimal Surface Porous Structures

Based on the advantages of triply periodic minimal surface (TPMS) porous structures, extensive research on NiTi shape memory alloy TPMS scaffolds has been conducted. However, the current reports about TPMS porous structures highly rely on the implicit equation, which limited the design flexibility. In this work, novel shell-based TPMS structures were designed and fabricated by laser powder bed fusion. The comparisons of manufacturability, mechanical properties, and shape recovery responses between traditional solid-based and novel shell-based TPMS structures were evaluated. Results indicated that the shell-based TPMS porous structures possessed larger Young’s moduli and higher compressive strengths. Specifically, Diamond shell structure possessed the highest Young’s moduli of 605.8±24.5 MPa, while Gyroid shell structure possessed the highest compressive strength of 43.90±3.32 MPa. In addition, because of the larger specific surface area, higher critical stress to induce martensite transformation, and lower austenite finish temperature, the Diamond shell porous structure exhibited much higher shape recovery performance (only 0.1% residual strain left at pre-strains of 6%) than other porous structures. These results substantially uncover the effects of structural topology on the mechanical properties and shape recovery responses of NiTi shape memory alloy scaffolds, and confirm the effectiveness of this novel structural design method. This research can provide guidance for the structural design application of NiTi porous scaffolds in bone implants.

Design of thermally programmable 3D shape memory polymer-based devices tailored for endovascular treatment of intracranial aneurysms

Despite recent technological advancements in endovascular embolization devices for treating intracranial aneurysms (ICAs), incomplete occlusion and aneurysm recanalization remain critical challenges. Shape memory polymer (SMP)-based devices, which can be manufactured and tailored to patient-specific aneurysm geometries, possess the potential to overcome the suboptimal treatment outcome of the gold standard: endovascular coiling. In this work, we propose a highly porous patient-specific SMP embolic device fabricated via 3D printing to optimize aneurysm occlusion, and thus, improve the long-term efficacy of endovascular treatment. To facilitate device deployment at the aneurysm via Joule-heating, we introduce a stable, homogeneous coating of poly-pyrrole (PPy) to enhance the electrical conductivity in the SMP material. Using an in-house pulse width modulation circuit, we induced Joule-heating and characterized the shape recovery of the PPy-coated SMP embolic devices. We found that the employed PPy coating enables enhanced electrical and thermal conductivity while only slightly altering the glass transition temperature of the SMP material. Additionally, from a series of parametric studies, we identified the combination of catalyst concentration and pyrrole polymerization time that yielded the shape recovery properties ideal for ICA endovascular therapy. Collectively, these findings highlight a promising material coating for a future coil-free, personalized shape memory polymer (SMP) embolic device, designed to achieve long-lasting, complete occlusion of aneurysms.

3D-printed injectable nanocomposite cryogel scaffolds for bone tissue regeneration

Cryogels are known for their high water content and interconnected macroporosity, two relevant features in tissue engineering approaches. The cryogel structure can support tissue growth as it allows nutrient and oxygen diffusion, removal of waste products, as well as an enhancement of cell infiltration and proliferation. Bioactive glass nanoparticles are biocompatible and clinically approved bioactive materials widely used as implants in the human body to repair or replace diseased or damaged bones. They are known to facilitate bone binding while stimulating bone growth. Indeed, the combination of cryogels with bioactive nanoparticles has already demonstrated promising results for bone regeneration. Although the developed biomaterials succeed in bone regeneration, they lack suitability for minimal invasive procedures or patient-specificity. Here, we demonstrate a freeform 3D printed nanocomposite cryogel, resorting to an ink composed of functionalized gelatin and bioactive glass nanoparticles with methacrylate groups. Complex structures with multiple layers were 3D printed in a xanthan gum supporting bath. The developed 3D printed nanocomposite cryogels demonstrate the ability to recover their shape without any permanent damage, withstanding up to 65 % compression upon injection. Additionally, they stimulate the differentiation of human adipose-derived stem cells into the osteoblast lineage, therefore promoting bone tissue growth. We further demonstrated their suitability for minimal invasive therapeutics by filling a reproduction of a maxillofacial defect. The developed 3D-printed nanocomposite cryogels offer robust shape-recovery properties, easy injectability, tailored geometry into patient-specific injuries, and high osteogenic bioactivity, showcasing its versatility for bone regeneration purposes.

4D printing of the ferrite permanent magnet BaFe12O19 and its intelligent shape memory effect

4D printing of the barium ferrite permanent magnets provides new opportunities for intelligent structure and process innovation of permanent magnets. In this paper, the preparation, filament manufacture, and intelligent printing methods with shape memory/recovery effect are studied for the barium ferrite permanent magnet. Firstly, the PLA/TPU/BaFe12O19 composite materials are developed by adding different contents of ferrite permanent magnet BaFe12O19 powders into the PLA/TPU (polylactic acid/thermoplastic polyurethane) matrix. The weight percentage of the BaFe12O19 is 10–40 wt% with an interval of 10 wt%. Secondly, the composites are pelletized and further extruded to manufacture composite filaments with different BaFe12O19 contents. Scanning electron microscope (SEM) is used for the microstructural analysis. The BaFe12O19 particles are uniformly distributed in the PLA/TPU matrix, and the slight particle agglomeration phenomenon occurs in the composite filaments with 30 and 40 wt% BaFe12O19. The thermal properties of PLA/TPU/BaFe12O19 composite filaments are analyzed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). After adding the barium ferrite particles, the glass transition temperature of the composite filaments changes within a small range from 53.5 °C to 55 °C. Thirdly, the filaments are successfully manufactured to magnet parts by the fused deposition modeling (FDM) 3D printing for the tests of mechanical, magnetic, and shape recovery properties. With the increase of the barium ferrite content, the magnetic properties show a rising trend, proving the feasibility of using 3D printing technology to make magnets. The shape memory effects of the printed parts are excellent and the shape recovery ratios show a rising trend, from 85.6 % to 88.9 % under heat contact stimulus and from 88.9 % to 97.8 % under electromagnetic induction non-contact stimulus. Relatively, the parts hold fast response speed due to direct contact with the heat source under the heat stimulus, and the parts hold higher shape recovery ratios due to more heats produced under the electromagnetic induction stimulus. The addition of magnetic particles is helpful to improve the heat transfer ability and provide the possibility of non-contact heat transfer. The magnetic parts with complex structure and intelligent shape memory/recovery effect can be achieved for special application requirements. As a result, the research of this paper expands preparation methods of new magnetic composite materials, explores the manufacture process for intelligent magnets and lays a solid foundation for the development of new permanent magnet devices.

The polytannic acid‐Fe(III) functionalized graphene oxide in poly (propylene carbonate) nanocomposite enabling enhanced shape memory, UV‐resistance, and self‐healing performance

AbstractPolypropylene carbonate (PPC) is a promising candidate of substrate materials used for human wearable devices due to its excellent ductility and biocompatibility. However, PPC is prone to be deformed, thus resulting in poor shape recovery property. One effective solution is to construct composites. In this work, poly (tannic acid) with Fe3+ (FPTA) @ graphene oxide (GO) was added to fabricate FPTA@GO/PPC composites. The results show that FPTA@GO can construct a tight network of hydrogen bonds into PPC to improve shape memory and self‐healing properties. By varying the FPTA@GO content, the composites exhibit tunable shape memory property. Dynamic mechanical analysis confirms that FPTA@GO/PPC composites show better shape memory properties compared with pure PPC. 5 wt% FPTA@GO/PPC composite demonstrates superior shape fixation ratio (Rf = 99%) as well as superior shape recovery ratio (Rr = 94%) in 37°C. Moreover, 5 wt% FPTA@GO/PPC composite possesses great self‐healing efficiency of 81% and tensile strength (17.2 MPa). The UV absorption capacity of 5 wt% FPTA@GO/PPC is increased by 150% (far and mid‐UV regions) and 400% (near‐UV region) compared with pure PPC. Thus, FPTA@GO/PPC composites have potential applications in developing shape memory, self‐healing, and UV‐resistant materials for human wearable devices.

Atomistic simulations of structure and motion of twin interfaces reveal the origin of twinning in NiTi shape memory alloys

Shape memory alloys (SMAs) are functional materials featuring unique shape recovery properties that make them suitable for key industrial applications. The essential process controlling this fascinating performance is microstructural twinning. Some twin systems are much more frequently observed than others, yet a fundamental mechanistic understanding of this evidence is missing, which hampers SMA design. Here, we consider the prototypical NiTi SMA to show that the emergence of twinning can be strongly affected by twin interface mobility. In this work, we adopt an integrated approach combining crystallographic theory, state-of-the-art atomistic modelling, topological model, and validation with high-resolution transmission-electron micrographs. Atomistic simulations confirm that the occurrence of twins is dictated by the driving force for twin boundary motion rather than interfacial energy. Moreover, our findings settle long-standing questions by elucidating the propagation mechanisms of twin interfaces, which the established martensite crystallography and kinetic theories could not address conclusively. The newly discovered understanding of the role of interface mobility in twin formation can be used to predict variant selection and guide the design of SMAs with improved functional performance.

Shape recovery and energy absorption properties of 3D printed continuous ramie fiber reinforced thin‐walled biocomposite structures

AbstractContinuous fiber reinforced composites are widely used in thin‐walled structures due to their high specific strength and stiffness. In this work, continuous natural fiber was introduced into thin‐walled biocomposite structures via 3D printing technique to enhance energy absorbing properties and promote ecological compatibility. The effects of varying configurations of printing speed, layer thickness, and path optimization on the deposition quality of continuous ramie fiber and polylactic acid matrix were explored. The results showed that reduced printing speed (100 mm/min) and optimal layer thickness (0.25 mm) effectively minimized structural forming defects. In addition, further enhancements in the printing quality could be achieved by smoothing the path with rounded corners. Based on optimal printing strategies, different configurations of thin‐walled biocomposite structures were fabricated. Lateral monotonic and cyclic load tests were performed to investigate their energy absorption and shape recovery capacities. When the loading displacement was 10 mm (strain was 20%), the circular structure presented good shape recovery capability, with measured recovery ratio remaining above 89%. The hexagonal structure showed a similar variation in shape recovery ratio as the quadrangular structure, both remaining above 75%. Moreover, the specific energy absorption of all the structures converged after two cycles, indicating their remarkable and stable repeatable load‐bearing capacity.Highlights Continuous ramie fiber reinforced thin‐walled structures were prepared. The deformation patterns of structures under lateral compression were analyzed. Energy absorption and shape recovery radio were studied under cycle loading. Printed structures exhibited great and stable repeatable load‐bearing capacity.

A novel method for preparing anisotropic negative Poisson's ratio composite foam with excellent structural stability and shape recovery property

Negative Poisson's ratio (NPR) foams are usually prepared by the transformation of positive Poisson's ratio (PPR) foams with honeycomb cell structure, which face a challenge for restraining the increase in volume and Poisson's ratio once they are stimulated by high temperature. In addition, they cannot restore to its reentrant structure after large deformation. In this work, a novel method for preparing anisotropic NPR foam with excellent structural stability and shape recovery property is established by synergistically controlling hierarchical pore structures. The obtained NPR silicone rubber/polylactic acid (SR/PLA) composite foam has large-size holes (auxetic structure) with concave hexagonal shape and small-size honeycomb cells, which are generated by 3D printing technology and salt leaching method, respectively. The auxetic structure with the orientation characteristic is directly formed rather than transformed from the honeycomb cell structure. Therefore, the dimension and Poisson's ratio value of the NPR SR/PLA composite foam keep constant after high temperature heating treatment, meaning an excellent structural stability. Furthermore the excellent shape recovery property of the NPR SR/PLA composite foam endows it restore to the initial auxetic structure after large deformation to a large extent. It should be mentioned that the anisotropy of the NPR SR/PLA composite foam further will bring a broad application prospect in the field of human body cushioning materials, such as prosthetic lining sleeve.

Mechanically interlocked [c2]daisy chain backbone enabling advanced shape-memory polymeric materials

The incorporation of mechanically interlocked structures into polymer backbones has been shown to confer remarkable functionalities to materials. In this work, a [c2]daisy chain unit based on dibenzo-24-crown-8 is covalently embedded into the backbone of a polymer network, resulting in a synthetic material possessing remarkable shape-memory properties under thermal control. By decoupling the molecular structure into three control groups, we demonstrate the essential role of the [c2]daisy chain crosslinks in driving the shape memory function. The mechanically interlocked topology is found to be an essential element for the increase of glass transition temperature and consequent gain of shape memory function. The supramolecular host-guest interactions within the [c2]daisy chain topology not only ensure robust mechanical strength and good network stability of the polymer, but also impart the shape memory polymer with remarkable shape recovery properties and fatigue resistance ability. The incorporation of the [c2]daisy chain unit as a building block has the potential to lay the groundwork for the development of a wide range of shape-memory polymer materials.

Preparation and Properties of Soft-/Hard-Switchable Transparent Wood with 0 °C as a Boundary

Transparent wood has excellent optical and thermal properties and has great potential utilization value in energy-saving building materials, optoelectronic devices, and decorative materials. In this work, transparent wood with soft-/hard-switchable and shape recovery capabilities was prepared by introducing an epoxy-based polymer with a glass transition temperature of about 0 °C into the delignified wood template. The epoxy resin was well filled in the pore structure of the delignified wood, and the as-prepared wood exhibited excellent transparency; the optical transmittance and haze of the transparent wood with a thickness of 2.0 mm were approximately 70% and 95%, respectively. Because the glass transition temperature of the epoxy-based polymer was about 0 °C, the prepared transparent wood was rigid below 0 °C and flexible above °C; meanwhile, the transparent wood exhibited shape change and shape recovery properties. Incorporating optical transparency and soft-/hard-switchable ability into the transparent wood opens a new avenue for developing advanced functional wood-based materials.

Force-activated ratiometric fluorescence switching of tensile mechano-fluorophoric polyurethane elastomers with enhanced toughnesses improved by mechanically interlocked [c2] daisy chain rotaxanes

The extended and contracted conformations of [c2] daisy chain rotaxanes were introduced in mechano-fluorophoric polyurethane (PU) scaffolds via step-growth polymerizations to obtain distinct mechanical and optical features upon external forces. Particularly, the inherent ductile and stretchable capabilities along with notable toughnesses of PU films containing very low contents of [c2] daisy chain moieties with long-range sliding motions were obviously improved. Furthermore, appealing Förster resonance energy transfer (FRET) behaviors with the optimum energy transfer efficiency ca. 26.02% between blue-emitting tetraphenylethylene (TPE) and yellow-orange-emitting rhodamine derivatives could be acquired in the PU film under tensile conditions, which could be assigned to the incorporation of bi-fluorophoric daisy chain rotaxane into PU frameworks to gain reversible dual fluorescence switching behaviors upon stretching and relaxation processes. Additionally, the stretching deformations of PU films were inspected by X-ray diffraction (XRD) and FTIR techniques for the verification of correlated morphological properties of stretching states in the oriented daisy chain rotaxane-based PU films. Interestingly, impressive shape recovery and reversible ratiometric mechano-fluorophoric fluorescence switching properties of [c2] daisy chain rotaxane-based PU films could be attained upon heating, suggesting potential applications of PU films comprising artificial molecular muscles with glorious mechanical and optical behaviors in advanced polymer networks.

Characterization of Anisotropic Shape Memory Behavior of Thermoresponsive Components in 4D Printing.

Four-dimensional (4D) printing has emerged as a promising manufacturing technology in recent years and revolutionized products by adding shape-morphing capabilities when exposed to certain stimuli. Increasing research attention has been dedicated to studying the shape memory behaviors of the 4D fabricated structures. However, in-depth discussions on quantifying the influence of process parameters on shape fixity and recovery properties are limited, and the anisotropy induced by the layer-wise fabrication nature is significantly underreported. To further exploit the shape memory property of 4D printed structures, it is essential to investigate the process-induced anisotropic shape memory behaviors. In this study, the effects of critical process parameters on anisotropy in shape memory properties are mathematically quantified; meanwhile, the feasibility of tailoring the anisotropy of 4D printed parts is examined with joint consideration of total build time. Different scanning patterns are experimentally analyzed for their influence on anisotropic behaviors. It is found that the Triangle scanning pattern often leads to the best shape memory behaviors in different directions. The outcome of this study confirms the existence of anisotropy in both shape fixity and shape recovery ratios. In addition, the results also reveal that a smaller scanning angle tends to minimize the anisotropy and total fabrication time while ensuring satisfactory shape memory performance. Furthermore, layer thickness shows negligible effects on anisotropy, while the scanning angle and shape memory temperature suggest the opposite.