Low Shape Recovery Research Articles

Analysis of thermal‐active bending and cyclic tensile shape memory mechanism of UHMWPE/CNT composite

AbstractThe exploitation of shape memory ultra‐high‐molecular‐weight polyethylene (UHMWPE) can expand the application field as an intelligent material; however, low shape recovery ratio limits its further practical application. Herein, carbon nanotubes (CNT) are used to improve shape memory property of UHMWPE. Interestingly, after analyzing the bending and cyclic tensile shape memory properties of UHMWPE/CNT composite, it is found that bending shape memory property of UHMWPE/CNT composite is improved dramatically, while cyclic tensile shape memory property is not. The optimal bending shape memory property with shape fixity ratio of more than 95% and shape recovery ratio of about 94% can be obtained at 115°C. However, the shape recovery ratio in cyclic tensile shape memory test is only changed from 62.72% to 65.63%, and the shape fixity ratio is reduced from 85.82% to 82.83%. The main reason for this phenomenon is the possible difference in thermal‐active bending and cyclic tensile shape memory mechanism: CNT engender forced deformation in bending shape memory, while it does not exhibit in cyclic tensile shape memory. The outstanding bending shape memory property of UHMWPE/CNT composite make it possible for more promising applications.

Polyethylenimine‐Based Shape Memory Polyurethane with Low Transition Temperature and Excellent Memory Performance

AbstractFlexible shape memory polyurethanes (SMPUs) are the favorable candidates as a coating or substrate for wearable smart textiles, electronics, and biomedical applications. However, conventional SMPUs (e.g., 1,4 butanediol (BDO)‐based) are not suitable in these applications due to high rigidity, poor mechanical properties, low shape recovery, and high transition temperature. Herein, a polyethylenimine (PEI)‐based SMPU with low transition temperature and tailored properties are reported. The synthesized SMPU are characterized, and their properties are compared with BDO‐SMPUs. The chemical structure of PEI is explored to improve thermal and mechanical properties and to assess their effect on shape memory behavior. The bulky nature of PEI plays a critical role in lowering transition temperature and introduces flexibility in the structure at room temperature. A drop in Young's modulus is found from 13.6 MPa in BDO‐SMPU to 6.2 MPa in PEI‐SMPU. Simultaneously, tensile strength is increased from 3.77 MPa in BDO‐SMPU to 11.85 MPa in PEI‐SMPU. Owing to the improved mechanical properties in PEI‐SMPU, 100% shape recovery is observed, which displays a reproducible trend in ten repetitive cycles due to the presence of reversible physical crosslinks. Therefore, it is envisioned that this can serve as a potential shape memory material in smart wearable technologies.

Thermo-Sensitive Shape Memory Polymer Nanocomposite Based on Polyhedral Oligomeric Silsesquioxane (POSS) Filled Polyolefins

ABSTRACTShape memory polymer nanocomposite of EOC-EPDM blend has been prepared by the incertion of two different types of POSS nanoclay namely octaisobutyl POSS and trisilanolIsobutyl POSS. Compared to TSIB POSS, OIB POSS shows significant impact upon the shape memory behaviour of the blend. With increasing amount of POSS nanoclay, reduction in percentage shape fixity is lower for the nanocomposite containing OIB POSS rather than the TSIB POSS containing nanocomposite. On the other side, presence of closed cage like structure also results higher shape recovery ratio and lower shape recovery induction time for OIB POSS containing nanocomposite.

Enhancement in Mechanical and Shape Memory Properties for Liquid Crystalline Polyurethane Strengthened by Graphene Oxide.

Conventional shape memory polymers suffer the drawbacks of low thermal stability, low strength, and low shape recovery speed. In this study, main-chain liquid crystalline polyurethane (LCPU) that contains polar groups was synthesized. Graphene oxide (GO)/LCPU composite was fabricated using the solution casting method. The tensile strength of GO/LCPU was 1.78 times that of neat LCPU. In addition, shape recovery speed was extensively improved. The average recovery rate of sample with 20 wt % GO loading was 9.2°/s, much faster than that of LCPU of 2.6°/s. The enhancement in mechanical property and shape memory behavior could be attributed to the structure of LCPU and GO, which enhanced the interfacial interactions between GO and LCPU.

Shape memory polybenzoxazines based on a siloxane-containing diphenol

A siloxane-containing diphenol is synthesized from 1,1,3,3-tetramethyldisiloxane and o-allylphenol, followed by the Mannich condensation with aniline, methylamine, and formaldehyde yielding two siloxane-containing benzoxazines. The onset polymerization temperature of aniline-based benzoxazine is higher than that of the methylamine counterpart. The dynamic mechanical properties of the polybenzoxazines depend on the structure of the starting primary amines. Both polybenzoxazines exhibit one-way dual-shape memory behavior in response to changes in temperature, and they show excellent shape fixity ratios in bending, tension, and tensile stress–strain tests, high shape recovery ratios in bending and tension tests, but relatively low shape recovery ratios in tensile stress–strain test. The network chain segments including the alkylsiloxane units serve as a thermal control switch based on the glass transition temperatures (39 and 53 °C) for the polybenzoxazines. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1255–1266

Thermomechanical characterization of a shape memory polymer based self-repairing syntactic foam

While the current self-healing approaches such as micro-capsules, hollow fibers, thermally reversible covalent bonds, ionomers, incorporation of thermoplastic particles, etc., are very effective in self-healing micro-length scale damage, self-healing of structural scale or macro-length scale damage remains one of the grand challenges facing the self-healing community. We believe that self-healing of structural damage may need multiple steps, at least two steps: close then heal (CTH), similar to the biological healing of wounds in the skin. In a previous study [1], it has been proven that the confined shape recovery functionality of a shape memory polymer (SMP) based syntactic foam can be utilized to repair structural damage such as impact damage repeatedly, efficiently, and almost autonomously. The purpose of this study is to investigate the effect of various design parameters on the closing efficiencies of both the pure SMP and the SMP based syntactic foam. A systematic test program is implemented, including glass transition temperature ( T g) determination by dynamic mechanical analysis (DMA), isothermal compressive constitutive behavior at various temperatures, and stress-controlled uniaxial compression programming and shape recovery. During thermomechanical cycle testing, two stress levels are utilized for programming and three confinement conditions (fully confined, partially confined, and free) are investigated for shape recovery. It is found that the programming stress is restored under confined recovery conditions, which helps in fully closing the crack; the foam shifts the T g higher and increases the stiffness at temperatures above the T g; higher programming stresses lead to slightly higher shape fixity but lower shape recovery in free recovery cases; a higher programming stress also results in a higher peak stress for confined recovery conditions; while the peak stress recovered is controlled by thermal stress, the final stress recovered is controlled by the programming stress, which is stored and recovered using an entropic mechanism. This study lays a solid foundation for using shape memory polymer based composites to self-repair macro-length scale damage.

Temperature profiles in a Ti–45Ni–5Cu (at%) shape memory alloy developed by the Joule heating

Spatially varying temperature profile generated by the Joule heating of a Ti–45Ni–5Cu (at%) alloy wire was investigated by experiment and analysis. The Joule heating is a simple and effective method to fabricate shape memory alloy (SMA) actuators for proportional control. By applying a voltage drop of 1.338 V, a temperature gradient between 504 K and 413 K was obtained experimentally over a length of 30 mm SMA wire. A closed-form expression derived from a simple analytic model was suggested to predict the temperature profile, and the result of the finite element analysis was also provided. Finally, it is demonstrated that the Joule heated specimen shows a low shape recovery rate of 0.02%/K and a unique Lüders-type deformation with a progressively increasing stress level from 340 MPa to 380 MPa.

Magnetic shape memory ribbon produced by rapid quench method

Fe-Mn-Si based alloy, one of the shape memory material has slight magnetization in addition to shape memory effect (SME). We tried to improve the magnetization on the ribbon specimen produced by rapid quench method. Though the magnetization curve shows only slight magnetization as cast, it has clear SME and this worked at lower shape recovery temperature than the sheet type specimen. By contrast, after annealing in air at 600°C the magnetization increases markedly while the ribbon becomes hard and fragile.We recognized that the oxidation on the surface brought by annealing caused the increase of magnetization.In order to be compatible both of the magnetization and SME we prepared 0.5 mm thick sheet. Oxidation by annealing was applied only on the surface and then the specimen was found to have both properties.

Shape Memory Behavior and Mechanical Properties of NiMn and Ni(Mn,X) Alloys

AbstractPotential high transition temperature shape memory alloys based on NiMn have been studied with emphasis on the shape recovery, transformation temperatures and mechanical properties. Binary NiMn, which has been reported to be brittle, has a low shape recovery, but this can be increased with Al or Ti additions. Also, the transformation temperature can be changed and the room temperature ductility improved by ternary element additions. The various substitutional solute characteristics affecting the shape recovery, the transformation temperatures, and the ductility have been examined.