Microstructural and mechanical property evolutions of shape memory polyurethane during a thermodynamic cycle

To better understand the shape memory behaviors of synthesized shape memory polyurethane (SMPU) sealant with a tailored transition temperature (T_t) for concrete pavement joints, the thermal and dynamic mechanical properties of SMPU were first characterized to determine the shape memory switching temperature of SMPU. Then the microstructural and mechanical property evolutions of SMPU in the original, programmed, and recovered states during a five‐step thermodynamic cycle were discussed, respectively. The results indicate that the tailored T_t of prepared SMPU can be used as the shape memory switching temperature to match its working temperature. Further, the programming causes the phase separation in SMPU, leading to an obvious anisotropy. The SMPU has satisfactory shape memory performance. The orientation of molecular chains in soft segments is confirmed along the stretching direction. The oriented molecular chains can restore to the naturally curled state during the free recovery. Finally, the programming improves the mechanical properties of SMPU. The recovered SMPU shows a slight decrease in mechanical properties because of the partially impaired crystal structures and broken molecular segments during the programming and recovery. It is concluded that the synthesized SMPU with the specially tailored T_t is suitable to use as a sealant of concrete pavement joints. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45703.     

Dependency of the shape memory properties of a polyurethane upon thermomechanical cyclic conditions

The relationship between the shape memory properties and thermomechanical cyclic conditions was investigated with a type of shape memory polyurethane (SMPU). The thermal and dynamic mechanical properties of the polyurethane were examined by using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). It was found that the SMPU exhibited good shape memory effects (SMEs) at deformation temperatures ranging from T_g to T_g + 25 °C. The strain recovery ratios increased with the increase of deformation speed and with the decrease in maximum strain. The recovery ratios also increased with increasing fixing speed. Therefore, in practical applications, in order to obtain better SMEs, the SMPU should be cooled to its frozen state as soon as possible after being deformed at a high temperature. The ‘fixity’ dramatically increased with the decrease in fixing temperature. To obtain optimal SMEs, the polymer has to be reheated up to the temperature at which the polymer deformed. In addition, the recovery ratios of the SMPU could increase slightly with the increase of recovery time. Copyright © 2004 Society of Chemical Industry     

The effect of soft segment content and linker length on shape recovery and mechanical properties of laterally linked polyurethane copolymer

Lateral flexible linking of shape memory polyurethane (SMPU) by a polyethyleneglycol (PEG) linker through the allophanate linking method was studied, while adjusting the soft segment content and PEG length. The SMPU was composed of 4,4′‐methylenebis(phenylisocyanate) (MDI), poly(tetramethyleneglycol) (PTMG), 1,4‐butanediol (BD), and PEG‐200 as a linker. A second MDI was used to connect the carbamate group of the SMPU chain and PEG. The impact of soft segment content and PEG length on the mechanical properties and shape recovery of two series of SMPU were compared. In the best case, a 545% increase in maximum stress compared to a linear polymer was attained. The flexibly crosslinked SMPUs behave similarly to natural rubber in their stress–strain curve, but their tensile mechanical properties surpassed those of natural rubber. Shape recovery went up to 96%, which is among the best SMPUs tested so far, and shape recovery remained above 90% after four cyclic tests. The extraordinary shape memory results are analyzed and discussed together with DSC and IR data. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Shape Memory Polyurethane as a Drilling Fluid Lost Circulation Material

Drilling fluid loss is a major problem with serious economic and environmental consequences. The use of traditional lost circulation materials (LCMs) to seal wide fractures increases the risk of bit nozzle plugging. In this work, smart LCMs based on shape memory polyurethane (SMPU) are proposed for the first time. SMPU can be programmed to recover at temperatures suited to a given well. As such, SMPU smoothly passes through the bit nozzles, while effectively seal wide fractures once activated. The SMPU is prepared by two step pre-polymerization and characterized by Fourier transform infrared spectra, X-ray diffraction, and differential scanning calorimeter. The SMPU is programmed by changing and fixing the original shape to a temporary shape through a thermo-mechanical process. The shape memory behavior of SMPU is analyzed by tensile apparatus. Compatibility of SMPU with WBMs is determined from mud rheology and filtration tests. Fracture sealing efficiency and shape recovery of SMPU are evaluated by a modified particle permeability apparatus fitted with a model fracture. The results confirm high sealing and shape recovery attributes of SMPU. The plug formed at 114 kg m^–3 SMPU and 80 °C experiences a sealing pressure of 100 bar with 71.5 cm³ cumulative fluid loss.     

Multiple conjugate electrospinning method for the preparation of continuous polyacrylonitrile nanofiber yarn

Continuous polyacrylonitrile nanofiber yarns were fabricated by the homemade multiple conjugate electrospinning apparatus, and the principle of yarn spinning was studied. The effects of the applied voltage, flow rate, spinning distance, and funnel rotary speed on the diameter and mechanical properties of nanofiber yarn were analyzed. The diameter of the nanofibers decreased with increasing applied voltage and the flow rate ratio of the positive and negative needles (F_P/F_N), whereas the diameter of nanofibers increased with increasing overall flow rate and needle distance between the positive and negative. Subsequently, the diameter of the yarns increased first and then decreased with increasing applied voltage, F_P/F_N, and needle distance. However, the diameters of the yarns increased dramatically and then remained stable with increasing overall flow rate. The nanofibers were stably aggregated and continuously bundled and then uniformly twisted into nanofiber yarns at an applied voltage of 20 kV, an overall flow rate of 6.4 mL/h, a needle distance of 18.5 cm, and an F_P/F_N value of 5:3. With increasing funnel rotary speed, the diameters of the nanofibers and yarns decreased, whereas the twist angle of the nanofiber yarns gradually enlarged. Meanwhile, an increase in the twist angle brought about an improvement in the yarn mechanical properties. Nanofiber yarns that prepared showed diameters between 70 and 216 μm. Nanofiber yarns with a twist angle of 65° showed a tensile strength of 50.71 MPa and an elongation of 43.56% at break, respectively. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40137.     

Fabrication of continuous nanofiber yarn using novel multi‐nozzle bubble electrospinning

A novel multi‐nozzle bubble electrospinning apparatus, including spinning unit, metering pump, constant flow pump, metal funnel and yarn winder, was designed for the preparation of continuous twisted polyacrylonitrile nanofiber yarns, and the principle of nanofiber yarn spinning was studied. An innovative spinning unit consisting of nozzle and air chamber was used to improve the production of nanofibers. Double conjugate electrospinning was developed using two pairs of oppositely charged spinning units to neutralize the charges. The effects of applied voltage, air flow rate, overall solution flow rate and funnel rotary speed on the fiber diameter, production rate and mechanical properties of the nanofiber yarns were analyzed. Nanofibers could be aggregated stably and bundled continuously, then twisted into nanofiber yarns uniformly at an applied voltage of 34 kV, air flow rate of 1200 mL min^?1 and overall solution flow rate of 32 mL h^?1. With an increase in the funnel rotary speed, the twist angle of the nanofiber yarns gradually increased when the take‐up speed was constant. The yarn tensile strength and elongation at break showed an increasing trend with increasing twist angle. Nanofiber yarns obtained using this novel method could be produced at a rate from 2.189 to 3.227 g h^?1 with yarn diameters ranging from 200 to 386 µm. Nanofiber yarns with a twist angle of 49.7° showed a tensile strength of 0.592 cN dtex^?1 and an elongation at break of 65.7%. © 2013 Society of Chemical Industry     

Design and development of graphene oxide/shape memory nanocomposite based on hexamethylene diisocyanate mixing segment

Shape memory polymers are remarkable materials renowned for their distinctive ability to fix and recover their original shape in response to specific stimuli. However, the lower shape fixity (SF) of conventional thermally triggered shape memory polyurethane (SMPU) has limited its broad application potential. This study investigates the transformative influence of graphene oxide (GO) nanofillers when incorporated into a linear diisocyanate-based mixing segment SMPU (SMPU-GO). The lower SF issue of SMPU is ingeniously addressed by leveraging the interactive properties of GO with the 4,4′-methylene bis-phenyl diisocyanate hard segment and the introduction of additional physical cross-links via hexamethylene diisocyanate mixing segment. At 1 wt.% GO incorporation, the modulus increased by 178%, an 8% increase in tensile strength, while the elasticity was maintained. Excellent improvement in SF and shape recovery (SR) was attained at 1 wt.% GO incorporated SMPU, and the SMPU nanocomposite showed the highest SF (65%) and SR (100%) at 70°C temperature and 50% strain.     

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

Flexible 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.     

硅烷偶联纳米SiO2改性形状记忆聚氨酯

本文采用经硅烷偶联剂表面处理的纳米SiO2粒子制备了形状记忆聚氨酯／SiO2纳米复合材料。并用SEM、DSC、TGA对其进行分析,考察其形状记忆行为及力学性能。结果表明,形状记忆聚氨酯中加入纳米SiO2后,其热稳定性能提高了,形状回复起始温度(Ti)提高约10℃,形状回复速率提高约3．8倍,而力学性能有所下降。     

液化MDI基形状记忆聚氨酯扩链剂的选择研究

在选定液化MDI和聚己二酸丁二醇酯(PBAG)软段原料的前提下，采用双酚A(FA)、乙二醇(ED)、1，4-丁二醇(BD)、1，6-己二醇(HD)、一缩乙二醇(DE)扩链剂合成了一系列形状记忆聚氨酯(SMPU)；用FT-IR、DSC对样品的结构进行了分析，并考察了它们的形状记忆性能和力学性能。结果发现，扩链剂对SMPU有一定影响，用FA、ED扩链的SMPU具有较好的微相分离，而DE扩链的具有较好的软段结晶性能；FA、HD的SMPU具有较低的形状回复温度和较快的形状回复速率，HD、DE扩链的则具有较好形状固定性能，FA、DE扩链的SMPU循环使用性能较好；同时FA、HD的SMPU具有较高的弹性模量与力学强度。     

Structure changes during tensile deformation and mechanical properties of a twisted carbon nanotube yarn

The small-angle X-ray scattering measurements during tensile deformation have been performed for studying the structure and mechanical property relationships of twisted carbon nanotube (CNT) yarns. The tensile strength distribution and the diameter changes during tensile deformation have also been measured. The orientation distribution of the CNTs in the yarn has been determined and its changes during tensile deformation have been related to the variation of the tensile modulus with the twist angle. The tensile modulus and Poisson’s ratio of the yarns decreased with increasing twist angle, whereas the tensile strength of the yarn showed a maximum at the twist angle of 25°. At this twist angle, the distribution width of the tensile strength was minimum indicating the higher uniformity of the yarn structure.     

Study on the liquefied‐MDI‐based shape memory polyurethanes

A series of liquefied‐MDI‐based polyurethanes having shape memory behavior, with various soft segments, chain extenders, and micro‐phase separation promoters were synthesized. Their morphology and properties were investigated in terms of thermal properties, dynamic mechanical properties, and shape recovery behavior. The results indicate that the soft segment formed with longer chain segment incline to crystallize during the cooling scans and the resulting SMPU have the higher crystallinity. Meanwhile, the chain extenders, which can enhance the polarity of hard segment, incline to have excellent shape memory properties with bigger storage modulus in rubbery state too. It was also found that the micro‐phase separation promoters have great influence on the shape memory behavior due to the enhancement of micro‐phase separation of SMPU. Furthermore, it was proved again that SMPU with longer soft segment and lower hard segment contents usually showed good shape memory behavior. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Effect of cationic group content on shape memory effect in segmented polyurethane cationomer

To illustrate the importance of cationic groups within hard segments on shape memory effect in segmented polyurethane (PU) cationomers, the shape memory polyurethane (SMPU) cationomers composed of poly(ε‐caprolactone) (PCL), 4,4′‐diphenylmethane diisocyanate (MDI), 1,4‐butanediol (BDO), and N‐methyldiethanolamine (NMDA) or N,N‐bis(2‐hydroxyethyl)isonicotinamide (BIN) were synthesized. The comparison of shape memory effect between NMDA series and BIN series was made. The relations between the structure and shape memory effect of the two series of cationomers with various ionic group contents were investigated. It is observed that the stress at 100% elongation is reduced for these two series of PU cationomers with increasing ionic group content. Especially for NMDA series, the stress reduction is more significant. The fixity ratio and recovery ratio of the NMDA series can be improved simultaneously by the insertion of cationic groups within hard segments, but not for the BIN series. Characterizations with DSC and DMA suggest that the crystallibility of soft segment in SMPU cationomers was enhanced by incorporation of ionic groups into hard segments, leading to a relative high degree of soft segment crystallization; compared with the corresponding nonionomers, incorporation of charged ionic groups within hard segments can enhance the cohesion force among hard segments particularly at high ionic group content. This methodology offers good control of the shape memory characteristic in thin films and is believed to be beneficial to the shape memory textile industries. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 545–556, 2007     

Tensile fatigue behavior of polyamide 66 nanofiber yarns

Nanofiber yarns with twisted and continuous structures have potential applications in fabrication of complicated structures such as surgical suture yarns, artificial blood vessels, and tissue scaffolds. The objective of this article is to characterize the tensile fatigue behavior of continuous Polyamide 66 (PA66) nanofiber yarns produced by electrospinning with three different twist levels. Morphology and tensile properties of yarns were obtained under static tensile loading and after fatigue loading. Results showed that tensile properties and yarn diameter were dependent on the twist level. Yarns had nonlinear time‐independent stress–strain behavior under the monotonic loading rates between 10 and 50 mm/min. Applying cyclic loading also positively affected the tensile properties of nanofiber yarns and changed their stress–strain behavior. Fatigue loading increased the crystallinity and alignment of nanofibers within the yarn structure, which could be interpreted as improved tensile strength and elastic modulus. POLYM. ENG. SCI., 55:1805–1811, 2015. © 2014 Society of Plastics Engineers     

A relative study of the effect of static and dynamic vulcanization upon shape memory nature of thermally actuated peroxide crosslinked polyolefinic blends

Thermally actuated peroxide crosslinked shape memory polyolefinic blends of EOC and EPDM has been developed. Effect of static and dynamic vulcanization process on the shape memory character has been pursued in detail. Shape memory properties study at 60°C reveals that statically vulcanized blend and dynamically vulcanized blend are superior in terms of shape fixity character and shape recovery behavior, respectively. Breakage of lower percentage of crystalline structure gives higher percentage of shape fixity for the statically vulcanized blend. In a parallel way, statically vulcanized blend shows better stress–strain properties also. On the other hand, higher degree of crosslinked network structure results higher shape recovery percentage for the dynamically vulcanized blend. Similar to shape recovery characteristics, dynamically vulcanized blend shows better thermal stability and higher percentage of gel content.     

聚已内酯/聚氨酯共聚热致形状记忆材料的合成与表征

以聚己内酯（PCL）为软段,甲苯2,4-二异氰酸酯（TDI）为硬段,通过预聚合和扩链反应制得含结晶软段的聚氨酯形状记忆材料（SMPU）,通过差热分析、红外分析及X射线衍射对其微结构进行了表征。结果表明,随着硬段含量的增加,SMPU的结晶熔融温度上升,结晶度下降,形状回复率减小,形状固定率始终保持在98 %左右;随着PCL相对分子质量的提高,SMPU的结晶熔融温度减小,结晶度增大,回复响应温度逐渐降低。     

Failure mechanism of polyamide 66 nanofiber yarns under fatigue and static tensile loading

This work aims at fractography of polyamide 66 nanofiber yarns. The yarns are produced with three twist levels via electrospinning. In order to study the fracture modes of nanofiber yarns, fatigue, and static tensile tests including monotonic, low cycle fatigue, and postcyclic monotonic tensile tests are performed. It is observed that the catastrophic failure of yarns is associated with axial splitting in the three categories. The nanofibers within the yarn structure show a ductile fracture and buckle after tensile stress release. In comparison of postcyclic monotonic tensile tests with other categories, nanofibers show severe plastic buckling in response to release of the same applied force. Fractography studies reveal that twisting causes construction of a layered structure in the yarns which is similar to the ideal yarn structure as well. Applying cyclic loading causes the separation of these structural layers which is more considerable under higher number of cycles. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41925.     

Morphological and mechanical properties of drawn poly(l‐lactide) electrospun twisted yarns

In this study, the continuous twisted PLLA yarns were produced using an electrospinning device consists of two oppositely charged nozzles. The electrospinning process was performed at different twist rates. The electrospun twisted yarns were drawn at different extension ratios of 50% and 100% and their morphological and mechanical properties of post‐drawn yarns were investigated. The morphological studies at all twist rates shown that uniform and smooth fibers without any bead were formed. Increasing the twist rate up to 240 rpm resulted to a decrease in the average diameter of the fibers in the yarn structure. After uniaxially drawing of the yarns, the average diameter of fibers and thus the yarn diameter decreased. The post‐drawing process enhanced the crystallinity of the fibers in the yarn structure. Furthermore, by increasing the extension ratio, the tensile strength and modulus of yarns increased, while the elongation at break (%) decreased. POLYM. ENG. SCI., 58:1091–1096, 2018. © 2017 Society of Plastics Engineers     

形状记忆聚氨酯的发展现状及应用前景

介绍了形状记忆聚氨酯的合成研究进展、性能和形状记忆原理以及应用状况，并详细介绍了其在纺织和医学方面的应用前景。