Influence of heat treatment on the properties of shape memory fibers. I. Crystallinity,hydrogen bonding,and shape memory effect

Shape memory fibers (SMFs) were prepared via melt spinning. The fibers underwent different heat treatments to eliminate internal stress and structure deficiency caused during melt spinning. The influences of heat treatments on the SMF crystallinity, molecular orientation, hydrogen bonding, and shape memory behavior were studied. It was found with increasing heat‐treatment temperature, the soft segment crystallinity, crystallite dimension, and microphase separation increased, and the hydrogen bonding in the hard segment phase increased. Low temperature heat treatments decreased the shape recovery ratios while increasing the shape fixity ratios as a result of internal stress releasing and molecules disorientation. High temperature heat treatments increased the hard segment stability. Increasing heat‐treatment temperature resulted in the improvement of both the shape recovery and fixity, because it promoted the phase separation. The results from DSC, DMA, XRD, and FTIR were used to illustrate the mechanism governing these properties difference. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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     

Nanoclay-tethered shape memory polyurethane nanocomposites

The study investigated shape memory properties of nanoclay-tethered polyurethane nanocomposites. Polyurethanes based on polycaprolactone (PCL) diol, methylene diisocyanate, and butane diol and their nanocomposites of reactive nanoclay were prepared by bulk polymerization in an internal mixer and the values of shape fixity and shape recovery stress were determined as function of clay content. The melting point of the crystalline soft segment was used as the transition temperature to actuate the shape memory actions. It was seen that clay particles exfoliated well in the polymer, decreased the crystallinity of the soft segment phase, and promoted phase mixing between the hard and soft segment phases. Nevertheless, the soft segment crystallinity was enough and in some cases increased due to stretching to exhibit excellent shape fixity and shape recovery ratio. A 20% increase in the magnitude of shape recovery stress was obtained with the addition of 1 wt% nanoclay. The room temperature tensile properties were seen to depend on the competing influence of reduced soft segment crystallinity and the clay content. However, the tensile modulus measured at temperatures above the melting point of the soft segment crystals showed continued increases with clay content.     

Shape‐memory polyurethane/multiwalled carbon nanotube fibers

Shape‐memory polyurethane/multiwalled carbon nanotube (SMP–MWNT) composites with various multiwalled carbon nanotube (MWNT) contents were synthesized, and the corresponding SMP–MWNT fibers were prepared by melt spinning. The influence of the MWNT content on the spinnability, fracture morphology, thermal and mechanical properties, and shape‐memory behavior of the shape‐memory polymer was studied. The spinning ability of SMP–MWNTs decreased significantly with increasing MWNT content. When the MWNT content reached 8.0 wt %, the fibers could not be produced because of the poor rheological properties of the composites. The melt‐blending, extrusion, and melt‐spinning processes for the shape‐memory fiber (SMF), particularly at low MWNT contents, caused the nanotubes to distribute homogeneously and preferentially align along the drawing direction of the SMF. The crystallization in the SMF was promoted at low MWNT contents because it acted as a nucleation agent. At high MWNT contents, however, the crystallization was hindered because the movement of the polyurethane chains was restricted. The homogeneously distributed and aligned MWNTs preserved the SMF with high tenacity and initial modulus. The recovery ratio and recovery force were also improved because the MWNTs helped to store the internal elastic energy during stretching and fixing. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

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     

聚甲醛纤维制备工艺及其性能的研究

以聚甲醛(POM)切片为原料,在200～210℃进行熔融纺丝制得POM长丝。利用DSC、Olympus偏光显微镜、单纱电子强力仪测定了POM纤维结晶度、熔点、取向度和纤维的机械性能;研究了后处理对POM纤维性能的影响以及拉伸对POM纤维耐酸碱性能的影响。结果表明:(1)POM纤维耐碱性良好,拉伸可以提高POM纤维结晶度、取向度、断裂强度以及耐酸性,但是使断裂伸长率减小;最佳拉伸温度在110℃左右,拉伸倍率在6～8之间。(2)热定形温度对POM纤维结晶度和熔点影响不大,延长热定形时间使POM结晶度、熔点降低;最佳热定形条件为在140℃下热定形4min。(3)经过拉伸热定形后的POM纤维的最大断裂强度和断裂伸长率分别为7.41cN/dtex和19.2%。     

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。     

Melt spinning of metallocene catalyzed polypropylenes. II. As‐spun filament structure and properties

The melt spinning of metallocene catalyzed isotactic polypropylene (miPP) resins was investigated. The as‐spun filament properties from six miPP resins were studied with melt flow rates (MFR) between 10 and 100, and a Ziegler–Natta catalyzed isotactic polypropylene (zniPP) resin with a MFR of 35 was studied for a comparison. Generally, as the molecular weight increased the filament density increased, the birefringence decreased, the tensile strength decreased, and the elongation to break increased. As the spinning speed increased, the density, birefringence, tensile strength, and crystalline and noncrystalline orientation functions generally increased. However, the low MFR miPP and the zniPP resin had decreases in the birefringence and tensile strength with an increase of the spinning speed. The miPP resins were found to have breaking tensile strengths up to 50% higher than the zniPP resin at similar spinning speeds. The observed fiber properties were explained based on the nature and orientation of noncrystalline portions of the fibers. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3237–3247, 2001     

Shape memory segmented polyurethanes: dependence of behavior on nanocellulose addition and testing conditions

The response of synthesized shape memory segmented polyurethanes (PUs) was affected by the addition of cellulose nanocrystals, as well as by the various conditions selected to carry out thermomechanical cyclic tests. The PUs were synthesized from an α‐hydro‐ω‐hydroxy‐poly(ethylene oxide), tolylene‐2,4‐diisocyanate and 1,4‐butanediol as chain extender. Nanocomposites were prepared by mixing a suspension of cellulose nanocrystals in N,N‐dimethylformamide with the thermoplastic PU dissolved in the same organic solvent. The thermal properties of the neat PU and resulting composites were examined using differential scanning calorimetry. It was found that cellulose addition increases the PU soft segment melting and crystallization temperatures and the degree of crystallinity of this phase. Shape memory behavior was studied using cyclic thermal tensile tests. Both neat PU and composites exhibit shape memory properties, with fixity and recovery values that depend on heating temperature, imposed deformation, deformation rate and nanofiller addition. Copyright © 2011 Society of Chemical Industry     

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     

Polyurethanes having shape memory effects

Segmented polyurethanes (PUs) were prepared from polycaprolactone diols (PCLs), 4,4′-diphenylmethane diisocyanate, and 1,4-butanediol, and tested for shape memory effects. Effects of soft segment molecular weight (M_n = 2000, 4000 and 8000), soft segment content (50–90%), and maximum strain (_m = 100, 200, and 600%) on the cyclic tensile properties as well as the dynamic mechanical, and mechanical properties below (25°C) and above (65°C) the shape recovery temperatures were studied. With increasing soft segment contents: i) glassy state modulus increased and rubbery state modulus decreased; ii) hardness increased at room temperature, and decreased at 65°C; iii) recovery strain decreased with PCL 2000, and increased with PCL 8000 based PUs. On the other hand, the increase in soft segment length resulted in: i) increased rubbery state modulus as well as glass state modulus; ii) increased hardness at room and high temperatures; iii) increased recovery strain at high soft segment content. Tensile yielding became clear with increasing soft segment length and content. Strain upon cooling and unloading (_u) and residual strain (_p) increased, and recovery strain (_r) decreased with cycling. Among these, residual strain was most sensitive to the cycling. Most of the cycling effects were confined during the first one or two cycles. These results were interpreted in terms of soft segment-hard segment phase separation and soft segment crystallization.     

Influence of Graphene Oxide on Thermally Induced Shape Memory Behavior of PLA/TPU Blends: Correlation with Morphology,Creep Behavior,Crystallinity, and Dynamic Mechanical Properties

In this study, shape memory is thermally induced in a series of graphene oxide (GO) filled poly(lactic acid)/thermoplastic polyurethane (PLA/TPU) blends, prepared via melt mixing process, and their shape recovery and shape fixity are measured, and the results are correlated with morphology, dynamic mechanical properties, crystallinity and creep recovery behavior. Morphological analysis by scanning and transmission electron microscopy reveals that the blends are immiscible, and GO platelets are mainly localized in the TPU phase of the blends, which lead to smaller and more elongated TPU droplets with improved interfacial adhesion being responsible for the improved shape recovery performance compared to the unfilled blend. A systematic enhancement found in storage and Young's modulus, tensile strength, creep resistance and creep recovery, and cold crystallinity as a result of GO inclusion are in agreement with the improved shape recovery, shape fixity and overall shape memory performance of the filled systems. The developed PLA/TPU/GO nanocomposites with highly improved mechanical properties can be utilized as a new class of environmentally friendly shape memory materials for a broad range of applications.     

The Effect of Chain Extender Type on the Physical,Mechanical, and Shape Memory Properties of Poly(ε-Caprolactone)-based Polyurethane-ureas

A series of segmented polyurethane-ureas were synthesized, based on poly(ε-caprolactone), hexamethylenediisocyanate, and different chain extenders. The effect of chain extender on physical, mechanical, and shape memory properties of the polyurethane-ureas was examined at constant hard-to–soft–segment molar ratio. Polyurethane-urea from piperazine showed higher degree of phase separation, mechanical characteristics, shape fixity, and shape recovery temperature than polyurethane-ureas from aliphatic chain extenders. Furthermore, the degree of phase separation was decreased by increasing of chain length of aliphatic chain extenders. This was attributed to the dissimilar compatibility and flexibility of the hard segments, resulting from different chain extenders.     

乙烯基对苯二甲酸类甲壳型液晶高分子的形状记忆性能

通过自由基聚合合成了四种基于乙烯基对苯二甲酸类甲壳型液晶高分子（PBPCS,PMPCS,PDCHVT,PbiPCS）。TGA和DMA表征结果表明,这四种甲壳型液晶高分子均具有较好的热稳定性,其侧基末端基团刚性越大,相对应的片材在30～80℃温度区间刚性越大。形状记忆弯角回复测试表明：四种聚合物热压成型的片材都具有很好的形状记忆固定率,均接近100%。形状记忆回复效果与聚合物侧基的末端基团有关,侧基末端基团分别为对丁氧基苯基和环己基（对应聚合物分别为PBPCS和PDCHVT）时,片材显示出较好的形状记忆性能,形状记忆回复率分别为87%和100%。将PDCHVT通过熔融纺丝制成纤维,采用形状记忆循环DMA测试表征其形状记忆性能,结果表明PDCHVT纤维具有稳定的优异的形状记忆性能。     

Enhanced dynamic mechanical and shape‐memory properties of a poly(ethylene terephthalate)–poly(ethylene glycol) copolymer crosslinked by maleic anhydride

Dimethyl terephthalate (DMT) and ethylene glycol (EG) were used for the preparation of poly(ethylene terephthalate) (PET), and poly(ethylene glycol) (PEG) was added as a soft segment to prepare a PET–PEG copolymer with a shape‐memory function. MWs of the PEG used were 200, 400, 600, and 1000 g/mol, and various molar ratios of EG and PEG were tried. Their tensile and shape‐memory properties were compared at various points. The glass‐transition and melting temperatures of PET–PEG copolymers decreased with increasing PEG molecular weight and content. A tensile test showed that the most ideal mechanical properties were obtained when the molar ratio of EG and PEG was set to 80:20 with 200 g/mol of PEG. The shape memory of the copolymer with maleic anhydride (MAH) as a crosslinking agent was also tested in terms of shape retention and shape recovery rate. The amount of MAH added was between 0.5 and 2.5 mol % with respect to DMT, and tensile properties and shape retention and recovery rate generally improved with increasing MAH. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 27–37, 2002     

Characterization of nanocellulose‐ reinforced shape memory polyurethanes

BACKGROUND: Shape memory polymers are capable of fixing a transient shape and of recovering their original dimensions by the application of an external stimulus. Their major drawback is their low stiffness compared to smart materials based on metals and ceramics. To overcome this disadvantage, nanocellulose was utilized as reinforcement. RESULTS: Composites were prepared by casting stable nanocellulose/segmented polyurethane suspensions. The heat of melting of the polyurethane soft segment phase increased on cellulose addition. Composites showed higher tensile modulus and strength than unfilled films (53% modulus increase at 1 wt% nanocellulose), with higher elongation at break. Creep deformation decreased as cellulose concentration increased (36% decrease in 60‐minute creep by addition of 1 wt% nanocellulose). The nanocomposites displayed shape memory properties equivalent to those of the neat polyurethane, with recoveries of the order of 95% (referred to second and further cycles). CONCLUSIONS: It is possible to markedly improve the rigidity of shape memory polymers by adding small amounts of well‐dispersed nanocellulose. However, this improvement did not have substantial effects on the material shape fixity or recovery. Shape memory behavior seems to continue to be controlled by the polymer properties. Copyright © 2007 Society of Chemical Industry     

Development of shape memory polyurethane based on polyethylene glycol and liquefied 4,4′‐diphenylmethane diisocyanate using a bulk method for biomedical applications

In this study, a series of shape memory polyurethanes (SMPUs) were synthesized successfully by the bulk polymerization method from liquefied 4,4′‐diphenylmethane diisocyanate (L‐MDI), 1,4‐butanediol (BDO) and polyethylene glycol (PEG). The influence of the hard segment content (HSC) on the structure, morphology, properties and biocompatibility of PEG based SMPUs (PEGSMPUs) was carefully investigated. The results show that a microphase separation structure composed of a semicrystalline soft phase and an amorphous hard phase is formed in the PEG6000/L‐MDI/BDO system. Crystallization of the PEG soft segment is influenced by the hard segments. The PEG semicrystalline soft phase serves as a reversible phase while the L‐MDI?BDO hard segment acts as physical netpoints. Finally, a cyclic tensile test shows that all PEGSMPUs have good shape recovery (e.g. above 80%), whereas good shape fixity can only be achieved when the HSC is less than 35 wt%. The Cell Counting Kit 8 assay also demonstrates that only PEGSMPUs containing less than 40 wt% HSC have low cytotoxicity. It is thus concluded that PEGSMPUs bearing both good shape memory effects and good biocompatibility can be used as shape memory materials for biomedical applications when the HSC is less than 35 wt%. © 2014 Society of Chemical Industry     

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 influence of drawing,twisting, heat setting,and untwisting on the structure and mechanical properties of melt-spun high-density polyethylene fiber

The variation of crystalline morphology and mechanical properties of polyethylene fibers was studied as they were sequentially melt spun, drawn, twisted, heat set, and untwisted. Twisting of as-melt spun fibers was also investigated. The morphology was characterized using wideangle x-ray diffraction, small-angle x-ray diffraction, and scanning electron microscopy techniques. Drawing results in high crystalline orientation, fibrillation, and large increases in modulus and tensile strength. Effects due to variation of spinning conditions were noted. Twisting either as-spun or drawn fibers decreased the axial orientation, modulus, tensile strength, and usually also the elongation to break. The changes in these properties increased with twist angle. Twisting also caused transformation of a small fraction of the sample to the monoclinic form of polyethylene. Heat setting caused healing of voids generated during drawing and increased the perfection and periodicity of the stacking of lamellar crystals along the fiber axis. Heat setting also caused the monoclinic polyethylene to transform back to the orthorhombic form, and it increased the modulus and tensile strength. Untwisting returned the orientation in the fiber to essentially that which it would have if it had not been twisted, but untwisting also resulted in the formation of kink bands.     

Structure and properties development in poly(phenylene sulfide) fibers,part I: Effect of material and melt spinning process variables

The detailed research study of manufacturing PPS fibers using melt spinning and further enhancement of tensile properties by drawing and annealing experiments, a study lacking as of today in open scientific literature, was the focus of this research. This article discusses the effect of polymer molecular weight (MW) and melt spinning process variables on the structure and properties development in melt spun fibers manufactured from proprietary Fortron® linear PPS resins. Structure‐properties relationship was studied using several characterization tools like tensile testing, differential scanning calorimetry, polarized light optical microscopy, and wide‐angle x‐ray scattering. Changes in dynamic mechanical behavior of as‐spun fibers manufactured from resins of varying MW and different melt spinning take‐up speeds were also studied. The study showed that by a combination of higher MW of the polymer and spinning at higher take‐up speeds, tensile properties of as‐spun PPS fibers can be improved. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011