Effect of thermal expansion on shape memory behavior of polyurethane and its nanocomposites

The effects of thermal expansion on shape memory performance of shape memory polyurethanes and their nanocomposites with organoclay, carbon nanofiber (CNF), silicon carbide (SiC), and carbon black (CB) were evaluated. The shape memory test cycle involved tensile deformation at above the trigger temperature to initiate shape memory function, cooling to room temperature to fix the shape, and shape recovery induced by heating to above the trigger temperature. Phenomenological models were used to interpret the experimental data on coefficient of thermal expansion (CTE). It was found that Kerner model showed good fit for composites of SiC and CB, and Halpin model gave better fit for composites of organoclay and CNF. It was observed that thermal expansion exerts negative effect on recovered strain, the extent of which depends on the magnitude of temperature gradient, CTE, and the level of tensile strain. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1437–1449, 2008     

Bio‐based hyperbranched polyurethane/Fe3O4 nanocomposites as shape memory materials

The bio‐based shape memory polymers have generated immense interest as advanced smart materials. Mesua ferrea L. seed oil‐based hyperbranched polyurethane (HBPU)/Fe₃O₄ nanocomposites were prepared by the in‐situ polymerization technique. The transmission electron microscopy confirmed the homogeneous distribution of the Fe₃O₄ nanoparticles in polymer matrix, whereas Fourier transform infrared spectroscopic study revealed the presence of strong interfacial interactions between them. The incorporation of Fe₃O₄ (0 to 10 wt%) into the HBPU resulted in an increase in tensile strength (5.5–15 MPa) and scratch resistance (3–6 kg). The thermo‐gravimetric analysis indicated the improvement of thermal stability (240–270°C) of the nanocomposites. The nanocomposites exhibited full shape fixity, as well as almost full shape recovery under the microwave stimulus. The shape recovery speed increased with the increase of Fe₃O₄ nanoparticles content in the nanocomposites. Thus, the studied nanocomposites might be used as advanced shape memory materials in different potential fields. Copyright © 2013 John Wiley & Sons, Ltd.     

Ultra low density and highly crosslinked biocompatible shape memory polyurethane foams

We report the development of highly chemically crosslinked, ultra low density (～0.015 g/cc) polyurethane shape memory foams synthesized from symmetrical, low molecular weight, and branched hydroxyl monomers. Sharp single glass transitions (T_g) customizable in the functional range of 45–70 °C were achieved. Thermomechanical testing confirmed shape memory behavior with 97–98% shape recovery over repeated cycles, a glassy storage modulus of 200–300 kPa, and recovery stresses of 5–15 kPa. Shape holding tests under constrained storage above the T_g showed stable shape memory. A high volume expansion of up to 70 times was seen on actuation of these foams from a fully compressed state. Low in vitro cell activation induced by the foam compared with controls demonstrates low acute bio‐reactivity. We believe these porous polymeric scaffolds constitute an important class of novel smart biomaterials with multiple potential applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Investigation into stress recovery behavior of shape memory polyurethane fiber

In this work, the stress recovery behavior of shape memory polyurethane (SMPU) fiber was investigated. The as‐spun SMPU fibers were subjected to various programing‐recovery conditions. It was observed that recovering at 100 °C generated higher recovery stress than recovering at 150 °C. It was also found that, while hot‐drawn programed fiber has higher recovery stress than cold‐drawn programed fiber if recovered at 100 °C, cold‐drawn programed fiber has higher stabilized recovery stress than hot‐drawn programed counterpart when recovered at 150 °C. A morphological model was proposed based on the results from differential scanning calorimetry, Fourier transform infrared spectrometry, and X‐ray diffraction to understand the physics behind the different stress recovery behaviors. It is found that SMPU experiences different phase transitions and phase separations under different programing and stress recovery conditions. It is concluded that the two sequential phase separations taking place at 100 and 150 °C are primarily responsible for the differences in the stress recovery behavior. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1429–1440     

Polypropylene/carbon nanotube magnetic composites obtained using carbon nanotubes from sawdust

Magnetic polypropylene (PP) nanocomposites with different loadings (from 0.5 to 20 wt %) of carbon nanotubes with iron (CNT‐Fe) were fabricated using the melt‐mixing method. The carbon nanotubes were synthesized by pyrolysis of sawdust from the furniture industry. The morphological characterization shows homogenous dispersion of the filler in the polymer matrix. The addition of only 0.5 wt % CNT‐Fe already results in ferromagnetic behavior in the diamagnetic polymer matrix. The thermal properties were investigated using thermogravimetric analysis and differential scanning calorimetry. The results show an increase in the maximum degradation, crystallization, and melting temperatures of the nanocomposites compared with neat PP. The nanocomposites showed improvement in terms of mechanical and oxygen permeability properties. A very significant result of the work is the high remnant magnetization and coercivity values of the nanocomposites at room temperature whereas most of the works on similar systems show magnetic properties only at very low temperatures.     

Effects of graphene quantum dot (GQD) on photoluminescence,mechanical, thermal and shape memory properties of thermoplastic polyurethane nanocomposites

A group of shape memory polyurethane‐based nanocomposites containing graphene quantum dot nanoparticles (GQDs) were prepared via in‐situ polymerization method. GQD nanoparticles were synthesized by a facile and rapid microwave‐assisted method and characterized by Fourier‐transform infrared spectroscopy (FTIR), X‐ray diffraction pattern, field emission scanning microscopy, transmission electron microscopy, and fluorescence analysis. Chemical structure and hydrogen bonding index (HBI_[C=O]) of the nanocomposites were analyzed via FTIR spectra. The results show that the incorporation of GQDs in PU matrix reduces HBI_(C=O) of nanocomposites. Crystalline structure and thermal properties of the nanocomposites were investigated by differential scanning calorimetry. As results indicate, nucleation effect of GQDs raises crystallinity content of the samples. Mechanical examinations indicate that incorporation of GQDs improves Young's modulus of the nanocomposites, while their elongation at break values are reduced. In addition, shape memory analyses reveal that the presence of GQDs in PU matrix increases the shape fixity ratios in nanocomposites.     

Synthesis and design of polyurethane and its nanocomposites derived from canola-castor oil: Mechanical,thermal and shape memory properties

The recent global pandemic and its tremendous effect on the price fluctuations of crude oil illustrates the side effects of petroleum dependency more evident than ever. Over the past decades, both academic and industrial communities spared endless efforts in order to replace petroleum-based materials with bio-derived resources. In the current study, a series of shape memory polymer composites (SMPC's) was synthesized from epoxidized vegetable oils, namely canola oil and castor oil fatty acids (COFA's) as a 100% bio-based polyol and isophorone diisocyanate (IPDI) as an isocyanate using a solvent/catalyst-free method in order to eventuate polyurethanes (PU's). Thereafter, graphene oxide (GO) nanoplatelets were synthesized and embedded in the neat PU in order to overcome the thermomechanical drawbacks of the neat matrix. The chemical structure of the synthesized components, as well as the dispersion and distribution levels of the nanoparticles, was characterized. In the following, thermal and mechanical properties as well as shape memory behavior of the specimens were comprehensively investigated. Likewise, the thermal conductivity was determined. This study proves that synthesized PU's based on vegetable oil polyols, including graphene nanoparticles, exhibit proper thermal and mechanical properties, which make them stand as a potential candidate to compete with traditional petroleum-based SMPC's.     

Waterborne polyurethane nanocomposites having shape memory effects

UV curable waterborne polyurethane/silica nanocomposites were designed and synthesized with functionalized silicas, where the functionalization was made with allyl isocyanate. The incorporated silica particles gave triple effects of multifunctional chemical cross‐links, reinforcing fillers, and stress relaxation retarders. Consequently, functionalized silica incorporated into the polymer chains showed significantly improved mechanical and thermal properties than the simple addition of unmodified silica. Notably, over 99% shape fixity and shape recovery with minimum cyclic hysteresis were obtained for the repeated cycles at 1% loading of the modified silica. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Multiwalled carbon nanotube/polymer nanocomposites: Processing and properties

Nanocomposite materials were prepared with an amorphous poly(styrene‐co‐butyl acrylate) latex as a matrix with multiwalled carbon nanotubes (MWNTs) as fillers. The microstructure of the related films was observed by transmission electron microscopy, which showed that a good dispersion of MWNTs within the matrix was obtained. The linear and nonlinear mechanical behavior and the electrical properties were analyzed. Mechanical characterization showed a mechanical reinforcement effect of the MWNTs with a relatively small decrease of the elongation at break. The composite materials exhibited an elastic behavior with increasing temperature, although the matrix alone became viscous under the same conditions. The electrical conductivity of the composite filled with 3 vol % MWNTs was studied during a tensile test, which highlighted the late damage of the material. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1186–1197, 2005     

Effect of nanotube functionalization on the properties of single‐walled carbon nanotube/polyurethane composites

A commercially available aliphatic thermoplastic polyurethane formulated with a methylene bis(cyclohexyl) diisocyanate hard segment and a poly(tetramethylene oxide) soft segment and chain‐extended with 1,4‐butanediol was dissolved in dimethylformamide and mixed with dispersed single‐walled carbon nanotubes. The properties of composites made with unfunctionalized nanotubes were compared with the properties of composites made with nanotubes functionalized to contain hydroxyl groups. Functionalization almost eliminated the conductivity of the tubes according to the conductivity of the composites above the percolation threshold. In most cases, functionalized and unfunctionalized tubes yielded composites with statistically identical mechanical properties. However, composites made with functionalized tubes did have a slightly higher modulus in the rubbery plateau region at higher nanotube fractions. Small‐angle X‐ray scattering patterns indicated that the dispersion reached a plateau in the unfunctionalized composites that was consistent with the plateau in the rubbery plateau region. The room‐temperature modulus and tensile strength increase was proportionally higher than almost all increases seen previously in thermoplastic polyurethanes; however, the increase was still an order of magnitude below what has been reported for the best nanotube–polymer systems. Nanotube addition increased the hard‐segment glass transition temperature slightly, whereas the soft‐segment glass transition was so diffuse that no conclusions could be drawn. Unfunctionalized tubes suppressed the crystallization of the hard segment; whereas functionalized tubes had no effect. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 490–501, 2007     

Nanodiamond tethered epoxy/polyurethane interpenetrating network nanocomposite: Physical properties and thermoresponsive shape-memory behavior

In this study, a novel blend of polyurethane and diglycidyl 1,2-cyclohexanedicarboxylate epoxy was prepared and reinforced with various content of functional nanodiamond (0.1–5 wt%). According to morphological analysis, diglycidyl 1,2-cyclohexanedicarboxylate/polyurethane/nanodiamond nanocomposite revealed beaded-twine network structure due to entrapment of nanodiamond aggregates into epoxy/polyurethane interpenetrating networks. Exclusive self-assembled nanodiamond-tethered interpenetrating network was due to physical inter-linking of nanodiamond with blend components. There was a 47% rise in tensile strength and an 80% increase in Young’s modulus of diglycidyl 1,2-cyclohexanedicarboxylate/polyurethane/nanodiamond 5 nanocomposite relative to neat polymer. The original shape of diglycidyl 1,2-cyclohexanedicarboxylate/polyurethane/nanodiamond 5 was 95% recovered using heat-induced shape-memory effect.     

Preparation of electrospun shape memory polyurethane fibers in optimized electrospinning conditions via response surface methodology

Herein, the electrospinning method, as an effective approach, was utilized to fabricate poly (ε‐caprolactone)‐based polyurethane (PCL‐based PU) fibers. PCL was synthesized by ring‐opening polymerization, and characterized by proton nuclear magnetic resonance (¹H NMR) and Fourier‐transform infrared (FTIR) spectroscopies. Afterward, PU was prepared by step‐growth polymerization. The effects of solution concentration and solvent type on fibers' diameter were investigated. Scanning electron microscopy (SEM) images revealed that the optimum solution was N, N‐dimethylformamide(DMF): chloroform with a ratio of 60:40. In addition, results showed that bead‐less nanofibers could be achieved by a concentration of 5 w/v% (polymer to solvent). Various optimum practical parameters, such as applied voltage, feeding rate, and needle‐to‐collector distance, were obtained and compared with the results of response surface methodology (RSM). On the other hand, the mechanical evaluations indicated that the porous structure of scaffolds caused them to possess lower mechanical properties, as well as shape fixity ratios than those of bulk samples.     

Actuation design for high‐performance shape memory polyurethanes

A series of shape memory polyurethanes were synthesized from poly(tetramethylene glycol), 4,4‐methylene diphenyl diisocyanate, and 1,3‐butanediol. The prepolymers with different molecular weights (M_c) were capped with 2‐hydroxyl ethylacrylate or 3‐aminopropyltriethoxysilane (APTES) and crosslinked by UV curing or a sol–gel reaction. Variations of the crosslinker functionality (f), subchain density (N), and hard segment content (HSC) produced systematic variations of the glass transition temperature (6–45 °C), accompanied by changes in the mechanical, dynamic mechanical and shape memory properties. More than 95% of shape fixity and 98% of shape recovery up to the fourth cycles were obtained with APTES crosslinked 3000M_c with 30% of HSC. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1473–1479     

Water vapor permeability of shape memory polyurethane with amorphous reversible phase

Shape memory thermoplastic polyurethanes (TPUs), based on amorphous soft segment from the reaction of hexamethylene diisocyanate and 1,2‐butane diol, and the crystalline hard segment from 4,4′‐methylenediphenyl diisocyanate and 1,6‐hexanediol, were modified by hydrophilic segments, diol‐terminated poly(ethylene oxide) or dimethylol propionic acid (DMPA). Differential scanning calorimetry, dynamic mechanical testing, tensile testing, and the measurement of shape memory effect, water swell, and water vapor permeability were carried out to examine these TPUs. The hydrophilic segment increased the hysterisis in shape memory effect by reducing the crystallinity of the hard segment. The neutralized DMPA unit enhanced the sensitivity of the thermoresponsive water vapor permeability (WVP) by amplifying the increase of WVP at the temperature range above the glass transition temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3009–3017, 2000     

Two-way shape memory behavior of styrene-based bilayer shape memory polymer plate

In this work, a bilayer shape memory polymer (SMP) composite plate with two-way shape memory behavior is simulated, in which two types of styrene-based SMPs with well-separated glass transition temperatures are assembled in parallel. The finite element (FE) software ABAQUS is selected to exhibit the two-way shape memory effect during the shape recovery step and the Generalized Maxwell Model with the WLF equation is applied to characterize the temperature-dependent properties of the SMP bilayer plates. The effect factors of axial predeformation, thermal expansion coefficient and plate thickness are all considered for the two-way shape memory behavior of the styrene-based bilayer SMP plate. After that, a smart gripper composed of four SMP composite plates is proposed to realize grabbing and releasing functions for one-step and staged heating recovery. The FE results provide some necessary theoretical guidelines for future soft smart structural designs and optimization.     

Organic‐montmorillonite modified shape memory epoxy composite

A series of organic‐montmorillonite (OMMT) modified shape memory epoxy (SMEP) composites were prepared for the purpose of application on space deployable structures. Tensile test, dynamic mechanical analysis (DMA), X‐ray diffraction (XRD), scanning electron microscope (SEM), and fold‐deploy shape memory test methods were used to characterize the mechanical, structure, and shape memory properties of these materials. The results showed addition of OMMT could improve the composites' toughness, tensile strength, transition temperature, and shape recovery speed, while shape recovery ratio was unaffected. Composite with 3wt%. OMMT had the optimum combination property. It could fully recover its original shape in about 2 min at 185°C under the maximum bending angle of 180°. Its elongation at break and tensile strength were increased by 835 and 17.4%, respectively, compared to that of neat SMEP. The transition temperature also slightly increased. Copyright © 2010 John Wiley & Sons, Ltd.     

High strength toughened epoxy nanocomposite based on poly(ether sulfone)‐grafted multi‐walled carbon nanotube

Hydroxyl terminated poly(ether sulfone) (PES) has been grafted on multi‐walled carbon nanotube (MWCNT). The grafting reaction was confirmed by different characterization techniques such as Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and transmission electron microscopy. The extent of the grafting was found to be around 58 wt%. Hybrid nanocomposite of epoxy with the modified MWCNT was also prepared. Effect of grafting on the mechanical, thermal, and viscoelastic properties was studied. Dynamic mechanical studies show an increase in the storage modulus for the nanocomposite prepared using PES‐grafted MWCNT compared with neat epoxy system. PES‐grafted MWCNT–epoxy nanocomposite induces a significant increase in both tensile strength (26%) and fracture toughness (125%) of the epoxy matrix. Field emission scanning electron micrographs of fractured surfaces were examined to understand the toughening mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Thermal degradation and flammability behavior of polypropylene/clay/carbon nanotube composite systems

This paper reports on the testing and development of a polypropylene (PP) nanocomposite systems with improved flame retardancy. The work utilizes the unique properties of sepiolite nanoclay (Sep) in combination with carbon nanotubes (CNTs) in order to develop an optimized ternary nanocomposite system. Thermogravimetric analysis (TGA) showed significant improvements to the residual char towards the later stages of the thermal ramp. The pyrolysis combustion flow calorimeter (PCFC) was employed to screen the various PP composites with respect to their potential flammability performance. The heat release capacity, which is an indicator of a materials fire hazard, did not show any reduction with the addition of nanofillers using the apparatus standard testing procedure. However, this changed by switching to a lower burn within the PCFC's furnace; this diverted oxygen to the pyrolysing sample. Using the results gained from TGA and the PCFC, the optimized ternary nanocomposite system (10 wt.% Sep + 2 wt.% CNT) was compounded on a larger scale and tested in the cone calorimeter. This showed a significant reduction of 82% in peak heat release rate in comparison to unfilled PP. Comparisons were also made between the testing of these samples in the PCFC and cone calorimeter. The main objectives were to develop a flame‐retardant PP nanocomposite ternary system as well as assessing the PCFC with traditional techniques. Copyright © 2012 John Wiley & Sons, Ltd.     

Shape memory behavior of carbon nanotube‐reinforced trans‐1,4‐polyisoprene and low‐density polyethylene composites

Shape memory composites of trans‐1,4‐polyisoprene (TPI) and low‐density polyethylene (LDPE) with easily achievable transition temperatures were prepared by a simple physical blending method. Carbon nanotubes (CNTs) were introduced to improve the mechanical properties of the TPI/LDPE composites. The mechanical, cure, thermal, and shape memory properties of the TPI/LDPE/CNTs composites were investigated in this study. In these composites, the cross‐linked network generated in both the TPI and LDPE portions acted as a fixed domain, while the crystalline regions of the TPI and LDPE portions acted as a domain of reversible shape memory behavior. We found that CNTs acted as not only reinforced fillers but also nucleation agents, which improved the crystalline degree of the TPI and LDPE portions of the composites. Compared with the properties at the other CNT doses, the mechanical properties of the TPI/LDPE composites when the CNT dose was 1 phr were improved significantly, showing excellent shape memory properties (R_f = 97.85%, R_r = 95.70%).     

Microstructural design for enhanced mechanical property and shape memory behavior of polyurethane nanocomposites: Role of carbon nanotube,montmorillonite, and their hybrid fillers

The recent rapid development of technology has demanded smart materials with tailoring a bridge between macro properties and sophisticated micro and nano characteristic. Principally, shape memory polymers (SMPs) will come to play as an indispensable part of numerous aspects of human activity. Nevertheless, the low mechanical strength and thermal conductivity of SMPs have primarily restricted their applications. To impart shape memory behaviour and mechanical properties, we fabricated a series of composites by a feasible and commercial melt-mixing method. Thus, a series of fast heat-actuated shape memory polymer composite with greatly enhanced stretch-ability, mechanical stiffness, dynamic-modulus, rheological qualities, recovery and fixity ratio was prepared by incorporating multi-walled carbon nanotubes (CNT), montmorillonite (MMT) and CNT:MMT hybrid into thermoplastic polyurethane (TPU). Noteworthy, CNT-based specimens exhibited superior mechanical properties than those of MMT-based samples, and interestingly, the hybrid composites featured a synergistic effect due to the sacrificial role of MMT nanoplatelets for adjusting the dispersion of CNT nanotubes. Microstructural observations indicated that the crystallization percentages of the composites were generally higher than that of pristine TPU; therefore, the shape-memory performance of the specimens improved notably in the case of the hybrid composites owing to creating more interfacial zone with CNT:MMT nanoparticles as compared to other simple composites. This study proved that the simultaneous incorporation of CNT and MMT nanoparticles not only granted outstanding mechanical properties, but also improved the overall shape memory behaviour of the composites by systematical localization of the nanoparticles without any functionalization or modification.