Composites of carbon nanofibers and thermoplastic polyurethanes with shape‐memory properties prepared by chaotic mixing

Composites of carbon nanofibers (CNFs), oxidized carbon nanofibers (ox‐CNFs), and shape‐memory thermoplastic polyurethane (TPU) were prepared in a chaotic mixer and their shape‐memory properties evaluated. The polymer was synthesized from 4,4′‐diphenylmethane diisocyanate, 1,4‐butanediol chain extender, and semicrystalline poly(ε‐caprolactone) diol soft segments. The shape‐memory action was triggered by both conductive and resistive heating. It was found that soft segment crystallinity and mechanical reinforcement by nanofibers produced competing effects on shape‐memory properties. A large reduction in soft segment crystallinity in the presence of CNF and stronger mechanical reinforcement by well‐dispersed ox‐CNF determined the shape‐memory properties of the respective composites. It was found that the maximum shape recovery force, respectively, 3 and 4 MPa, was obtained in the cases of 5 and 1 wt% CNF and ox‐CNF, respectively, compared with ～1.8 MPa for unfilled TPU. The degree of soft segment and hard segment phase separation and thermal stability of the composites were analyzed. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers.     

Celite‐mediated linking of polyurethane block copolymers and the impact on the shape memory effect

Celite, a porous inorganic material with enormous surface area and hydroxyl groups on the surface, was used as a cross‐linker of polyurethane (PU) copolymer chains to improve its shape memory and mechanical properties. PU copolymers with different Celite contents were prepared and characterized by IR, DSC, and universal testing machine. The glass transition temperature of PU copolymers was maintained around 20°C independent of Celite content. The shape memory and mechanical properties were dependent on when Celite was added during the polymerization reaction. The reaction in which Celite was added at the middle stage of polymerization showed the best shape memory and mechanical properties. The best shape recovery of PU was found at 0.3 wt % Celite and increased to 97% even after the third cycle. Likewise, the shape retention also maintained a remarkable 86% after three cycles. The reasons underlining the high shape recovery and shape retention by adopting Celite as a cross‐linker are discussed in this article. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Recent advance on constitutive models of thermal‐sensitive shape memory polymers

Shape memory polymers (SMPs) are a novel class of shape memory materials which can store a deformed (temporary) shape and recover an original (permanent) shape under a shape memory thermomechanical loading–unloading cycle. The deformation mechanisms of SMPs are very complicated, but the SMPs also have a lot of advantages and the widespread application value and prospect. So developing proper constitutive models that describe thermomechanical properties of SMPs and the shape memory effect is very challenging and of great theoretical and application value. Based on the deformation mechanisms and considerable experimental investigations of SMPs, researchers have developed many constitutive models. This article investigates the deformation mechanism and introduces the recent research advance of the constitutive models of thermal‐sensitive SMPs. Special emphases are given on the micromechanical constitutive relations in which the deformation is considered being based on the microstructure of the SMPs. Finally, the lack of research and prospects for further research are discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Thermostimulative shape‐memory effect of reactive compatibilized high‐density polyethylene/poly(ethylene terephthalate) blends by an ethylene–butyl acrylate–glycidyl methacrylate terpolymer

High‐density polyethylene (HDPE)/poly(ethylene terephthalate) (PET) blends were prepared by means of melt extrusion with ethylene–butyl acrylate–glycidyl methacrylate terpolymer (EBAGMA) as a reactive compatibilizer. The effects of the EBAGMA and PET contents, recovery temperature, and stretch ratio on the thermostimulative shape‐memory behavior of the blends were studied. The results show that the addition of EBAGMA to the HDPE/PET blends obviously improved the compatibility and the shape‐memory effects of the blends. The response temperature was determined by the melting point of HDPE, and the shape‐recovery ratio of the 90/10/5 HDPE/PET/EBAGMA blend reached nearly 100%. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

A smart hollow filament with thermal sensitive internal diameter

In this work, a thermoplastic shape memory polyurethane was prepared via melt polymerization and a corresponding shape memory hollow fiber was fabricated via melt spinning. The fiber mechanical properties, especially shape memory effect, were characterized by static tensile, thermo‐mechanical cyclic tensile testing. The hollow fiber switching temperature was the melting transition temperature of the soft segment phase at about 41°C. The tenacity of the hollow fiber was about 1.14 cN/dtex, and breaking elongation was 682%. The shape fixity ratio was above 87% and the recovery ratio was above 89%. The internal diameter of the hollow fiber could be noticeably changed and the deformed fiber cross‐section could be well fixed. Once heated above the soft segment phase melting transition temperature, the hollow fiber internal hole recovered to its original diameter. The results from differential scanning calorimetry, X‐ray diffraction, and dynamic mechanical analysis were used to illustrate the mechanism governing the mechanical properties and shape memory effect especially. Due to the changes of the hollow fiber and the internal diameter affect of the physical properties of the prepared products, this fiber may be used in smart textiles for thermal management, or as stuffing of pillows and mattresses, which can adjust to body contours. Furthermore, the findings suggest that this kind of hollow fiber with thermal sensitive internal diameter could be used in smart filtration, drug‐controlled release, and liquid transportation in vivo. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Synthesis and characterization of poly(carbonate urethane) networks with shape‐memory properties

In this study, a series of shape‐memory polyurethanes were prepared from polycarbonate diol (PCDL) with a molecular weight of 2000, trimethylol propane, and isophorone diisocyanate (IPDI). The properties of crosslinked poly(carbonate urethane) (PCU) networks with various compositions were investigated. The chemical structures and thermal properties were determined with Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. FTIR analysis indicated that PCU had the structures of IPDI and PCDL and the amido formyl ester in polyurethanes. The gel content of PCU showed that PCU could be effectively formed as crosslinked polyurethane networks. The glass‐transition temperatures of the PCU networks increased slightly with decreasing soft‐segment content in the networks. The values of Young's modulus in the networks at 25°C increased with decreasing soft‐segment content, whereas the tensile stress and breaking elongation decreased significantly. PCU showed shape‐memory effects with a high strain fixity rate. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Synthesis of poly(cyclooctene) by ring‐opening metathesis polymerization: Characterization and shape memory properties

Cis‐cyclooctene was polymerized via ring‐opening metathesis polymerization using a well‐defined ruthenium catalyst (Grubbs' type) under varying reaction conditions. Control over molecular weight was achieved by the inclusion of a chain transfer agent and its influence on the behavior of the obtained polymers was evaluated. The resulting polymers were characterized by means of differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical thermal analysis. Taking into account their thermal behavior, samples of appropriate molecular weight were subjected to a suitable treatment by chemical crosslinking to obtain a material showing thermally induced shape memory effect. The material recovers its original shape after several cycles of deformation into different shapes. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Hyperbranched‐modified epoxy thermosets: Enhancement of thermomechanical and shape‐memory performances

In this study a complete characterization of the thermomechanical and shape‐memory properties of epoxy shape‐memory polymers modified with hyperbranched polymer and aliphatic diamine was performed. Focusing on the mechanical properties that are highly desirable for shape‐memory polymers, tensile behavior until break was analyzed at different temperatures and microhardness and impact strength were determined at room temperature. As regards shape memory performance, the materials were fully characterized at different programming temperatures to study how this influenced the recovery ratio, fixity ratio, shape‐recovery velocity, and switching temperature. Tensile testing revealed a peak in deformability and in the stored energy density at the onset of the glass transition temperature, demonstrating that this is the best programming temperature for obtaining the best shape‐memory performances. The Young's moduli revealed more rigid structures in formulations with higher hyperbranched polymer content, while microhardness showed higher values with increasing hyperbranched polymer content due to the increased crosslinking density. Impact strength was greatly improved as the aliphatic diamine content increases due to the energy dissipation capability of its flexible structure. As regards the shape‐memory properties, increasing the programming temperature has a minor effect on formulations with a lower hyperbranched polymer content and worsens these properties when the hyperbranched polymer content is increased. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44623.     

Soybean Oil-Based Polyurethane Networks: Shape-Memory Effects and Surface Morphologies

Vegetable oil‐based shape‐memory polyurethane networks are an emerging class of bio‐based functional materials with great potential applications. In this study, a series of different structural soybean oil polyols were synthesized, and utilized to fabricate polyurethane networks by reacting with 1,6‐diisocyanatohexane. The soybean oil‐based polyurethanes (SOPUs) were characterized with differential scanning calorimetry (DSC), dynamic mechanical tests (DMA), tensile testing, shape‐memory testing, and atomic force microscopy (AFM). It was found that SOPUs with a preserved triglyceride structure were fixed in a temporary shape at ?20 °C, while others were fixed in temporary shapes at 4 °C. Although the recovery speeds were different, all the samples could completely regain their permanent shapes at 37 °C (human body temperature). Furthermore, different SOPUs exhibited different surface structures, which might provide the materials with additional values.     

Shape memory property and underlying mechanism by the phase separation control of poly(ϵ‐caprolactone)/poly(ether‐b‐amide)

In this work, a poly(?‐caprolactone)/poly(ether‐b‐amide) blend with weight ratio 35/65 was prepared by solution mixing and compression molding. A simple and sensible method to control the phase separation structure was introduced by adjusting the temperature and time for the process of phase separation. Samples with obviously different morphology were obtained and the microstructure was studied by phase contrast optical microscopy, SEM and DSC. The shape memory properties were measured using dynamic mechanical analysis. The results show that the shape memory performance of the blend is closely related to the phase morphology, and the blend with co‐continuous structure has a better shape memory property. A model is put forward to illustrate schematically the microstructural evolution during the shape memory process. © 2018 Society of Chemical Industry     

Fourier transform infrared study of supramolecular polyurethane networks containing pyridine moieties for shape memory materials

Non‐covalent interactions are increasingly used in the molecular self‐assembly of well‐defined structures. In this study, a series of pyridine‐containing polyurethanes (PUPys) were synthesized with diisocyanates and pyridine derivatives. Fourier transform infrared spectroscopy was employed to identify the vibration frequencies and investigate the relationship of shape memory effect (SME) and hydrogen‐bonded supramolecular structure of the PUPys. The results show that a large fraction of strong hydrogen bonds are formed in the urethane group as well as in the pyridine ring. Moreover, the hydrogen bonding in the pyridine ring not only shows a response to a temperature stimulus in the PUPys, but is also responsive to moisture in N,N‐bis(2‐hydroxylethyl)isonicotinamine (BINA)/hexamethylene diisocyanate (HDI)‐based PUPys; the hydrogen bonds in the urethane group have a higher dissociation temperature and show little response to moisture absorption. Accordingly, the PUPys are expected to show thermally induced and moisture‐induced SMEs. Finally, the shape recovery process of films in the shape of flowers induced by temperature and moisture supports the idea that the PUPys could be used as thermally induced shape memory polymers (SMPs), and the BINA/HDI‐based PUPys could be used as moisture‐sensitive SMPs. Copyright © 2009 Society of Chemical Industry     

The influence of heat treatment on the properties of shape memory fibers. II. Tensile properties,dimensional stability,recovery force relaxation,and thermomechanical cyclic properties

Shape memory fibers (SMFs) were prepared via a melt spinning process. The fibers were subject to different heat treatments to eliminate internal stress and structure deficiency caused during the melt spinning process. The influences of heat treatments on the SMF thermal properties, molecular orientation, tensile properties, dimensional stability, recovery force relaxation, and thermomechanical cyclic properties were studied. It was found that the heat treatments increased soft segment crystallinity and phase separation while decreased molecular orientation. The low‐temperature heat treatment increased the breaking elongation, shape fixity ratios, and decreased boiling water shrinkage while shape recovery ratios were decreased. High‐temperature treatment increased both the shape recovery ratios, fixity ratios, recovery stress stability and at the same time decreasing the fiber mechanical strength. The results from differential scanning calorimetry, molecular orientation apparatus, and cyclic tensile testing were used to illustrate the mechanism governing the mechanical properties and shape memory effect. To obtain comprehensive outstanding properties, the SMF is expected to be treated at a high temperature because of the hard segment high glass transition temperature. Unfortunately, the heat treatment could not be conducted at a too high temperature because the SMF became too tacky and soft due to the melting of the soft segment phase. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Numerical study of smart honeycomb core using shape memory polymers

Shape memory polymers (SMPs) attract widespread attention because they are able to maintain a temporary deformation after unloading and recover the initial shape under high temperature conditions. Based on a three‐dimensionally constitutive equation of SMPs, a finite element program is followed by compiling user‐defined material subroutine, which describes the shape memory behavior of thermo‐mechanical experiment. A honeycomb core using SMP is designed, which has the ability to recover the initial shape after deformation and be used as a smart core for sandwich structures. To prove their advantages in the engineering application, a series of thermodynamic behaviors of the SMP honeycomb core are simulated, including loading at high temperature, cooling, unloading at the low temperature, and recovering original shape on heating. Shape memory behaviors of tensile, compressive, bending, and locally sunken deformations are demonstrated and the effect of time and temperature on the recovery process is discussed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45672.     

Short glass fiber reinforced radiation crosslinked shape memory SBS/LLDPE blends

The poly(styrene‐b‐butadiene‐b‐styrene) triblock copolymer (SBS) and linear low density polyethylene(LLDPE) were blended and irradiated by γ‐rays to prepare shape memory polymer(SMP). Various amounts of short glass fiber (SGF) were filled into SMP to form a novel shape memory SGF/SBS/LLDPE composite. The effect of SGF on the shape memory SGF/SBS/LLDPE composite was studied in terms of mechanical, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) and shape memory effects. It is found that the SGF act as reinforcing fillers and significantly augment the glassy and rubbery stated moduli, tensile strength and shape memory properties. When SGF content is <2.0 wt %, full recovery can be observed after only several minutes at different temperatures and shape recovery speed reduces as the SGF content increases. The shape recovery time decreases as the temperature of the shape memory test increases and the shape recovery rate decreases with increment of cycle times. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40691.     

Thermal and shape memory properties of cyanate/polybutadiene epoxy/polysebacic polyanhydride copolymer

A novel thermal‐induced shape memory polymer was synthesized by copolymerization of a new kind of epoxy resin‐polybutadiene epoxy (PBEP), bisphenol A‐type cyanate ester (BACE), and polysebacic polyanhydride (PSPA) with different mass ratios. Fourier transform infrared spectroscopy (FTIR), bending test, dynamic mechanical analysis (DMA), and shape memory property were investigated systematically. It was found that the PSPA significantly enhanced the bending strength and flexural modulus. The DMA results showed that the glass transition temperature reduced with increasing content of PSPA. Furthermore, the shape memory tests proved that the copolymer possessed excellent shape memory properties. The shape recovery time decreased with increasing content of PSPA and temperature. The shape memory rate increased as the PSPA content increased. The shape recovery ratio decreased with increasing cycle times. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42045.     

Influence of the soft segment nature on the thermomechanical behavior of shape memory polyurethanes

Shape‐memory polyurethanes (SMPUs) represent a highly interesting class of materials due to their applications in different sectors such as biomedical, textile, and aerospace. Moreover, it is possible to synthesize a variety of polyurethanes with different molecular architectures just choosing properly the chemical structure of their components. In this work, it is described the influence of the soft segment on the thermomechanical properties and shape memory behavior of shape memory polyurethanes. The synthesis, based on two‐step polymerization, was prepared by two different soft segments: poly(oxytetramethylene) glycol (PTMG) or poly(ethylene glycol) (PEG). Depending on the molecular architecture achieved, the materials present different properties that were studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Furthermore, the shape memory response of thermally activated SMPUs is determined qualitative and quantitatively by visually monitoring the shape recovery process and by thermomechanical analysis (TMA), respectively. All developed compositions have shown good shape memory behavior, with recovery ratios higher than 99.8%. POLYM. ENG. SCI., 58:238–244, 2018. © 2017 Society of Plastics Engineers     

Melt spun thermoresponsive shape memory fibers based on polyurethanes: Effect of drawing and heat‐setting on fiber morphology and properties

Thermoresponsive shape memory (SMP) fibers were prepared by melt spinning from a polyester polyol‐based polyurethane shape memory polymer (SMP) and were subjected to different postspinning operations to modify their structure. The effect of drawing and heat‐setting operations on the shape memory behavior, mechanical properties, and structure of the fibers was studied. In contrast to the as‐spun fibers, which were found to show low stress built up on straining to temporary shape and incomplete recovery to the permanent shape, the drawn and heat‐set fibers showed significantly higher stresses and complete recovery. The fibers drawn at a DR of 3.0 and heat‐set at 100°C gave stress values that were about 10 times higher than the as‐spun fibers at the same strain and showed complete recovery on repeated cycling. This improvement was likely due to the transformation brought about in the morphology of the permanent shape of the SMP fibers from randomly oriented weakly linked regions of hard and soft segments to the well‐segregated, oriented and strongly H‐bonded regions of hard‐segments. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2172–2182, 2007     

Sequential Motion of 4D Printed Photopolymers with Broad Glass Transition

The term “4D printing” refers to the development of stimulus‐responsive structures through 3D printing of active smart materials, typically shape memory polymers. A noteworthy aim of this research field is to obtain objects able to display complex shape‐shifting motions, such as sequential transformations over time. In this work, this peculiar response is studied on a commercial photopolymer, printed by stereolithography and featuring, on the basis of its inherent broad glass transition, the so‐called “temperature‐memory effect” (TME). The TME, that is, a response in which the shape memory effect occurs on a region controlled by the deformation temperature, is studied in shape memory cycles where the deformation temperature is systematically varied, so to provide a correlation between deformation and recovery temperatures. This also allows to properly select two temperatures at which deforming a specimen along a multistep history, so as to finally separate each recovery process on the temperature and time scales. This sequential recovery is studied in double folded bars, with arms deformed at different temperatures, and on a properly designed self‐locking clamp. The obtained results are promising for the realization of smart temperature‐responsive structures printed with one single polymer and capable of multiple shape transformations.     

Shape‐memory polymer networks with Fe3O4 nanoparticles for remote activation

Shape‐memory polymers (SMPs) have recently shown the capacity to actuate by remote heating via the incorporation of magnetic nanoparticles into the polymer matrix and exposure to an alternating magnetic field. In this study, methacrylate‐based thermoset SMP networks were synthesized through free‐radical polymerization with varying amounts of Fe₃O₄ magnetite (0, 1, and 2.5 wt %). Furthermore, the chemistry of the networks was controlled to maintain a constant glass transition temperature (T_g) while varying the degree of chemical crosslinking. Remote heating of the networks was shown to be a direct function of the nanoparticle concentration and independent of the chemistry. Magnetite reinforcement was shown to influence the thermomechanical properties of the networks; increasing Fe₃O₄ concentrations led to decreases in T_g and rubbery modulus. However, networks with a higher degree of crosslinking were more resistant to thermomechanical changes with respect to magnetite concentration. Strain to failure was shown to decrease with the addition of nanoparticles and the free‐strain shape‐memory cycle was investigated for all of the networks. Networks with lower degrees of crosslinking and high magnetite concentrations showed a significant amount of irrecoverable strain. Last, the use of remotely heated shape‐memory materials is discussed in light of potential biomedical applications. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Triple‐shape‐memory polymer films created by forced‐assembly multilayer coextrusion

Triple‐shape‐memory polymers are capable of memorizing two temporary shapes and sequentially recovering from the first temporary shape to the second temporary shape and eventually to the permanent shape upon exposure to a stimulus. In this study, unique three‐component, multilayered films with an ATBTA configuration [where A is polyurethane (PU), B is ethylene vinyl acetate (EVA), and T is poly(vinyl acetate) (PVAc)] were produced as a triple‐shape‐memory material via a forced‐assembly multilayer film coextrusion process from PU, EVA, and PVAc. The two well‐separated thermal transitions of the PU–EVA–PVAc film, the melting temperature of EVA and the glass‐transition temperature of PVAc, allow for the fixing of the two temporary shapes. The cyclic thermomechanical testing results confirm that the 257‐layered PU–EVA–PVAc films possessed outstanding triple‐shape‐memory performance in terms of the shape fixity and shape‐recovery ratios. This approach allowed greater design flexibility and simultaneous adjustment of the mechanical and shape‐memory properties. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44405.