Shape Recoverability Research Articles

Self-Recoverable Dual-Network Silicon Elastomer Applied in Cell Adhesives

Dual-network filler-free silicon elastomers with excellent mechanical properties and high tensile strength (>3 MPa) were fabricated through physical and chemical cross-linking via Fe3+ ion coordination and aza-Michael addition, respectively. Dynamic mechanical analysis and loading–unloading tests were used to analyze the sacrificial bonds and dual networks in the elastomers. Continuous-wave electron paramagnetic resonance results revealed the incomplete coordination and similar chemical environment of Fe­(III) ions in the elastomers. The coordination bonds dissipated energy, and the chemical cross-linking maintained the networks; they enhanced tensile strength jointly. The elastomers demonstrated favorable self and shape recoverability due to the dynamic reversibility of the coordination bonds and the stress of the chemical cross-linking in the reshaped elastomers. Such elastomers could be used as intelligent and sustainable materials. Moreover, the materials exhibited good biocompatibility and can be utilized as a type of cell adhesive in bioimaging.

Deployment performance of shape memory polymer composite hinges at low temperature

Shape memory polymer composite hinges, adapted for possible space applications, were successfully designed and fabricated, and performance tests at room temperature confirmed their full recoverability in our previous studies. Since shape memory polymer composite hinges are intended for space applications, they should be able to operate at low temperature. Even though the deployment of the hinge at room temperature triggered by the stimulation of a heating element has been quite promising, a suitable design for a shape memory polymer composite hinge with a heating element is more important at low temperatures because shape memory polymer composite hinges lose much heat to the environment. The recoverability of shape memory polymer composite hinges and the impact of the heating element design on the deployment time at low temperature are brought to light in this article. A shape memory polymer composite hinge with an attached heating element was fabricated as in our previous studies. The necessary power and supply power for deployment of the shape memory polymer composite hinge at a low temperature of –10°C were calculated, and a finite element analysis for the heating process was performed with the supply power. A folding and deployment test of the shape memory polymer composite hinge at –10°C was performed to show its shape recoverability. However, the shape memory polymer composite hinge did not deploy to its original shape. To determine the reason, measurements of temperature distribution were done using an infrared camera and thermocouples. The results revealed that the low temperature along the two side edges of the shape memory polymer composite tape prevented full deployment of the shape memory polymer composite hinge, which also revealed the need for design modification. The folding and deployment test of our modified shape memory polymer composite hinge demonstrated a nearly full deployment.

Origami-inspired shape memory dual-matrix composite structures

A sandwiched morphing structure is developed using an Origami-inspired shape memory dual-matrix composite core and shape memory polymer composite skins. The geometric parameters of the morphing structure are designed to have a zero Poisson’s ratio. In addition, an analytical model is developed to analyze the three-dimensional morphing structure easily. The shape memory dual-matrix composites are fabricated with woven fabrics based on the shape memory polymers, and an epoxy matrix is used to ensure a flexible and shape-recoverable structure. The shape recoverability of the shape memory polymer composite skins is verified by measuring the shape recovery ratio at various temperatures. Based on the tensile tests for the shape memory polymer composite skins and shape memory polymer hinges, it is found that the morphing structure can be highly flexible depending on temperature. Finally, the bending and shape recovery behaviors of the morphing structure are demonstrated.

Twinning Organosuperelasticity of a Fluorinated Cyclophane Single Crystal

Superelasticity is characterized by unique mechanical behavior, i.e., spontaneous shape recoverability by diffusionless plastic deformation and has been considered to be a property of specific allo...

How Water Can Affect Keratin: Hydration‐Driven Recovery of Bighorn Sheep (Ovis Canadensis) Horns

AbstractKeratin is one of the most common structural biopolymers exhibiting high strength, toughness, and low density. It is found in various tissues such as hairs, feathers, horns, and hooves with various functionalities. For instance, horn keratin absorbs a large amount of energy during intraspecific fights. Keratinized tissues are permanent tissues because of their basic composition consisting of dead keratinized cells that are not able to remodel or regrow once broken or damaged. The lack of a self‐healing mechanism presents a problem for horns, as they are under continued high risk from mechanical damage. In the present work, it is shown for the first time that a combination of material architecture and a water‐assisted recovery mechanism, in the horn of bighorn sheep, endows them with shape and mechanical property recoverability after being subjected to severe compressive loading. Moreover, the effect of hydration is unraveled, on the material molecular structure and mechanical behavior, by means of synchrotron wide angle X‐ray diffraction, Fourier transform infrared spectroscopy, nanoindentation, and in situ and ex situ tensile tests. The recovery and remodeling mechanism is anisotropic and quite distinct to the self‐healing of living tissue such as bones.

Versatile Shape Recoverability of Odd-Numbered Saturated Long-Chain Fatty Acid Crystals

Superelasticity, which has been known dominantly in association with a specific kind of alloy called shape-memory alloys, involves a peculiar recoverability after plastic deformation. However, meta...

High‐Strain Shape Memory Behavior of PLA–PEG Multiblock Copolymers and Its Microstructural Origin

ABSTRACTShape memory properties of two thermoplastic multiblock copolymers composed of poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) having different PEG‐segment lengths of 6 and 11 kDa were studied. The performance as a shape memory polymer at high strain level (600%) and its interrelations with shape‐programming conditions, molecular orientation, and microstructural changes are elucidated. A significant contribution of strain‐induced crystallization of PLA segments to the improvement of temporary shape fixation was evidenced upon increasing draw ratio and/or shape‐holding duration as well as programming temperature (within certain range) without largely sacrificing the shape recoverability. Series of microstructural characterizations reveal the occurrence of fibrillar‐to‐lamellar transformation upon shape recovery (at 60 °C) of the samples programmed at 40 °C, generating shish–kebab crystalline morphology. Such phenomenon is responsible for the high‐strain shape memory effect of these materials. The unprecedented formation of shish–kebab structure at such relatively low temperature (instead of the melting temperature range) in solid state observed in these copolymers as well as their high‐strain shape memory functionality would bestow the promising future for their practicability in diverse areas. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 241–256

Shape retainability and reusability investigation of bottle-shaped SMP mandrel

Shape memory polymer (SMP) mandrel, a kind of mandrel for fabrication of composite structures, has attracted great attention due to the advantages of low cost, simple operation and easy demoulding. In this paper, a type of styrene-based SMP with good shape retainability and recoverability were synthesized and examined by uniaxial tensile test and dynamic mechanical analysis test. Then, a bottle-shaped smart mandrel with shape memory effect was prepared by inflation method. Short/long-time shape retainability and reusability were characterized. Finally, the SMP mandrel was used to form a bottle-shaped composite structure model by filament winding technology. The excellent demoulding property and good reusable ability of SMP mandrel provided potential applications in the composite part production.

Mechanical properties and shape memory effect of thermal-responsive polymer based on PVA

In this study, the effect of content of glutaraldehyde (GA) on the shape memory behavior of a shape memory polymer based on polyvinyl alcohol chemically cross-linked with GA was investigated. Thermal-responsive shape memory composites with three different GA levels, GA-PVA (3 wt%, 5 wt%, 7 wt%), were prepared by particle melting, mold forming and freeze–drying technique. The mechanical properties, thermal properties and shape memory behavior were measured by differential scanning calorimeter, physical bending test and cyclic thermo-mechanical test. The addition of GA to PVA led to a steady shape memory transition temperature and an improved mechanical compressive strength. The composite with 5 wt% of GA exhibited the best shape recoverability. Further increase in the crosslinking agent content of GA would reduce the recovery force and prolong the recovery time due to restriction in the movement of the soft PVA chain segments. These results provide important information for the study on materials in 4D printing.

Enabling Simultaneous Extreme Ultra Low-k in Stiff, Resilient, and Thermally Stable Nano-Architected Materials

Low dielectric constant (low-k) materials have gained increasing popularity because of their critical role in developing faster, smaller, and higher performance devices. Their practical use has been limited by the strong coupling among mechanical, thermal, and electrical properties of materials and their dielectric constant; a low-k is usually attained by materials that are very porous, which results in high compliance, that is, silica aerogels; high dielectric loss, that is, porous polycrystalline alumina; and poor thermal stability, that is, Sr-based metal-organic frameworks. We report the fabrication of 3D nanoarchitected hollow-beam alumina dielectrics which k is 1.06-1.10 at 1 MHz that is stable over the voltage range of -20 to 20 V and a frequency range of 100 kHz to 10 MHz. This dielectric material can be used in capacitors and is mechanically resilient, with a Young's modulus of 30 MPa, a yield strength of 1.07 MPa, a nearly full shape recoverability to its original size after >50% compressions, and outstanding thermal stability with a thermal coefficient of dielectric constant (TCK) of 2.43 × 10-5 K-1 up to 800 °C. These results suggest that nanoarchitected materials may serve as viable candidates for ultra low-k materials that are simultaneously mechanically resilient and thermally and electrically stable for microelectronics and devices.

Electrical actuation and shape memory behavior of polyurethane composites incorporated with printed carbon nanotube layers

In this study, a novel digitally controlled spraying-evaporation deposition modeling process was used to deposit carbon nanotube (CNT) layers on shape memory polymer (SMP) films. This system is able to produce CNT layers with variable thicknesses and arbitrary patterns on defined locations. The study on the electrical properties of composite films revealed that the composites with 50 CNT layers exhibited the lowest sheet resistance of 28.7 Ω/sq. Accordingly, the resistive heating performance of the fabricated composites was investigated at different power densities. The maximum surface temperature can be adjusted by simply tuning the numbers of printed CNT layers. Particularly, the even temperature distribution or temperature gradient could be realized on the surface of composites through modifying the design of printed CNT patterns. The study on the shape memory effect of the CNT/SMP composites indicated that the CNT/SMP composites can be actuated by the resistive heating of the printed CNT layers without an external heater. The shape recoverability of the composites was approximately 100% taking 30s under 40 V. It is worth noting that through programming the number of printed CNT layers at different locations, the shape recovery rate could be controlled and localized actuation with the desired recovery ratio was achieved. The high efficiency of heating coupling with wide adjustability of surface temperature and shape recovery ratio at specified locations make the fabricated CNT/SMP composite a promising candidate for various heating and actuation applications.

Shape memory effect of thermal-responsive nano-hydroxyapatite reinforced poly-d-l-lactide composites with porous structure

In this study, the effect of the inclusion of nano-hydroxyapatite (nano-HAp) and the porous structure on the shape memory behavior of poly-d-l-lactide (PDLLA) composites was investigated. Porous, thermal-responsive shape memory composites, PDLLA/nano-HAp, were prepared by a newly developed polymer coagulation, particulate leaching, and cold compression moulding technique. The inclusion of nano-HAp to PDLLA led to a higher shape memory transition temperature, improved mechanical strength and larger recovery force as compared with the neat PDLLA. The nanocomposite with 10 wt% of nano-HAp exhibited the best shape fixity and recoverability with the shortest recovery time. Further increase in the filler content of nano-HAp would reduce the recovery force and prolong the recovery time due to restriction in the movement of the amorphous PDLLA chain segments. The morphologies, thermal properties and shape memory behavior were measured by scanning electron microscopy, thermal gravimetric analyzer, differential scanning calorimeter, dynamic mechanical analysis, physical bending test and cyclic thermo-mechanical test. These results provide important information for designing biomedical devices using this novel type of bioabsorbable shape memory polymer composite.

A Multiscale Investigation on the Mechanism of Shape Recovery for IPDI to PPDI Hard Segment Substitution in Polyurethane

Shape memory thermoplastic polyurethane (SMTPU) containing isophorone diisocyanate (IPDI) in hard segment has excellent shape recoverability even after large strain deformation. However, the underlying mechanism of shape recovery remains unclear. In this study, 1,4-phenylene diisocyanate (PPDI) in the polyurethane is gradually substituted by IPDI, and multiscale effects are examined by normal and dichroic Fourier transform infrared spectroscopy (FTIR), small-angle X-ray scattering (SAXS), single-molecule force spectroscopy (SMFS), and mechanical test. Contradictory to the traditional conclusion, the degree of microphase separation decreases as the content of IPDI increases, while the macroscopic shape recoverability is largely improved. With dichroic FTIR and SAXS, we find that the morphology of hard phases changed from lamellar-like to fibrillar-like, which is more stable under stretching. SMFS experiments discover that IPDI could increase the elasticity of polymer chain and could endow the hard phases w...

Influence Of Conducting Polymer On Mechanical, Thermal And Shape Memory Properties Of Polyurethane/polythiophene Blends And Nanocomposite

Polyurethane/polythiophene (PU/PTh) blends and nanocomposites were prepared by solution mixing and in situ polymerization, respectively and were investigated for mechanical, thermal, electrical and shape memory properties. Formation of blends and composites was supported by FTIR analysis. Surface morphology of prepared samples was clarified by scanning electron microscopy (SEM). Homogeneous morphology of the composites was observed compared to blends due to well dispersion of NH2 functionalized MWCNTs attributed to introduction of urea linkages between the functionalized nanotubes and the NCO-terminated PU. Smooth morphology of the composites resulted for the significant improvement of the mechanical properties. Thermal stability of the blends and composites was found increased with PTh content. According to differential scanning calorimetry (DSC), an increase in glass transition, melting and crystallization temperature was observed for composites with PTh addition. Maximum shape recoverability (92 %) was exhibited by the PU/PTh composite with 1 wt. % PTh loading. Copyright © 2016 VBRI Press.

Coaxial carbon@boron nitride nanotube arrays with enhanced thermal stability and compressive mechanical properties.

Vertically aligned carbon nanotube (CNT) arrays have aroused considerable interest because of their remarkable mechanical properties. However, the mechanical behaviour of as-synthesized CNT arrays could vary drastically at a macro-scale depending on their morphologies, dimensions and array density, which are determined by the synthesis method. Here, we demonstrate a coaxial carbon@boron nitride nanotube (C@BNNT) array with enhanced compressive strength and shape recoverability. CNT arrays are grown using a commercially available thermal chemical vapor deposition (TCVD) technique and an outer BNNT with a wall thickness up to 1.37 nm is introduced by a post-growth TCVD treatment. Importantly, compared to the as-grown CNT arrays which deform almost plastically upon compression, the coaxial C@BNNT arrays exhibit an impressive ∼4-fold increase in compressive strength with nearly full recovery after the first compression cycle at a 50% strain (76% recovery maintained after 10 cycles), as well as a significantly high and persistent energy dissipation ratio (∼60% at a 50% strain after 100 cycles), attributed to the synergistic effect between the CNT and outer BNNT. Additionally, the as-prepared C@BNNT arrays show an improved structural stability in air at elevated temperatures, attributing to the outstanding thermal stability of the outer BNNT. This work provides new insights into tailoring the mechanical and thermal behaviours of arbitrary CNT arrays which enables a broader range of applications.

Investigation on material removal rate, surface and subsurface characteristics in wire electro discharge machining of Ti50Ni50-xCux shape memory alloy

TiNiCu shape memory alloys have superior properties as compared with NITINOL due to their greater ductility, reduced hysteresis temperature range, and quick actuation response. The present article investigates the surface and subsurface modifications occurring due to wire electro discharge machining of Ti50Ni50-xCux shape memory alloy. The machining experiments were performed considering the pulse on time, pulse off time, and servo voltage as the process parameters. The influence of these parameters was studied on the material removal rate, surface roughness, recast layer thickness, microhardness, and phase changes in the machined surface. Longer pulse on time causes greater discharge energy, hence leading to higher material removal rate, surface roughness, and recast layer thickness. The machined surface hardness increased up to 900 Hv, which is about 59% increase with respect to the base material for longer pulse on time due to the recast layer thickness and the formation of oxides. A phase change on the machined surface was observed to cause the shape recoverability of the alloy. The microstructure, composition through EDAX, and the phase changes of the machined surface are also discussed in the article.

Dual self-healing abilities of composite gels consisting of polymer-brush-afforded particles and an azobenzene-doped liquid crystal.

We prepared the composite gels from polymer-brush-afforded silica particles (P-SiPs) and an azobenzene-doped liquid crystal, and investigated their inner structure, dynamic viscoelastic properties, thermo- and photoresponsive properties, and self-healing behaviors. It was found that the composite gels had a sponge-like inner structure formed with P-SiPs and exhibited good elastic property and shape recoverability. The surface dents made on the composite gel could be repaired spontaneously at room temperature. Moreover, the composite gel exhibited a gel-sol transition induced by the trans-cis photoisomerization of the azo dye, and the transition could be used as a mending mechanism for surface cracks. Consequently, we successfully developed a material exhibiting two types of self-healing abilities simultaneously: (1) spontaneous repair of surface dents by means of the excellent elasticity of the composite gel and (2) light-assisted mending of surface cracks by photoinduced gel-sol transition.

Origin of highly recoverable shape memory polyurethanes (SMPUs) with non-planar ring structures: a single molecule force spectroscopy investigation

In this work, SMPUs with non-planar ring structures in the hard segments display a low degree of phase separation but excellent shape recoverability (shape recovery rate ∼99% with 500% strain). The accepted wisdom is that there are two criteria for SMPUs possessing good shape recoverability: (i) high degree of phase separation forming physical crosslinks; (ii) strong physical interactions between hard segments which keep physical crosslinks stable under external stress. However, our results are completely against the accepted wisdom since the asymmetrical non-planar ring structures will depress the micro-phase separation and physical interactions in the hard phase. Thus, the excellent shape recovery could not be attributed to the phase morphology. Based on such results, single molecule force spectroscopy was adopted to study the properties of single polymer chains with non-planar ring structures. We found that the single chain elasticity was largely improved by non-planar rings. It is highly possible that the excellent shape recovery property originates from the elastic non-planar ring structures absorbing the external stress which stabilizes the physical crosslinks. Much work needs to be done in the near future to confirm this assumption.

The modification of graphene with alcohols and its use in shape memory polyurethane composites

AbstractGraphene prepared by the thermal reduction of graphite oxide was modified by reactions with methanol or 1‐butanol using aqueous HBF4 solution as a catalyst. Results showed that the reaction created hydroxyl groups on the graphene and at the same time reduced the number of defects. Gravimetry, thermogravimetry, X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy showed that the alcohols had reacted with epoxide groups on graphene. Raman spectroscopy showed that the defects in the graphene were repaired through other accompanying reactions. The reinforcing effect of graphene, observed in the tensile properties and the shape memory behavior of graphene/polyurethane composites, was increased when the graphene was modified with methanol. However, decreases in density and glass transition temperature were evident for the composites made with alcohol‐modified graphene. These results show that the newly created hydroxyl groups on graphene produce effective covalent bonds with the polyurethane chains of the matrix; however, the increased number of bonds restricts the rearrangement of the matrix molecules for dense packing. The covalent bonds between graphene and polyurethane chains enhanced shape recoverability and reduced the hysteresis brought about by repeated thermomechanical cycles. Copyright © 2012 Society of Chemical Industry

Stretch along the craniocaudal axis improves shape recoverability of the spinal cord

The spinal cord is physiologically stretched along the craniocaudal axis, and is subjected to tensile stress. The purpose of this study was to examine the effect of the tensile stress on morphological plasticity of the spinal cord under compression and decompression condition. The C1–T2 spinal column was excised from 4 rabbits. The laminae and lateral masses were removed. After excision of surrounding structures, a small rod was placed on the spinal cord. The rod was connected with a pan of the scale balance. Varying the weight between 0 and 20g on the other scalepan, the indentation of the rod was measured. Then, the spinal cord was cut transversely to remove longitudinal tensile stress. The samples were measured again with the same protocol at point 10mm caudal to each pre-measured point on the spinal cord. The shape recovery rate was calculated. The length of the spinal cord decreased by 9.7% after the separation. The maximum indentation was 2.1mm (mean) at 20g, and did not differ between the separated and un-separated cords. The recovery rate was not significantly different between the separated and un-separated cords until 3g. At the load under 3g, the recovery rate after the separation was significantly lower than that before the separation. The tensile stress along the craniocaudal axis in the spinal cord did not affect the spinal cord deformation in response to the compression, but it did affect the shape recoverability after the decompression.