Shape Memory Polyurethane Composites Research Articles

Polyurethane shape memory filament yarns: Melt spinning, carbon-based reinforcement, and characterization

The aim of this work was to develop and characterize polyurethane-based shape memory polymer filament yarns of a suitable diameter and thermo-mechanical performance for use in tailored multi-sectorial applications. Different polymer compositions – pure shape memory polyurethane and shape memory polyurethane composites with 0.3 and 0.5 wt.% of multi-walled carbon nanotubes or carbon black as additives – were studied. Filaments were obtained using a melt spinning process that allowed the production of the permanent and temporary shape of the shape memory polyurethane filament. Two drawing speeds (20 and 32 m/min) were studied. Characterization techniques such as the tensile test, differential scanning calorimetry, and dynamic mechanical analysis were used to investigate the shape-memory effect of the filaments. Pure and additive shape memory polyurethane filament yarns of a controlled diameter were produced. The results indicated that the pure shape memory polyurethane on the temporary shape had the highest tensile strength (234 MPa). Filaments with carbon black revealed a significant strain (335%) in the permanent shape with respect to the other filaments. The melt spinning process influenced the soft segment glass transition temperature (Tgs) significantly, with a decrease in the temporary shape (first heating) as compared to the permanent shape (second and third heating). However, only the 0.5% multi-walled carbon nanotubes additive clearly influenced the filament, increasing the Tgs by 10°C. The additives also influenced the shape-memory effect, obtaining an increased fixity ratio (up to 97%) with the multi-walled carbon nanotubes additive and an increased recovery ratio (up to 86%) with the carbon black additive.

Electroactive shape memory polyurethane composites reinforced with octadecyl isocyanate-functionalized multi-walled carbon nanotubes.

Shape memory polymers (SMPs) have a wide range of potential applications in many fields. In particular, electrically driven SMPs have attracted increasing attention due to their unique electrical deformation behaviors. Carbon nanotubes (CNTs) are often used as SMP conductive fillers because of their excellent electrical conductivities. However, raw CNTs do not disperse into the polymer matrix well. This strictly limits their use. In this study, to improve their dispersion performance characteristics in the polymer matrix, hydroxylated multi-walled carbon nanotubes (MWCNT-OHs) were functionalized with octadecyl isocyanate (i-MWCNTs). Polyurethane with shape memory properties (SMPU) was synthesized using polycaprolactone diol (PCL-diol), hexamethylene diisocyanate (HDI), and 1,4-butanediol (BDO) at a 1:5:4 ratio. Then, electroactive shape memory composites were developed by blending SMPU with i-MWCNTs to produce SMPU/i-MWCNTs. The functionalized i-MWCNTs exhibited better dispersibility characteristics in organic solvents and SMPU composites than the MWCNT-OHs. The addition of i-MWCNTs reduced the crystallinity of SMPU without affecting the original chemical structure. In addition, the hydrogen bond index and melting temperature of the SMPU soft segment decreased significantly, and the thermal decomposition temperatures of the composites increased. The SMPU/i-MWCNT composites exhibited conductivity when the i-MWCNT content was 0.5 wt%. This conductivity increased with the i-MWCNT content. In addition, when the i-MWCNT content exceeded 1 wt%, the composite temperature could increase beyond 60°C within 140 s and the temporary structure could be restored to its initial state within 120 s using a voltage of 30 eV. Therefore, the functionalized CNTs exhibit excellent potential for use in the development of electroactive shape memory composites, which may be used in flexible electronics and other fields.

Preparation of shape memory polyurethane/modified cellulose nanocrystals composites with balanced comprehensive performances

AbstractIn this work, we first synthesized modified cellulose nanocrystals (MCNC) with 2,4‐toluene diisocyanate and 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide via a two‐step in‐situ polymerization method, and then incorporated it with shape memory polyurethane (SMPU) to fabricate composite. It was found that the urethane chains and phosphorus‐containing flame retardant were covalently grafted onto the cellulose nanocrystals surface, and the MCNC exhibited good dispersion level in the SMPU matrix. Due to the crystallization‐induced and flame retardant effects of the prepared MCNC, it could be acted as a multifunctional reinforcer in SMPU composites. Noticeably, the most balanced improvements in crystallization, shape memory, mechanical, thermal stability, and flame retardancy properties of composites were observed with the incorporation of 3 wt% MCNC. Shape memory experiment demonstrated that the SMPU‐MCNC3 composite exhibited admirable thermal induced shape memory behavior, and its shape fixity and shape recovery ratios were maintained above 90% after five cycles. More importantly, the SMPU‐MCNC3 composite showed well balanced mechanical (σb = 13.1 MPa, εb = 189.3%), limiting oxygen index (23.8%), and UL‐94 rating (V‐2) among the prepared polyurethane films. In addition, the representative SMPU‐MCNC3 showed obviously reductions in heat and smoke production during combustion as compared to the neat SMPU. The present work offers a promising route to fabricate shape memory polyurethane composites with versatile functions.

“Rigid-stretchable” unity of shape memory composites with fluorescence via crystallinity tailoring for anti-counterfeiting application

Strength and stiffness improvement without flexibility loss in shape-memory polymers is particularly significant in creating desirable functionality and broadening practical applications. Based on crystallinity tailoring strategy, a “rigidity-stretchability”-united approach is presented to construct shape-memory polyurethane composites (SMPUs) with specialized assembly for the integrated mechanical robustness and reliable deformability. Cellulose with different geometries were prepared as tailoring agent of crystallinity and structural modifier to assemble dual-networked structures of hybridized ionic- and hydrogen-bonding for the first time. Surface modification and morphology control of these celluloses were developed to manipulate the crystallization behaviors of soft segments in PU. Specifically, functionalized nanocrystal cellulose (f-NCC) with high crystallinity and needle-like geometry promoted a strong heterogeneous nucleation on the PU crystallization (the highest crystallinity, 31.3%) and contributed to 58% and 47% increase in tensile strength (48.9 MPa) and Young's modulus (237.2 MPa) without deteriorating the existent outstanding flexibility (εmax = 756.14%), thereby featuring favorable shape memory and fluorescence. Furthermore, the f-NCC based films showed the highest shape memory properties (Rr = 99.7%, Rf = 99.4%) than functionalized cellulose from cotton pulps and functionalized microcrystal cellulose based ones. The “rigid-stretchable” unity of SMPUs would lay a significant foundation of applications in the fields of larger strain sensors and show great potential in information hiding and storage fields.

NIR-light-induced thermoset shape memory polyurethane composites with self-healing and recyclable functionalities

To endow thermoset shape memory polymers with self-healing and recyclable functionalities is highly desirable. In this study, we fabricated Diels-Alder crosslinked polyurethane/functionalized reduced graphene oxide composites (DAPU-FRGOs) through in situ polymerization. The incorporated FRGO with urethane chains could achieve good compatibility with the DAPU matrix, forming physical and chemical crosslinking structures in the composite. And the most effective improvements of mechanical, thermal stability, crystallization-triggered, and photothermal effect were achieved for the DAPU/FRGO2 composite with 2 wt% FRGO. Owing to the outstanding crystallization-induced and photo-thermal effects of FRGO and the thermal reversible behavior of Diels-Alder bonds, the DAPU-FRGO2 composite showed rapid shape memory (<10 s, shape-fixity and recovery ratios were 92.3% and 94.1%, respectively) and self-healing (<120 s, healing efficiencies for tensile strength and elongation at break were 89.3% and 90.4%, respectively) under near-infrared light. Meanwhile, the DAPU-FRGO2 composite exhibited good repeatability of self-healing and shape-memory functionalities, and its shape memory parameters and healing efficiencies were maintained above 92% and 85% after three-cycles, respectively. Further recycling test demonstrated that the mechanical properties of the recycled DAPU-FRGO2 composite could be recovered to more than 82% of the original sample. This work provides a heuristic perspective for designing durable light-responsive thermoset shape memory composite materials.

Shape memory polymer composites embedded with hybrid ceramic microparticles

Smart polymeric composites have been engineered as advanced materials with superior physical properties while maintaining relatively low manufacturing cost, lightweight, and stimuli-responsive properties. We here report a unique smart composite with shape recoverability and celadon-like color. The samples were prepared by injection molding of in situ polymerized composite pellets, which consist of shape memory polyurethane (SMPU) and hybrid ceramic micro-particulates of silicon carbide and clay. The morphological, rheological, thermomechanical, and shape memory-recovery properties of the SMPU and SMPU composites were investigated. Especially, the resulting hybrid composite of SMPU/SiC/clay shows green celadon-like color, good formability, high thermal stability, and 1.8 times higher tensile strength compared to the pure SMPU. The smart hybrid composite reported here shows the possibility of ceramic-like polymeric materials.

Development of hybrid shape memory polyurethane composites for endovascular applications

Interventional radiology technique (IRT) is being increasingly used to embolize the blood vessels or prevent the flow of blood into the diseased part of a blood vessel in a human body. Shape memory polyurethane (SMPU) has been identified as an alternate embolic agent for IRT, which can overcome vital issues such as non-target embolization, incomplete occlusion, coil migration, and others. However, its inherent lack of radiopacity and bioinertness restrict potential applications of it in IRT. The objective of the present work is to study the effect of combining SMPU and radiopaque filler materials on the mechanical, thermal, shape recovery characteristics, and radiopacity. Hybrid composite materials having Barium sulphate (BaSO4) nanoparticles and hydroxyapatite (HaP) nanoparticles in the SMPU matrix are fabricated using a twin-screw extruder, and their properties are compared with that of the composites containing only single filler. The addition of the filler in SMPU is not found to affect its shape-holding characteristics. However, the shape recovery characteristics of the hybrid composites are decreased by 20–30% during the 1st cycle, which was then recovered to more than 90% during the 2nd recovery cycle. It is observed that the shape recovery rate of the SMPU can be fine-tuned by changing its programming temperature. It is also noted that the tensile strength of composites is decreased with an increase of filler concentration due to their agglomeration and voids in the composites. In case of 10 wt% hybrid composites, the reduction of tensile strength is found to be in the range of 24–28% irrespective of individual filler concentration, and it showed better mechanical properties than individual filler at the same concentration. The radiopacity of the composites is observed to be increased with filler concentration, where BaSO4 played a dominant role in comparison to that of hydroxyapatite nanoparticles. Hence, the developed SMPU hybrid nanocomposites can be explored as embolic agents for IRT due to their suitable mechanical properties, actuation profiles, and enhanced radiopacity.

UV–vis–NIR light-induced bending of shape-memory polyurethane composites doped with azobenzene and upconversion nanoparticles

Recently, shape-memory materials (SMMs) have attracted considerable attention due to their superior properties and potential applications. Up to now, the UV–vis–NIR photochemical response for SMMs still remains a challenge. Here, UV–vis–NIR photo-activated composites are successfully prepared via incorporating an azobenzene compound 4-cyano-4′-pentyloxyazobenzene (5CAZ) and upconversion nanoparticles (UCNPs) into shape-memory polyurethane (SMPU) matrices. The mechanically-stretched composite films exhibit photomechanical bending deformations when exposed to UV/vis/NIR light and shape recovery on heating. Furthermore, the biomimetic finger bending can be successfully achieved via light irradiation on different regions of the film, which would pave a path towards biomimetic actuators and functional devices owing to the unique UV–vis–NIR photochemically responsive and shape-memory properties.

Polydopamine Particle-Filled Shape-Memory Polyurethane Composites with Fast Near-Infrared Light Responsibility.

A new kind of fast near-infrared (NIR) light-responsive shape-memory polymer composites was prepared by introducing polydopamine particles (PDAPs) into commercial shape-memory polyurethane (SMPU). The toughness and strength of the polydopamine-particle-filled polyurethane composites (SMPU-PDAPs) were significantly enhanced with the addition of PDAPs due to the strong interface interaction between PDAPs and polyurethane segments. Owing to the outstanding photothermal effect of PDAPs, the composites exhibit a rapid light-responsive shape-memory process in 60 s with a PDAPs content of 0.01 wt%. Due to the excellent dispersion and convenient preparation method, PDAPs have great potential to be used as high-efficiency and environmentally friendly fillers to obtain novel photoactive functional polymer composites.

The effect of liquid crystal fillers on structure and properties of liquid crystalline shape memory polyurethane composites II: 4-hexadecyloxybenzoic acid

The effect of liquid crystal fillers on structure and properties of liquid crystalline shape memory polyurethane composites II: 4-hexadecyloxybenzoic acid

The impact of liquid crystal fillers on structure and properties of liquid-crystalline shape-memory polyurethane composites I: 4-dodecyloxybenzoic acid

Aiming to elucidate the influence of liquid crystal fillers on the structure and properties of liquid-crystalline (LC) shape-memory polyurethane (SMPU) composites, a series of LC-SMPU composites are successfully prepared by adding various concentrations of 4-dodecyloxybenzoic acid (DOBA) to shape-memory polyurethane containing 40 wt% hard segment content (SMPU40). The structure, morphology, thermal properties, liquid crystalline properties, and shape memory properties are systematically investigated. The results demonstrate that DOBA is successfully incorporated into the polymer matrix of SMPU40 without interrupting the dimerization structure of DOBA that allows it to exhibit LC properties. The incorporated DOBA not only improves the crystallizability of the soft segment but also promotes the crystallizability of the hard segment. The isothermal crystallization kinetics further reveal that the crystallization rate of the hard segment decreases as the DOBA content increases, and the crystallization of the LC-SMPU composites nucleates in a similar manner. The LC-SMPU composites are composed of a DOBA phase and an SMPU phase, which further forms soft phase–hard phase microphase separation structures. The composite tends to only maintain the nematic LC properties of DOBA due to the interruption of polyurethane chains. However, the LC-SMPU composites exhibit good triple-shape memory effects. The first step of strain recovery is associated with the melting transition of soft segment crystals of the polyurethane matrix, and the second step of strain recovery is resulted from the melting transition of the DOBA crystals.

Shape memory-enhanced water sensing of conductive polymer composites

Electrically conductive polymer composites have been widely utilized as the sensory materials being capable of response to various types of stimuli. A shape memory-enhanced water sensing was unprecedentedly reported for polymer composites comprising of carbon nanotubes (CNTs) and shape memory polyurethane (SMPU). The morphological, thermal, mechanical and micro-structural properties of the CNT–SMPU composites were investigated. Two swelling processes were specifically designed to measure the water sensing of the composites. It was found that the shape memory thermo-mechanical programming enlarged magnitudes of the resistivity variation, which may be attributed to the combination of the swelling effect and the re-orientation/local movements of the CNTs along with stress/strain energy release. The findings may greatly benefit the applications of the smart polymers in the fields of sensory materials and flexible electronics.

Chemo-responsive shape memory effect in shape memory polyurethane triggered by inductive release of mechanical energy storage undergoing copper (II) chloride migration

In this study, 10% weight fraction of copper (II) chloride (CuCl2) was embedded into shape memory polyurethane (SMPU) by dissolving it in a solvent mixture of tetrahydrofuran and N,N-dimethyl formamide. It is found that CuCl2 particles migrate; they are released from the polymer in the water-driven shape recovery process of SMPU composites. SMPU composites, after various immersion times in water, were characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. Experimental results support that hydrogen bonding between polyurethane macromolecules and water molecules is the driving force, resulting from the inductive decrease in the glass transition temperature. Furthermore, the release of the stored mechanical energy in SMPU is demonstrated by means of tracking the migration of CuCl2 particles via x-ray diffraction and scanning electron microscopy tests. This study focuses on the mechanism of release of the stored mechanical energy of a polymer, which is identified as the driving force for the chemo-responsive shape memory effect and inductive decrease in glass transition temperature of SMPU in response to the water.

Temperature sensing of conductive shape memory polymer composites

A novel strategy of stimulus sensing is unprecedentedly presented for conductive shape memory polymer composites. Temperature stimulation makes the composites recover not only the shapes but also the conductivity as the composites experience “memorizing” process from the temporary shapes to the original shapes. Silver nanowires (AgNWs) are incorporated into the surface layer of shape memory polyurethane (SMPU). The AgNW–SMPU composites are stretchable and electrically conductive, particularly exhibiting reversible strain-dependent conductivity. The resistance increases and is subsequently stabilized at relatively higher value as the composites are extended and fixed in a typical shape memory programming. Temperature stimulation enables the composites to recover the shapes, along with large decrease in resistance. The findings disclose that the conductive shape memory composites are able to respond to external stimulus by variation of the electrical signals in a desired manner, which may greatly benefit the development of smart polymer materials in flexible electronics and sensors fields.

Multi-stimuli responsive carbon nanotube–shape memory polymeric composites

Multi-walled carbon nanotubes (CNTs) are incorporated into shape memory polyurethane (SMPU) by a transfer method, which not only endow the composites with electrical conductivity but also improve the hydrophilicity due to the polar groups on the surface of the modified CNTs. The morphological and structural properties as well as the water contact angles (WCAs) of the CNT–SMPU composites are investigated by multiple tools. Furthermore the composites exhibit multi-stimuli responsive shape memory behaviors on exposure to water and electrical stimulations, which are studied by combining the experimental measurements and modeling analysis. The findings may greatly benefit extension of the carbon and smart polymer materials in the flexible electronics and sensors fields.

The synergistic effect of the combined thin multi-walled carbon nanotubes and reduced graphene oxides on photothermally actuated shape memory polyurethane composites

We evaluated the synergistic effect of the hybrid-type nanocarbon, consisting of 1D thin-walled carbon nanotubes (TWNTs) and 2D reduced graphene oxide (RGO), on the shape memory performance of hyperbranched polyurethane composites. The shape recovery of the resulting composites was activated via a photothermal process using a near-infrared laser. The best laser-induced shape recovery performance was achieved for the composites with a 7/3 of TWNT/RGO ratio and a 1wt.% of nanocarbon content. Such result can be explained by good dispersion of TWNTs and RGO in the hyperbranched polymer as well as three-dimensionally enhanced interconnection between carbon nanotubes and graphenes. The optically active TWNTs with a high optical absorption section exhibited high ability of transferring laser-induced thermal energy to polymer matrix whereas RGO provided a high mechanical property to polymer matrix. The tensile modulus and electrical conductivity of the composites also showed a similar dependence on the TWNT/RGO composition ratio as the photothermal shape recovery. Our study demonstrated an effective conversion from light, thermal to mechanical work by irradiating shape memory polymer composite containing hybrid-type fillers using a near-infrared laser.

Development of liquid-crystalline shape-memory polyurethane composites based on polyurethane with semi-crystalline reversible phase and hexadecyloxybenzoic acid for self-healing applications

Novel liquid crystalline shape memory polyurethane composites display liquid crystalline properties, triple-shape memory behaviours and self-healing behaviours.

Insights into liquid-crystalline shape-memory polyurethane composites based on an amorphous reversible phase and hexadecyloxybenzoic acid

Liquid-crystalline polymers and shape-memory polymers are attractive to many researchers. This paper proposes a strategy to prepare liquid-crystalline shape-memory polyurethane (LC-SMPU) composites having both liquid-crystalline properties and shape-memory properties. A series of LC-SMPU composites are successfully prepared by doping different concentrations of 4-hexadecyloxybenzoic acid (HOBA) into shape-memory polyurethanes (SMPUs). The results show that the HOBA is physically mixed with the polyurethane matrix. The doped HOBA is categorised as HOBA captured by the polyurethane chains and free HOBA in the LC-SMPU composite. A higher content of HOBA results in a higher fraction of free HOBA, and more HOBA molecules enter into the polyurethane matrix at higher annealing temperatures. The resulting LC-SMPU composites keep their intrinsic liquid-crystalline properties and the crystallisation properties of HOBA. The LC-SMPU composites contain a two-phase, separated structure: a HOBA crystalline phase and the polyurethane matrix. Thus, the LC-SMPU composites show a two-stage decrease in modulus. Finally, shape-memory tests show that the LC-SMPU composites exhibit a triple shape-memory effect; in the first step, strain recovery is associated with the glass transition of the amorphous polyurethane matrix, and in the second step, strain recovery results from the melting transition of the HOBA crystal dopants.

Electro-activated surface micropattern tuning for microinjection molded electrically conductive shape memory polyurethane composites

Shape memory polymers with surface micropatterns have seen rising demand for high value applications such as adjustable adherence surfaces, dynamic micro-geometries for cell culture studies and switchable information carriers. Recently, microinjection molding has emerged as an efficient way to manufacture devices which contain surface micro-features using a wide range of polymers with high accuracy. In this study, shape memory polyurethane–carbon nanotube composites were prepared by twin-screw melt extrusion and subsequently processed using microinjection molding to obtain components with surface micropatterns. Then an electro-activated surface micropattern tuning system was developed which could recover the original micropatterned surface of the components after a thermal deformation by applying a current which heats the component using resistive heating. In order to optimize the technique, three key areas were investigated in this work: conductivity of the microinjection molded microparts, the retention of shape memory micropatterns on the surface of microparts during annealing treatment, and the macroscopic area shrinkage of microparts after thermal treatment. It has been found that the electrical conductivity of microinjection molded parts is relatively low due to the high shear rates prevalent in the process. An annealing treatment improves the electrical conductivity by several orders of magnitude, but can be detrimental to the dimensional stability of the micropatterns, which depends significantly on the micro-injection molding parameters, especially the mold temperature. Increasing the mold temperature, melt temperature, injection speed and injection pressure result in better retention of the micropattern and improved dimension stability during annealing treatment. This work demonstrates the potential of electro-activated surface micropattern control for microinjection molded electrically conductive shape memory polymer composites, which could be a promising technology for a range of application areas including electro-adjustable adherence, information storage, and anti-counterfeiting technology.

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