Shape Fixity Ratio Research Articles

Distributed sensing based real‐time process monitoring of shape memory polymer components

AbstractShape memory polymer (SMP) materials have the capacity to undergo large deformations imposed by mechanical loading, hold a temporary shape, and then recover their original shape upon exposure to a particular external stimulus. The fiber reinforced shape memory polymer composites (SMPCs) with enhanced structural performances give a boost to breakthrough technologies for large‐scale engineering applications. This article presents a novel technique for distributed optical fiber sensor (DOFS) embedded SMPCs intended for real‐time process monitoring of large‐scale engineering applications such as deployable space structures. Herein a carbon fiber reinforced SMPC was tested under a three‐point flexural shape memory process and the DOFS data were acquired through optical backscatter reflectometry. Experiments were conducted in a temperature controlled thermal chamber coupled with a 10 kN electromechanical testing system. DOFSs offered unique advantages for spatially distributed dynamic temperature and strain measurements during the shape memory process. Compared to the standard test method dynamic mechanical analysis, larger samples can be tested effectively by using a single DOFS with large strain levels and shape complexity. The proposed technique demonstrated the ability of embedded DOFSs for in‐situ shape memory characterization such as shape fixity ratio, shape recovery ratio and recovery rate. This technique will eliminate the challenges hindering the process monitoring and performance evaluation of large SMPC components operating in their real working environments.

Multi-attribute parametric optimisation of shape memory polymer properties for adaptive orthopaedic plasters

Fall-related injuries and road accidents often demand orthopaedic emergency care and management in hospitals. Early and effective fracture treatment immobilisation techniques are essential for stabilising the musculoskeletal system of the patient. During past decades, shape memory polymers (SMPs) have rapidly been developed and they have exhibited their potential to replace traditional plaster of Paris or wood casting orthopaedic treatment practices. In this context, SMPs must match all the operational conditions of traditional materials while keeping SMP's superior qualities such as adjustability and ease of removal for observation and reapplication. In order to achieve their significantly delicate shape memory effect and shape fixity properties, SMP needs an accurately engineered material characterisation process. Therefore, this research comprises and extensive evaluation of shape memory polymer composites’ (SMPCs’) thermo-mechanical properties under ASTM/ISO standards for extended use in the orthopaedic medical field.The robust Taguchi (L18) optimisation approach combined with a multi-responses grey relational study was employed to achieve optimum SMPC fabrication parameters for orthopaedic applications while prioritising the required thermo-mechanical and shape memory properties. It was determined that the number of fabric layers is the most decisive parameter (P-value 0.013, relative contribution > 37.86%) for achieving SMPs’ delicate and crucial properties for biomedical applications. In this study, it was found that one layer of glass fibre (SMP weight fraction > 89%) provided the optimum thermo-mechanical and shape memory properties. Moreover, optimised control variables assured the self-life of the SMP casting material by retaining a programmed shape for 30 days (shape fixity ratio >98%), which is an exclusive property for an orthopaedic device. This study provides first-hand pre-engineered experimental program optimisation techniques to meet the rapid growth of SMPC orthopaedic non-invasive smart plasters and adaptive devices.

Tensile and shape memory properties of polylactic acid/ethylene-vinyl acetate blends

Thermally-induced shape memory polymers (SMPs) can be fabricated to contain two different components in the structure, namely a stable polymer network and a reversible switching unit, which will enable the shape fixity and shape recovery of SMP. In this work, the effect of the blend ratios of ethylene-co-vinyl acetate and polylactic acid (EVA/PLA) blend on the thermo-responsive and tensile properties were investigated. EVA/PLA blends with different blend ratios were prepared using an internal mixer and then compression moulded to the desired shape for testing. The shape memory properties such as shape fixity ratio (Rf) and shape recovery ratio (Rr) were tested based on bending tests at different deformation and recovery temperatures. Meanwhile, the tensile properties were tested based on ASTM D638 Type IV. The results revealed that EVA85/PLA15 which consists of 85 wt% of EVA and 15 wt% of PLA and possessed morphological characteristic of PLA domains in EVA continuous phase exhibited the optimum Rf and Rr where the glass transition temperature (Tg) of PLA as switching temperature. Besides that, EVA85/PLA15 blend also showed the most desirable tensile properties with high elongation at break and reasonably good tensile strength.

Cyanate esters as a high performing shape memory polymer: A review

Cyanate Ester (CE) based Shape memory polymers (SMPs) are one of the emerging SMPs with a considerable potential for space applications. As a thermoset polymer with excellent mechanical and thermal properties due to its 3D polycyanurate network, CEs have an added qualification with its shape memory property. From the perspective of shape memory characteristics, by co-curing with epoxies and modifiers, several properties of CEs including toughness, glass transition temperature (55 0C to 178 0C), durability (up to 30 shape memory cycles) can be tailored to achieve desired properties for a particular application. Herein the effects of using modifiers such as Polyethylene glycol (PEG), Polybutadiene epoxy (PBEP) were reviewed. While readily available chemicals such as PEG and PBEP has shown shape fixity (Rf) and recovery ratios (Rr) over 95%. Most of the reviewed CE SMPs have retained more than 90% Rf &Rr even after 30 shape memory cycles. Many fundamental research work has used fillers and reinforcements such as carbon clack, graphene and carbon fibres for the electro-activation of SMPs. It has been reported that carbon black and short carbon fibre reinforced CE SMPs are very effective for rapid activation of SMPs. Also, it has been reported that, Graphene can effectively substitute carbon black with much lower quantities and reduce the weight of the components significantly. Furthermore, it has been revealed that the effect on CE based SMPs by space mimicking conditions such as high vacuum, ultraviolet (UV), gamma and atomic oxygen were insignificant on the shape memory properties of CE SMPs.

3D-printed NIR-responsive shape memory polyurethane/magnesium scaffolds with tight-contact for robust bone regeneration

Patients with bone defects suffer from a high rate of disability and deformity. Poor contact of grafts with defective bones and insufficient osteogenic activities lead to increased loose risks and unsatisfied repair efficacy. Although self-expanding scaffolds were developed to enhance bone integration, the limitations on the high transition temperature and the unsatisfied bioactivity hindered greatly their clinical application. Herein, we report a near-infrared-responsive and tight-contacting scaffold that comprises of shape memory polyurethane (SMPU) as the thermal-responsive matrix and magnesium (Mg) as the photothermal and bioactive component, which fabricated by the low temperature rapid prototyping (LT-RP) 3D printing technology. As designed, due to synergistic effects of the components and the fabrication approach, the composite scaffold possesses a homogeneously porous structure, significantly improved mechanical properties and stable photothermal effects. The programmed scaffold can be heated to recover under near infrared irradiation in 60s. With 4 wt% Mg, the scaffold has the balanced shape fixity ratio of 93.6% and shape recovery ratio of 95.4%. The compressed composite scaffold could lift a 100 g weight under NIR light, which was more than 1700 times of its own weight. The results of the push-out tests and the finite element analysis (FEA) confirmed the tight-contacting ability of the SMPU/4 wt%Mg scaffold, which had a signficant enhancement compared to the scaffold without shape memory effects. Furthermore, The osteopromotive function of the scaffold has been demonstrated through a series of in vitro and in vivo studies. We envision this scaffold can be a clinically effective strategy for robust bone regeneration.

Thermal design and analysis of a flexible solar array system based on shape memory polymer composites

Smart structures based on shape memory polymer composites (SMPCs) have attracted extensive attention because of their unique self-deployment behavior. This study investigated the thermal design and analysis of the SMPC flexible solar array system (SMPC-FSAS). The thermal design ensured the reliability of the deployment functionality and desirable structural temperatures. The fundamental properties of the shape-memory materials were obtained by dynamic mechanical analysis and shape fixity ratio tests. Thermal experiments, including the thermal balance test (TBT), thermal vacuum test and thermal cycle test were conducted to verify the safety and reliability of the SMPC-FSAS. Additionally, the temperature distribution of the SMPC-FSAS was simulated using numerical analysis. Notably, a geostationary satellite carrying the SMPC-FSAS was successfully launched into a geosynchronous orbit and controlled deployment was accomplished for the first time. The prediction of the numerical model was consistent with the TBT and on-orbit data, thus validating the accuracy of the numerical method. The research in this work has important reference significance for ultra-large SMPC-FSAS in the future.

Engineering the shape memory parameters of graphene/polymer nanocomposites through atomistic simulations: On the effect of nanofiller surface treatment

This paper aims to comprehend the mechanisms underlying the shape memory behavior of polylactic acid infused with graphene functionalized by four groups of –OH, –CH3, –NH2, and tethered polymer layer. Applying molecular dynamics simulation, it is revealed that the graphene surface treatment enhances the shape fixity ratio of nanocomposites monotonically by increasing the physical cross-linking points within the polymer matrix. The improvement would be even more pronounced by increasing the coverage degree of small functional groups and grafting density of the covalently bonded polymer chains. Monitoring the key parameters illustrates that contrary to the OH groups, which improve the shape recovery value, the other functional groups degrade it by prohibiting the polymer chains mobility. Attempts to explore the governing mechanism demonstrate that shape fixity is improved when the difference between the potential energy variations in the loading and unloading stages increases. Interestingly, shape recovery is only under the influence of conformational entropy, and it is not affected by the potential energy. As such, we also probe variations of the radius of gyration during the recovery stage to address the role of different functionalization procedures on the reported shape recovery parameter.

Dual-functional polymer blends with rapid thermo-responsive shape memory and repeatable self-healing properties

Although dynamic chemical interaction plays a crucial role for designing sustainable materials with extended lifespans and functionality, the preparation complexity and lack of acceptable mechanical strength limit their practical application. Herein, tunable thermo-responsive shape memory and self-healing bifunctional blends are fabricated by melt blending of poly(ethylene-co-methacrylic acid sodium salt) (EMNa) and poly(ethylene-co-methyl acrylate) (EMA) copolymers. To realize the complementary advantages, the ionic interactions in the EMNa ensure the structural integrity and self-healing repeatability of the blends, meanwhile the highly elastic EMA as the dispersed phase is capable to provide the main driving force during the shape recovery process. The shape recovery behavior can be well controlled by varying the EMA content. The shape recovery time shortens with increasing EMA content, and a U-shaped EMNa with 30 wt% of EMA (EMNa/EMA-30) specimen could recover to a plate shape within 8 s. Interestingly, the EMNa/EMA-30 exhibits repeatable self-healing capability, and its tensile strength can reach 19.4 and 15.9 MPa after one and three healing cycles, respectively. The self-healed EMNa/EMA-30 still exhibits excellent reusability and endurance, and the shape fixity and shape recovery ratios are both above 95% after 15 shape memory cycles. The rapid thermo-responsive shape memory effect, superior self-healing property, and prominent programmability make the EMNa/EMA blends promising in various cutting-edge applications, such as self-deployable structures, artificial muscles, and wearable devices.

Photo-Dissociable Fe3+-Carboxylate Coordination: A General Approach toward Hydrogels with Shape Programming and Active Morphing Functionalities.

An extendable double network design for hydrogels with programmable external geometries and actuating trajectories is presented. Chemically cross-linked polyacrylamide as the first network penetrated with linear alginate chains is prepared for demonstration. The coordination of Fe3+ ions with carboxylate groups in alginate chains acts as the second network, and its dissociation through photoreduction is utilized to realize the photoresponsive shape memory property; the shape fixity ratio and shape recovery ratio both exceed 90%. The gradient dissociation of Fe3+-carboxylate coordination under UV facilitates 3D programming of hydrogel geometry. On another aspect, the resulted cross-linking gradient differentiates the extent and rate of solvent-induced volume change of the PAAm network, endowing the hydrogel with photo-programmable solvent-driven actuating behavior. Furthermore, by inducing the formation of Fe3+-carboxylate coordination within the entire network for shape programming and cross-linking gradients in specific regions as active joints, hydrogels with designed actuating behaviors based on specific 3D shapes are realized. The shape memory and active morphing functionalities enabled by photo-dissociable Fe3+-carboxylate coordination in PAAm hydrogel can be generally extended to other hydrogels.

Tuning Microphase Separation, Thermal Characteristics, and Physicomechanical Properties of Shape Memory Polyurethanes by Incorporation of Isocyanate‐Modified Fe3O4 Magnetic Nanoparticles

AbstractFe3O4 magnetic nanoparticles (MNPs) are subsequently modified with (3‐aminopropyl) triethoxysilane (APTES) and octadecyl isocyanate (OD) to yield MNPs‐g‐OD. Fourier‐transform infrared (FTIR), X‐ray diffraction (XRD), thermogravimetric analysis (TGA), and vibrating sample magnetometer (VSM) verified successful modification of MNPs with three different ratios of OD. Shape memory polyurethane (SMPU) nanocomposites containing 4 wt% of the neat, APTES‐grafted, and OD‐grafted MNPs are prepared through solution casting, where a strong correlation is achieved between the microstructure, crystallinity, thermal, and mechanical properties of the nanocomposites. Phase mixing degree of the soft and hard phases is increased by hydrogen bonding between the OD and polyurethane (PU) chains, causing to decrease of crystallization ability of PU chains. Field‐emission scanning electron microscopy (FE‐SEM) images show that high dispersion of MNPs‐g‐OD reduces the number and size of their aggregation sites in the PU matrix. The nanocomposites show high Young's modulus, because of increasing hydrogen bonding index of carbonyl groups with incorporation of MNPs‐g‐OD into the PU matrix. The nanocomposites show lower Young's modulus with increasing the degree of crystallinity. Shape memory studies indicated that all the samples have considerable shape memory abilities at 50 °C and room temperature. However, shape fixity ratios show a close dependence on the degree of crystallinity at room temperature.

Effect of shear thickening gel on structure and properties of flexible polyurethane foam

Compression and low-velocity impact properties of shear thickening gel/polyurethane foam (STG/PUF), a soft protective material, have been reported. In order to have a deeper understanding of the relationship between the structure and properties of this material, we utilized Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Proton nuclear magnetic resonance (1H-NMR) to study and prove that STG and polyurethane (PU) did not produce new chemical bonds during the blending process, which belongs to physical blending. Because the molecular chains of STG and PU are intertwined, the composites are endowed with higher thermal stability, safety protection performance, and shape memory performance. For instance, Young’s modulus (E) of the STG/PUF was automatically increased by 57% when stimulated by external forces at different rates and the Emax over 200 kPa, demonstrating a pronounced shear thickening effect. Shape memory tests showed that the shape fixity ratio of STG/PUF gradually increases with the increase of STG content, and the shape recovery ratio remained at 100%. More importantly, the STG/PUF was flexible and comfortable, had excellent mechanical properties and a longer service life, and can be extended to various human protective equipment.

Study on electroactive shape memory behaviors of polyvinyl alcohol/waterborne polyurethane/short carbon fiber composites

AbstractThe properties of electroactive shape memory composites fabricated from polyvinyl alcohol (PVA) and waterborne polyurethane (WPU) filled with short carbon fibers (SCFs) were studied. The addition of WPU effectively enhanced the elongation property and reduced the glass transition temperature. The elongation at break of the composites doubled, with a maximum content of WPU at nearly 50%. The tensile strength of the samples increased first and then decreased, reaching the maximum when the content of WPU was 10%. At the same time, the sample with WPU content of 10% showed the best electrical properties and the speed of response to external stimuli was faster. However, with the increase of WPU, the shape fixity ratio and the shape recovery ratio of the composites slightly dropped. Further, a reciprocating deformable robot was designed without adding any other materials, which could move forward under the action of alternating electric field. The above results indicated that the composites blended with an appropriate content of WPU could offer the possibility for the potential application in the field of soft robot.

4D Multimaterial Printing of Programmable and Selective Light‐Activated Shape‐Memory Structures with Embedded Gold Nanoparticles

Abstract4D printing is based on 3D printing of objects that can change their shape upon a proper triggering. Here, a novel approach is reported for fabricating programmable 3D printed objects composed of shape‐memory polymers (SMPs) that are activated by light. The light activation of the movement and shape morphing are based on combining gold nanoparticles (AuNPs) as photothermal converters with acrylate‐based printing compositions that form an SMP with tunable transition temperatures. The shape change of the printed objects is triggered by remote irradiation with a low‐cost LED light at a wavelength specific to the surface plasmon resonance of the embedded AuNPs. The light is converted to heat which enables the shape transition when the temperature reaches the Tg of the polymer. Excellent SMP properties are achieved with shape fixity and recovery ratios over 95%. This material composition and triggering approach enable fabricating programmable light‐activated 3D printed structures with a dual transition while tuning the concentration. Furthermore, numerical simulations performed by finite‐element analysis result in the excellent prediction of the shape‐memory recovery. The presented approach can be applied in remotely controlling morphing, mainly for applications in the fields of actuators and soft robotics.

Efficient inductively heated shape memory polyurethane acrylate network with silane modified nanodiamond@Fe3O4 superparamagnetic nanohybrid

Multifunctional magnetic shape memory polymer (SMP) nanocomposites with high sensitivity was synthesized through inclusion of silane functionalized [email protected]3O4 (S-NDF) hybrid nanoparticle into polycaprolactone (PCL) based polyurethane acrylate (PUA) matrix followed by in situ crosslinking of the matrix. Highly biocompatible and superparamagnetic nanodiamond(ND)@Fe3O4 nanohybrids were synthesized through in situ co-precipitation method. The morphological analysis suggested that S-NDFs filled PUAs (2 to 9 wt% loadings) well interacted with both soft and hard domains of the matrix. The base polymer and the nanocomposites presented excellent shape fixity ratio (above 97%) and shape recovery ratio (above 99.5%) in hot water. Furthermore, PUA/S-NDF nanocomposites with nanohybrid loading of 9 wt% exhibited excellent shape recovery rate (above 96%) in a very small magnetic field (H = 0.76 kA.m−1) as well as faster magnetic responsiveness compared to Fe3O4 loaded nanocomposites. Moreover, PrestoBlue cell viability assay suggested biocompatibility of the base polymers and the nanocomposites.

4D-Printed Resins and Nanocomposites Thermally Stimulated by Conventional Heating and IR Radiation

The shape memory (SM) capabilities of nanocomposites based on two photocurable acrylated/methacrylated resins, doped with carbon nanotubes (CNTs), and manufactured by digital light processing 3D printing were investigated. The mechanical properties and glass transition temperature (Tg) can be tailored in a broad range by varying the weight ratio of the two resins (Tg ranging from 15 to 190 °C; Young’s modulus from 1.5 to 2500 MPa). Shape fixity (SF) and recovery (SR) ratios are strongly influenced by the temperature being significantly higher at temperatures close to the Tg. The results confirm that the SF strongly depends on the stiffness of chain segments between cross-linking points, whereas the SR mainly depends on the cross-link density of the network. CNT addition barely affects the SF and SR in the conventional oven, whereas the recovery speed using IR heating is significantly increased for the doped nanocomposites due to their higher IR absorbance.

4D printing high temperature shape-memory poly(ether–ether–ketone)

Shape-memory polymers have exhibited extensive attractive properties for practical utility and applied potential, such as in clothing industry and for medical implants, which demands soft materials amenable to change shapes at low temperature. Whereas, potential applications in aerospace engineering require materials with higher modulus and higher transition temperature to ensure their effectiveness under harsh environments. As an engineering thermoplastic polymer material, poly(ether–ether–ketone) (PEEK) is a promising candidate that meets the stringent requirements for aerospace applications. Yet, additive manufacturing of PEEK polymers, to bring out the full potential of the materials, remains challenging and unfulfilled because of the intrinsic very high processing temperature and high modulus. In this paper, we present that 4D printing of the high temperature PEEK polymer with outstanding shape memory properties. We systematically investigate the effect of different printing parameters and heat treatment conditions on the corresponding shape memory performance, which has been rarely reported. The highest shape fixity ratio and highest shape recovery ratio that we can achieve in the 3D-printed PEEK samples is 99% and 87.8%, respectively, with Young’s modulus of 3.23 GPa and good thermal stability below 520 °C. Furthermore, the incomplete shape recovery processes have been exploited to fabricate more complex programming of shape changes and to demonstrate the potential for practical applications, and models of PEEK have been designed and printed to demonstrate the outstanding shape memory properties and for potential applications in high temperature environment.

Nanocomposite electrospun fibers of poly(ε-caprolactone)/bioactive glass with shape memory properties

Electrospun fibers of shape memory triethoxysilane-terminated poly(epsilon-caprolactone) (PCL-TES) loaded with bioactive glasses (BG) are here presented. Unloaded PCL-TES, as well as PCL/BG nanocomposite fibers, are also considered for comparison. It is proposed that hydrolysis and condensation reactions take place between triethoxysilane groups of the polymer and the silanol groups at the BG particle surface, thus generating additional crosslinking points with respect to those present in the PCL-TES system. The as-spun PCL-TES/BG fibers display excellent shape memory properties, in terms of shape fixity and shape recovery ratios, without the need of a thermal crosslinking treatment. BG particles confer in vitro bioactivity to PCL-based nanocomposite fibers and favor the precipitation of hydroxycarbonate apatite on the fiber surface. Preliminary cytocompatibility tests demonstrate that the addition of BG particles to PCL-based polymer does not inhibit ST-2 cell viability. This novel approach of using bioactive glasses not only for their biological properties, but also for the enhancement of shape memory properties of PCL-based polymers, widens the versatility and suitability of the obtained composite fibers for a huge portfolio of biomedical applications.

DLP 4D‐Printing of Remotely, Modularly, and Selectively Controllable Shape Memory Polymer Nanocomposites Embedding Carbon Nanotubes

AbstractAn in‐depth investigation on novel electro‐activated shape memory polymer composites (SMPCs) for digital light processing 3D‐Printing, consisting of a poly(ethylene glycol) diacrylate/poly(hydroxyethyl methacrylate) matrix embedding multi‐walled carbon nanotubes (CNTs), is reported here. The composition of the photocurable (meth)acrylate system is finely tuned to tailor the thermomechanical properties of the matrix, whereas the effect of CNTs on the photoreactivity and rheological properties of the formulations is investigated to assess the printability. Electrical measurements confirmed that the incorporation of CNT into the polymeric matrix enables the electrical conductivity and thus the possibility to remotely heat the nanocomposite using the Joule effect. The feasibility to drive a shape memory cycle via Joule heating is proved, given that the high shape fixity (Rf) and shape recovery (Rr) ratios achieved (Rf ≈ 100%,Rr > 95%) confirmed the significant electrically‐triggered responsiveness of such CNT/SMPCs. Finally, it is shown how to activate a modular and selective electro‐activated shape recovery, which may ultimately envisage the 4D‐Printing of remotely and selectively controllable smart devices.

Triple shape memory effect of ethylene-vinyl acetate copolymer/poly(propylene carbonate) blends with broad composite ratios and phase morphologies

In this work, a triple-shape memory polymer (SMP) was prepared by chemically crosslinking melting blending from ethylene-vinyl acetate copolymer (EVA) and poly(propylene carbonate) (PPC). The EVA/PPC blends combine the glass transition of PPC (~25.6 °C) and melting transition of EVA (~72.2 °C) to function as the shape memory mechanisms. The triple-shape memory effect (triple-SME) and its dependence on composite ratios and micro-phase morphologies were systematically investigated. The results show that EVA/PPC blends with different composite ratios and morphologies all exhibit excellent triple-SMEs, with shape fixing ratio over 90% and shape recovery ratio close to 90%. The key to realize such triple SME is the certain matching of the modulus of the two components. Specifically, the modulus of glassy PPC is much higher than the modulus of semi-crystalline EVA, whereas the modulus of viscoelastic PPC is lower than the modulus of semi-crystalline and elastic EVA. This work provides a comparatively simple and efficient method to design and prepare high-performance triple-SMP.

Shape memory behavior of unidirectional pultruded laminate

This study investigates the shape memory behavior of epoxy-based pultruded flat laminates reinforced with unidirectional glass fibers. To evaluate the shape memory performance of the composite material, a series of tests by bending flat specimens to the shape programming angle has been conducted. The shape fixity and shape recovery ratios for the pultruded composite specimens with 90° fiber orientation constituted 78.4% and 92.6%, respectively, and 13.3% and 91.9%, for specimens with 0° fiber orientation. The results obtained in this study show that the shape fixity and recovery ratios of the composite material are only slightly dependent on the shape programming angle and the number of shape memory cycles (up to 10 cycles). The study also determined the shape fixity and shape recovery ratios for cured resin specimens, which constituted 99.7% and 98.2%, respectively. To lay the foundation for further studies on the simulation and optimization of the pultrusion process, the resin cure kinetics, the thermomechanical and thermophysical properties of the cured resin, and the mechanical characteristics of the pultruded shape memory polymer composite were analyzed.